HomeMy WebLinkAboutDrainage Policy and Design Standards..
Stormwater Management Plan
for the
City of College Station
Drainage Policy
and
Design Standards
Stormwater Management Plan
for the
City of College Station
Drainage Policy
and
Design Standards
TABLE OF CONTENTS
CHAPTER 13: FLOOD HAZARD PROTECTION
Section 1: Purpose and Authority ............................................................................................................... 13-1
Section 2: Definitions ..................................................................................................................................... 13-2
Section 3: General Provisions and Applicability ..................................................................................... 13-8
Section 4: Administration .............................................................................................................................. 13-9
Section 5: Special Provisions ....................................................................................................................... 13-12
Section 6: Variances ...................................................................................................................................... 13-1 7
Section 7: Enforcement and Penalties ...................................................................................................... 13-19
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CHAPTER13
FLOOD HAZARD PROTECTION
SECTION 1: PURPOSE AND AUTHORITY
A. AUTHORITY
This chapter is adopted under the authority of the constitution and laws of the State of
Texas, including particularly Chapter 231, Acts of the 40th Legislature, Regular Session, as
heretofore or hereafter amended, compiled as Article 974a, V.A.C.S ., the provisions of Sec-
tion 4 of the Municipal Annexation Act as heretofore or hereafter amended, compiled as
Article 970a, V.A.C.S., and in Article 1175, V.A.C .S., as heretofore or hereafter amended,
and pursuant to the provisions of the Charter of the City of College Station, Texas.
(Ordinance No. 1728 of October 22, 1987)
8. PURPOSE
It is the purpose of this chapter to protect, maintain, and enhance the public health, safety,
and general welfare by establishing minimum requirements and procedures to control the
adverse impacts associated with the increased stormwater flows generated by development.
It is also the purpose of this chapter to enhance the public health, safety and welfare by
furthering the goals and objectives of the City of College Station Comprehensive Plan and
all of its elements. The following objectives will minimize public and private losses due to
flooding, erosion, and sedimentation .
(1) To protect human life and health;
(2) To minimize the expenditure of public money for costly flood and erosion control
projects;
(3) To minimize the need for relief and rescue efforts associated with flooding and
generally undertaken at the expense of the general public;
(4) To minimize the damage to public facilities and utilities such as water and sewer
lines, streets, and drainage structures;
(5) To help maintain a stable tax base for the City by providing for the sound use and
development of all areas in such a manner as to minimize future areas of flooding;
(6) To establish review, approval, and permit procedures for the methods of handling,
conveying, and disposing of stormwater flows within the corporate limits and
extraterritorial jurisdiction of the City, and insure the review by the appropriate
authority of the design, analysis, construction and maintenance of all drainage
facilities according to the provisions of this ordinance and the Drainage Policy and
Design Standards;
(7) To restrict or prohibit development which is dangerous to health, safety, or property
during flooding conditions, or causes unacceptable increases in water surface
elevations or velocities;
(8) To require that uses vulnerable to floods, or flooding, including public and private
facilities which serve such uses, be protected against flood damage at the time of
initial construction;
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(9) To provide authoritative guidance in the alteration of any natural stream course,
flood plain, or their associated protective barriers which are involved in the
accommodation of floodwaters ;
(10) To prevent the construction of barriers which will divert stormwater flows and subject
other lands to increased flood hazard ;
(11) To provide authoritative guidance in the modification of ground cover to minimize
erosion and sedimentation;
(12) To ensure that potential property owners are notified if the property is included in an
area of special flood hazard;
(13) To control filling, grading, dredging, and other development which may increase
flood damage.
(Ordinande No. 2277 of November 13, 1997)
SECTION 2: DEFINITIONS
ACCELERATED SOIL EROSION -means the erosion that results from increased slope
and/or exposure of the land surface that occurs as a result of man's activities.
ANTICIPATED DEVELOPMENT -means full potential urbanization of the contributing
watershed in compliance with the Comprehensive Plan and the reasonable assumption that
in considering the effects of a proposed development in a floodplain area that there will be
an equal degree of encroachment extending for a significant reach on both sides of the
stream or water course .
APPEAL -means a request for a review of the Administrator's interpretation of any provi-
sions of this ordinance or a request for a variance.
AREA OF SHALLOW FLOODING -means a designated AO, AH, or VO Zone on a commu-
nity's Flood Insurance Rate Map (FIRM) with a one percent or greater chance of flooding to
an average depth of one to three feet where a clearly defined channel does not exist, where
the path of flooding is unpredictable and where velocity flow may be evident. Such flooding
is characterized by ponding or sheet flow .
AREA OF SPECIAL FLOOD HAZARD -is the land adjacent to a clearty defined channel
within a community subject to a one percent or greater chance of flooding in any given year.
The area may be designated as Zone A on the Flood Hazard Boundary Map (FHBM). After
detailed ratemaking has been completed in preparation for publication of the FIRM, Zone A
usually is refined into Zones A, AE, AH, AO, Al-99, VO , V1-30, VE, or V.
BASE FLOOD -means the flood having a one percent chance of being equalled or
exceeded in any given year ("The 100 Year Flood").
CERTIFICATE OF COMPLIANCE -means a letter signed by the Administrator indicating
compliance with all plans and specifications applicable to the subject project and completion
of all stormwater management and soil erosion protection measures.
CITY -refers to the municipal corporation, City of College Station, Texas .
CITY ATTORNEY -means the person employed as City Attorney of the City of College
Station, Texas.
CITY COUNCIL -shall mean the duly and constitutionally elected governing body of the City
of College Station, Texas.
CITY ENGINEER -means the person employed as City Engineer of the City of College
Station, Texas, or his assign .
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CONSTRUCTION PLANS -means the construction documents required to accompany th e
final plat according to the Subd ivision Control Ord i nance of the City of College Station , or
the building and site plans required for the issuance of a building permit by the City of Co l-
lege Station.
CRITICAL FEATURE -means an integral and readily identifiable part of a flood protection
system, without which the flood protection provided by the entire system would be com-
promised .
DETENTION -the temporary storage and controlled release of stormwater flows .
DETENTION FACILllY -means a permanent facility for the temporary storage of storm -
water flows which is designed so as not to create a permanent pool of water.
(Ordinance No. 1728 of October 22, 1987)
DEVELOPMENT -means any man-made change to improved or unimproved real estate ,
including but not limited to buildings or other structures, mining , dredging , filling, grading ,
paving, excavation, clearing , or drilling operations.
(Ordinance No. 174-0 of February 25, 1988)
DRAINAGE AREA -means that area, measured in a horizontal plane, which contributes
stormwater flows by gravity flow along natural or man-made pathways to a single designated
point along a pathway.
DRAINAGE EASEMENT -means an interest in land granted to the City for maintenance of a
drainage facility, on which certain uses are prohibited; and providing for the entry and
operation of machinery and vehicles for maintenance .
DRAINAGE FACILllY -means any element necessary to convey stormwater flows from its
initial contact with earth to its disposition in an existing watercourse; said drainage facilities
shall consist of both public and private storm sewers (closed conduits), streets, improved
channels constructed in conformity with the adopted Drainage Policy and Design Standards,
unimproved drainageways left in their natural condition , areas covered by drainage
easements for the purpose of providing concentrated or overtand sheet flow, and all
appurtenances to the foregoing , includ ing inlets, manholes , junction boxes, headwalls,
energy dissipaters, culverts, etc.
DRAINAGE RIGHT-OF-WAY-means an area of land dedicated to the City for the purposes
of carrying and containing stormwater flows, constructing drainage facilities, and/or allowing
entry or operation of maintenance equipment.
EARTH CHANGE -means a man-made change in the natural cover or topography of land ,
including cutting or filling activities, which may result in or contribute to soil erosion or
sedimentation.
EASEMENT -means a grant of reservation by the owner of land for the use of such land by
others for a specific purpose or purposes, and which must be included in the conveyance of
land affected by such easement.
(Ordinance No. 1728 of October 22, 1987)
ELEVATED BUILDING -means a non-basement building (i) built, in the case of a building in
Zones Al-A30, AE, A, A99, AO, AH, 8 , C , X and D, to have the top of the elevated floor, or in
the cases of a building in Zones V1-30, VE or V , to have the bottom of the lowest horizontal
structural member of the elevated floor elevated above the ground level by means of pilings,
columns (posts and piers), or shear wall parallel to the flow of the water and (i i) adequately
anchored so as not to impair the structural integrity of the building during a flood of up to the
magnitude of the base flood . In the case of Zones Al-A30, AE, A, A99, AO, AH , B , C , X and
D, "elevated building" also includes a building elevated by means of fill or solid foundation
perimeter walls with openings sufficient to facilitate the unimpeded movement of
floodwaters . In the case of Zones V1-30, VE , or V, "elevated build i ng" also includes a
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building otherwise meeting the definition of "elevated building", even though the lower area
is enclosed by means of breakaway walls if the breakaway walls meet the applicable
standards of the National Flood Insurance Program regulations .
(Ordinance No. 17-40 ot February 25, 1988)
ELEVATION -means the vertical distance from a datum, usually the NGVD, to a point or
object. If the elevation of point A is 802 .46 ft., the point is 802.46 ft ., above some datum .
ENCROACHMENT -means an intrusion, obstruction, or other infringement on an area
reserved for a specific purpose such as an easement or floodway.
EROSION -means the process whereby the surface of the earth is broken up and carried
away by the action of wind , water, gravity, ice, or a combination thereof.
EXCAVATION -means any act by which soil or rock is cut into, dug, quarried, uncovered,
removed , displaced, or relocated purposely by man and shall be taken to include the con-
ditions resulting therefrom .
EXISTING CONSTRUCTION -means structures for which the "start of construction" com-
menced before the effective date of the FIRM. "Existing construction" may also be referred
to as "existing structures".
EXISTING DEVELOPMENT -means any development as defined above which existed or
was permitted prior to the date on which this ordinance became effective.
EXTRA TERRITORIAL JURISDICTION -within the terms of the Texas Municipal Annexation
Act, means the unincorporated area, not a part of any other city, which is contiguous to the
corporate limits of the City of College Station, the outer boundaries of which are measured
from the extremities of the corporate limits of the city, outward for such distances as may be
stipulated in the Texas Municipal Annexation Act, in which area, within the terms of the act,
the City may enjoin the violation of its subdivision control ordinance .
FEDERAL EMERGENCY MANAGEMENT AGENCY (FEMA) -is an agency of the Federal
Insurance Administration which administers the National Flood Insurance Program .
FLOOD OR FLOODING - a temporary rise in the level of water which results in inundation of
areas not ordinarily covered by water from :
(1) the overflow of inland or tidal waters .
(2) the unusual and rapid accumulation or runoff
o f surface waters from any source .
FLOOD FREQUENCY -means the statistically determined average for how often a specific
flood level or discharge may be equalled or exceeded .
FLOOD HAZARD BOUNDARY MAP (FHBM) -means an official map of a community,
issued by the Federal Insurance Administration, where the areas within the boundaries of
special flood hazards have been designated .
FLOOD INSURANCE RATE MAP (FIRM) -means an official map of a community, on which
the Federal Insurance Administration has delineated both the areas of special flood hazards
and the risk premium zones applicable to the community.
FLOOD INSURANCE STUDY -is the official report provided by the Federal Emergency
Management Agency. The report contains flood profiles, the water surface elevation of the
base flood, as well as the Flood Hazard Boundary Floodway Map.
FLOODPLAIN OR FLOOD-PRONE AREA -means any land susceptible to being inundated
by water from any source (see definition of flooding).
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FLOOD PROTECTION SYSTEM -means those physical structural works for which funds
have been authorized , appropriated , and expended and which have been constructed
specifically to modify flood ing in order to reduce the extent of the areas within a community
subject to "special flood hazard" and the extent of the depths of associated flooding . Such
systems typically include hurricane t idal barriers, dams, reservoirs, levees or dikes. These
specialized flood modifying works are those constructed in confonnance with sound engi-
neering standards .
FLOODWAY -means the channel of a riyer or other watercourse and the adjacent land
areas that must be reserved in order to discharge the base flood without cumulatively
increasing the water surface elevation more than one foot.
FLOODWAY FRINGE -means that part of the base floodplain outside the floodway.
FUNCTIONALLY DEPENDENT USE -means a use which cannot perfonn its intended pur-
pose unless it is located or carried out in close proximity to water. The tenn includes only
docking facilities, port facilities that are necessary for the loading or unloading of cargo or
passengers, and ship building and ship repair facilities , but does not include long-tenn stor-
age or related manufacturing facilities .
GRADING -means any act by which soil is cleared , stripped, stockpiled, excavated , scari-
fied , filled or any combination thereof.
HIGHEST ADJACENT GRADE -means the highest natural elevation of the ground surface
prior to construction next to the proposed walls of a structure .
LAND USE -a use of land which may result in an earth change, including , but not limited to ,
subdivision, residential, commercial, industrial, recreational, or other development, private
and public highway, road and street construction, drainage construction, logging operations ,
agricultural practices, oil and gas exploration, exploitation, extraction and mining .
LEVEE -means a man-made structure , usually an earthen embankment, designed and con-
structed in accordance with sound engineering practices to contain , control , or divert the flow
of water so as to provide protection from temporary flooding.
LEVEE SYSTEM -means a flood protection system which consists of a levee, or levees,
and associated structures, such as closure and drainage devices, which are constructed and
operated in accordance with sound engineering practice.
LOT -means a tract or parcel of land which is physically and legally undivided and is shown
on a duly recorded plat.
LOWEST FLOOR -means the lowest floor of the lowest enclosed area (including base-
ment). An unfinished or flood resistant enclosure, usable solely for parking of vehicles,
building access or storage, in an area other than a basement area , is not considered a
building's lowest floor, provided that such enclosure is not built so as to render the structure
in violation of the applicable non-elevation design requirements of this ordinance.
MANUFACTURED HOME -means a structure, transportable in one or more sections, which
is built on a pennanent chassis designed to be used with or without a permanent foundation
when connected to the required utilities. For floodplain management purposes the tenn
"manufactured home" also includes park trailers, travel trailers, and other similar vehicles
placed on a site for greater than 180 consecutive days. For insurance purposes, the tenn
"manufactured home" does not include park trailers, travel trailers, and other similar vehi-
cles.
MANUFACTURED HOME PARK OR SUBDIVISION -means a parcel (or contiguous
parcels) of land divided into two or more manufactured home lots for rent or for sale.
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MEAN SEA LEVEL -means the National Geodetic Vertical Datum (NGVD) of 1929 or other
datum, to which the base flood elevations shown on a community's Flood Insurance Rate
Map are referenced .
NATIONAL GEODETIC VERTICAL DATUM (NGVD) -means the nationwide reference
surface for elevations throughout the United States made available to local surveyors by the
National Geodetic Survey with the establishment of thousands of benchmarks throughout the
continent. It was obtained through a least-squares adjustment in 1929 of all first-order
leveling in the United States and Canada. The adjustment included the 26 tide stations, and
thus referenced the NGVD to MSL.
NATURAL -the cover and topography of land before any manmade changes, or in areas
where there have already been manmade modifications, the state of the area and topogra-
phy of land at the date of the adoption of this ordinance.
NEW CONSTRUCTION -means, for floodplain management purposes, structures for which
the "start of construction" commenced on or after the effective date of a floodplain man-
agement regulation adopted by a community.
PERSON -an individual, firm, corporation, partnership, or association .
PLANNING AND ZONING COMMISSION -means the duly appointed Planning and Zoning
Commission of the City of College Station, Texas.
PLAT -the final plat as described in the Subdivision Control Ordinance of the City of College
Station.
PRIMARY DRAINAGE SYSTEM -means the system of natural watercourses, improved or
channelized watercourses; and all closed conduits, culverts, bridges, detention facilities, and
retention facilities associated with the watercourses; all of which are shown or indicated in
the Drainage Policy and Standards.
REGISTERED PROFESSIONAL ENGINEER - a person duly authorized and licensed under
the provisions of the Texas Engineering Practice Act, to practice the profession of engi-
neering.
RETENTION -means the storage of stormwater flows in a facility which has a permanent
pool of water.
RETENTION FACILITY -means a facility that provides for the storage of stormwater flows
by means of a permanent pool of water or permanent pool in conjunction with a temporary
storage component.
SECONDARY DRAINAGE SYSTEM -means the system of conveyance of rainfall from the
point that it becomes concentrated flow to the point where it reaches the primary drainage
system. This system includes all swales, ditches, minor channels, streets, gutters, inlets, cul-
verts, detention or retention facilities, or other means of conveyance of stormwater flows.
SEDIMENT -means soils or other surficial materials transported or deposited by the action
of wind, ice, or gravity as a product of erosion .
SITE PLAN - a site development plan submitted for approval as described in the Zoning
Ordinance of the City of College Station .
START OF CONSTRUCTION -includes substantial improvement, and means the date the
building permit was issued, provided the actual start of construction, repair, reconstruction,
placement, or other improvement was within 180 days of the permit date. The actual start
means the first placement of permanent construction of a structure on a site, such as the
pouring of a slab or footings, the installation of piles, the construction of columns , or any
work beyond the stage of excavation, or the placement of a manufactured home on a foun-
dation. Permanent construction does not include land preparation, such as clearing, grading,
and filling; nor does it include the installation of streets and/or walkways; nor does it include
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/
excavation for basement, footings, piers, or foundations or the erection of temporary forms ;
nor does it include the installation on the property of accessory building s, such as g a rages or
sheds not occupied as a dwell ing unit and not part of the main structure .
STORMWATER MANAGEMENT -includes all ordinances, standards, plans, and studies to
insure the timely and effective construction of:
(1) a system of vegetative and structural measures that control the increased volume
and rate of surface runoff caused by man-made changes to the land ; and
(2) a system of vegetative, structural , and other measures that reduce or eliminate pol-
lutants that might otherwise be carried by surface runoff.
STRIPPING -any activity which removes or significantly disturbs the vegetative surface
cover, including clearing and grubbing operations .
STRUCTURE -means a walled and roofed building, including a gas or liquid storage tank,
that is principally above ground, as well as a manufactured home.
SUBDIVISION CONTROL ORDINANCE -means that chapter of the Code of Ordinances of
the City of College Station entitled or pertaining to "Subdivisions".
SUBSTANTIAL IMPROVEMENT -means any repair, reconstruction, or improvement of a
structure, the cost of which equals or exceeds 50 percent of the market value of the structure
either: (I) before the improvement or repair is started, or (2) if the structure has been
damaged and is being restored, before the damage occurred. For the purposes of this
definition, "substantial improvement" is considered to occur when the first alteration of any
wall, ceiling , floor, or other structural part of the building commences , whether or not that
alteration affects the external dimensions of the structure. The term does not , however,
include either: (I) any projects for improvement of a structure to comply with existing state or
local health, sanitary, or safety code specifications which are solely necessary to assure safe
living conditions , or (2) any alterations of a structure listed on the National Register of
Historic Places or a State Inventory of Historic Places .
VARIANCE -means the mod ification of the minimum stormwater management requirements
for specific circumstances such that strict adherence to the requirements would : (I) result in
unnecessary hardship and (2) not fulfill the intent of the Ordinance .
VIOLATION -means the failure of a structure or other development to be fully compliant
with the community's floodplain management regulations . A structure or other development
without the elevation certificate, other certifications, or other evidence of compliance re-
quired by this chapter is presumed to be in violation until such time as that documentation is
provided.
WATER SURFACE ELEVATION -means the height, in relation to the National Geodetic
Vertical Datum (NGVD) of 1929 (or other datum if specified), of floods of various magnitudes
and frequencies in the floodplains of coastal or riverine areas .
WATERCOURSE -means any natural or artificial stream, river, creek, ditch, channel, canal ,
conduit , culvert, drain, waterway , gully , ravine , or wash, in and including any area adjacent
thereto which is subject to inundation by reason of overflow of flood water.
ZONING -means a method of land use control requiring the categorization of land use of
every tract of land within a particular jurisdiction according to a zoning ordinance or code and
usually in accordance with a land use plan which is intended to preserve the quality of life
and orderly development of that jurisdiction .
ZONING ORDINANCE -refers to the duly enacted Zoning Ordinance of the City of College
Station, Texas, adopted by reference in the Code of Ordinances.
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SECTION 3: GENERAL PROVISIONS AND APPLICABILITY
A. LANDS TO WHICH THIS CHAPTER APPLIES
This chapter shall apply to all lands within the corporate limits and the extraterritorial juris-
diction of the City of College Station, Texas as adjusted from time to time by the action of
the City Council.
B. ESTABLISHMENT OF CITY DRAINAGE POLICY AND DESIGN STANDARDS
All design, construction plans, as-built construction plans, plats, zoning change requests, site
plans, building permit applications , and all items relating to stormwater flows shall be in
accordance with the approved Drainage Policy and Design Standards. This document shall
be available at the office of the Administrator and shall be maintained by him as a single
document. Proposals for amendment to or revision of the Drainage Policy and Design
Standards shall be submitted to the City Council by the Administrator.
C. ESTABLISHMENT OF DEVELOPMENT PERMIT
Issuance of a duly approved development permit shall be required prior to any development,
as defined herein, to insure conformance to the provisions and requirements of this chapter.
(Ordinance No. 1728 of October 22, 1987
(1) Exception -The following uses shall be exempt from the permitting requirements of
the ordinance but must otherwise meet all of the requirements of the ordinance and
the Drainage Policy and Design Standards:
(a) Customary and incidental grounds maintenance , landscaping, and garden-
ing .
(b) Improvements by a homeowner on property used as their principal residence
where that property lies outside of the designated Area of Special Flood
Hazard .
(c) Uses by a landowner of their property for bona fide agricultural purposes
where that property lies outside of the designated Area of Special Flood
Hazard .
(Ordinance No. 1740 of February 26, 1988)
D. SEVERABILITY
Should any section or provision of this chapter be declared by the courts to be unconstitu-
tional or invalid, such decision shall not affect the validity of the chapter as a whole , or any
part thereof, other than the part so declared to be unconstitutional or invalid.
E. COMPLIANCE
No structure, permanent or otherwise , or land, shall hereafter be located , substantially
improved, platted, replatted, subdivided, have its use changed, or have its drainage pattern
changed, without full compliance with the terms and provisions of this chapter, the Drainage
Policy and Design Standards , and other applicable regulations. The provisions of this
chapter shall apply to and be binding on any and all persons, firms, or corporations who
singly or jointly seek to develop, redevelop, grade, regrade, excavate, landfill , berm , dike , or
otherwise change existing land within the corporate limit of the City, or its extraterritorial ju-
risdiction .
(1) Interpretation. In the interpretation and application of this chapter, all provisions
shall be : (I) cons idered as minimum requirements; (2) liberally constructed in favor
of the governing body, health, and safety, and (3) deemed neither to li mit nor repeal
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any other powers granted under State statutes . The Administrator shall interpret and
apply this chapter. If the Administrator determines that the meaning of a word ,
provision, or requirement is unclear, or that its application to a particular
circumstance is uncertain, or upon written request for an interpretation by any in-
terested party, the Administrator shall prepare a written interpretation or determina-
tion, setting forth the reasons, explanations, and conclusions regarding the inter-
pretation . Appeals of interpretations made under this section shall be made in
accordance with Section 4 of this chapter.
(2) Abrogation and Greater Restriction . This chapter is not intended to repeal, abrogate,
or impair any existing easements, covenants, or deed restrictions; however, where
this chapter and another conflict, or overlap, whichever imposes the more stringent
restrictions, under the interpretation of the Administrator, and subject to the provi-
sions of interpretation defined above, shall apply. ·
(3) Warning and Disclaimer of Liability. The degree of flood protection accorded by the
requirements of this chapter is considered reasonable for regulatory purposes and is
based on scientific and engineering considerations which are consistent with
accepted engineering principles and practice. Floods from stormwater flows can and
will occur which are greater than those indicated or referenced under this chapter,
and water surface elevations may be increased by man-made or natural causes
beyond the scope of this chapter. This chapter does not imply that land outside the
areas of special flood hazard, or uses permitted within such areas will be free or
protected from flooding or flood damages. This chapter shall not be construed as
warranting any such protection or freedom. This chapter shall not create liability on
the part of the City, the Administrator of this chapter, or any office or employee of
the City, or for any flood damages resulting from reliance on or compliance with the
provisions and requirements of this chapter or any administrative decision lawfully
made thereunder.
(4) Exception. Any development in an approval process prior to adoption of this chapter
shall be excepted from the provisions and requirements of this chapter; however,
said development shall be required to comply with all applicable provisions and
requirements, and all applicable ordinances and regulations in effect prior to
adoption of this chapter. Developments shall be considered in an approval process
given one or more of the following conditions apply:
(a) An acceptable application for building permit has been filed with all
requirements for that application having been met within 90 days of the date
of adoption of this ordinance;
(b) An acceptable final plat, with all construction drawings, has been submitted,
subject to the requirements of the Subdivision Control Ordinance, within 180
days of the date of adoption of this ordinance;
{c) An acceptable site plan has been submitted, subject to the requirements of
the Zoning Ordinance, within 90 days of the date of adoption of this ordi -
nance.
SECTION 4: ADMINISTRATION
A. DESIGNATION OF ADMINISTRATOR
The City Engineer shall implement, administer, and oversee the provisions, terms, condi-
tions and requirements of this chapter and shall maintain as his guideline for administration
the purposes of this chapter.
8 . DUTIES AND RESPONSIBILITIES OF THE ADMINISTRATOR
The duties and responsibilities of the Administrator shall include but not be limited to the
following :
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(1) maintain and hold open for public inspection all records pertaining to the provisions
of this chapter;
(2) review, and approve or deny all applications for development permits. as required by
this chapter, for compliance with the terms and provisions of this chapter;
(3) assure that adequate inspection of construction permitted under the terms and pro-
visions of this chapter are carried out in accordance with the permitted plan ;
(4) maintain, update, and provide to interested parties at a reasonable cost the effective
City Drainage Policy and Design Standards;
(Ordinance No. 1728 of October 22, 1987)
(5) assure that adequate maintenance of drainage pathways, including altered or relo-
cated waterways, is provided such that capacity for carrying stormwater flows is
maintained;
(Ordinance No. 1740 of February 25, 1988)
(6) provide interpretation, where required, of boundaries of Areas of Special Flood
Hazard, location of floodway, and water surface elevations, when disputes arise
during review. (The initial determination of the above is the responsibility of the
engineer working on the particular project .)
[T) provide information to the Zoning Board of Adjustments , Municipal Court, or City
Council, as applicable on all variance requests, administrative appeals , enforcement
actions, and proposed amendments to the Drainage Policy and Design Standards as
required;
(8) review and utilize any acceptable new flood study data in accordance with the
Drainage Policy and Design Standards ;
(Ordinance No. 1728 of October 22, 1987)
(9) notify adjacent communities and the Texas Water Commission prior to any alteration
or relocation of a watercourse , and submit evidence of notification to the Federal
Insurance Administration ;
(Ordinance No. 1740 of February 25 , 1988)
(10) interpret the terms and provisions of this chapter, as required, as they apply to each
project, in accordance with the stated purpose of this chapter; and
(11) review permits for proposed development to assure that all necessary permits have
been obtained from those Federal, State , or local governmental agencies (including
Section 404 of the Federal Water Pollution Control Act Amendments of 1972, 33
U.S.C . 1334) from which prior approval is required.
C. ADMINISTRATIVE APPEALS
All rulings, requirements, and interpretations of the Administrator shall be final and binding
on the parties thereto unless said rulings, requirements, and interpretations are appealed to
the Municipal Court. Any person aggrieved thereby shall submit a formal appeal by filing a
written notice of appeal with the Administrator within ten working days of the action
prompting the appeal. The notice of appeal shall specify all rulings being appealed , and
shall include the following :
(1) specific sections of this chapter related to the appealed ruling or pertinent thereto ;
(2) a brief summary of all facts material to the appeal ;
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(3) grounds for the appeal.
A hearing by the Municipal Court on the appeal shall take place at a regularly scheduled
court meeting .
The ruling of the Municipal Court shall be finally binding upon all parties involved . Any party
aggrieved by said final ruling may seek relief in a court of competent jurisdiction. The
appeal of a ruling or requirement shall stay the enforcement of said ruling or requirement
unless the Administrator certifies to the Court that by reason of fact a stay would , in his opin-
ion, cause an immediate public hazard or impair life or property. In such case enforcement
shall not be stayed other than by a restraining order from a court of competent jurisdiction .
D . PERMIT PROCEDURES
Prior to the issuance of a development permit, the following requirements shall be met:
(1) A preliminary conference shall be held with the Administrator or his designated rep-
resentative .
(2) An application for a Development Permit shall be presented to the Administrator, on
forms available in his office, and shall include one or more of the following: con-
struction plans, drainage plan, erosion and sediment control plan , engineering
drainage design and analysis documentation, drainage report, and location of the
foregoing in relation to Areas of Special Flood Hazard; each of which shall conform
to the City Drainage Policy and Design Standards.
(3) The Administrator shall review the required information and application form and
shall take one of the following actions:
(a) approve the development permit,
(b) disapprove the development permit,
(c) require additional information or an engineering conference with the appli-
cant or his engineer.
The applicant shall be notified in writing of the action prescribed above . If the
development permit has been disapproved, the specific reasons for disapproval shall
be indicated in the notification. If additional information is required of the applicant,
the specific requirements shall be indicated in the notification. A final determination
of the approval or disapproval of the development permit, considering the additional
information, shall be made and written notification to the applicant given within ten
working days after receipt of said additional information.
(Ordinance No. 1728 of October 22, 1987)
(4) Any proposal which alters the floodways of the following special drainage areas:
• The entirety of Carter's Creek,
• The main channel and south fork of Lick Creek,
• Wolf Pen Creek from SH 6 By-Pass to the confluence
with Carter's Creek and The Brazos River
shall receive written notice of approval or disapproval of the development permit
from the Floodplain Administrator within sixty (60) working days after receipt of the
proposal.
(Ordinance No. 2277 of November 13, 1997)
13-11
Rev. 12197
E. CONDITIONS OF APPROVAL
Approval or denial of a development pennit by the Administrator shall be based on all of the
provisions of this chapter and the following relevant factors :
(1) The danger to life or property due to flooding or erosion damage;
(2) The susceptibility of the proposed facility and its contents to flood damage and the
effect of such damage on the individual owner;
(3) The danger that materials may be swept onto other lands to the injury of others;
(4) The compatibility of the proposed use with existing and anticipated development;
(5) The costs of providing governmental services during and after flood conditions ,
including maintenance and repair of streets and bridges, and public utilities and
facilities such as sewer, gas, electrical, and water systems .
(6) The expected heights, velocity, duration, rate of rise and sediment transport of the
flood waters and the effects of wave action, if applicable, expected at the site;
[!) The necessity to the facility of a waterfront location, where applicable ;
(8) The availability of alternative locations, not subject to flooding or erosion damage ,
for the proposed use;
(9) The relationship of the proposed use to the comprehensive site plan for that area .
SECTION 5: SPECIAL PROVISIONS
A. NATIONAL FLOOD INSURANCE PROGRAM
The National Flood Insurance Program (NFIP) is a federal program enabling property owners
to purchase flood insurance. This program is based on an agreement between local
communities and the federal government that if a community will implement programs to
reduce future flood damages, the federal government will make flood insurance available
within the community as a financial protection against flood losses . The United States
Congress established the NFIP with the National Flood Insurance Act of 1968 and later
modified and broadened the program. The NFIP is administered by the Federal Emergency
Management Agency (FEMA). The following provisions are in accordance with the
requirements for participation in the NFIP.
B. ESTABLISHMENT OF AREAS OF SPECIAL FLOOD HAZARD
The basis for identification of the Areas of Special Flood Hazard shall be the most current
engineering report entitled "Flood Insurance Study for the City of College Station , Texas" as
accepted by FEMA and the Federal Insurance Administration, including the accompanying
Flood Insurance Rate Maps and Flood Hazard Boundary Maps showing the floodway. The
current report is dated January 2, 1981; however, any revisions, amendments, or updated
studies accepted by FEMA and the Flood Insurance Administration shall be adopted by ref-
erence and are hereby declared to be a part of this chapter. The most current Flood
Insurance Study shall be used for all detenninations of flood insurance rates, floodways, and
additional revisions as they become available . The current Flood Insurance Study, FIRM,
FHBM, along with any revisions or amendments shall be maintained and made available by
the designated Administrator of the Stonnwater Management Program .
C . REVISION OR AMENDMENT OF FLOOD INSURANCE STUDY
Any revision or amendment to the Flood Insurance Study which is requested by a land owner
in the City shall be submitted to the designated Administrator of the Stormwater
13-12
Rev. 12197
Management Program in accordance with the requirements set forth in the Drainage Policy
and Design Standards. All requests for map amendment or map revision must be approved
by the Administrator in writing prior to their submission to FEMA. If modification of any
watercourse is involved, an effective Conditional Letter of Map Amendment shall be on file
with the Administrator prior to any development. All submittals to FEMA shall be made at no
cost to the City.
D . METHODS OF REDUCING FLOOD DAMAGE
In order that the purposes of this chapter shall be accomplished in accordance with Section
1, the following methods, measures, requirements, and practices may be utilized by the
Administrator in accordance with the Drainage Policy and Design Standards:
(1) Limit peak stonnwater flows after development to that which existed before devel-
opment;
(2) Limit, control, or prevent changes in the path of stonnwater flows across or away
from a site or development;
(3) Limit, control, or prevent alterations to existing watercourses and drainage facilities
either inside or outside existing Areas of Special Flood Hazard;
(4) Limit, control, or prevent the alteration of natural or developed Areas of Special
Flood Hazard, channels, or protective barriers which are necessary to accommodate
flood waters;
(5) Limit, control, or prevent the use of existing or proposed drainage easements such
that the easement remains useful for its intended purpose;
(6) Limit, control, or prevent the construction of barriers which may increase flood haz-
ards to other lands or facilities;
[!) Establish or cause to be established adequate drainage easements and/or rights of
way to control development and limit potential flood damage and protect existing or
proposed drainage facilities;
(8) Limit, control, or prevent dumping of refuse, fill, garbage, grass clippings, brush,
waste concrete, or other materials in existing drainage facilities including swales,
streets, inlets, ditches, stonn sewers, culverts, etc.;
(9) Limit, control, or prevent filling, grading, clearing, dredging, paving, benning, or
other earthwork which may increase stonnwater flows, change drainage pathways,
increase erosion and sedimentation, or increase flood hazard or damage from
flooding;
(10) Limit, control, or prevent development which is dangerous to health, safety, or
property by causing increases in water surface elevations, velocities, or flowrates
which exist prior to such development;
(11) Limit, control, or prevent development which is vulnerable to flood damage or
require that said development shall be adequately protected against said flood
damage at the time of construction;
(12) Require adequate maintenance of drainage facilities such that they retain their
capacity for conveyance of stonnwater flows .
E. SPECIAL PROVISIONS FOR AREAS OF SPECIAL FLOOD HAZARD
In all areas of Special Flood Hazard where base flood elevations have been determined in
accordance with this chapter, the following provisions shall apply:
13-13
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(1)
(2)
(3)
(4)
(5)
(6)
CJ)
(8)
All new construction, any substantial improvement to a structure, and appurtenances
shall be securely anchored to prevent flotation , collapse or lateral movement ;
All new construction , any substantial improvement to a structure, and appurtenances
shall be constructed in such a manner as to minimize flood damage ; and, all
electrical, heating, ventilation, plumbing, and air conditioning equipment and other
seivice facilities shall be designed and/or located so as to prevent water from
entering or accumulating within the components during conditions of flooding ;
New and replacement sanitary sewage systems, including but not limited to septic
tanks and drain fields, package treatment plants, etc., shall be designed to minimize
or eliminate infiltration of flood waters into the system and discharges from the
system into flood waters ;
New and replacement water supply systems including wells, treatment plants , dis-
tribution facilities, etc ., shall be designed to prevent infiltration of flood waters into
the system;
Solid or liquid waste disposal sites or systems shall be designed and located to avoid
contamination from them during flooding and to avoid impairment of their operation
during times of flooding;
All new construction or any substantial improvement of any residential structure shall
have the lowest floor, including any basement, at an elevation at least one foot
above the base flood elevation, and certification that this standard has been satisfied
shall be submitted to the Administrator, said certification shall bear the dated seal
and signature of a registered professional engineer or registered public suiveyor on
the form provided by the Administrator;
All new construction or any substantial improvement of any commercial , industrial ,
or other non-residential structure shall either have the lowest floor, including base-
ments, elevated at least one foot above the base flood elevation or the structure with
its attendant utility and sanitary facilities shall be floodproofed so that the structure
and utility and sanitary facilities shall be watertight and impermeable to the intrusion
of water in all areas below the base flood elevation, and shall resist the structural
loads and buoyancy effects from the hydrostatic and hydrodynamic conditions.
Certification that this standard has been satisfied shall bear the dated seal and sig-
nature of a registered professional engineer on the form provided by the
Administrator;
For all new construction and substantial improvements, fully enclosed areas below
the lowest floor that are subject to flooding shall be designed to automatically
equalize hydrostatic flood forces on exterior walls by allowing for the entry and exit
of floodwaters. Designs for meeting this requirement must either be certified by a
registered professional engineer or architect or must meet or exceed the following
minimum criteria: A minimum of two openings having a total net area of not less
than one square inch for every square foot of enclosed area subject to flooding shall
be provided. The bottom of all openings shall be no higher than one foot above
grade . Openings may be equipped with screens, louvers, or other coverings or
devices provided that they permit the automatic entry and exit of floodwaters ;
(Ordinance No. 1728 of October 22, 1987)
(9) In areas of special flood hazard where base flood elevations ~~v~ not been estab-
lished base flood elevation data shall be generated for subd1v1s1on proposals and
other proposed development, including ma~ufactur~d home parks and subdivisions
which are greater than 50 lots or 5 acres, whichever 1s less.
(Ordinance No. 17.W of February 25, 1988)
13-14
R ev . 12197
F. SPECIAL PROVISIONS FOR MANUFACTURED HOMES IN AREAS OF
SPECIAL FLOOD HAZARD
The following provisions are required in all Areas of Special Flood Hazard where base flood
elevations have been determined:
(1) No manufactured home shall be placed in a floodway;
(2) All manufactured homes shall be anchored to resist flotation, collapse, or lateral
movement and shall meet the following requirements:
(a) over-the-top ties shall be provided at each of the four comers of the manu-
factured homes:
(b) on manufactured homes of 50 feet in length or less, one additional over-the-
top tie shall be provided approximately at the mid point;
(c) on manufactured homes of over 50 feet in length, two additional over-the-
top ties shall be provided at intermediate locations;
(d) frame ties shall be provided at each of the four comers of the manufactured
home ;
(e) on manufactured homes of 50 feet in length or less, four additional frame
ties shall be provided at intermediate locations;
(f) on manufactured homes of over 50 feet in length, five additional frame ties
shall be provided at intermediate locations;
(g) all components of the anchoring system for manufactured homes shall be
capable of carrying a force of 4800 pounds without sustaining permanent
damage.
(3) For new manufactured home developments; expansions to existing manufactured
home developments; existing manufactured home developments where repair,
reconstruction, or improvement of the streets, utilities, or building pads in which the
cost of repair, reconstruction, or improvement exceeds half of the value of the
streets, utilities, and building pads before the repair, etc. has commenced; and for
manufactured homes not placed in a manufactured home development, the follow-
ing are required:
Rev. 12197
(a)
(b)
(c)
stands or lots shall be elevated on compacted fill or on pilings such that the
lowest floor of the manufactured home will be above the base flood eleva-
tion and the elevation of the center of the stand shall be no more than one
foot below the base flood elevation. Certification that this standard has been
satisfied shall be submitted to the Administrator; said certification shall bear
the dated seal and signature of a registered professional engineer or
registered public surveyor on the form provided by the Administrator.
adequate surface drainage and access for a hauler shall be provided.
if a manufactured home is elevated on pilings:
(i)
(ii)
(iii)
lots shall be large enough to permit steps;
piling foundations shall be placed in stable soil no more than ten feet
apart;
reinforcement shall be provided for pilings more than six feet above
the existing or finished ground level.
13-15
G . SPECIAL PROVISIONS FOR FLOODWAYS
Located within Areas of Special Flood Hazard establ ished in Section 5-B are areas desig-
nated as floodways . The floodway is an extremely hazardous area due to the velocity of
flood waters which carry debris, potential projectiles, and the potential for erosion; therefore,
the following provisions shall be required:
(1) Encroachments shall be prohibited, including fill, new construction , substantial
improvements of existing construction, structures, manufactured homes, or other
development. Variances requested on this standard shall be accompanied by a
complete engineering report fully demonstrating that the encroachmerits shall not
result in any increase in water surface elevation or flood hazard upstream , within, or
downstream of the encroachment location . The engineering report shall conform to
the requirements of the Drainage Policy and Design Standards and shall bear the
dated seal and signature of a registered professional engineer;
(Ordinance No. 1728 of October 22, 1987)
(2) Exemptions for the requirements of Section 5-G may be made in the following
cases :
(a) Customary and incidental routine grounds maintenance, landscaping and
home gardening which does not require a building permit, zone change
request, or variance from the provisions of the Zoning Ordinance;
(b) Emergency repairs of a temporary nature made on public or private property
which are deemed necessary for the preservation of life, health, or property,
and which are made under such circumstances where it would be impossible
or impracticable to obtain a development permit.
(c) Temporary excavation for the purpose of maintaining, or repairing any public
street, public utility facility, or any service lines related thereto;
(Ordinance No. 1740 of February 25 , 1988)
(3) All new construction or substantial improvements of existing construction shalf be
subject to the methods of flood hazard reduction outlined in Section 5.
(Ordinance No. 1728 of October 22, 1987)
(4) When a regulatory floodway has not been designated, no new construction, sub-
stantial improvements, or other development, including fill, shall be permitted within
zones designated Al-A30 and AE on the community's FIRM, unless it is demon-
strated that the cumulative effect of the proposed development, when combined with
all other existing and anticipated development, will not increase the water surface
elevation of the base flood more than one foot at any point within the community.
(Ordinance No. 1740 of February 26, 1988)
H. SPECIAL PROVISIONS FOR AREAS OF SHALLOW FLOODING
Located within the Areas of Special Flood Hazard established in Section 5-B are areas des-
ignated as Areas of Shallow Flooding . These areas have special flood hazards associated
with base flood depths of 1 to 3 feet where a clearly defined channel does not exist and
where the pathway of flood waters is indeterminate and unpredictable ; therefore, the
following provisions shall be required :
(Ordi nance No. 1728 of October 22, 1987)
13-16
Rev. 12197
(1) All new construction or any substantial improvement of any residential structure shall
have the lowest floor, includ ing basements, elevated at least one foot above th e
depth number specified on the community's FIRM .
(Ordinance No. 174-0 of February 25, 1988)
(2) All new construction or any substantial improvement of any commercial, industrial ,
or other non-residential structure shall either:
(Ordinance No. 1728 of October 22, 1987)
(a) Have the lowest floor, including basements, elevated at least one foot above
the depth number specified on the community's FIRM; or
(Ordinance No. 174-0 of February 25, 1988)
(b) the structure with its attendant utility and sanitary facilities shall be flood-
proofed so that the structure and utility and sanitary facilities shall be
watertight and impermeable to the intrusion of water in all areas below the
base flood elevation, and shall resist the structural loads and buoyancy ef-
fects from the hydrostatic and hydrodynamic conditions. Certification that
this standard has been satisfied shall bear the dated seal and signature of a
registered professional engineer on the form provided by the Administrator.
(Ordinance No. 1728 of October 22, 1987)
(3) Adequate drainage paths to guide floodwaters around and away from proposed
structures shall be provided for all proposed structures on slopes in Zones AH or AO.
(Ordinance No. 17 4-0 of February 25, 1988)
SECTION 6: VARIANCES
A. GENERAL
The Zoning Board of Adjustments may authorize a variance to the provisions and
requirements of this chapter when, in their opinion, undue hardship on the owner will
result from strict compliance with those requirements, and when either of the
following criteria are met:
(1)
(2)
(3)
There are special circumstances or condition affecting the land involved
such that strict compliance with the provisions and requirements of this
chapter will deprive the applicant of the reasonable use of his land; or,
The variance is necessary for the preservation and enjoyment of a
substantial property right of the applicant; or,
Variances may be issued for the reconstruction, rehabilitation, or restoration
of structures listed on the National Register of Historic Places or the State
Inventory of Historic Places, without regard to the procedures set forth in the
remainder of this section .
(Ordinance No. 2277 of November 13, 1997)
B . PREREQUISITES FOR GRANTING OF VARIANCES
Upon consideration of the factors noted above , the specific provisions. and requi~ements of
th is chapter, and the intent of this chapter, the Board may grant vanances subject to the
following prerequisites:
13-17
Rev . 12197
C.
(1) A variance shall only be granted upon a determination that the variance is the
minimum necessary, consid ering the flood hazard, or pot ential fl ood damage, to
afford relief to the applicant ;
(2) The effect of the variance will not be detrimental to the public health , safety, or
welfare, or injurious to other property in the City;
(3) The effect of the variance will not increase water surface elevations, flow velocities ,
or alter drainage pathways to the extent that there will be any threat to public safety,
extraordinary public expense, increase in nuisance flooding, or be detrimental to
other porti"ons of the major or minor drainage systems ;
(4) The effect of the variance will not prevent the orderly subdivision of other land,
upstream or downstream of the subject property, in the City, and;
(5) No variance shall be allowed within any designated floodway if any increase in water
surface elevation would occur during the base flood discharge .
(6) Variances may be issued for new construction and substantial improvements and for
other development necessary for the conduct of a functionally dependent use pro-
vided that (i) the criteria and procedures outlined in this Section for obtaining a
variance are met, and (ii) the structure or other development is protected by meth-
ods that minimize flood damage during the base flood and create no additional
threats to public safety .
VARIANCE PROCEDURES
The granting of variances shall be subject to and in conformance with the following proce -
dures and requirements :
(1) The Zoning Board of Adjustments shall hear and render judgment on any requests
for variances from the requirements of this chapter.
(2) The Administrator shall maintain a record of all actions involving appeals, and vari-
ance requests, and shall report all variances to the requirements of this chapter to
the Federal Insurance Administration upon request.
(3) Upon consideration of the above criteria, and prerequisites, and the intent of this
chapter, the Board may attach such conditions to the granting of any variance as it
deems necessary to further the purpose and objectives of this chapter.
(4) All requests for variance shall be judged solely on the merits and special conditions
of the case. The granting of a variance based upon the facts and evidence of one
request for variance shall have no bearing on the consideration of any other request
for variance.
(5) Before consideration by the Board, a formal request for variance shall be submitted
to the office of the Administrator. The request shall contain sufficient information to:
(a) define specifically which provision or requirement according to this chapter
that allegedly causes the undue hardship;
(b) detail specifically what measures shall be taken to obtain the minimum relief
from said hardship;
(c) define specifically the effects of the variance in terms of water surface
elevations, flood velocities, etc . which pertain to the prerequisites required
by this chapter; and
(d) identify any special conditions which should be considered as criteria for
granting said variance .
13 -18
Rev. 12197
,,
After review of the request, the Administrator may require additional information
which he/she deems necessary to fully document the prerequisites required by this
chapter for the granting of a variance . This information shall be provided by the
applicant prior to placement of the variance request on the agenda for consideration
by the Board.
(6) Any applicant to whom a variance is granted shall be given written notice that the
structure will be permitted to be built with a lowest floor elevation no more than 3
feet below the base flood elevation, and that the cost of flood insurance will be
commensurate with the increased risk resulting from the reduced lowest floor ele-
vation.
(1) Any person or persons aggrieved by the decision of the Zoning Board of Adjust-
ments may appeal such decision to the courts of competent jurisdiction.
SECTION 7: ENFORCEMENT AND PENAL TIES
A. NOTIFICATION OF NONCOMPLIANCE
If at any time development, as defined by this chapter, takes place for which an approved
development permit has not been issued, or development occurs which does not conform to
the plans and specifications upon which the issued development permit was based, the
Administrator shall issue a written Notice of Noncompliance to the owner. The notice shall
include a description of the nature and location of each alleged violation of this chapter, and,
if readily apparent, the measures required to bring the development into compliance with the
provisions and requirements of this chapter. The notice shall give a specific time allowance
to the owner during which he must take one of the following steps to bring the development
into compliance with this chapter:
(1) An acceptable application for a development permit must be filed with the office of
the Administrator for the entire scope of development taking place or proposed for
the site;
(2) The item or items which are not in compliance with the terms, conditions, and pro-
visions of this chapter shall be corrected, added, or improved until they are in com-
pliance with this chapter, at which time the owner shall request, in writing, a re-
inspection by the Administrator;
(3) Modified construction plans shall be submitted to the Administrator which detail, in
an acceptable manner, the remedial, additional, or corrective measures which must
be taken to bring the development within the provisions and requirements of this
chapter; or
(4) An acceptable variance request, subject to the provisions of this chapter, shall be
submitted to the office of the Administrator which shall have the effect, if granted, of
removing the requirements for which the development was determined to be in
noncompliance.
The time allotment for these actions shall be reasonable, and shall be determined
according to the number, nature, and severity of the noncomplying items. In no case
shall that time period exceed thirty (30) calendar days from the date of notification .
If, in the opinion of the Administrator, a condition exists which is hazardous to the
immediate safety of the public, he may seek remedies outside the scope of this
section .
B . REVOCATION OR SUSPENSION OF DEVELOPMENT PERMIT
Upon expiration of the time allotment for remedial or corrective measures, the Administrator
shall take one of the following courses of action;
13-19
Rev. 12197
(1) If, in the opinion of the Administrator, the owner has made, or is making , a good faith
effort to remedy the offending situation, the Administrator may extend the time
allotment previously granted if such extension is requested by the own er , or
(2) Where an approved development pennit has been issued for the development, the
Administrator shall suspend the development pennit. Written notice of said suspen-
sion, along with the tenns and requirements for reinstating the development pennit,
shall be delivered to the owner.
Upon suspension of a development pennit, all portions of the work being done on the prop-
erty that are regulated by this chapter shall cease . These activities include , but are not
limited to grading, excavation, fill , benning, stripping, clearing , paving, placement of any
storm sewer, drainage structure, inlet, or appurtenance thereto; any work within a defined
Area of Special Flood Hazard, or placement of any structure, temporary or pennanent, or
any obstruction within the Area of Special Flood Hazard.
Upon finding no approved development permit exists for the work or property in question or
suspending an existing approved development pennit, the Administrator shall issue a stop
work order for all items of work on the subject property covered by this or other ordinances,
any work permitted, licensed, or otherwise regulated by the City, or any work subject to
inspection or approval by the City. The Administrator may enter property to inspect and
verify that the requirements of this ordinance are being met.
All notices required by this section shall be served upon the parties concerned either per-
sonally or by certified mail, addressed to the individual contracting parties , or permit holder,
at the address given on the pennit application.
(Ordinance No. 1728 of October 22, 1987)
C . PENALTY IMPOSED
Upon the finding of a violation of this ordinance, the Administrator may file a complaint in the
Municipal Court of the City of College Station against any person, firm, corporation , or other
legal entity. In the event that the judge finds any person , firm , corporation , or other legal
entity to be in violation of the tenns , conditions, or requirements of this chapter, or provisions
or conditions pursuant thereto , he shall find said person firm, corporation , or other legal
entity gu i lty of a misdemeanor; and upon conviction shall impose a fine pursuant to the
General Penalty set out in Chapter 1, Section 5, of this Code of Ordinances . Prosecution
under this provision shall not be a bar to any other relief for violation of this chapter.
(Ordinance No. 2030 of September 9, 1993)
D. FINE NOT EXCLUSIVE PENAL TY
In addition to a fine, the City may institute appropriate actions or proceedings at law or equity
for the enforcement of the provisions of this chapter or adopted Drainage Policy and Design
Standards , or to correct violations thereof, and, if applicable, appropriate actions or
proceedings at law or equity against any surety company, escrow holder, or any third party
who has affirmatively acted as surety or guarantor for the faithful performance of the permit
holder's work.
(Ordinance No. 1728 of October 22, 1987)
13-20
Rev . 12197
Drainage Policy
and
Design Standards
I.
I I.
II I.
IV.
V.
VI.
VI I.
VI I I.
IX.
x.
XI.
XI I.
TABLE OF CONTENTS
Introduction
City of College Station Drainage System Policies
Methods of Calculating Stormwater Runoff
Street Drainage and Flows
Storm Drain Inlets
Storm Drainage Systems
Open Channel Flow
Culverts and Bridges
Detention Facilities
Erosion and Sedimentation Control
Drainage Plan Submittal Requirements
Development Permitting Process
5
16
29
37
46
52
63
70
76
89
97
Table
III-1
II I-2
III-3
III-4
VI-1
VII-1
VI I-2
VIII-1
LI ST OF TABLES
Runoff Coefficients for Use in the Rational Formula
Velocities (v) of Runoff Flow for Determining the
Time of Concentration (tc)
24 Hour Rainfall Depths for Selected Storm Return Periods
Curve Numbers and Percent Impervious Area for
Various Land Cover and Land Use
Coefficient of Loss, K.
Manning's Roughness Co~fficients
Maximum Design Velocities for Various Surface Treatments
Values of Entrance Loss Coefficients "K " e
; ;
23
24
25
26
51
59
60
68
Figure
II-1
II-2
II-3
I I I-1
I I I-2
IV-1
IV-2
IV-3
IV-4
IV-5
V-1
V-2
V-3
V-4
V-5
VII-1
VII-2
VIII-1
IX-1
X-1
X-2
X-3
X-4
X-5
X-6
XI I-1
XI I-2
LI ST OF FI GU RES
Carter Creek Drainage Basin (Partial)
White Creek Drainage Basin
Lick Creek Drainage Basin
Intensity Duration Frequency Curves
for the City of College Station
Hydrograph by Triangular Approximation
Nomograph for Flow in Triangular Channels
Gutter Flow Curves for 28' Street-6" Parabolic Crown
Gutter Flow Curves for 39' Street-7" Parabolic Crown
Gutter Flow Curves for 47' Street-7" Parabolic Crown
Gutter Flow Curves for 57' Street-9" Parabolic Crown
Inlet Capacity Type A-1
Inlet Capacity Type A-2
Capacity for Curb Opening Inlets of Grade
Ratio of Intercepted to Total Flow for Inlets on Grade
Inlet Type B-1 and B-2
Section Through Curb Inlet with Gutter Depression
Typical Channel Cross Sections
Schematic of Stream Section Showing Encroachments
Schematic of Typical Culvert Structure
Simplified Method of Determining Storage Volume
Temporary Straw (or Hay) Bale Barrier
USSR Type III Stilling Basin
USBR Type IX Stilling Basin
USBR Type VI Stilling Basin
Design Data -USSR Type Stilling Basins
St. Anthony Falls Stilling Basin
Application Form Development Permit
For Areas Outside Flood Hazard Areas
Application Form Development Permit
In Established Flood Hazard Areas
; ; ;
13
14
15
27
28
32
33
34
35
36
41
42
43
44
45
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99
I. Introduction
A. Purpose
The standards adopted in this document are intended to
imp 1 ement the intent of the adopted Stormwater Management
Ordinance and to encourage uniformity in the analysis, design and
construction of drainage facilities for their intended purpose.
B. Source of Authority
These standards are a part of the Stormwater Management
Ordinance adopted by the City Council of the City of College
Station as Chapter on the day of , 1986.
C. Definitions
Unless specifically defined below, words or phrases used in
these standards shall be interpreted so as to give them the
meaning they have in common usage and to give these standards
their most reasonable application.
ANTICIPATED DEVELOPMENT. Full potential urbanization of the
contributing watershed in compliance "11ith the Comprehensive Plan
and the reasonable assumption that in considering the effects of a
proposed development in a floodp 1 ai n area that there wi 11 be an
equa 1 degree of encroachment extending for a significant reach on
both sides of the stream or watercourse.
BASE FLOOD The flood having a one percent chance of being
equal led or exceeded in any given year, also known as "100 year"
flood.
BASIN A region or area whose surface stormwater runoff flows
ultimately to a particular point in a watercourse or body of
water.
CFS means cubic feet per second
CITY. City of College Station
DATUM. Any level surface to "11hich elevations are referred (for
example, mean sea level). Also called datum plane, although not
actually a plane.
DETENTION. The temporary storage and contro 11 ed rel ease of storm
water flows.
DETENTION FACILITY. A permanent facility for the temporary
storage of stormwater flows which is designed so as not to create
a permanent pool of water.
DRAINAGE EASEMENT. An interest in land granted to the city for
the maintenance of a drainage facility, on which certain uses are
prohibited; and providing for the entry and operation of machinery
and vehicles for maintenance.
DRAINAGE FACILITIES. Includes all elements necessary to convey
storm water runoff from its initial contact with earth to its
disposition in an existing watercourse, and said drainage
facilities shall consist of both public and private storm sewers
(closed conduits), improved channels constructed in conformity
with the adopted Ora i nage Policy and Design Standards, uni mp roved
drainageways left in their natural condition, areas covered by
drainage easements for the purpose of providing concentrated or
overland sheet flow and all appurtenances to the foregoing,
including inlets, manholes, junction boxes, headwalls,
dissipaters, culverts, etc.
DRAINAGE RIGHT OF WAY. An area of land dedicated to the city for
the purposes of carrying and containing stormwater flows,
constructing drainage facilities, and/or allowing entry or
operation of maintenance equipment.
ELEVATION. The vertical distance from a datum, usually the NGVD,
to a point or object. If the elevation of point A is 802.46 ft.,
point A is 802.46 ft. above some datum.
ENGINEER. A Registered Profess i ona 1 Engineer du 1 y authorized and
licensed under the provisions of the Texas Engineering Practice
Act, to practice the profession of engineering.
EROSION. The process whereby the surface of the earth is broken
up and carried away by the action of wind, water, gra~ity, ice, or
a combination thereof.
EXISTING CONDITION. The natural condition of the basin that
exists or existed at the time of a particular drainage analysis.
EXTRATERRITORIAL JURISDICTION (ETJ). Within the terms of the
Texas Municipal Annexation Act, means the unincorporated area, not
a part of any other city, which is contiguous to the Corporate
Limits of the City of College Station, the outer boundaries of
which are measured from the extremities of the corporate limits of
the city, outward for such distances as may be stipulated in the
Texas Municipal Annexation Act, in which area, within the terms of
the act, the City may enjoin the violation of its subdivision
control ordinance.
F.H.A. Federal Housing Administration.
FLOOOWAY The channe 1 of a river or other watercourse and the
adjacent 1 and areas that must be reserved in order to discharge
the base flood without cumulatively increasing the water surface
elevation of the base flood more than one foot.
HYDRAULICS. A branch of science that deals with pr act i ca 1
applications (such as the transmission of energy or the efforts of
flow) of liquid (such as water) in motion.
HYDROLOGY. A science dealing with the properties, distribution
and circulation of water on the surface of the land, in the soil
and underlying rocks, and in the atmosphere.
LOWEST FLOOR. The lowest floor of the lowest enclosed area
(including basement). An unfinished or flood resistant enclosure,
usable solely for the parking of vehicles, building access or
storage, in an area other than a basement area, is not considered
a building's lowest floor, provided that such enclosure is not
built so as to render the structure in violation of the applicable
non-elevation design requirements of this ordinance.
MEAN SEA LEVEL (MSL) The average height of the surf ace of the
sea for all stages of the tide taken over a 19-year period.
NATURAL. The cover and topography of 1 and before any man-made
changes, or in areas where there have already been man-made
modifications, the state of the area and topography of land at the
date of the adoption of this standard.
3
RETENTION FACILITY. A facility that provides for the storage of
stormwater flows by means of a permanent pool of water or a
permanent pool in conjunction with a temporary storage component.
SEDIMENTATION. Deposition of detached soil particles or rock
fragments after being transported from their site or origin by
runoff water.
STRUCTURE A walled and roofed building that is principally above
ground, as well as a manufactured home.
SURVEYOR. A Registered Public Surveyor or Registered Land
Surveyor as licensed by the State of Texas.
TAMU. Texas A&M University.
TSDHPT. Texas State ·.Department of Highways and Public
Transportation.
ULTIMATE DEVELOPMENT. See ANTICIPATED DEVELOPMENT.
WATERCOURSE Any depression, storm sewer, or culvert serving to
give direction to a current of stormwater.
WATERSHED. See BASIN.
4
II. City of College Station Drainage System Policies
A. Genera 1
For purposes of regulation, the drainage system of College
Station shall be split into geographical and functional
groupings. The drainage system consists of all natural and man-
made f ea tu res that co 11 ect or receive concentrated flows of
stormwater. Examples are natural swales or man-made channels,
streets, storm sewer, minor streams, and major streams.
Functional division consists of separation of the
components into the primary and secondary drainage systems.
The primary system consists of major streams that convey
collected stormwater through and out of the city. The
secondary system consists of a 11 mi nor streams, streets, storm
sewers, and swales that collect stormwater and convey it to the
primary system.
From a hydrologic standpoint, the secondary system is
sensitive to short duration, high intensity rainfall events.
Flood effects occur suddenly and dissipate quickly, usually
within a period of a few hours. The primary system is
sensitive to longer duration, moderate intensity rainfall
events. Flood events occur over a longer period, with a slower
rise to and fall from peak flows and flood elevations. This
fundamental difference between the primary and secondary system
forms the basis for strategies to control stormwater and its
effect within each.
Geographical division involves separating the various
streams and drainage areas into basins having similar
characteristics in terms of land cover, pattern of development,
governmental jurisdiction, proposed land uses, and system
interconnection. Recognition of these differences allows for
rational formulation of policies and standards tailored to
specifics rather than generalities.
5
B. Principles for System Policy
1. Secondary System
Stormwater prob 1 ems in the secondary drainage system
are site specific and scattered throughout the city.
Typically they result from inadequate design of streets,
storm sewers, and collection channels. Examples are
excessive ponding in streets at low points, overflow of
streets on to private property, excessive storm flows in
streets restricting or blocking traffic capacity, etc.
Causes for problems in the secondary drainage system
are listed as follows:
Inadequate capacity for design flows.
Inadequate allowance for increases in design flows
due to future development.
No provision for containing and controlling the
discharges from the 100 year rainfall event and
ultimate development conditions.
Failure to control discharges of new developments
that exceed the capacity of the receiving secondary
system, existing or proposed.
The results are creation of nuisance problems and
situations where damage to pub 1 i c and private property can
occur. Remed i a 1 measures may range from expensive capita 1
improvements to situations where remedies are infeasible
from a practical standpoint.
The policy of the City shall be to attempt to prevent
the formation of these problems at the design and
development stage. Central to this strategy are the
performance standards for drainage design contained in
subsequent · chapters and the "pathway concept" for
containing the discharges from the base flood.
Under this policy performance standards are set for
design rainfall events. The emphasis at the performance
level is to mitigate the nuisance aspect of storm drainage
to reason ab 1 e occurrences . An ex amp 1 e of a performance
standard would be "Design the street to carry the discharge
6
from a 10 year rainfall event leaving one lane at the
center open". Subsequent chapters contain similar
perfonnance standards for the various portions of the
secondary and primary drainage systems.
Add it i ona 11 y the secondary system is evaluated and
designed for the stormwater conditions that will exist up
to the 100 year rainfall event and ultimate development
within the applicable basin. A pathway for this stormwater
shall be identified and provided from its source to its
discharge into a main channel of the primary drainage
system. The designated pathway shall be wholly contained
within designated drainage rights of way and easement,
except in areas of existing development. The purpose at
this leve 1 is to prevent the creation of hazardous
situations to life, and damage to public and private
property. Some performance standards have been set at this
level, in an attempt to prevent the fonnation of hazardous
conditions, but the major emphasis is on control and
confinement to known pathways.
2. Primary System
Stonnwater problems in the primary drainage system
result from floodwaters rising out of the banks of natural
streams and inundating adjacent natural floodplains.
Typical problems described are flooding of building
structures, overflow of bridges and culverts cutting off or
restricting traffic access, and damage to public and
private infrastructure (utilities, roads, etc.).
Causes for problems in the primary system are listed as
follows:
Inadequate capacity of crossing structures and
failure to allow for overflow.
Placing the finish elevation of the lowest floor of
a structure adjacent to the primary system below the
existing or ultimate 100 year flood elevation.
Inadequate or out-dated engineering studies that
form the basis of the regulatory flood elevations.
7
Failure to allow for increased discharge and
resulting flood elevations in upstream developing
area; or failure to control and limit the increased
discharges in upstream developing areas.
Improper or ineffective alterations to natural
channels that "transfer" flood problems to upstream
or downstream areas.
The results are creation of hazards to 1 ife and damage to
public and private properties. Remedial measures usually
involve large capital impro·vements to channelize streams,
create large detention basins, or build larger crossing
structures.
As a first step to dea 1i ng with these prob 1 ems, the
city has adopted a comprehensive hydrologic and hydraulic
engineering study. This identifies the flood discharges
and flood elevations within the primary system, for
existing and ultimate development conditions. Ultimate
development conditions reflect the drainage situation as
expected if the development within the city follows that
projected in the adopted comprehensive 1 and use p 1 an, or
Plan 2000. In theory, the existing and ultimate flood
conditions are no longer unknowns.
The policy of the city is to encourage the rapid
conveyance of stormwater through and out of the City within
the primary system. The 1 owes t floor of a 11 structures
adjacent to the primary system shall be kept above the
ultimate 100 year flood level, and out of the existing 100
year flood path defined as the "floodway". Modification to
the floodway shall be restricted to projects engineered and
constructed in conformance to a comprehensive masterplan
along regulatory channel reaches, to eliminate sporadic and
uncoordinated site specific improvements. Minor
encroachments in the f 1 oodway fringe wi 11 be a 11 owed for
individual sites and developments. Crossing structures
shall be designed to accommodate the range of ultimate
design flows through and over them to eliminate formation
of hazards and damage to property.
0
To implement this policy, design standards and a
stormwater management ordinance have been adopted. Site
specific requirements are varied along each channel reach
to recognize the differences related to basin development
conditions, expected increases in flood elevations, and
potential damages.
C. City of College Station Recognized Drainage Basins and Channel
Reaches
Figures II-1 through II-3 are maps of the drainage basins
within and adjacent to the City and its extra-territorial
jurisdiction (ETJ). Basins are divided · to recognize the
relationships of geography, land cover, political
jurisdiction, and proposed development relative to their
effects on existing and ultimate flood flows and elevations.
~ithin each basin, streams are designated as part of the
primary system and individual reaches of these streams are
identified for regulation.
Basin identification is schema~ic in the figures. A land
area is defined as being a part of a given basin if stormwater
that falls upon it travels overland by natural or man-made
paths and initially enters the main channel of the primary
system of that basin. The primary system and channel reaches
are established by physical landmarks such as stream
confluences and crossing structures.
The following is a list of basins keyed to the appropriate
figure.
1. Burton Creek Main Channel and Trib. A (II-1)
2. Wolf Pen Creek and Tributaries (II-1)
3; Bee Creek and Tributaries (II-1)
4. Bee Creek Trib. "A" and Minor Sub-Tributaries (II:...l)
5. Bee Creek Trib. "B" and Minor Sub-Tributaries (II-1)
6. Carter's Creek and Minor Tributaries (II-1)
6a. Carters Grove Tributary (II-1)
7. White Creek and Tributaries (II-2)
8. Lick Creek and Minor Tributaries (II-3)
9. Spring Creek and Tributaries (II-3)
10. Alum Creek and Tributaries (II-3)
D. Special Drainage Basin and Channel Reach Design Standards
The following special considerations shall be applied to
areas where they are deemed necessary to protect 1 if e and/ or
property for existing or ultimate expected flood conditions.
These requirements shall be considered to supersede the genera l
requirements of the remainder of these standards.
The necessity of detention facilities shall be determined
by the subsequent requirements of these standards except the
following areas shall automatically incorporate detention into
the design of new deve 1 opments. The rat i ona 1 e for t hese more
stringent requirements reflect the direction of stormwater
flows across political boundaries for the Burton Creek Basin,
the demonstrated potential for the creation of a flood prone
area due to limited crossing structure capacity for the Carters
Grove Basin, and the reasonable assumption of lack of capacity
for the Secondary System in the older areas of the city to
absorb increased flood flows (Wolf Pen Creek and Bee Creek
Basins).
Basin Basin
No. Name
6a Carters Grove
2 Wolf Pen Creek
3 · Bee Creek
1 Bu rt on Creek
Location
Descri~tion
A 11
North of Harvey Rd.,
West of Texas Ave.
Upstream of Trib.
II B II Confluence
A 11
Rainfall Event
Detained
100 Year Ultimate
25 Year Ultimate
25 Year Ultimate
100 Year Ultimate
The elevation of the lowest habitable floor ·of a structure
adjacent to a watercourse of the primary system shall be
located at least 1 foot above the base flood elevation, except
along the following designated channel reaches the minimum
lowest floor elevation shall be located further above the
existing base flood e l evation, or .5 feet above the base flood
elevation associated with the ultimate basin development
condition, whichever is higher:
10
Stream Channel Reach
Bee Creek III
Bee Creek Trib. "A" I. II
Bee Creek Trib. "B" I (Downstream FM 2818)
I (Upstream FM 2818)
II, III
Lick Creek I, II, III, IV
Spring Creek I. I I
North Fork
South Fork
E. Easement and Right of Way Requirements
Minimum Floor
Elevation
Base Flood +3'
Base Flood +2'
Base Flood +2'
Base Flood +4'
Base Flood +2'
Base Flood +3'
Base Flood +3'
Base Flood +3'
Base Flood +3'
The detenni nation of the necessary areas and extents of
rights of way and easements shall be determined by the
Administrator of the Stormwater Management Program. The
criteria for determination shall be based upon the anticipated
amount and spread of stormwater flows, the possibility of
increased flows at some time in the future, the concurrent uses
to be associated with the designated areas. the required space
for the appropriate maintenance equipment and personnel, and
the access necessary to conduct maintenance activities.
The following minimum standards shall be used in
determining drainage easements and rights of way.
11
l, The minimum width of any drainage easement or right of
way shall be 15 feet.
2. The minimum width for any storm sewer easement or right
of way shall be 15 feet, and the centerline of the storm
sewer shall be at least 5 feet from both sides of the
easement
3. The minimum width for any easement or right of way for
an open channel shall be the design top width of the
channel plus 5 feet from the edge of one side and plus 15
feet f ram the edge of the other side to be assumed for
maintenance access use.
4, If access into a drainage easement · is not available
from public right of way, then an access easement with a
width of 15 feet or more sha 11 be provided from a pub 1 i c
right of way •
5, All easement and right of way widths shall be
sufficient to cover the land areas that will be part of the
designated pathways of the Secondary System.
6. All easement and right of way widths in the Primary
System shall be sufficient to cover the designated floodway
for the existing base flood plus the additional minimum
width requirements for an open channel.
The purpose of the easement or right of way is to pro vi de
the necessary space for stormwater flows and maintenance
activities for drainage facilities. Other uses for these spaces
that are inconsistent with these purposes are prohibited.
Examples of prohibited uses include cons .truction of fences or
other obstructions, placing of structures within these areas,
or uses that alter the required shape, configuration, or
surf ace treatment needed for these purposes.
12
LEGEND
BASIN REFERENCE NUMBER D
REGULATORY CHANNEL REACHES ~ 0 ~
PRIMARY DRAINAGE SYSTEM
SECONDARY DRAINAGE SYSTEM
BASlN BOUNDARY
SUB-BASIN BOUNDARY
r---
/ COtl£G£
I
I
ST.iriQ;;---........:__ crn----r----~ LIM1r$--...1 :z CITY LlMITS-
--- -) J_~ i§ ,
I
I
r--I
I ----J
\.
\ CARTER
\.
'
FIGURE 11-1
CREEK DRAINAGE BASIN
f ,U) /
___ -i....._.._-1__-+,~-/
:_ lj /
13 /
\ ! 1:
{PA P--2< I LLBO:>_. ~-_J
7 FM ?!j4 PAGE 13
J
I
I
I
LEGEND
BASIN REFERENCE NUMBER D
' ' '
REGULATORY CHANNEL REACHES~ 0 -_.;.:j . -O'?'
PRIMARY DRAINAGE SYSTEM
SECONDARY DRAINAGE SYSTEM
BASIN BOUNDAR Y
-~O ·
o~~ ,,,,_ __ ~
'--~~~~~~~~~~~~~~~~~~~
SUB-BASIN BOUNDARY
FIGURE IT-2
WHITE CREEK DRAINAGE BASIN
PA GE 14
I
I
FIGURE II-3
LEGEND
~-----l REFERENCE NUMBER D
/LATORY CHANNEL R E ACHES~ 0 -ti
--!\RY DRAINAGE SYSTEM
riDARY ·. l?R~INAGE SYSTEM
< BOUNDARY
BASIN BOUNDARY
~ //~ ...... --Ro "-....._ . ...... ---.,,,---.... -). ,,,,.,..,. ________ _..),.
,. _,
,.
/
/
LICK CREEK DRAINAGE BASIN
PA GE 15
/
/
I
III. ~ethods of Calculating S t o r mwater Runoff
A. Gen eral
The two types of analyses most of ten required are the
computation of the peak discharge at a specific locati.on an d
the compu tation of a hyd rograph at a specific location. r ... 0
methods are recommended for computation of peak discharges and
three methods are recommended for .computation of hydrographs.
Other methods of pro\·en use may be submitted to the
Administrator for approval.
B. The Rational Formula
1. Discussion of Variables
The formula shall be expressed as
Q = ciA
where the variables are defined below.
a. "Q" is the discharge in exact units of acre-inches per
hour and accepted to be equivalent to units of cubic
feet per second (cfs). This value is taken as the peak
or highest discharge expected at a designated design
point.
b. "c" is a coefficient, having no units, that represents
the average runoff characteristics of the land cover
within the drainage area delineated for a designated
design point.
c. "i" is the rainfall intensity :..n units of inches per
hour (in I hr.).
d. "A" is the area of lan d in acres that contributes
storlIIW'ater runoff t~at passes through or at a
designated design point.
2. Assumption and Limitations
a. The Rational Formula shall only be used to estimate
peak discharges at specific designated design points.
b. The contributing area "A" of ·runoff shall not exceed 50
acres, except for d esign of enclosed storm sewer
systems.
1 i:.
c.
d.
The Rational Form ula s hall not be used t o estimate
storlI!W'ater discharges of the Prim ary Sy stem. Its use
is strictly limited to the Secondary D~ainage Sy stem.
The Rational Formula shall be limited to use in
determining the peak discharge from areas of overland
o r street flow, and concentrated flows in street
gutters, storm sewer, and man-made channels.
3. Application
a. Intensity Duration, Frequency Curves for the Cit y of
College Station
(1) Figure III-1 shall be used to select the rainfal l
intensit-y "i" for use in the Rational Method.
(2) Input consists of the storm return period (tr) in
years and the time of concentration (tc) in
minutes.
b. Runoff Coefficients
( 1) Table III-1 shall be used to select the runoff
coefficient "c" for the appropriate land cover and
land use. Linear interpolation shall be used to
choose specific values within the ranges given.
(2 ) For areas that consist of different types of land
cover or land use, a weighted average runof f
coefficient shall be computed using the following
equation.
c • ----~---------------------
where:
A= A1 + A2 + .... +Ax' the total drainage area
c 1 , c 2 , .... cx are the runoff coefficients for sub-
areas
A1 , A2 .... Ax are the areas of land cover or land
use that correspond to the runoff coefficient c 1 ,
l 7
c is the runoff coefficient for use in :h e
formula for the Rational ~ethod.
(3) The runoff coefficient "c " shall be determined
using the "land cover" when using t!"le ratio nal
formula to compute the peak discharges within o r
out of specific sites and developments.
(4) The runoff coefficient "c" may be determined fr om
the "land use" when using the rational formula t o
compute the peak discharge from more than one site
or development.
c. Determination of Time of Concentration
(1) The time of concentration (t ) shall be defined as c
the maximum length of time required for water to
travel from the uppermost point in the drainage
basin to the designated design point.
( 2) Table III-2 shall be used to determine velocities
of flow for various reach segments along t he
travel length used to determine the time o r
concentration. Linear interpolation shall be used
to derive specific values within the given range.
(3) The time of concentration need not be taken as
being less than 10 minutes.
C. Soil Conservation Service (SCS) ~ethods -Urban Hydrology For
Small watersheds (Technical Release No. 55)
1. Discussion of Methods
The SCS methods are e mpirically derived relationships
that use precipitat ion , land cover, and physica l
characteristics of drai nage basins to calculate runoff
amounts, peak discharges, and h ydrographs. Of the various
methods available, the ~ollo w1ng two are adopted for use:
a. Chart Method -u se d :o d etermine the peak stormwater
discharges and the e r :ec t o f development on those peak
discharges at a des 15n ated design location.
18
b. Tabular ~eth od used to deter mi ne a hyd r ograph of
stormwater di sc ha rges at a d esignated design location .
2. Assumptions and Li mi tations
a. The accepted methods from Technical Release ~o . 55 are
fo r use in determining stormwater discharges and
hydrographs in the Secondary Drainage System only.
b. Application of these methods shall be in 5:trict
conformance with the instructions and recommendations
given in Technical Release ~o. 55 and the latest
updates and revisions issued by Soil Conservation
Service, except as superseded
requirements of this section.
or altered by t h e
c. The drainage area for application of these methods
shall not exceed 2000 acres.
3. Application
a. 24 Hour Rainfall Depths for the City of College Station
(Table III-3) shall be used to select the rainfall
depth for selected storm return periods. This value
shall be used for the variable "P" as input to all
equations, graphs, and tables as applicable. A Type II
rainfall distribution shall be used to determine
incremental totals.
b. Hydrologic Land Cover Parameters (SCS Curve Numbers)
(1) Table III-4 shal l be used to select values of
curve numbers for various types of land cover and
land use. The value( s) chosen shall be used for
the variable "C~" as input to all equations
(2)
graphs, and tables as applicable.
several types of
composite curve
using the same
methodology outlined in Section III. B. 3. b.
Where a drainage area consists of
land cover and /o r land use, a
number shall be determined
c. Determination of Peak Discharges -The TR-55 Chart
Method
( 1) Calculations :nust include the appropriate factors
and modifications fo r watershed shape, waters hed
L9
slope, and urbanization (percent of i mper v i ou s
area and percent of hyd rau lic lengt h modified).
(2) Where a drainage area consists of several types o f
land cover and/or land use, a ccmposite percent of
impervious area shall be det ermined using the sa me
methodology outlined in Section III. B. 3. b.
d. Determination of Time of Concentration
Two methods, the Lag ~cthod and the Upland ~ethod, are
available in ''TR-55" and shall be used.
D. Hydrograph Methodology
Three methods of deter~ining a hydrograph are accepted for
use. These are the Triangular Approximation, Tabular Method of
SCS Technical Release No. 55, and the SCS Dimensionless Unit
Hydrograph.
1. Discussion of Methods
a. Triangular Approximation The hydrograph is
constructed assuming the peak discharge occurs at time
equal to the computed time of concentration and o ne
third of the flow volume occurs befo~e the peak
discharge is reached. figure III-2 illustrates the
method.
b. Tabular Method of SCS Technical Release So.55 -The
hydrograph is computed by an empirical method that
relates drainage area, land use, and time of
concentration.
c. SCS Dimensionless L'nit Hydrograph -The hydrograph is
computed using basin area, land cover, lag, and
precipitation as modifiers to a dimensionless unit
hydrograph.
2. Assumptions and Limitati ons
a. Triangular Approximation ~ethod
(1) The peak discharge (Q p) shall be determined by the
Rational Formula.
(2) The drainage area analyzed by this method sha ll
not exceed 50 acres .
20
( 3 ) Th i s :n eth o d s h a ll onl y a p pl y to ana lysi s or :'.:e
secondar y d rainage s ystem.
(4) This met hod shall not be used to c onstruct a
hydrograph for a natural watercourse, stream, or
channel.
b. Tabular ~ethod o f SCS Technical Release No. 55
(1) The method shall be applied according t o th e
instructions and limitation outlined in Techni.c al
Release No. 55, · and the latest updates a nd
revisions issued by the Soil Conservation Service.
( 2) This method shall only apply to analysis of the
Secondary drainage s ystem.
c. SCS Dimensionless Cnit Hydrograph
(1) This method is adopted for use in the Primar y
System and is the method used in
analysis for the adopted College
Study.
the hydrologic
Station flood
(2) The method shall be used to compute hydrographs at
(3)
locations in the Primary System where the adopte d
College Station flood Study does not determine a
hydrograph.
The method shall only be applied using the
Generalized Computer P~ogram, HEC-1, Flood
Hydro graph Package developed by the Hydrologic
Engineering Center of the U.S. Army Corps of
Engineers.
(4) Data from the adopted College Station Flood Stud y
shall be used with only the necessar y
modifications to account for the location of t he
hydrograph desired. Typically this will involve
deletion of data for areas outside of o r
downstream of the study location, and modification
of the most downstream drainage basin and/o r
routing reach.
3. Computer Analysis and Si~ulation
a. A comprehensive h yd r ologi c :nodel o f the Primary Sy stem
has been adopted by r ;-i e City of College Station. The
21
model is applied using Generalized Computer Pr o gram,
HEC-1, Flood Hydrograph Pack age of the U.S. Ar my Corps
of Engineers.
b. The model uses the following ;nethods available i n HEC-
1:
( l) Precipitation is computed using the 24 hou r
rainfall depths (see Table III-3) distr ibu ted
according to the Soil Conservation Service Type II
Distribution.
(2) Basin Hydrographs are computed using the SCS
Dimensionless Unit Hydrograph Method.
(3) Routing of hydrographs is computed by ~ormal Depth
Storage and Outflow ("Channel Routing").
c. Amendment of the adopted flood study will be made by
the Administrator. as conditions in the drainage basins
change.
The model consists of analyses of the 10 year, 25 year, 50 year
and 100 year storms for two basin conditions. The existi ng
c ondition analysis reflects the existing land uses and channe l
conditions in the basins. The ultimate condition anal ysis
reflects the fully developed basin conditions defined by the
adopted Comprehensive Land Use Plan for development within the
city and the existing channel and floodplain conditions at the
time of the study. ~o allowance for channelization was made to
determine the ultimate condition flood discharges or
elevations.
22
Table III-1
Runoff Coefficients For Use In The Rational Formula 1
Category
Land Cover:
Natural Woodlands
Natural Grasslands
Landscaped Areas
Impervious Areas
(Pavements, Rooftops, etc.)
Land Use:
Low Density Residential
Medium Density Residential
High Density Residential
Business/Commercial
Industrial
0 -li.
.12 -.18
.2S -.3S
.30 -.40
.90
.so -.60
.SS -.6S
.60 - . 7S
.6S -.75
. 75 -.85
Average Slope (7.)*
-3.5%
.18 -.25
.35 -.45
.40 -.70
.90
. 50 - . 60
.55 -.65
. 60 - . 75
.65 -.75
.75-.85
*For slopes in excess of 5.S% use the maximum value given.
1 From the "Hydraulic Manual" of the TSDHPT
23
3. 5 -5. Si.
.25 -.30
.45 -.55
. 70 - . 85
.90
.50 -.60
. 55 - . 65
.60 - . 75
.65 -.75
.75-.85
Table III-2
Velocities (v) of Runoff Flow for D~tfrmining the
Time of Concentration (t )
Reach Description
Overland or Sheet Flow:
Natural Woodlands
Natural Grasslands
Landscaped Areas
Pavements
Concentrated Flow:
0 -3i.
v(ft/sec)
0 -1. 5
0 -2. 5
0 3.0
0 -8.5
Slope (i.)
4 -7%
v(ft/sec)
1.5-2.5
2.5 -3.5
3.0 -4.5
8.5 -13.5
8-11i.
v(ft/sec)
2.5 -3.25
3.5 -4.25
4.5 5.5
13.5 -17.0
Street or Gutter Flow -Use procedure from Section IV.
Storm Sewer Use procedure from Section VI.
Op en Channel Use procedure from Section VII.
Natural Channels 0 - 2 2 - 4 4 - 7
1 From the "Hydraulic Manual" of the TSOHPT
24
> 127.
v (ft /sec )
>3.25
>4.25
>5. 5
>17.0
Storm
Table III-3
24 Hour Rainfall Depths for 1Selected
Storm Return Periods
Return Period, II t II 24 Hour Rainfall Depth,
(Years) r (Inches)
2 4.5
5 6.2
10 7.4
25 8.8
50 9.8
100 11 . 0
From National Weather Service lsohyetal Charts For
Selected Return Periods
25
"P"
Table III-4
Curve Numbers and Percent Impervious Area
For Various Land Cover and Land Use
Category
Land Cover:
Natural Woodlands
Natural Grasslands
Landscaped Areas
Impervious Areas
(Pavements, Rooftops, etc.)
Land Uses:
Low Density Residential
Medium Density Residential
High Density Residential
Business/Corrmercial
Industrial
Percent Impervious i.
0
0
0
100
38
52
65
85
72
Curve "iumber
75
75
77
98
84
87
91
94
92
From "Urban Hydrology For Small Watersheds, Technical Release No. 55"
of the Soil Conservation Service
26
l I i i I I I i I I ! /
i I
I : I
f I ·1 ~16URE II[· I I I I I I I I I i I
I INT[N ~IT~ NJA lT10N FRt:' :lUf!NCY ~ C IR~E!c I I
I ; i I fr(') t :Tllt:'1 CIT t n• cru 1 t:'~[I ~TAT n• ! !
I
I I i I I I ' I I
I I
I ' I : I I I '
=----==-i /r I
,-~-~ ,,_____ r-----0
C:I I
~·-~ I
a10 "": -.... I
~. It'!
32: <" ~ !•
4ii -'j' j -·· ---/; 5:::2 j, ' '-'',,, -: I ~a:
;?~ I I I
~er ' I
z~ "I\. ' J
""" I-~ I "kJ I '\.. g,,,j ,, l..... J "l ~: I
~:~ ' /' l I I
"i I I ~-~-
~:~ I I -~~ ""' ' I I I •!
IX>
~ -.la/ ~/~ j j ~ +
i: ~ M_ /~~ j; ~
-~ u 7~ 1!lJ ~/j!/ I -•
)Iii: :d . _,/ j I ~/ft)/ .... --· I i I + :/ I I --J ·--~, .:/: .§/ I I i f;
~>; -ti ., i J. , I :/, .. -I 1'1 · -o:' : I J ~ . H a-. I I I 1fl i I I " !'....
--~1 I I "' a ,, I I l : -----~ .' I I I ! ... ' !';:? ~· ----~ill a I - -/ /'I I J ~ ---I
; --,-, ' /'... / /' I /~ .., i : ...;
~~I : ~//-E-1 ~ +·
i::·_ J . --;
' . / / ~ -------..
~ .0 -/A · -------....
~ / ! / // '.../' / ~E ---
I ~--m -:::-~~ . ./ . __._ ---
·--------·---_ _ _ t--r r f --r 1 ---
I I I • N G 2 .~I co • N 0
t •JU /"UI\ I ·~111u11u1 110,UIO~
Q
(.' -
c
0 -a
t() ...
c.o c
s c.;
0
N
c c:
<.,;
FIGURE m-2
HY5ROGRAPH BY TRIANGULAR APPROXIMATION
DISCHARGE
0
(cfs)
_o_=_ q_e_~a~-----
t = time of concentration, t c
t: 0
28
t=3t-c
I
I
I
Time, t 'min)
I V. Street Drainage and Flows
. .\. General
The design flow of water in streets shall be related to th e
extent and frequency of interference to traffic and the ch ance
o f flood damage to surrounding pr op erties. In ter ference to
traffic is regulated by design limits of the spread of water
into traffic lanes. Flooding of surrounding prcpe•ties i s
regulated by limiting the depth of flow at the curb.
B. Performance Standards and Limitations
1. Velocity of Flow
The maximum velocity of street flow shall not exceed 10
feet/second. A minimum velocity shall be maintained to
insure cleaning at low flo ws by keeping the minimum gutter
slope to .004 ft/ft. (0.4%).
2 . Allowable Depth of Flow
The depth of flow shall be limited to the top of curb
for a design storm having a · return period of 10 years .
Design flows for storms with an average return period up to
100 years shall be confined within the limits of the stree t
right-of-way until discharge into adjacent drainage
easement, which is part of the designated pathway of the
Secondary System or directly into a main channel of the
Primary Drainage System.
3. Grades and Cross-slopes
Street grades and cross-slopes shall be consistent with
the "Standard Specification For Street Construction" fo r
the City of College Statio n.
4. Allowable Water Spread
a. Residential Streets -The flow of water in resident ial
streets shall be limit ed to the top of crown or the top
of curb, whichever i s less, for the design storm.
Stor!IIW'ater shall be removed from the streets by inlets
or openings into adj acent drainage systems. These
shall be placed at low poi nts or µl aces where
29
calcu lations i ndicat e th e allowable wa ter sp rea d e r
depth is met or exceed e d . The design storm shall ha ve
a return period of 10 years.
b. Collector and Commercial Streets -The flow of water in
collector and co mmercial streets shall be limited such
that one 12 foot traffic la-ne at the center oi the
street will remain clear for the design storm.
Stormwater shall be remo ved from the street ty inlets
or openings into adjacent draina~e systems. These
shall be placed at low points or places where
calculations indicate the allowable water spread o r
depth is met or exceeded. The design storm shall have
a return period of 10 years.
c. Arterial and Parkway Streets -The flow of water in
arterial and parkway streets shall be limited such that
one 12 foot traffic lane each direction at the center
of the street will remain clear for the design storm.
Stormwater shall be removed from the street by inlets
or openings into adjacent drainage systems. These sha l l
be placed at low points or places where calculati on s
indicate the allowable water spread or depth i s met o r
exceeded. The design storm shall have a return period
of 10 years.
C. Design Procedure
1. Straight Crown
Flows in streets which have a straight crown will be
calculated using the follow ing equation for triangular
channels:
z 1/2 8/3
Q ~ .56 n S y ;;here,
Q = gutter discharge (cfs)
z ~ reciprocal of the c r o wn slope (ft/ft)
S = street or gutter sl o pe (ft /ft)
n = manning's r oughnes s co efficient
Y = depth of flow (ft )
Figure IV-1 is a nornograph that provides a dire ct
solution to the above eq u at io n. When flow s over concrete
or asphalt pavement are bei n g calculated, the value of
shall be taken as .018.
2 . Parabolic Crowns
,, ,,
n
Flows in streets which ha ve a parabolic crown become
complicated and difficult to solve exactly for each design
case. Design curves have been prepared which allow a quick
method to determine depth of flow, street capacities, and
spread of water. Figures IV-2 through IV-5 are street flow
curves for 28, 39, 47, and 57 foot streets respect iv el y .
~ ..
2
~ c a::
100
500
400
300
200
IOO
IO
IO
70
IO
50
40
30
zo
10
EQUATIONs Q•Q~ (~ ~ 11"5
Z•A!CIPftCM.. O' TRAHSVE"3E SLOP!
n•COEf'FlCIOT ~ ~s IN MA*we'S
FOMIJLl
t•MAOI: ~ ~NEL IN F'T./Ft
y•OE'TH AT CUM Of' DEEPEST POINT IN FT.
EXAMP'L! (S.. dalf\ff lfMa)
a.v£N• t•O.OS
Z•24 J ZA-•1200 n•.OI
Q•2.0 CFS
F1N01 1•0.22
100
70
&O
30
10
2 .0
.IO (10%GMO£)
.oe 1.0
.01
.oe .90
.70 ·°' .SO
.04 .~
---------IO------.03 .•o ______ , _________ _
---• 1 ---------------
INl'!!!yCTIC!!S
---
~ 5
• 3
.I
.ar
.09
.03
.OI
.Ot
I. COMCCT '.f. MnO WITH ~ (1) AHO COMCCT
OCICMM• ( Q) Wln4 DOTH ( r ). T1'4D! TWO U.:S
lilUl'T ln!Ml:CT AT TIMNINI LINE Fat
COW\.11'1 9QJJ1'10N.
FIGURE lll:-1
NOMOGRAPH FOR FLOW
IN TAtANGULAR CHANNELS
?... '.::\.
---
(,) .Of
.ooe
.004
.~
.30
~ .oe
~ .07
DI
.05
.04
.03
.02
.01
II
0
'2 ·•9!1
90
80 J..
70
60
E
~--
50 =
40
30
=-
20
I
6 ~-
5
I
.001 .002
...
I .. ~ --~ -
' -~ .006 .008 Ot .02
So 2 STREET GRADE -FT./FT.
~~
FIGURE Til: -2
(N=0 .016)
GUTTER FLOW CURVES FOR
--1 _..,....,..,..r..,. r 11 r"IAOAO'°'I Ir"" f"'Ot"'\WtJ
---
-~ -~ .Q8
I_•:
90
a:
7_
s_
" s:
9
a ·
7
6 c:c
s
(/)
u... 4 u
I
~
0 3
_J
u...
a::
UJ ;-2 ;-
::::)
<.!)
II
0
.00
-
.002
-'
r
.004 .ooe .ooe ·°' .02
So = STREET GRADE -FT. I FT.
FIGURE ISl-3
( N• 0 .016)
GUTTER FLOW CURVES FOR
39' STREET -7• PARABOLIC CROWN
·-:i , I -------------
.. _:.;,
-
I I I I
.04 .0 6 )a
.o·
I
i 11 .IL
I " . I
•
-
~
'
--
~ i iii!
-
----+--
~o~ S3A~nJ MO -l.:2 ~~J..J..11~
(910'0=N)
~-E: 3etn~1~
<;ct_
.J..=' FLJ -30Y~H> J.33~.l.S : OS
zo· 10 · eoo · too· .oo·
I
I
I .,
~
~
~ -. ~
' ""' ...
•r "' ---~
---
~
-
Ii
zoo·
I
'•
--
·-
IOO "
~
--
·-
____,
=-=:J
I
z
t
•
--
9
l
•
6
:;1
0
0
0
0,
---
0
0
l
&
06
001
0
II
..,, ,..
0
E
I
(') ..,,
c.o
(I) ....
<.J
I
~
0
..J ....
a::
u.I .,_ .,_
::>
C)
" 0
...
""'re ·••• ..
12 .1•9
100
90
8 0
70 ~
so ~-
50 ,__ -
30
z:
.
9
s
7
6
5
" !-;;;
3
z
I
.001 .002
. . --.. _____
... -..-
-
I I
.004 .00. .008 .0 1 .02
So • STREET GRADE FT./FT.
FIGURE IV-5
( N • 0.016)
GUTTER FLOW CURVES FOR
....
57 1 STREET - 9 u PARABOLIC CROWN
~
. ----
.04 .0 6 .08
-_______________________ __j_
V. Storm Drain Inlets
A. General
The purpose of a storm drain inlet is to intercept excess
surface runoff and direct it into another component of th e
drainag e system. Inlets are typically of the curb opening t ype
for streets and grate type for area drainage. Curb inlets
occur at low points or on grade, and can either have a
depressed throat or be flush with grade. Grate inlets occur in
low po in ts and may or may not b'e depressed. Some guidelines
for inlets located on streets are given below:
1. Inlets on all streets other than residential streets shall
be recessed when a 4 inch depression or greater is used in
the inlet.
2. Inlet details shall conform to the College Station Standard
Specification For Street Construction.
3. Curb opening inlets shall have a minimum length of 5 feet.
B. Types of Inlets
Inlets are classified into two groups: inlets in sumps
(Ty pe A) and inlets on grade (Type B). These are further
subdivided as follows:
Inlets in Sumps
-Curb Opening (with or without gutter depression) Type A-1
-Grate Inlet Type A-2
Inlets on Grade
-Curb Opening with gutter depression
-Curb Opening without gutter depression
C. Inlet Location Guidelines
Type B-1
Type B-2
1. Inlets shall be located consi~tent with the requirements of
the College Station Standard Specifications For Street
Construction.
2. Inlet location shall conform to the requirements of Section
IV of this guide.
37
3 . Inlets shal l be lo cated as feasible to limit co nf l i c ts
(c aused by the inlet its e lf o r associated storrnwater) wit h
vehicle, bic ycle, o r pedestr ian traffic.
4 . Inlets shall be located on streets to prevent concentrated
storrnwater fl ows f rom c r oss ing traffic lanes, except at
intersections and driveway entries. Typical loca t ions for
these c onditions are at transitions to superelevated
sections, ends of long traffic islands, or medians in
superelevated sections.
D. Inlet Sizing
1. Inlets in Sumps -These inlets are placed at low points to
relieve ponding of surface water. For purposes of design,
inlets with greater than a 5" gutter depression on streets
with less than a one percent ( 1%) grade may be considered
as inlets in sumps. Inlets at bridge and culvert structures
shall be oversized to acc orruno date 12 5% of the design st o r~
from the roadway.
Under no circumstances shall inlets at low points in
streets allow water to pond above a depth of 18 inches
above gutter flowline for d esign storms up to 100 years and
ultimate basin development conditions. Where this is
exceeded a overflow outlet o r swale within the street or
adjacent drainage easement shall be provided to handle
excess flows, or the inlet and receiving facilities shall
be oversized as necessar y .
a. Curb Opening Inlets, Type A-1 -Unsubmerged inlets are
considered to function a s a rectangular weir with a
coefficient of disc~ar~e o f 3.0.
found by the f oll o w1 n~ e q ua t i on:
1 ' 1 Q = 3 .0 L; ~'-, wh ere
Q is the capac::v Ln cfs
Their capacity is
L is the leng:h ~f :~e o pening which water
enters into :~e :~~e~
38
-y is the total depth of water o r head on the
inlet.
Figure V-1 i s a graphical sol utio n of the ab ove
equation.
Because of the tendency for cu rb opening ~nlets in
sumps to collect debris, their ~a lculated capacity .
shall be reduced by ten percent (1 0%) to allow fo r
clogging.
b. Grate Inlets, Type A-2 -These inlets are considered to
function as an orifice with a coefficient of discharge
of 0.60. Their capacity is based on the following
equation:
1/2 Q = 4.82 A y , where g
-Q is the capacity i n cf s
A is the clear opening area in sq. ft. g
- y is the total depth of water or head on the
inlet in ft.
Figure V-2 is a graphical solution of the above
equation.
Because of the tending for grate inlets to collect
debris, their calculated c apacity shall be reduced by
twenty-five percent (25%) to allow for clogging, except
where used as controlling devices in detention basins.
2. Inlets on grade
a. Curb Opening Inlets without gutter depression, Type B-1
The capacities of t hese inlets are based upon the
weir equation, reduced to account for street grade and
cross-flow effects. The head, "y", shall be taken as
the depth of flow at t he upstream end of the opening
determined from sec ti on IV of the guide. Figures V-3
and V-4 shall be used to determine the capacities of
these inlets on grad e, with the value for "a" set equal
to 0.
b. Curb Opening Inlets with gutt er depression, Type B-2
The same guideline s and c riteria apply as f o r
those inlets listed above i n V. D. 2. a., except the
value "a" shall be taken as the gutter depression. The
gutter depressi o n is de f ined as the dif fere nce i ~
elevation f r orn th e nor mal gutter g rade to the p a ve~en r:
grade at the t h roat or e n tr y of t h e inlet. (see Figu re
V-5)
c . Cse of Figures V-3 and V-4
The following definit i ons are g iven:
Qa = the total flow in the street approaching t h e in~e r:
(cfs)
y = depth of flow, Qa, at the upstream end of t he
inlet (ft)
La = length of inlet required to intercept the f lo w
Qa (ft)
L = length of inlet specified (ft)
Q = total flow captured by the inlet (cfs)
Typically, "Qa" and "y" have been previously co mputed.
Entering figure V-3 with these values and the selected
gutter depression, the value Qa/La is selected, from
which La can be determined. The inlet size ( "L") c a n
then be selected.
If L > La no further calculations are necessar y as a ll
flow is captured by the inlet. If L < La, then f i gure -
4 is used with the values of L/La and a/y as inpur:s c o
obtain Q/Qa. From the value Q/Qa, Q can be compute d
and the flow past the inlet is given by Q-Qa.
.,,. ..........
1i'.'iiS
, ___
9 .0
8 .0
7.-
6 .... ~ ~ ~
~.o
4 .-
3 .;
12 -1••
6 ....
s. ...
4 .:
3 ....
z ....
-.v
-·-o.;
I-0.8
~ 0 .8
(!)
~ o.s z w
~ 0.4 -
1-w
..J
z 0.3
~
~ o.z
~
0
J: -I-.v
a. w
0
1.0
. i
I
I • • • .. ,. .. -.,,. ·---I -
'· ·'
I I
80 10
Q/AQ RATIO OF DISCHARGE TO AREA OF
FIGURE 7-2
I • ' '
'
. "' . ' .. .. .. "
I
IS 20 30
GRATE (CFS/SQ. FT. )
INLET CAPACITY TYPE A-2
42
. '
40
..,,.r ·••,.,.
~
1.00
.9 ....
.a ....
. 7_
so
Jt::::::::::::::========t=====::±:====t:::::j==:f:::::±==C:::C:J====~~~-~·----=t==+=:+=~~~.~·:::;:==+==~==:i:::::t=t::::+:::::::t
D7
~ .06 ~
u
a: w a.
>-
1-
~ a. .
4 u ,,
0
..-::: 0
0
II
02
I
7
.
01-r-~~~~~t-,""'---'-+~;._.t-.._t-~~ir---+--+1'-r1-+-1~~~--'~t---.i..... ....... .-+-.;...;J~+--~f---+-----.,~..--1;-,
0 .1 .02 03 ·""' ~ .oe 01 ce 10 .20 .30 Ml .so .60 .ao
y s DEPTH OF FLOW IN APPROACH GUTTER {FT.)
FIGURE 1Z'-3
CAPACITY FOR CURB OPENING INLETS ON GRADE
43
I
.... .,. .......
. 2 2 9 0 I
I I
I
I
i
i
I
I
I
I I
I I
I
I l
I I
I
I
I
I
.-I i :~::::::::::;,,====~-~-~:+:::::::::::~::::::::+:::::::::+:::::::~·:::::::::;:::l ::::::::t::::::= __ ~===~:::::::;:;;;;;~~~~~=~~::;!!~ -·---~,-----11
--
r
I CJ If ... ,, / / ,. --I
---··
I /'fl.I .r .r --,
i
,, , [•f /'f i
I I
----TI,,,, .rr
-· ... .. 'W•<I#
I / I 'ii' ./' ./" I L a I .,... .. , DI r•----I I I
-I T ,,.,, ,, f •a --M l!U,,_ -.... . .
I I J II / ._ ,,,.. . ---;--r-' , -----, / r ,,.~ ,, / • ---· . ~ -•• ~ •· · l-----+--_-_-+-----11-~,1-,~~~~~~,'l"'-,_.,,.'+-----+-----1----.. ri:,~=-=, ... ~~ • ...,,i+---._~,r~-.rl•~L-.~rt-1 -~r-----+----.l------'
_.. , , ., , ./l
J•I, "'./'. ~a , ... 11 T-'1•1 I
I ' ~i \. t
V ~ ... · \1 '9 ..... ,,.I .
...... 61 --..... ,....._._._ ,,,.,,
---------I. ~.. -t----t----t---r~-~-· ~-' "I'll'' I • -r-..,. ••• - • ·--
auttar 1 I t ·I.._ '--t----+-----+-----'------. -·.
I~ I .. NIG1IWA~ ,.. ... _ -!
llJ I I' II ' .&J r • I f\I
. .
' . I I
I : I I
I ; i
-----+-----~!..__.I _-+-+--'--+--' --+-----..,r----~-----+-----+-----+-----.-..----+-----------1 I
1------+-1 ___ -i_., ___ ,""-1r-t'....,.---r----1-----+----;'·~----1------+----+------<1---~·--+----~-----~·-----<'r-----+-'.....,'-+--+-+-'~-.-+----+-----~-----+-----1------+-----+------<-------+-----·-----~ I
~========~' ==:::.-_-_-_-_--1i-_-_-_-_-_-_-_-_-i_"""""'':::::::::::::t::::::::::::::::t~~"'".~""''4·.~~~:_,+~ .. -1~~:~.-~,~~~;:::;4 ~:::;_":_-r_ ... ~~~:t:::::.:::~::::::::~;.::::::::r::::::::+::::::::~·,.:_-_-.:_-_ -_
I
,... • -'"" - -•"' ~r .-. ' r --~ ---l'.""1-q.-~-~-IRH•~•r-r-1-i-~0t-<A .... i-.11·1----t1
---------
1 n '"''v'' \,,j .. ,,._,",so;.'"""'°''''-''"''"' .. ._:''-"'., ·----1 ...:.:...:~i '-""--.:+==-.......:c....::+.:....:.....:-==-'--l~--=%..:l'-f------+-----
L=---~'--·---+----,.......l ---rlil"1n~11l~r-,~.-~1,..__~T...,"rl<T .... 1.;1n~f~~·!:~-~~~-.---~!::!E•f_,-.~--l"ii.!lr_1·~1i"r~.P~t,_~.ri;_.-=ih~==~----~:~~~.:~:::::::_-_+;::_-~:_::--+~--=====---~ -~-
I ------+-----;----1'-----1-----+-----------,-·-----+---+------+-----+---+-.. --
1 ----4----~----r-'----.-----+: __ --, ____ .;.._ ____ ;__1' ---~:-----+-----+-------·-
I -l ! ~· i _J __ -=: : ---+-------+---_.----:: == -r-----.-----t-·-----1----~ ------1 ---1 -----+I----+-----"'f-----r----I I ---------!----+-----+. -----, I _._---+-----+----+-I ______ -··-I ' ___ _._ _______ ~ __ _._ ___ _._ ___ ~--~I----_.__-------
I , 11
WATER SUR~CE NORMAL Pt4VEMENT SLOPE
:. .-. ..
..... ...
.....
. .
FIGURE .Jl:-S
SECTION THROUGH CURB INLET WITH
GUTTER DEPRESSION
45
APPROACH DEPTH
GUTTER DEPRESSION
VI . St o rm Drainage Systems
:\. General
Storm Drain Systems are c o nduits for the collecti o n and
tr:rnsport of surface waters to desired points of d i scharge.
Design is accomplished by application of the Manning Equation
either directly, or through charts and nomographs that are
based upon it. The following general conditions apply to t he
design:
1. The system must be designed to accommodate all intercepted
flow for the design storm at each inlet and opening that
allows stormwater into the s ystem.
2. The system shall preferably operate "flowing full" and
within the theoretical ;imits of open channel flow for the
required design flows,
3. Design and construction shall take into account any future
additions and resulting flows added to the system. ~o
existing system shall have flows added or directed to it
that exceed its theoretical design capacity.
4. The system shall be evaluated with associated drainage
systems for the flow conditions that will result from a 100
year rainfall event and ultimate basin development
conditions. Design shall be revised as required to prevent
formation of any conditions that could be considered
hazardous to life or property, or that create conditions
inconsistent with the rel{uirements of other sections of
these standards.
B. Initial Design Considerations
1. Minimum and Maximum Velocities
Minimum velocities are necessary to prevent excessive
deposits of sediment that could lead to clogging. The
minimum design velocity for c onduits flowing full shall be
2.5 ft/sec.
~ximum velocities are '.1ecessary to prevent excessi ve
erosion of the inverts. T:-i e maximum design velocity fo r
conduits flowing full shal l be 15 ft/sec.
46
2. Roughness Coefficients, "n"
Selection o f a r o ughness c oefficient should reflect t h e
average condition present during the life of the conduit.
Items to consider are erosion of the interior surface,
displacement of joints, and foreign material and deposits.
!he following values shall be used for the materia l s l ~ste d
below:
Reinforced Concrete -.014
Ductile Iron or Steel (Smooth) -.010
Corrugated Metal -.024
PVC -.008
3. Guidelines for Location of Manholes and Junction Boxes
a. Junction boxes shall be provided at all changes in
conduit size, grade, or alignment, and at conduit
intersections.
b. Manholes shall be provided at intervals not to exceed
300 feet for conduits 54 inches in diameter or
smaller. For conduits greater than 54 inches i~
diameter, the interval between openings shall not
exceed 500 feet.
4. Minimum and Maximum Grades
a. The minimum grade for conduits shall be that necessary
to produce the minimum accepted velocity.
b. In order to prevent formation of a hydraulic jump
outside the conduit, the maximum grade along the
outfall shall be less than the calculated grade
necessary to cause supercritical flow, except where an
energy dissipation structure is provided.
5. Minimum Pipe Diameter
No conduit which will become an integral part of the
public storm sewer system shall have a diameter less than
18 inches, except for short laterals to adjacent inlets may
be 12 inches. Conduits of 24 inches or less diameter shall
be assumed to have 25% reduced cross-sectional area for
design purposes.
47
6. ~iscellaneous
a. Design shall attempt to increase the velocit y i n th e
downstream direction.
b. Pipe sizes shall increase in the downstream direction,
regardless of additional capacity developed by
increased grade.
c. Provide an elevation drop at all inlets, !llanholes, and
junction boxes equal to the change in pipe diameter or
.1 feet minimum.
d. Pipe shall be preferably placed on the design friction
slope.
C. Hydraulic Design Requirements
1. Flow Assumptions and ~anning's Equation
Design shall be by application of the Continuity
equation and Manning'£ Equation as follows:
Q = AV
1.49 2/3 1/2
Q = A R , where
n
Q = flow in cfs
. 1 . f 2 A = cross sectiona area in t
V ~ velocity of flow in ft /sec
n = roughness coefficient of conduit
R • hydraulic radius = A/wp in ft.
Wp • wetted perimeter in ft
sf • friction slope of c onduit in ft/ft
Capacity of a given size conduit is based upon an
assumption that i t 1s ''flo wing full". Thus, R is
equivalent to the c r os s sectional ar.ea divided by the
inner circumference, ·-hi ~e a value for n and Sf must be
chosen~
48
2 . Head Losses and Friction Losses
Head Losses computed at junct ions, inlets, and manholes
shall be determined using the fo llowing equation:
v 2 -v 2
2 1
h . = k . (-------------) , where
J J
2g
h = J
head loss in ft. at structures
vl = velocity at upstream entrance of structure
v2 velocity at downstream exit of structure
k . = structure coefficie'nt of loss (Table VI-1) J 2 g = 32.2 ft/sec
Head Losses for open channel flow conditions are foun d
by the following equation:
hf = Sf L , where
hf = head loss due to friction in ft.
sf friction slope (normally equal to the slope of the
conduit, S ), in ft./ft.
0
L = length of conduit in ft
Head Losses for pressure flow conditions are found
by the following equation:
L v 2
hf = SfL + f-, where
D 2g
v = velocity of flow in conduit in ft/sec
D =-diameter of conduit in ft
f = friction factor (Darcy-Weisbach)
All other variables are as previously defined.
3. Computation of Hydraulic Grade Line
All designs shall verif y the elevation of the hydrauli c
grade line by calculation along the length of the system
for two conditions. The theoretical hydraulic grade line
shall be verified as being at least .5 foot below the inlet
o pening elevation, gutter e le vation, or ground surface fo r
4 9
t he design st o r m. Th e hyd r aulic g ra d e line s hall als o '.:l e
calculated for the 100 year st o r m and ultimate ba s in
development conditio ns, and must be kept within t he limits
specified in all oth er sect ions of t h ese standards.
4 Allowance for Surcharging
Design of the system and evaluati o n of hydraul ic gr ad e
lines shall take into account the tailwater elevat i on a t
the outlet or final discharge point.Discharge at f ree
outfalls shall assume a starting water surface elevation a t
the soffit (inside top) of the conduit. Where outlets ma y
be submerged, t h e starting water surface elevation shall be
taken as the depth of water at that location or the soffit,
whichever is highest :
Table VI-1
Coefficient Of Loss, Kj*
Inlet on Main Line
Inlet on Main Line with
Branch Lateral
Junction or Manhole on Main Line
with 45 Degree Branch Lateral
Junction or Manhole on Main Line
with 90 Degree Branch Lateral
Inlet or Manhole at Beginning of Line
Conduit on Curves for 90 Degree*
Curve radius • diameter
Curve radius • (2 to 8) diameter
Curve radius ~ (7 to 8) diameter
0.50
0.25
0.50
0.25
1. 25
0. 50
0.40
0.25
** Where bends other than 90 Degree are used, the 90 Degree bend
coefficient can be used with the following percentage factor
app 1 i ed:
60 Bend -85%; 45 Bend -70%; 22-1/2 Bend -40%
* From City of Austin Drainage Criteria Manual
51
VII . Open Channel Flow
A. General
Analysis of open channels is d one to determine the depth
and velocity of a given flow for a established cross-sectior ..
Typical uses are to determine the tailwater elevat ion at a
culvert structure, flood elevation for selected discharge of
natural streams and watercourses, and discharge capacities for
existing man-made channels.
Design of open channels involves the selection of a cross-
section, finish surface treatment, and alignment to accommodate
a given design discharge. A successful channel design results
in a stable structure that does not develop excessive sediment
deposits or erosive cuts, maintains a stable cross-section, and
is not damaged by entry of uncontrolled surface flows.
B. Determination of Water Surface Profiles
1. ~ethods of Analysis
a. Manning's Equation
The equation is expressed as follows:
1 .49 2/3 1/2
Q = A R s , where
n
Q • the discharge in cf s
n •Manning's Roughness coefficient
A • cross-sectional area representing the depth of
flow in ft 2
R • hydraulic radius = A/W.P. in ft.
W.P.=-wetted perimeter of channel section for area
"A" in ft.
S • slope of channel bed in ft/ft
The equation is applied to a single cross-section and
assumes steady, unif o rm flow in the channel as well as
a uniform channel cross-section and slope. Because of
this, its use shall tie lim ited to man-made and natural
channels in the secondary drainage system.
52
b. Standard-Step Procedure
This procedure shall be us e d in anal yzing natural
or man-made channels of the primary drainage system.
It may also be applied to open channels i n the
secondary drainage system.
The procedure invol ves application of Bernoulli's
Equation to a series of stream cross-sections using the
continuity equation, the velocity head, and ~fanning' s
Equation as inputs. A . detailed description is beyond
the scope of this Design Standard.
The method shall be applied using the HEC-2 water
Surface Profiles Computer Program developed by the
Hydraulic Engineering Center of the U.S. Army Corps of
Engineers, or other computer analysis program employing
the same meth0dology. The application shall be
according to the recommendations contained in the
user's manual for the program.
2. Discussion of Parameters
a. Channel Section
Cross-section(s) should be representative of the
channel reach being studied.
b. ~nning's Roughness Coefficients ("n" values)
Selection of values for "n" shall fall within the
range of values and descriptions given in Table VII-1.
c. Channel Slope
The slope of the c hannel shall be taken as the
average slope along the reach being studied.
3. Determination of Flow Character
In order to prevent fo rmation of supercritical flow
areas and hydraulic ju mps exce pt where planned, flow must
be kept within the limits 'J f subcritical flow. To do this,
design flow depth mus t )e 5reater than critical depth .
Critical depth can f .'J und from the following
relationship:
53
c.
= , where
g B
Q = discharge in cfs
g 32.2 ft /sec 2
A = cross-sectional area of flow in ft 2
B = top width of flow in f~.
The terms A and B can be expressed as functions of each
other or the depth of flow, d, for c.hannels of uniform
cross-section. For
curve of critical
constructed.
non-uniform cross-sections, a rating
shall be depth versus discharge
Once the discharge Q, area A, and depth d are
determined, the slope necessary to produce these conditions
in a channel can be computed from ~nning's Equation.
Design of Open Channels
1. Physical Considerations
a. Cross-Section Geometry
Refer to Figure
""1II-I
minimum standards.
Alternate channel sections are acceptable provided they
meet the intent of these standards.
b. Minimum and Maximum Grades
Minimum grade shall be .004 ft/ft (.40%) for earth
or vegetative lined channels to prevent formation of
standing water. ~ximum grade shall be less than .7 of
the calculated critical slope values for the range of
flows. An exception is for channels specificall y
designed to function with supercritical flow, and that
possess the necessary linings and energy dissipation
structures to contain the resulting high velocities and
hydraulic jump (Ref. Sec ti on X). Channels designed to
function with supercritical flow are limited to
straight sections having a minimum grade greater than
1.3 of the calculated c ritical slope values for t h e
range of flows.
54
c. Hori z onta l Curv e s
?ro vi d e addi t ion a l ch anne l bank heigh t and surface
treatment o n ou ts i de cu rves to preve~t er o s io n an d
overtopping caused by d esign flows.
d . Erosion Protect io n ~e a s u res
1. Channel ear t h tan k s and inv erts shall be s ee d e d o r
sodded acc o rd i ng t o the re q ui rements of th e Ci t v
of College Station Standard Specific at~o n Fo r
Street Construction.
2. Design velocities shall be less than the
recommended velocity of flow for the proposed
surface treatment used (Table VII-2). where
multiple surface treatments are used, the limit i ng
velocity shall be the minimum recommended value of
all t ypes of sur f ace treatment being used.
3. All locations where stormwater enters t he channe l
shall have erosion and/or energy d i ssipation
measures to protect the channel lining an d
section.
4. Channels designed to function with supercrit ical
flow shall be lined appropriately to handle th e
resulting veloc i t i es and possess energ y
dissipation struct ures.
2 . Flow Consideration
a. Design Flows
1. Channel capacity shall be determined for the
discharge from a 25 year storm for the ultimate
basin deve lo pment c onditions.
2. Channels sha ll be designed to flow eit her
subcritical o r s u percritical for · the range of
discharges resu l t ing from the 5 year, 10 year, ,-_:::i
year, and 10 0 year storms for existing an d
ultimate basin de vel o pment conditions.
3. Concrete i nverts o f trapezoidal channels shall be
designed t o carr y 1/3 the peak discharge of a 5
year s to r ~ fJ r th e ul timate basin d evelo pme nt
c ondit i ons, ~ith ~o r e q u i rement for freeb o ar d .
55
b. Vel ocit y Limitatio n
1 . Velocit y of f l o w s hall no t be l ess t han 2 .5 fr /sec
f or t he design st o r m, except for tempo r a r y e ar th
channels.
2 . \i . .. aximum velocities fo r t he design flow shall t:le
less than t he values given in Table VI I -2 fo r :he
type of surface treatment specified.
c. Freeboard Requirements
Channels shall be designed with a dd itiona l
freeboard equal to .20 times the design depth of flo w,
but not less than .5 feet.
D. ~edification to Natural Watercourses
1. General
a. All modifications to natural channels shall meet t h e
requirements of open channel sections.
b. Modifications shall be defined as permanent physica l
changes
surf ace
in the alignment,
treatment of the
cross-section geometr y , an d
channel and overbank are 3 s.
Maintenance and removal of excess vegetation are not
c onsidered permanent c h anges.
c. Changes to channel sections are encouraged to be par t
of a masterplan for t he modification of a c omple te
length of a regulatory reach of the Primary System.
Changes to short reaches of channel must demonstrate
compatibility to similar modifications along the leng th
of the regulatory reac h .
d. Minor encroachments, co nsisting of fill a nd e ar th
changes in the existing defined floodway fringe areas,
are permitted on a i ndivi dual site basis without regar d
to a masterplan f or t he regulatory reach of the
channel. Any encroac hment shall meet all requireme nts
listed in the foll owin g secti9ns.
2. Determination of Floodwa y a nd Floodplain Areas
a. for natural s t reams ?a rt of the Primary Dra ina~e
System, a comprehens iv e ~y dra u l ic model re f erred to ::..::
the College Station Flood St ud y has been adopted . T~~s
56
stud y shall be us e d a s th e source of floodway and
floodplain areas for streams and ch annels of th e
Primary System.
b. For streams and channels not part of the Primary
Drainage System, floodway and floodplain areas shall be
determined by extending the adopted flood stud y using
the standard step procedure. ~ew flood discharges, as
required, shall be computed using the methods outlined
in Section III of the guide.
c. The floodway shall be determined using an encroachment
method based upon equal conveyance reduction from each
side of the channel overbank. An allowable rise of one
foot in water surface elevation for the 100 year flood
will be permitted. However, the limits of encroachment
shall not extend-_ into the designated channel area (See
Figure VII-2).
3. Design Consideration
a. Water Surface Profiles
Proposed changes in channel section or alignment
shall not increase the existing or ultimate flood
elevations as established within or upstream of the
area of modification by the adopted Flood Study. For
streams or channels outside of the Primary System,
existing and ultimate flood elevations shall be
established by extending the adopted Flood Study as per
VII. D. 2. b. above.
b. Hydrologic Analysis for System Impacts
When · existing channels are straightened, improved
in cross-section, and/or lined their efficiency
increases. This results in lower basin travel times
and t-imes of concentration, as well as loss of over bank
storage capacity. All of these cause higher flood
discharges and resulting higher flood elevation
downstream of the area of improvement. Any changes to
channels within the Primary System shall be accompanied
by a reanalysis of t'."l e hyd r o logic model (both current
condition and ultimate co ndition) of the adopted Flood
57
Study . The ch anges will ~e reflect ed in the routing
reaches and lag factors for affected channel reaches
and basins.
Downstream impacts shall be reviewed to prevent
dama ge to existing property and structures. Key i terns
to identify are the effect of higher discharges at
bridges and culverts, and the change in flood
elevations. Channel improvements shall not cau se a
rise in flood discharges :hat will exceed crossing
structure capacity and shall not raise the ultimate 100
year flood elevations.
c. Redefinition ·cf Floodpla i n and Floodway Areas
When existing channels are straightened, improved
in cross-section and/ or lined, existing floodplain and
floodway areas are altered. These changes shall be
made part of the adopted Flood Study and submitted to
the required authorities. Redefinition shall follow
the methodology for floodway determination outlined in
VII .D. 2. c. above.
d. Transition Sections
The modified channel section shall be preceded and
followed by transitions to the existing upstream and
downstream channels. These transitions should be
gradual to prevent the formation of excess energy
losses and turbulence in the flow, or the creation of
inappropriate velocities in the upstream or downstream
channels. Abrupt ch an ges in section, profile, or
alignment must be accompani ed by the appropriate energ y
dissipation struct u res.
58
Tab l e VII-1
Manning's Roug hness Co effic i ents 1
NATURAL STR EAM CHANNELS
I . Mi nor Streams
I I.
A. Fa i rly regular section
1. Some grass and weeds; little or no brush
2. Dense gro~th of weeds, depth of flow
materially greater than weed height
3. Some weeds, light brush on banks
4, Some weeds, heavy brush on banks
5. Some weeds, dense willows on banks • • .
6. For trees within channels with branches submerged
high stage, increase all values above by .•.•
B. Irregular section with pools, slight channel meander,
use 1A to SA above, and increase all values by
Fl ood Plain (adjacent to natural streams)
A. Pasture·, no brush
1. Short grass
2. Tall grass
B. Heavy weeds, scattered brush
C. Wooded
. .
at
. . .
.
.
.
Mi n.
. 0.030
. 0 ,035
G.035
0.050 . 0.060
. 0.010
. 0.010
. 0.030
. 0.035
. . o. 050
0.075
This varies depending on undergrowth, height of foliage on trees,
etc. The area of "n" • 0. 10 and greater i ndicates an extremely
heavily wooded condition.
LIN ED CHANNELS
1 . Metal corrugated . . . . . . . .
2. Neat cement 1 i ned .
3. Concrete . . . . .
4, Cement rubble . . .
GRASS COVERED SMALL CHANNELS, SHALLOW DEPTH
1. No rank growth •
2. Rank growth
UNLINED CHANNELS
1. Earth, straight and uni form
2 . Dredged . . . . . . . . . .
3. Winding and sluggish . . . .
4. Stony beds, weeds on bank
5. Earth bottom, rubble s i des
. . . . .
. . . .
. . . .
. 0.021 . 0.012 . 0.012 . . 0.017
. 0.035
. . 0.040
. 0.017 . 0.025 . 0.022 . 0.025
0 .028
~a x .
0 .03 5
0 .05 0
0 .050
0.070
0 .08 0
0.020
0 .020
0 .035
0 .050
0 .J7Q
0. 12 0
0 .024
0 .018
0 .018
0 .0 30
0 .045
0 .'J SO
0 .02 5
0 .0 33
0 .)30
'J . :i:.J
' . --... -_.
Fr om "Hy drau l i c Manual" of State Depar tm ent of High"Ways and Public T rans p or t a t1 s~
59
Table VII-2
Maximum Design Velocities f~r Various
Surf ace Treatments
Surface Treatment
Exposed Earth*
Grass -Seeded
Grass -Sodded
Impermeable
(Concrete, Gunite, Etc.)
*Temporary Channels Only
Maximum Design Veloc i ty
(ft/sec)
3.0
4.5
6.0
10,0
From "Erosion and Sediment Control Guidelines for Developing Areas in Texas"
by the Soil Conservation Service
60
4' -o· .. , ...
h • 6 MINIMUM -VEG[TATIV[ COVE"
h•2MINIMUM -LINED COVE"
TRIANGULAR CHANNEL
NOTE: CONC"[T[, ,_[IN'O"CING ANO JOINTS SHALL
I[ AS ~ER STANOA,_O CONC"[T[ SIDEWALK DETAIL.
4' -o· MIN •
LOW FLOW GUTTER SECTIONS
.20~ -
-... FALL ... ,... 111
20 .... .
4' -o· ...... . I.
TRAPEZOIDAL CHANNEL
.... 1 ...
. . .
"
"•I MINIMUM -VEGETAT IVl
COV!R
"• 0 MINIMUM -LIN!O COVE
TYPICAL CHANNEL CROSS SECTIONS
FIGURE VTT-
61
LIMIT OF LIMIT OF
LT. ENCROACHMENT RT. ENCROACHMENT
T
FLOOD WAY
FRINGE
LEFT OVERBANI<
NATURAL WATER SURFACE
+ 1' RISE
NATURAL WATER SURFACE
I
T
FLOOD WAY
FRINGE
CHANNEL RIGHT OVERBANI<
X -DESIGNATED CROSS-SECTION POINTS BY STATION ANO ELEVATION
\J -DEFINED LOCATION OF CHANNEL BANKS BY STATION
SCHEMATIC OF STREAM SECTION
SHOWING ENCROACHMENTS
FIGURE E-2
62
VIII. Culverts and Bridges
A. General
1. Design discharge sha11 be based upon t he ultimate
development conditions that are projected to exist in the
applicab1e drainage basin served by the structure.
2. The design discharge shall be that caused by the "25 year"
storm or greater, except for structures 1ttithin th e
secondary drainage system on residential streets may use a
design discharge caused by the "10 year" storm.
3. Structures shall additionally meet the criteria established
in 8.2. below for discharges up to "100 year" storm,
4, Discharge of str~et or storm drain flow at structures shall
be made in such a manner as to prevent erosion or scour of
adjacent channel or embankments. Typically, a concrete
apron or basin shall be provided to receive the discharge.
5. Figure VIII-1 shows a schematic of the terms used to
describe a typical culvert structure.
6. Structures within established areas of special flood hazard
as defined by the ordinance shall meet all the requirements
for those areas.
B. Design Limitations and Performance Criteria
1 . Maximum Operating Headwater
a. Culverts and bridges shall be designed to limit
upstream headwater to e 1 evat ions that do not endanger
or cause flooding to adjacent structures or properties
for all discharges up to and including the 100 year
storm.
b. Upstream headwater e l evat i on for the design discharge
shall be no greater than th e minimum top of curb at the
crossing less 1 foot.
2. Allowable Over-Road Flow, )e pth , and Velocity
Discharges that exceed th e design discharge may be
carried over the roadway ;:irovided the following criteria
are met.
63
a. All over-road discharge shall be contained in suc h a
manner that it will be returned to the watercourse
without endangering adjacent properties or structures.
b. The maximum depth of flow shall be 2.0 feet, measured
from the lowest point in the roadway profile at the
crown of the section to the elevation of the operating
headwater.
c. The product of velocity (ft/sec) of the overflow
discharge times the maximum depth of flow (ft.) as
measured in b. above shall be less than 6. The
overflow velocity shall
continuity equation:
Q over
v • -, where
A
be determined from the
V • velocity in the overflow discharge, ft/sec
Q over • maximum discharge over the roadway,
ft/sec
A • area of the overflow section described by
the headwater elevation and roadway profile at
the crown.
3. Determination of Design Discharges
a. For structures over watercourses in the Primary System,
the design discharges shall be determined from the
adopted College Station Flood Study as per Section 111
of these standards.
b. For structures over watercourses in the Secondary
System, the design discharges shall be determined using
the appropriate methods outlined in Section III of
these .standards.
4. Maximum Discharge Velocities
The velocity of discharge through the structure shall
be 1 imited to the fol lowing values based upon immediate
downstream channel conditions. Downstream conditions will
64
be evaluated to the point where normal flow characteristics
are reestablished in the receiving channel, a distance not
less than 4 times the difference between the width of the
flow downstream mi nus the width of the structure opening.
This requirement need not apply to discharges expected for
rainfall events above the 5 year storm.
Downstream
Condition
Natural Channel Section or
Man-made Section with Seeded Cover
Man-made Section with Sodded Cover
Concrete, Paved, or Rock Riprap Cover
C. Physical Configuration
Maxim~m Allo~able
Discharge Velocitv
(ft/sec)
6
8
15
The following guidelines shall be used to select the type
of headwalls and endwalls for culvert structures. Orientation
of headwalls shall be with respect to the direction of flow
upstream and downstream of the structure. In no case shal 1
headwalls or wingwalls restrict the clear opening of the
structure.
1. Parallel Headwalls and Endwalls
a. Approach velocities are less than 6 ft/sec for the
design discharge
b. Approach channel is irregular and not well defined
c. Downstream channel protection is not necessary
2. Flared Headwalls and Endwalls
a. Approach velocities are greater than 6 ft/ sec for the
design discharge.
b. Channel is well defined and regular in cross-section.
D. Hydraulic Design consideration of Bridges and Culverts
The following items shall be addressed as part of the
engineering design and analysis of crossing structures.
Bridges shall be analyzed for hydraulic conditions using th e
65
HEC-2 Water Surface Prof i les computer program app l ied us in g th e
gu i delines and recommendations of the U.S. Army Corps of
En gineers.
Culverts may be analyzed us i ng the same method as for
bridges. Additionally, they may be analyzed using accepted
ch arts and nomographs for the type of structure ~nd ma t eria l
proposed for use. Chapter IV of the Hydraulic Manual prepared
by the Bridge Division of the Texas State Department of
High'#ays and Public Transportation contains a complete
treatment of culvert analysis and design, i nc l uding
nomographs. It shall be considered the standard for analys i s
for culverts by this procedure.
1 . Operating Considerat i on for Culvert Structures
The flo._ through a culvert structure is either limited
by conditions at the entrance to (Inlet Control) or at the
exit from (Outlet Control) the structure. The type of flo'#
for each discharge shall be determined.
2. Head'#ater and Tail'#ater Elevation
a. Tail'#ater elevations shall be determined using one o f
the methods described i n Section VI I of the guide for
open channels.
b. Head.,.ater elevations shall be determined by adding the
total head losses through the structure to t he
tail.,.ater elevation, for the given discharge.
3. Head Losses
The total head, H, on a structure is the sum of a ll
losses due to exit, frict i on, and entrance conditions for
the given discharge.
a. Entrance losses are caused by the narro-.ing of fl O'#
from the normal channel '#idth to the structure opening,
(predominant for bridges) and to the shape or condition
of the actual inlet or opening (predominant for
culverts): Channel l osses of this type must be
computed using a standard ste~ procedure as outlined in
Section VII. Entry lo sses s hall be computed using t he
follo-.ing equation:
66
he = Ke [~=z --~~] W'here
2g 2g
he = entrance head loss, feet
V2 = velocity of flow in culvert, ft/sec
Vl = ve 1 oc ity of flow in approach channel,
ft/sec
32.2 ft/sec 2 g ,.
Ke• entry loss coefficient from Table VIII-1
b. Exit losses are caused by the expansion of flow from
the structure opening to the normal channel width
downstream. The same equation for entrance losses
applies to those for exit losses except Ke may be taken
as .5 and V1 shall be defined as the velocity of flow
in the downstream receiving channel after full
expansion.
c. Friction losses are those that occur within the
structure i tse 1f. These can range from open channel
flow losses, pressures flow losses, to losses caused
by physical obstructions within the structure (bridge
piers as an example). All friction losses shall be
accounted for in the analysis and design of crossing
structures.
4. Erosion and Scour Protection
a. All culverts determined to be functioning under inlet
contra l for the design discharge sha 11 have a energy
dissipation structure at the outlet of the culvert or
meet the requirements of "b" below.
b. The velocity of flow for the design discharge in the
structure shall not exceed the requirements for the
downstream channel condition in VIII. B. 4. above.
67
Table VIII-1
Values Of Entrance Loss Coefficients "K 111
e
Typ e of Structure and Entrance Design
Box, Reinforced Concretg
Submerged Entrance
Parallel Wingwalls
Flared Wingwalls
Fr~e Surface Flow
Parallel Wingwalls
Flared Wingwalls
Pipe, Concrete
Projecting from fill, socket end
Projecting from fill, sq. cut end
Headwall or headwall and wingwalls
Socket and of pipe
Square-edge
End-Section conforming to fill slope
Pipe, or Pipe-Arch, Corrugated Metal
Projecting from fill (no Headwall)
Headwall or headwall and wingwalls
Square-edge
End~section conforming to fill slope
1 From City of Austin Drainage Criteria Manual
68
Value of K e
0.5
0.4
0.5 o. 15
0.2
0.5
0.2
0.5
0.5
0.9
0.5
0.5
HEADWATER
ELEVATION
(HW)
C I iJW U
UPSTREAM
HEADWALL
ROADWAY
CROWN
TOTAL HEAD "H"
ON STRUCTURE
SLOPE, S u
DOWNSTREAM
ENDWALL
TAILWATER
ELEVATION
{TW)
"WWW
TOTAL HEAD H = t LOSSES = EXIT LOSS + FRICTION LOSS + ENTRANCE LOSS
HW: TW + H
SCHEMATIC OF TYPICAL CULVERT STRUCTURE
FlGURE ~ -l
69
r
IX. Deten ti on Facil i ties
A. Genera 1
The purpose of a detention f aci 1 ity is to store excess
volumes of stormwater runoff and discharge it at a
predetermined controlled rate. Typical l y, this is done t o
li mit post development discharge rates to those that exis t ed in
existing conditions for a range of des i gn storms. The fo ll ow in g
sequence of design storms shall be used until the maximum
design storm is reached: 5 year, 10 year, 25 year, 50 year, 100
year.
Detention facilities may be site or development specific ,
their purpose being to protect im mediate downstream properties
and drainage systems. An example of this would be a detention
facility in a large parking area to avoid overwhelming adjacent
streets and storm sewers of t he secondary drainage system.
Typical methods utilize parking lots, landscaped areas, and
rooftops formed into depressions that drain dry between
rainfall events.
Detention facilities also may be regional, receiv in g
stormwater from many developments and sites. Here the l i mit i ng
capacity is that of the dra i nage system that traverses an
existing developed area. This type of facility requires a
l arge land area to develop the required storage and thus is
usually designed for multip l e uses compatible with its
stormwater purpose. Typically, these are combined with
greenbelt and recreation areas centered around a permanent
storage area designed to hold water between rainfall events.
B. Design Parameters
1. Design Storm
All detention facil i ties located in the secondary
drainage system that are s it e or development specific shal l
use a maximum design storm based upon the specif ic
detention requirements. Th ese requ i rements are based upon
the requirements of t'i e -:Jth er sec ti ons of these standard s
70
and the effect the development 111i 11 have on the receiving
facilities of the secondary drainage system relat i ve to
performance standards and path111ay requirements. In
addition, a design storm having a return period of 100
years shall be evaluated to check emergency overfl 0 ,.
requirements of the detention facility and the effect of
resulting flo""s on do'#nstream dratnage systems.
All detention facilities located in the primary
drainage system (regional) shall use a maximum design storm
having an average return period of 100 years or greater.
2. Delineation of Drainage Area
The land area contributing runoff to the proposed
detention facility shall be determined for the pre-
development existing condition and the post-development
expected condition.
3. Pre-development and Post-development Hydrographs
A pre-development hydrograph shall be determined
representing the drainage area and land cover conditions
that exist prior to the proposed development.
A post-development hydrograph sha 11 be determined
representing the drainage area and land cover conditions
that exist after ultimate development occurs 111ithin the
1 and area that contributes runoff to the detention
facility.
Hydrographs shall be determined using the appropriate
methods from Section III of the guide.
4. Determination of Sto~ge Volume
Storage vo 1 ume sha 11 be determined such that the peak
discharges of the ultimate development hydrographs for the
design storms from the detention facility shall be limited
to values less than the target discharges. The target
discharges shall be equal to or less than the peak
discharges of the pre-development hydrographs for the
design storms.
71
The required storage volume may be taken as th e
difference in area between the pre-and post-deve l opment
hydrographs as shown in Figure IX-1. The out let structure
shall be sized such that the design storage volume will
occur at a depth that corresponds to the target discharge.
This method shall be limited to detention facilities whose
hydrographs may be determined by the rational method. All
other detention facilities shall have the storage volume
determined from Storage-Routing Analysis procedures.
5. Storage-Routing Analysis
The basis of this method is the continuity equation
stated thus:
Iavg -Oavg • dS/dt, where
!avg • the average inflow over time period t
Oavg • the average outflow over time period t
dS • the change in storage
dt • the designated time period
Substituting subscripts of 1 and 2 for beginning and
end of time period respectively, and re-arranging terms
yields the final equation.
2s 1 2s 2
0 1 + I 2 ) + ( -o,). <---+ Oz)
dt dt
The use of this procedure ; s based upon several
assumptions:
The inflow hydrograph is known.
The starting conditions of storage volume and
outflow are known at the beginning of the routing.
The discharge rate at the outlet structure(s) is
only a function of the head available.
The relationship between depth and storage are
known.
The time period "dt" shal 1 be taken as less than or equal
to 1/St (time of concentrat i on). c
72
6. Outlet Structures
a. The design of o utlet structures shall cons id er th e
conditions for all design storms. The outlet structure
shall limit the peak d i scharge to less than the peak
discharge that existed under predevelopment conditions
for all design storms.
b. A 11 out 1 et structures sha 11 be designed to a 11 ow the
facility to be drained dry by gravity. This does not
include those faci 1 ities designed to have a permanent
storage component~
c. An emergency overflow outlet shall be provided with a
capacity to carry the peak discharge from a 100 year
storm for ultimate basin development conditions. This
discharge must be directed and limited to prevent
damage to adjacent properties and hazards to life. In
addition, this . discharge shall be evaluated for its
effect on the downstream receiving drainage systems,
and shall not exceed its capacity to control and
contain the ultimate condition discharge.
d. Analysis and design of outlet works shall use th e
methods and guidelines in Sections V -Storm Drain
Inlets, Section VI -Enclosed Storm Drainage Systems,
Section VII -Open Channel Flow, and Section VIII -
Culverts.
C. Physical Characteristics and Limi tations
1. Side and Bottom Slopes
a. Side slopes shall not exceed 4:1 for vegetative cover
and 2:1 for non-vegetat iv e cover.
b. Bottom slopes must be 20 :1 or steeper directed to the
low flow outlet for facili ties designed to drain "dry"
between rainfall even ts .
c. A low-flow invert shall be provided for all facilities
which have a vegetative c over on the bottom •. Design
shall be cons i stent with Section VII -Open Channel
Flow.
73
2. Emergency Overf l ow Requirements
3.
a. The geometry shall be that of a rectangular weir.
b. Surface treatment shall be consistent with the expected
velocities at ultimate conditions. Velocities shall be
1 imited or proper surface treatments provided as per
the recommendations in Sections VII -Open Channel Flow
and Section VIII -Culverts.
c. At least . 5 feet of freeboard above the water surface
elevation for ultimate conditions shall be provided
around the perimeter of the facility.
Storage Depths
The maximum storage depths for design and ultimate
conditions shall ~e limited as follows:
Facility Design Ultimate
Location Hydrograph Hydrograph
Parking Areas .83ft 1.Sft
Rooftops .Sft 1. Oft
Landscaped Areas 3.0ft 4.Sft
4. Permanent Storage Facilities
All facilities located on natural streams or water
courses that are designed with a permanent storage
component shall meet all criteria in terms of design and
construction for Dams and Reservoirs as required by the
Texas Water Commission.
5. Detention facilities shall have their design storage
capacity increased by 10% to allow for sedimentation,
except those located in parking lots and rooftops.
74
0 -IM
<:> a: < J:
(.)
Cl)
c
~
'\
'\
'\
'\
TIME, t
PRE-DEVELOPMENT HYOROGRAPH
'\
'\
'
SIMPLIFIED METHOD OF DETERMINING
STORAGE VOLUME
FIGURE IX-I
75
X. Erosion and Sedimentation Control
A. General
1. Measures to mitigate the effects of erosion and resu 1 ting
sedimentation are divided into two categories.
a. Temporary measures are designed to capture eroded
sed ime;1ts on areas that are incorr.plete or under
construction. The emphasis is on preventing sediments
from being transported and deposited, by ·#ind, water,
or actions of man, on adjacent properties, or into the
secondary or primary drainage system.
b. Permanent measures are designed to prevent erosion and
resulting sedimentation from occurring. In addition,
they promote the proper drainage of stonnwater.
c. A general guide and reference service for erosion and
sediment contra 1 methods and protect ion is put out by
the Soil Conservation Service. It is adopted as a
guide and can be obtained at the following address:
U.S. Dept. of Agriculture
Soil Conservation Service
P.O. Box 648
Temple, TX 76501
Request the publication entitled "Erosion and Sediment
Control Guidelines For Developing Areas in Texas".
2. Measures to prevent the movement of sediment by erosion or
action of man shall be implemented at all areas undergoing
development or construction. Positive steps shall be taken
by those conducting these activities to:
a. Prevent the transport of sediment from these areas onto
adjacent properties or any part of the primary and
secondary drainage system.
b. Promptly remove all sediment as a result of their
activities that enters onto adjacent properties or any
part of the primary or seconda!y drainage system.
76
B. Non-Permanent Erosion Control Measures
A method to contra 1 or contain sediment materi a 1 sha 11 be
used on areas newly developing or under construction.
1. Sediment Basins
Sediment basins may be used to trap runoff waters and
sediment from a development. The water is temporarily
detained or slowed down so that the major portion of the
sediment carried by water drops out and is retained in the
basin while the water is released automatically. These
consist of a dam or embankment, a pipe outlet, and an
emergency spillway. They are usually situated in natural
dra i nageways or at the 1 ow corner of the deve 1 opment. In
some situations where an embankment is not feasible, a
basin excavated below the earth surface may serve the same
purpose. Temporary basins serve only during the
construction stage and are eliminated when vegetation is
established and the area is stabilized.
The size of the structure will depend upon the
location, size of drainage area, soil type, and rainfall
pattern. Sufficient space for sediment should be provided
to store the expected sediment yield from the drainage area
for the planned life of the structure, or provisions made
for periodic cleanout of sediment from the basin.
State and local safety regulations regarding such
structures shall be observed concerning design, warning
signs, and fencing.
a. Basins shall not be installed where the following
conditions occur:
(1) failure of the structure would result in loss of
life, damage to buildings, highways, railroads, or
interruption of use or service of public
utilities;
(2) height of embankment e~ceeds 20 feet;
(3) the drainage area exceeds 200 acres.
b . Sedi ment Storage Capacity
The sediment storage capac i ty of a d ebris :J as in
sha 11 equa 1 or exceed the vo 1 ume of sediment to be
trapped in t!ie basin dur i ng its planned useful 1 ife.
The minimum capacity provided shall be in accordance
with criteria in the referenced guide.
The storage basin shall be cleaned out whenever
its functioning is impaired due to insuf f icient
capacity.
c. Principal Spillways
A pipe spillway is required on all basins. Th e
pipe spillway shall consist of a vertical pipe riser or
box riser jo~ned to a conduit which will extend through
the embankment and out 1 et be 1 ow the downstream toe of
the f il 1.
The pipe spi 1 lway shal 1 be proportioned to convey
not less than 0.2 cfs per acre of drainage area without
causing flow through the emergency spillway. The
minimum size pipe shall be 4 inches in diameter. The
vertical pipe riser or box riser shall have a cross-
sectional area at least 1.5 times that of the p i pe.
The pipe
controlling the
completely.
spillway shall have a method of
re 1 ease of water to drain the basin
d. Earth Emergency Spillways
A 11 basins sha 11 have an earth emergency spi 1 lway
unless the peak flow from the design storm is carried
through a pipe spil l way or other mechanical spillway.
The earth spillway s h a ll be excavated in undisturbed
earth or compacted
designed to be stable
Peak discharges
f 1 11 .
fo r th e
•o r
The
design
design
spillway shall be
flow.
of the
spillway shall be ::m p ut ed using
emergency
the methods for
detention facilit i es ~,, Sect i on VIII and shall be based
on the soil and :3 n ~1co ated cover conditions i n th e
drainage area d ur 1rg :"e expected 1 ife o f ':h e
structure.
For debris bas i ns with 20 acres or less watershed,
the combined capac i ties of pipe and emergency spi l lways
shall be sufficient to convey the peak discharge from
the 10-year, 24-hour frequency storm. For debris
basins with watersheds greater than 20 acres, the
combined c~pacity of pipe and t!mergency spillway shall
be adequate to convey the peak discharge from the 25-
year, 24-hour storm.
The top of embankment dam for a 11 debris bas; ns
shall be at least 0.5 foot higher than the stage
reached by the design storm.
The crest elevation of the emergency spillway will
be determined by the head required on the principal
spillway but shall be at least 1.0 foot above the crest
of the principal spillway.
e. Physical Requirements
( 1) Embankment
The minimum top width shall be as follows:
Height of Embankment Top Width
10 feet or less
10 -14
14 -20
(2) Side Slopes
6 feet
8 feet
9 feet
Side slopes shall have a maximum slope of 2.5:1.
(3) Site Preparation
Areas under the embankment and any structural
works shall be cleared and grubbed; all vegetation
and objectionable material shall be removed.
f. Final Disposal
After temporary structures have served their
intended purpose and the drainage area above is
properly stabilized, the embankment and resulting silt
deposits are to be leveled or otherwise disposed of in
accordance -.ith approved final grading plans. Denuded
areas will be revegetated.
2. Hay Bale Barriers
Barriers to sediment transport may be constr uc ted of
rectangular bales of hay or straw bound into stab 1 e uni ts .
The bales are placed in horizontal ro111s and securely ti ed
or staked in pl ace. A suggested method for anchoring hay
bale barriers on ground is sho111n in Figure X-1.
This method is considered effective by itseH when t he
follo1111ng conditions are met:
There is no concentration of wat~r in a c hanr.e 1
above the barrier.
Erosion occurs as the result of sheet flow.
Length of slope above the barriers is less than
100 feet.
Hay bale barriers may be used as retards to
concentrated flow encountered in small drainage channels
such as swales,
construction. A
temporary channels, and streets under
back-up method for containing sediment
shall be provided for this situation.
Hay bale barriers sha 11 be inspected on a daily basis
and shall be replaced or repaired due to damage o r
deterioration.
3. Additional Methods
a. Construct channels or swa 1 es to divert water around
areas susceptible to erosion. Temporary seed them to
protect from excessive wash.
b. Stockpile excavated material at uphill locations on the
site and away from areas of concentrated flow.
c. Provide "111ash-down" areas for vehicular traffic at
entry and exit l ocat i ons to the construction or
development site.
d. Remove sediment from streets, adjacent properties and
parts of the primary or secondary drainage system on a
daily basis.
e. Prevent wind born dust creation by watering down
exposed earth.
f. Other methods may be s ubmit ted to the Administrator for
approved use in li eu of :~ese given in these standards.
80
C. Permanent Erosion Control Measures
The fo 11 owing i terns sha 11 be i ncorporated into the design
and construction of permanent improvements to properties.
1, Land grading
a. The cut face of earth excavation which is to be
vegetated sha 11 not be steeper than 3 horizontal to 1
vertical. Unvegetated cut slopes shall be protected by
surface treatment to protect them from erosion.
b. The permanent exposed faces of fills shall be no
steeper than 3 horizontal to vertical.
c. Provisions are to be made to safely conduct surface
water to storm drains or suitable natural water courses
and to prevent surface runoff from damaging cut faces
and fill slopes.
d. Excavations shall not be made so close to property
lines as to endanger adjoining property without
supporting and protecting such property from eras ion,
sliding, settling, or cracking.
e. No fill is to be placed where it will slide, or wash
upon the premises of another or so p 1 aced adj a cent to
the bank of a channe 1 as to create bank fa i 1 ure or
reduce the natural capacity of the stream.
2. Surface Treatment
a. All areas that are graded and stripped of natural cover
sha 11 receive at 1 east a 6" finish 1 ayer of top soi 1 and
be reseeded. The result sha 11 be a reestab 1 i shment of
a protective vegetative cover capable of resisting the
erosive effects of surface flows.
b. Protective linings and t reatments shall be specified
for all channels and swales where design velocit i es
exceed those recommended in Section VII.
c. Energy dissipation structures shall be provided where
discharge velocities from structures exceed the
allowable velocities for th.e existing or proposed
surface treatment of receiving channels. Figure X-2
through X-5 shows recom~ended configurations for energy
8 1
- - -_______________ ___.
dissipation structures at structures and c hanne l s. Th e
examples given are for the suggested configuration
only. Reinforcing steel shall be designed to resist the
anticipated hydraulic, hydrostatic, dead, and live
loads for the structures.
Flow
verticcJl fcJce
£~BEDDING DETAIL
Angle first stake toward
previously laid bale-~~~~~
Wire or nylon
bound b.iles
placed cri the
contour
2 re-b.irs, steel pickets, or
2" x 2" stakes 1 1/2' to 2'
in groUtld
ANCHORING DETAIL
FIGURE~ -I
TEMPORARY STRAW (OR HAY) BALE BARRIER
81 -----·-----------------------
FIGURE X-2
USBR TYPE m STILLING BASIN
.,,,r-0111, End soil ·,
'' ,-·Boff1t p1tr1
- - -T-<" •• 0 5 0,
--·~ --"''/o' __ s,_r:J,
z 1 Slept --
' • •' : . 0 - ---a.a Ot ----- -'-• ~----------------
FIGURE X-3
USSR TYPE IX STILLING BASIN
c(
')
' ,.J <···············-L·-··-·-···--·--··
PLAN
t . • ' .
. ' .
' : ~ ·k
' 0 v
: :r-~~· 0
·•• -3" Fill«t ~ ~ ~-d · ~; ·tb
~-_.. ........... ;--1------
.. . "' . 8tddinq •• ,.J._ SECTION
STILLING 8ASIN OE.SIGN
FIGURE X-4
USBR TYPE ~ STILLING · BASIN
-0 ....
d
PLAN
0 • 0 •
SECTION
ALTERNATE
ENO SILL
s~T ST 11.uNG IAS"'1 ~
CAHAL STlt\JCTU'l(S , SMALL OUTL(T
~S AKO S~L ~I LLWAT1
1us1• 1111
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usu; OIM(HSIOflllS
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OtSC14AllG( LIMITS
•.
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COMHlllSO-0# (H(llCY LOSl(S
FIGURE x-~
&AH l.LJ A, "ON F Cit C.U. o& 1,, Ott
S,,LLIIAT ~
I lo&S IN IX I
FOi' wtt ,,. fl ow •Oft •"t r t •O ft r ., 'O.,.
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DESIGN DATA -USBR TYPE STILLING BASINS
D lRECTAlllCULAR llASllll D HAlf -,LU
'-1... -~+--r=-
111 m
I
IJI
t-~
D
SIOE WALL
(4) (51 16 1 EQUATION NUMIER
FLAAEO IASIN
HAlf -PlAlll
0 -91t '
45 · PAEFERREO
CHUTEILOCK' ) . ......,i..._~_l,~:.,,..__,.....J~Jl!!!:~~l~~~zt
I VARIES
FLOOR OR IAFFLE ILOCKS ENO Sill
0.07 '2
(1) W9 • IASUll WIOTH U'STREAM
(2) "ILOCKSATJl4Y1 !
L __ j t
'CUT OFF WALL
(JI UtWaz ~AGGREGATE ILOCK WIOTH ~ O.H Wu • (Cl • ILOCKS .AT l/C Y1 __!!. !
ISi Waz• w1 • 2L9/li
(II W93••1•2l111
••
FIGURE X-6
ST. ANTHONY FALLS STILLING BASIN
XI . Dr aina ge ?lan Submittal Re qu irements
. .\. Review Process
Th e development
crainage plan for
permitting process requires an app r oved
anv subdivision platting activity, anv
replatting process, any d evelopment requiring site pian review
only, and any development on exi.:;ting commercial, high-d.;nsit y .
o r industrial lot or tr~ct. The review process fo r ~nv
drainage plan involves a two-or three-step process. The
preliminary drainage conference with the Administrator and a
final drainage plan is required for all development permits.
If d esired, a preliminary drainage plan may be submitted.
B. Gen eral Drainage Plan Requirements
The submitted drainage plan must satisfy the requirements
of the City of College Station Stormwater Management Ordinance,
the requirements of the City of College Station Drainage Polic y
and Design S tandards, and the specific requirements of thi s
section of the Standards.
C . Preliminary Drainage Conference
1. Th e drainage design concept fo r the proposed development
shall be discussed with the Administrator prior to the
development of any specific design or plan preparati on.
The parties representing the proposed development shall
obtain all resources, plans, and references necessar y to
discuss the i terns below. Th e co nference shall address the
following information relative to the proposed development.
a. General Location
(1) Local streets, within and adjacent to the
development.
(2) Primary and secondary drainageways and dr aina ge
facilities in the vicinity.
(3) Names of surr o unding developments.
b . Propert y Description
(1) Acreage
(2) Type of l and cov er ()o th ex i sting and pr oposed )
(3) ~fame of O•wner and ty pe o f development
(4) Current zoning status and proposed change, if any
c . Primary Drainage Basin Description
(1) Basin drainage characteristics
d.
(2) Related previous drainage studies
(3) Flood Insurance ~aps
Secondary Drair.age Basin Description
onsite and off site drainage flow
anticipated
development.
impact on existing
Discussion of
patterns and
and ultimate
e. Drainage Facilit y Design
(1) General Concept
a.) Discussion of concept and typical drain age
patterns. This shall include the delineation
of the pathways for conducting the discharges
from the site to the Primary Drainage Sy stem.
b.) Discussion of co mpliance with offsi te runoff
considerations.
c.) Discussion of the content of tables, charts,
figures, o r drawings to be presented in the
report.
(2) Specific Details
a.) Discussions of d rainage and erosion problems
encountered and proposed solutions at
specific de si6n points both onsite and
off site.
b.) Discussion of d etention/retention storage and
outlet design, i f any.
c.) Discussi o n of 11ain tenance access and aspects
of the design.
d.) Discussi on of the areas to be set aside as
drainage ease~ent and right of way.
f. References -Refere!1ce a ll c riteria, master plans, and
technical informati o n Jsed in support of concept.
2 . ~pan completion o f th e prel i~in ar y d rainage co nfere nc e , :~e
Administrator :na y require t h e submission of a prel imi:-a r y
drainage report and plans. These shall be prepared a lon g
the lines of the requirements o f the Fi nal Draina ge Re po r :
a nd Drawings.
D. Final Drainage Report and Drawings
1. The purpose of the Final Drainage Report is to identif y a nd
define conceptual solutions to the problems which may occur
on site and off site as a result of the development. .-\1 1
reports shall be t y ped on 8 -1/2" x 11" paper and boun d
together. The report shall include a cover letter
presenting the proposed design for review and shall be
prepared by a Registered Professional Engineer licensed in
Texas. The report shall contain a certification sheet as
follows:
"I hereby certify that this report (plan) for :!:e
drainage design of (~ame of Development) was prepared by :n e
(or under my supervision) in accordance with the provisions
of the City of College Station Drainage Polic y and Desi gn
Standards for the owners thereof."
Registered Professional Engineer
State of Texas ~o.
(Affix Seal)
2. Report Contents
The Final Drainage Report shall be in accordance wit h
the following outline a nd contain the applicab le
information listed:
a. General Location and Description
(1) Location
a.) Local s treets within and adjacent to :~e
devel op ment
b.) Primar y Dra i:1 a ge Sy stem drainageways a i:a
facilities
c.) ~a.mes o f surr o un ding developments
(~ I
(2) Description o f Pr o pert y
a.) Acreage
b.) Land cover
c.) Primary and Secondary Sy stem
within the property
d.) General project description
b. Drainage Basins and Sub-Basins
(1) Primary Basin Description
drainagewa ys
a.) Reference to Primary Drainage System planning
studies such as flood hazard delineation
reports, and flood insurance rate maps.
b.) Primary Basin drainage characteristics.
( 2) Secondary Drainage Basin Description -Discussion
of offsite drainage flow patterns, and impact of
development o n existing and proposed pathways to
Primary Drainage System.
c. Drainage Design Criteria
( 1) Regulations -Discussion of the optional criteria
selected or the deviation from the Standards, if
any.
(2) Development Criteria Reference and Constraints
a.) Discussion of previous drainage studies for
the site in question that influence or are
influenced by the drainage design and how the
plan will affect drainage design for the
site.
b.) Discussion of th e drainage impact of site
constraints such as streets, existing
structures, and development or site plan.
(3) Hydrological Criteria
a.) Identif y design rainfall
b.) Identif y runoff calculation method
c.) Identify d ete ntio n discharge and storage
calculation metho d, if any
d.) Identif y ce si gn storm recurrence intervals
e.) Di sc us s i o n an d j us tifica t io n o f o t::er
c ri t eria o r c a lcul ation met hod s us e d t ha t are
not presented in o r re f ere nced bv th e
Standards.
(~) Hy draulic Criteria
a.) Identif y various c apaci ty re f erences
b.) Identif y detenti on /retent i on out l et ty pe , if
any.
c.) Discussion of o ther dra i nage facilit y desi gn
criteria used that are not presented i n t he
Standard s.
d . Drainage Facility Design
(1 ) General Concept
a.) Discussion o f c oncept and typical d rainage
patterns
b.) Discussion of compliance with off site runoff
c onsiderations
c .) Discussion of th e content of tables, ch arts,
figures , or drawings presented in t h e report
d.) Discussion of anticipated a nd pr o po sed
drainage patter n s.
(2) Specific Details
a.) Discussion of d rainage
and solutions a t specific
b.) Discussion o f detention
design, if an y
problems encountered
design points
storage and outlet
c.) Discussi o n o f :n a i ntenance access and aspect s
of the d es ig n
e. Conclusions
( 1) Compliance 1o1i th S und ards of the City o f Colle ge
Station Drai nage ?o lic y and Design Standards
(2) Effectiveness 0 f fra i ::.a ge design to control dama ge
from storrnwa t er ~~0 1o1s
(3) Explanation ·)t ::-.e eff ect iv eness of exist i ng a nd
proposed imp r o ~e ~e n:s ~ith regard to co ntr ol ling
the dischar ge s Jf ::-.e ~00 year storm wi t h ul tima t e
d evelopment c ond ~:i0 n s o f t he sec ondar y syste~
basins affected ~y :h e pr o po sed devel o pment.
a ~
-------------------------
f . References Reference all crit eria and tec~nic2 l
information used.
g. Appendices (whe re Applicable)
(1) Hydrologic Computations
a.) Land use assumptions
properties
regarding adjacent
b.) ~inor and ma i or storm runoff at specific
design points
c.) Historic and fully developed
computations at specific design points
d.) Hydrographs at critical design points
(2) Hydraulic Computations
a.) Culvert capacities
b.) Storm sewer capacity
c.) Street capacity
runoff
d.) Storm inlet capacity including inlet control
rating at connection to storm sewer
e.) Open channel design
f.) Detention area /v olume capacity and ou t let
capacit y calculations
E. Final Drainage Report Drawing Contents
1. Sheet #1 -General Location Map that
a. Depicts drainage flows entering and leaving the
development site;
b. Identifies major construction along path of drainage;
c. Illustrates general drainage flow within entire basin;
and
d. Is drawn at a scale of l "=500' to l "=2000' .
2. Sheet #2 -Floodplain Information
a. Copies of existing City of College Station 100-year
floodplain maps showing the location of the subject
development.
3 . Sheet #3 Drainage Plan '.-la p ( s ) o f th e p r o po se d
dev elopment at a scale of l "=20' to 1 "=2 00 ' on a .-,, ,, .;.-. x
drawing shall be includ ed.
following :
The plar. shall show the
a. Existing and proposed contour-s at 2-feet maxi:num
intervals.
b. Property lines, and easeme nt s with purposes noted.
c . Streets.
d. Existing drainage facilities, roadside ditches,
drainageways, gutter flow di rections, and culverts.
All pertinent information such as material, size,
shape, slope, and l ocation shall also be included.
e. Overall drainage area boundary and drainage sub-area
boundaries.
f. Proposed type of street flo w, roadside ditch, and/or
gutter flow directions.
g. Proposed storm sewers and open drainageways, including
inlets, manholes, culverts, and oth er appurtenances.
h. Proposed outfall point fo r runoff from the d evel oped
area and facilities to conv ey flows along the proposed
pathways to the final ou t f all points in the primary
drainage system wit ho ut damage to down stream
properties, or exceeding the capacities of the
receiving facilities as defined in these standards.
i. Routing and accumulation of flows at various critical
points for the minor st o rm runoff.
j . Path(s) chosen for com putation of time-of-
concentration.
k. Details of detention /retention storage . facilities and
outlet works.
1. Location and elevat ions of all defined floodplains
affecting the property.
m. Location and elevati on s o f all existing and proposed
utilities affected by o r affecting the drainage design.
n. Routing of o ffsice '.irainag e flow through the
development.
F . Construction Drawin g s and Sp ec ificatio ns
1. Where drainage im pro vements are to be con stru cted ~~
accordance with the approved Final Drainage Repor t, the
constructicn plans (on 24 " x 36 '' sheets ) and specifications
shall be submitted for re vi ew and approval p r i o r t o
2 .
construction. The plans and speci fic ations fo r t~e
drainage improvements will include:
a. Storm sewers, inlets, and outlets.
b. Culverts, end sections, and inlet/outlet protect io n.
c. Channels, ditches, and swales.
d. Erosion control facilities.
e. Detention pond g rading, pilot channels, outlets, and
landscaping.
f. Other drainage related structures and facilities.
g. ~aintenance access considerations.
h. Finished floor elevations of adjacent buildings.
i. Existing and Ultimate 100-year water surf ace
elevations.
j. Engineer certification
k. Approval by the City.
The information required for the d rawings and
specifications shall be in accordance with sound
engineering principles, these Standards and the City
requirements for subdivision designs. Construct ion
documents shall include geometric, dimensional, structural,
foundation, bedding, h yd raulic, landscaping, and other
details as needed to con struct the storm d ra inage
facility. The approved Final Drainage Plan shall be
included as part of the co nstruction documents for all
facilities affected by the dr ainage plan.
G. "As-Built Drawings"
As constructed finished plan s ("..\s-Builts") for all publ i c
improvements shall be attested to b y_ a professional engine er
registered in Texas and submitted to the City before the City
will accept the improvemen ts . The con struction d rawings are
ac ceptable, if they remain tru e after construction and 3re
attested to by a registere d engineer to represent 11 ..\s-B uil:''
con ditions consistent with the int ent of the approved p lan.
C\\,.,
XII. Development Permitting Process
The City of College Station Stormwater Management Ordinance
requires the acquisition of a development permit prior to beginning
"development" activities defined within the Ordinance. A blank
Development Permit Application Form (for areas outside established
flood hazard areas) is included in this section as Figure XII-l. A
blank Development Permit Application Form (for areas within
established flood hazard areas) is included in this section as
Figure XI I-2.
FOR OFFICE USE ONLY
CASE NO .:
DA TE SUBMITTED : ____ _
DEVELOPMENT PERMIT
MINIMUM SUBMITTAL REQUIREMENTS
__ $100.00 Development Permit Application Fee.
__ Drainage and erosion control plan, with supporting Drainage Report two (2) copies each.
__ Notice oflntent (N.O .I.) if disturbed area is greater than 5 acres .
APPLICANT (Primaiy Contact for the Project):
Name ---------------------------------------
Street Address -------------------City ______________ _
State Zip Code ______ E-Mail Address --------------
Phone Numb er Fax Number --------------------------------
PROPERTY OWNER'S INFORMATION:
Na,ne ---------------------------------------
Street Address City ______________ _
State _____ Zip Cod e ______ E-Mail Address
Phone Number Fa x Number --------------------------------
ARCHITECT OR EN GINEER'S INFORMATION:
Name ----------------------------------------
Street Address ------------------City ______________ _
State -----Zip Code _____ _ E-Mail Address --------------
Phone Numb er Fax Number --------------------------------
Application is hereby made for the following development specific site/waterway alterations :
ACKNOWLEDGMENTS:
I , _____________ , design engineer/owner, hereby acknowledge or affirm that:
111e information and conclusions contain ed in the above plan s and supporting documents comply with the current
requirements of the City of College Station , Texas City Code, Chapter 13 and it s associated Drainage Policy and Design
Standards .
·a condition of approval of this permit application, I agree to construct the improvements proposed in this application
,ording to these docum e nts and the requirements of Chapter 13 of the College Station City Code.
Property Owner(s)
DEVELOPMENT PERMIT
nnr.OUIT [)(\r 1./14 /99
Contractor
I o f2
..,ERTIFICATIONS: (for proposed alt e rations within desi g nated flood hazard areas .)
A. I, certify th a t an y nonres id ential structure on or propos ed to be on this site as part
of this application is des ignated to prevent da ma ge to the structure or its contents as a result of flooding from the 100 year
storm.
Engineer Date
B. I, certify that the finished floor elevation of the lowest floor, including any
basement, of any residential structure, proposed as part of this application is at or above the base flood elevation established
in the latest Federal Insurance Administration Flood Hazard Study and maps, as amended.
Engineer Date
C . I, certify that the alterations or development covered by this permit shall not
diminisb the flood-carrying capacity of the waterway adjoining or crossing this permitted site and that such alterations or
development are consistent with requirements oftl1e City of College Station City Code, Chapter 13 concerning
encroachments offloodways and of floodway fringes .
. 1gineer Date
D. I, do certify that the proposed alterations do not raise the level of the 100 year
flood above elevation established in the latest Fed eral Insurance Administration Flood Hazard Study.
Engineer Date
Conditions or comments as part of approval :----------------------------
In accordance with Chapte r 13 of the Cod e of Ordinances of the City of College Station, measures shall be taken to insure
that debris from construction , erosion, and sedim entation shall not be deposited in city streets, or existing drainage facilities .
A II development shall be in accordance with th e plans and specifications submitted to and approved by the City Engineer
the above named project. All of the applicabl e codes and ordinances of the City of College Station shall appl y .
DEVELOPM ENT PERMIT
DPERMIT.DOC 3n4/99
2 of2
,~.tfi~
Proj ec t Name: ___________ _
R evi ew D a te: ___________ _
COlllGl STATION Reviewe r : --------------
DEVELOPMENT PERMIT
1. Application Fee ($100) .
2. Application
3. Erosion Control Plan
4 . EPNNOI (> 5 acres)
5. Public Infrastructure Inspection Fee ($300)
6. Drainage Report
7 . HOA Covenants for maintenance of detention/retention ponds
8 . Easements Dedicated (separate instrument)
9. TXDOT permits issued
10 . Engineer's Estimates
(1-4 necessary for rough grading/clearing ....... all others necessary for full pennit)
o :\deve _ ser\forms\engr\chklsldoc 3/2 5/99 11of 12
Proj ec t Name : ____________ _
Re view D a te : ____________ _
Revi ewer: _____________ _
Drainage Reports
Location
D Primary drainage system identified
D Surrounding developments shown
D Local streets, drainage systems shown
Description
D Acreage of project
D Land cover described
D Primary/secondary systems within property shown
D General description
Drainage Basins
0 References to FIRMs if applicable
D Secondary system flow patterns and impact of development on existing system
0 Proposed pathways to Primary system shown or described
Design Criteria
D Any deviation from standards being requested?
D Discussion of site constraints and capacities (streets, existing structures, etc.)
D Discussion of any applicable previous drainage studies and how this plan will affect it
Hydrologic Criteria
D Rainfall/runoff calc ul a tion method described and calculations provided
D Detention discharge I storage calculation method I calculations
D Storm recurrence intervals used
D Discussion of other drainage facility design criteria used if not referenced in Standards
Hydaulic Criteria
D Identify capacity of systems used
D Specify velocities at critical points in system
0 Identify detention I retention outlet and routing
0 Discussion of othe r drainage facility design criteria used not referenced in Standards
Drainage Facility Design
D Discuss drainage patterns I flows (pre vs . post)
D Are tl1 ey draining water onto another property owner? -if so, is it whe re the wa ter flow ed before? and is it not
more than what flo wed there before? and no faster than it flowed before?
D Discuss erosion control measures employed
D Discuss detention pond design (sideslope, low flow channels, outlet works , freeboard, emergency spillway)
D Discuss maintenance access and responsibilities
Conclusions
D Verify compliance with DPDS (signed and sealed certification)
D Explain effectiveness of improvements with regard to controlling discharges of 5 -100 year storm by
a) Detaining
b) Accommodating runoff in existing I proposed easeme nts or ROW di sc harging in to primary system
c) Combina tion of a & b
o :\devc _ ser\fonns\engr\chklsl doc 3/25 /99 9 of 12
Project Name: ___________ _
Review Date: ____________ _
Reviewer : _____________ _
Drainage Reports(cont.)
Appendices
0 References (all criteria and teclmical information used)
0 Hydrologic computations
0 Land use assumptions for adjacent property
0 Minor/major storm runoff at specific points
0 Historidfully developed runoff specific points
0 Hydrographs at critical points
0 Hydraulic computations
0 Culvert capacities (headwater and tailwater assumptions)
0 Storm sewer capacities
0 Street capacities
0 Storm inlet capacities (inlet control rating)
0 Open channel design
0 Detention area I volwne capacity I outlet capacity
Attached Drawings
0 Location map with drainage patterns
0 Floodplain map if any
0 Drainage plan with topo (existing and proposed with arrows if needed)
0 Details of outlet structures
o:\deve _ ser\fonns\engr\chklst.doc 3/25/99 10of12