HomeMy WebLinkAboutFire Flow Analysisj;ATTIS ENGINEERIN~
ENGINEERS • CONSULTANTS
P.O. Box 13461, College Station, Texas 77841 * P: 979.575.5022 * F: 979.268.0150
Fire Flow Analysis
Dr. Privett Dental Development
October 2, 2006
FIRE FLOW DESIGN
The Fire Flow Analysis and Design for the above referenced project was developed in accordance
with 'Appendix B -Fire Flow Requirements for Buildings' and 'Appendix C -Fire Hydrant
Locations and Distribution ' of the 2000 International Fire Code (Attachment #1). Please see
Attachment #2 for project site plan and location of the proposed fire hydrant(s).
EXISTING WATER LINE DATA (Attachment #3 -Flow Test)
Location: 1111 Rock Prairie Road, CS, TX
Fire Flow Hydrant: N-130
~atic Hydrant: Q-008
Average Static Pressure 93 psi
Average Residual Pressure 92 psi
PROJECT CONST ANTS
Fire Area (s.£)
Type of Construction
12,200 s.£ (Phase 1 & Phase 2)
Type IHA
Required Fire Flow (per IFC)
Required Min. Residual Pressure
# of Hydrants Required (per IFC)
Project Water Line
1,500 gpm
20 psi
1
6" A WW A C909 PVC
PIPE FLOW CALCULATIONS
Pipe Dia. Flow Rate Velocity
(in.) (gpm) (fps)
6 1,500 17.02
ATTACHMENT #1
International Fire Code -Appendices 'B' & 'C'
..
Appendix B
FIRE .. FLOW REQUIREMENTS FOR BUILDINGS
SECTION B 101
GENE:RAL
BlOl.J Scope. The ~rocedure for determining tire-flow re-
quireme!nts for buildings or portions of buildings hereafter con-
structed $1'lall be m accordance with tl1is t1.ppendix. This
appendix; does not apply to s:ructures other than buildings.
SECTION B102
DEFINmONS
B 102.1 For the purpose of this appendix, certain terms are de-
fined as follows;
:nR.E AREA. The floor Hrca, in square feet, used to determine
the requirt>..tl tlTI". fl ow.
FIRE FLOW. The flr.iw rate of a water supply, measured at 20
pounds pu square inch (psi) ( 138 kPa) residu;il pr~sure. that is
available for fire fightmg.
SECTION B 103
MODIFICATIONS
:6103.l Decr~es. The flre chief is authori~ed to reduce lhe
fire-flow requirements for isolated buildings or a group of
buildings 111 .cw:al iu-ea~ u4 ::;u!llll wuuuw1.hic:~ whi;ri; I.hi; \li;,·c:l-
opment of full fire.flow requirementi; ls impractical.
B 103.:Z Increases. The fire chief is authorized to increase the
fire-flow rcquirementi where conditions indicate an unusual
su~ceptibility to group fires or conflagrations. An increase shall
not be rnoretrum twice that required for the building under ccn-
sideracion.
B103.3 Anas without water supply systems. For informa-
tion regarding water supplies for fire-fighting purposes in rural
itnd suburbiill areas in which adequate and reliable water supply
sy5tems do not ~ist, the code official i~ authorized to utilize
NFPA 1231 or the IFCI Urban Wild/and Interface Code.
SECTION 8104
FIRE AREA
B104.1 General. The fire area shall be the total floor area of all
floor levels withir. the exterior walls, and under the horizontal
projecrions of the root of a b\ll1'1mg used to protect storage or
use areas. except as modified in Sections Bl 04.2 and B 104.3.
lil04.2 Are~ $eparation. Portioni; Qfbuiidil\g~ which ucc sepa-
rated by fire walls without openings consttucted in accordance
with the lntematioriai Bui!ding Code are allowed to be consid·
ered as $ep!ll'ate fu-e ~as.
2000 INTERNATIONAL FIRE cooee
Bl04.3 Type IA and 'I)'pe IB construction. The fire area of
huilrlini:;q t"M~tr11r;~rl of Tyrie TA and 'iy[IP-TR M1H~1·mr1·inn
shall be the area of the thn!e largest successive floors.
Exception: Fire area for open parking garages shall be de-
rennint>.d by the area of t.'rle largest floor.
SECTION 6105
FIRE-FLOW REQUIREMENTS FOR BUILDINGS
BlOS.l One-and two-family dwellings. The min.imu.m fit~
flow reqtJiremenB for or..e-and two-family dwellings having a
fire area which does not exceed 3,600 :Jquare feet (344 m2) shall
be 1,000 gAllons per ni.i.m~te (3i85 L./min). Pirc flow And flow
duration for dwellings having a fire area in excess of 3,600
square fe.et (344 m2) shall not be less than that specified in Table
BIO~.L
Exception: A reductiou in required fire flow of 50 percent,
;is approved, is allowed where t.J.ie-. b1.1ilding is equipped
throughout with an e.pproved automatic sprinkler system in
accordance with Chapter 9 of the International Fire Code .
H 105.2 Buildings otl1er than one-and two-family dwellings.
The minimum fire flow and t1ow duration for buildings other
than one-and two-family dwellings shall be as specified in Ta-
ble Bl05.l.
Exception: A reduction in required fire flow of up to 75 per-
cent, ~ "PJ.lll..m;l.l, i~ <illvw~\J wl11:11 ~he: !Juilwug h; pruvidc<l
wilh an approved automatic sprinkler i.ystem installed in ac-
cordance wilh Section 903.3. l . l or 903.3 .1 .2 of the Inrerna -
riofllll Fire Cod~. The resulting fire flow shall not be less
than 1,500 gallons p~r minute (5678 U min.).
357
t>G/60 39\;;ld ~3S 1N3~d013A3Q 5808 %t>Et>9L5L5 ~::11 t>006 /1f /80
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APPENOIXB FIRE·FLOW REQUIREMENTS FOR BUILDINGS
TABLE B105.1
MINIMUM AEQUIRED FIRE FLOW AND FLOW OU RATION F'OR BUILDINGS -FIRE AREA (1;quano !Ht) f'lfll: !'I.OW F'LOW
Type IA 11nd 1a• Tlfpe llA and lllA8 Type IV •net V-J..• Typo llB and 11188 lYP9 V·EI" laAlinn• per m~nu\Q)" DUl'IATIQN (hQUr5)
0-22,700 0-12,700 0-8.200 ___ o-s.~_ 0-3.600 1 soo __
··-
22, 701-30,200 12,701-17,000 8,201· l0,900 5,901-7,900 3,6014,800 1.750
30.201-38,700 17,001-21,800 10,901-12,900 l 7,901 -9,800 4,801-6,200 ··-2.000 2.
38,701-48,300 21 ,801-24,200 12,901-17.400 9.&01-12.600 0.201-1.100 2,250 I
I 48,301-59,000 24,201-33,20C 17.401 -21,300 I 12,601-15,4-00 i ,701-9,400 2,500
59.00J-70,900 33,201-39,700 21,301-25,500 I 15,401-1&.400 9,4Q l-ll,300 2,750
70,901-83,700 39,701-47,100 25,501-30, 100 l~ 401-21,600 ll,301-L3.400 3,000 I
I
83,701 -97,700 47,101 -54,900 30.101-35.201) 2!,801 ·25.900 13,401-15,600 3,250
3 9i,70L-l 12,700 54,901-63,400 3S,201-40,600 25.901-29,3()() 15/iOl-18,000 3.SOO I
112,701·128,700 63,401· 72,400 40,601-46,400 29,301-33,500 18,001-20.600 3.750 I I
128,701-14-5,900 72.401-&2,lOO 46,401-52,500 33,SOl-37,900 20.601-23,300 4,000 J
14$,901-164:200 82, 101-92,400 52,501-59.100 37 ,90 l -42, 700 I
23,301-26.300 4,250 !
164,201-lSJ,4QO -92,401-103, 100 59,101-66.000 42,701-4i.700 26.301-29.300 4,500
ll!J,401-:203,700 lUJ,101·114.600 66,00 l-73,300 47.701-53,000 29.301-32.600 4.750 i
203,701-225.200 114,601-126.700 73 30l-81.JOO 53.001 -58.600 32.601-36,000 S.000
_:U.S.:Z.Ol-247,700 IZ0,701-139,400 81,101-69.200 :58,601-0!1,400 ~lti,001-J~.(>00 5,250
~l-271,200 139,401-1.52.600 89,201-97,700 65,40 l · 70.600 39.601-43,400 5.500
271,2.(H -295,900 I 52,601-166,500 97,701-106,SOO 70,601-77,000 43,401-47.400 :;;1;0 f------~-
295,96'1-Greater ; l66.50 l-Gn:11~ 106,!\01-115 ,800 7i,0Cl-83,700 47,401-51 . .500 6.000 4 ! 115,801-125,500 83,701·90,600 .51,501-5.:l,700 (),250 ,__.
--125.501-135,500 90,601-97,900 55,701-60,200 6,500
--135,SOJ 145,800 !)? ,')()l-l06,SOO 60,201·64,800 6, 750
--145,801-156,700 106.801-113,200 64,801-69,600 7,000
---I 'i6.701-167.900 113,201-1:?1,300 69,601-7'1,600 7,250 ---167,901-179,400 121.30l·129,600 74,601-79,800 7,500
--179.401.1 QI ,400 129.60t. I 3S.300 79,QOl-85,100 7,750
--191.40 I -Greater ; 3B.30 l ·OreateJ-S5,l0l-Greater 8,000
fur SI: I "quure foot = 0.09:!9 m2, 1 i;:~lon ~r :ninure = 3.7N5 Um, l pt:>und per square inch = 6.895 kPa.
a. Types of co11sm1ction are b;i~e-d on lhe lmema1io1tal Bui/1.'ini: Ct1de.
b. Mcll~ur~d at 20 p»i.
2000 INTERNATIONAL FIRE COOE®
95 t>Et"3L5L5 t>t :tt t>00~/tE/8 0
.. ~
AppendixC
FIRE HYDRANT LOCATIONS AND DISTRIBUTION
SECTION C101
GENERAL
CIOl.l Scope. Fire hydrants shall be provided Jn accordance
with thh appendix forlhe proteetion ofb\iildlngs, or portione of
buildings, hereafter constructed.
SECTION C102
LOCATION
Cl02.l Fire hydrant locatfon.s. Fire hydrants shall be pro·
vided along required fire appa:atus access roads and adjacent
publiG ~tret-.ts
SECTION Ct 03
NUMB!R Ofl' FIRE HYDRANTS
C 103.1 Fire hydrants available. The minimum number of fire
hydrants available to a building shall not be less than that listed.
in Table Cl05.l. The number of fire hydrants available to a
complex or subdivision shall not be less than that determined
by spacing requirements listed in Table C 105 .1 when applied to
fire apparatus access roads and perimeter public streets from
which fire operations could be conducted.
S!CTION C104
CONSIDERATION OF EXISTING FIRE HYDRANTS
Cl04.1 Existing fire hydrants. Existing fire hydrants on pub-
lic streets are allowe9 to be cun~ithm:.J ii~ avwlublt;. :EA.i~lini;
fire hydrants on adjacent properties shall not be considered
available unless fire apparatus acces5 ro<l.cs ex.tend between
properties and easements are established to prevent obstruction
of such roads.
SECTION C105
DISTRIBUTION O~ FIRE HYDRANTS
C105.l Hydrant spacina. n1c average spacing between fire
hydrants shall not exceed that listed in Table ClOS. l.
Exception: The fl.re chtef is aulhorized to accept a defi·
ciency of up to 10 percent where existing fire hydran:s pro-
vide all or a portion of the required fire hydrant service.
Regardlesi:; of~ average spacing, fire hydrants shall be lo-
cated such that all points on streets and access roads adjacent to
a buildin~ are within the distances listed in Table C105.1.
TABLE C105.1
NUMBER AND O(STRIBUTION OF FIRE HYDRANTS -
ARli·FLOW RliQUIRliMliNT MINIMUM NUMJHr-. (opml OFHVOR~NTS
l,7S0orless I
2,000-2,2.'\U 2
2500 3
3,000 3
I 3.500-4,000 4
4,500·5,000 5
5,500 6
6,000 6
6.500-7 .000 7
7,500 or more; 8 <)r more¢
!'or SI: I foot = 3D4. S mm. 1 ~all en pa minute = 3. 785 Um
~-Reduce by I 00 f::et for dead-end sln:ds or roll!JJ;.
AVERAGE SPACING MAXIMUM OISTANCE FROM
BElWEEN HYDRANTS"'~-• ANV !JOINT 01'1 $T~EET OR ROAD
(ftell) FROHTACIE TO A HVOFIAN'T"' -soo 250
4)0 I Zl~ I
' 450 ~ 22.5 ·-
400 I 22.S =t -350 210
300 I l80 I
' 300 180
250 I 150 '
250 I 150
I
200 I 120 '
b. Where streets are provided with median dividers wbich ca;1 be crossed by tire t\gh.tcr~ pulllni; hose Jinei;. orwh<"re arten<.tl ~!ri;eth <trt! prov1dea w11ll rour ormure
:nitfic lanes 11nd haves tral'ficcoun1 of more than 30,000 "'~hicles per ctay, hydr!lllt ~p11eins sh11ll 1tverase 500 feet on ~-uch •iJeof the streC"t un<: be J!'l'illgad on an ~1-
tematini: busis up to i1 fire-flow rcq11iremcnt of 7,000 gallons per minute an4 400 feet for hishcr fire-flew requirem~nL,.
c. Whoro new "'"""...,..; ... "'"° <1'10ndGd Qlong ~t1..,..1s where hydf.:u\ts ~re not ""'"fod fo' pt0cee1ion of otrt1cturo3 or similar fkc problom11, fire h;r<J••n"; •.;lull b• pro-
·;ided at spa.cing r.01 IQ e>1~cd 1,000feec10 provide for tca1'Sl)ortation h=tls.
d. ~educ~ by SC! fut fot dead-end stn:""' or road~.
e. One hydrnnt for euch 1,000 ~llllOtlS per mln~w. or fractior. there<lf.
2000 INTERNA.llONAL FIRE CODE~ 359
95 t>£t>'3L5L5 v::1t t>00~/1E/80
ATTACHMENT #2
Project Site Plan
ATTACHMENT #3
City of College Station Flow Test
FLOW TEST
CITY OF COLLEGE STATION
WATER SERVICES DEPARTMENT
Date: 20 SEPTEMBER 2006 Time: 0840
Location: 1111 ROCK PRAIRIE
Nozzle size: 2.5
Flow hydrant: ---""N"""'"-..;;;..13;;...;0'----
Pitot reading: 72.5
G.P.M. 1425
Static hydrant: Q-008
Static PSI: 93
Residual PSI: 92
Comments:
r '.
Item
No.
1.00
1.01
1.02
1.03
1.04
1.05
1.06
1.07
1.08
pATIISENGINEERIN~
ENGINEERS • CONSULTANTS
ENGINEER'S PRELIMINARY OPINION OF
PROBABLE COSTS
Privett Dental Development 6" Water Line Extension
GE#OllOOl -October 2006
Description Estimated Estimated ESTIMATED
Quantity Unit Cost TOTAL
COSTS
6" 0 AWWA CLASS 200 C909 PVC PIPE
611 0 A WW A CLASS 200 C909 PVC 245 LF $12.00 $2,940
PIPE -Non-Structural (includes
installation, testing, and cleanup.)
6110 AWWA CLASS 200 C909 PVC 30 L.F. $13.50 $405
PIPE -Structural (includes
installation, testing, and cleanup.)
6110 Resilient Gate Valve (includes 1 EA $800.00 $800
installation and cleanup.)
8110 D.I. Tee (includes installation and 1 EA $350.00 $350
cleanup.)
8" x 6" Reducer (includes installation 1 EA $225.00 $225
and cleanup.)
6110 45° D.I. Bend (includes 2EA $225.00 $450
installation and cleanup.)
6110 D.I. Tee (includes installation and 1 EA $250.00 $250
cleanup},.-.
4'-0,,,. American Darling Fire Hydrant 1 EA $2,200.00 $2,200
Assembly (includes installation and
cleanup.)
r ~
Item Description Estimated Estimated ESTIMATED
No. Quantity Unit Cost TOTAL
COSTS
1.09 1" Water Meter Service Connection 2 EA $500.00 $1,000
(includes installation and cleanup.)
1.10 Removal of Existing 8" 90° Bend 1 EA $300.00 $300
1.11 Removal of Existing Fire Hydrant 1 EA $300.00 $300
Assembly
1.12 4' Sidewalks (includes sand cushion & 36LF $4.50 $162
cleanup)
Sub-Total: $9,382
Contingency (10%) $938 --------
PRELIMINARY TOTAL CONSTRUCTION COST: $10,320
~OF~~\~ c:,-c.~1······•"5'''\._
ltl·· * ···.\1 ~··················~·I 2 JOE I. GATTIS ~ ................... . 1. ~··. 90964 .:,;. . Gattis, PE
TIS Engineering, LLC {.,, •/tcEN'Af.~•~q; ~,~~~C>N··~~·~
Tho fulloWU.g """"'""''" """"" ;, """1 °" ""'.,.;_., preHm""" opm;., ''""""'" oo-~IP. /?.7 ~'""' judgement at this time. Please note that the engineer does not have any control over contractor or supplier workloa and the ee to which inflation may
affect project costs between now and the bid date. During design and construction, additional features may become apparent as the work progresses, which
will result in an increase in cost.
j;ATTIS ENGINEERIN~
ENGINEERS + CONSULTANTS
P.O. Box 13461, College Station, Texas 77841 • P: 979.575.5022 • F: 979.268.0150
October 4, 2006
Josh Norton, EIT
Development Services
City of College Station
College Station, Texas 77845
Re: Drainage Addendum #1 to theDrainage Report for Privett Dental Development by
CSC Engineering & Consultants, Inc. Dated January 3, 2001 & July 27, 2001.
Dear Mr. Norton,
On behalf of Dr. Privett, Owner, Gattis Engineering is pleased to submit this Drainage Addendum # 1 to
the existing drainage report for the development referenced above. This addendum is necessary to clarify
the changes that have occurred to the Phase 2 development from the original site design. Please see
Attachment # 1 for the revised site layout.
As presented in the existing drainage report dated January 3, 2001 (See Attachment #2), both Phase 1 and
Phase 2 of the development were accounted for in the drainage design for the site and the detention pond.
Originally, the Phase 2 development was going to consist of two (2) single story buildings with each having
a footprint of 4,025 square feet totaling 8,050 square feet. The revised Phase 2 development now consists
of only one ( 1) single story building with an overall footprint of 7 ,560 square feet. This is a net decrease of
490 square feet of impervious area. With a weighted 'C' Factor of0.60 and Intensity (I) of 8.6 in/hr for a
10-Year Storm Event, this results in a decrease in storm water run-off equaling 0.058 cfs.
Phase 2 parking area has also decreased from the proposed 18,926 square feet to 18,332 square feet with a
net decrease of 594 square feet of impervious area. With a weighted 'C' Factor of0.60 and an Intensity (I)
of 8.6 in/hr for a I 0-Year Storm Event, this too results in a decrease in storm water run-off equaling 0.070
cfs.
As a result of the revised development changing from two buildings to one building in Phase 2, the combined
decrease in storm water production for the I 0-Year Storm event shall be approximately
0.128 cfs. In summary, the resultant changes are considered to be negligible and thus the existing storm
water drainage and detention design illustrated in the Drainage Report for Privett Dental Development by
CSC Engineering & Consultants, Inc is still applicable.
Please call me if you have any questions at 979-575-5022
Marek Brothers Construction
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JOEi.GATTiS
ATTACHMENT #1
Project Site Plan
ATTACHMENT #2
Drainage Report for Privett Dental Development
. ,__..
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Engineering & Env11onmental
Consultants, Inc
July 27, 200 1
Mr. Spencer Thompson
City of College Station Development Services
1101 Texas A venue
College Station, TX 77840
Re: Response to City of College Station Engineering Review Comments No. 1 Regarding the Drainage
Report for the Proposed Privett Dental Development ( 1-5000144)
College Station (Brazos County), Texas
Dear Mr. Thompson:
CSC Engineering & Environmental Consultants, Inc. (CSC), on behalf of ArchiTex (Dallas, Texas) has
prepared a response to the City of College Station's review comments dated June 27, 2001. The City of
College Station's comments are presented in italics, immediately followed by a response that addresses
the comm ent.
• The Drainage Report for the Development Permit is being reviewed at this time. The engineer will be
required to show calculations in order to utilize the storm sewer junction box. lf you are not
permitted to use the junction box, please plan on re-grading the site in order to drain runoff in
another manner.
Information presented in the CSC report entitled "Drainage Report for Privett Dental Development,
Coll ege Station, Texas, January 3, 200 l" indicates that existing flow to the referenced junction box during
the I 0-year storm event is approximately 10.5 cfs. The proposed development will result in an
incremental increase of 5.7 cfs to the junction box from the future parking area for the referenced storm
event. As a result, the total flow to the junction box will be on the order of 16.2 cfs (see attached
calcu lations). The capacity of the 24-inch storm drain at this location is approximately 26.7 cfs.
Therefore, it is our opi ni on that the proposed incremental flow increase from the development can be
accommodated by the existin g 24-inch storm drain.
Please contact either of the undersigned individuals at 778-2810 if you have any questions or need
additional information concerning this matter.
1t\.'3~~~
M. Fredenck Conlm, P.E.
Senior Engineer
3407 Tabor Road
Bryan, Texas 77808
Phone (979) 778-2810
Fax (979) 778-0820
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' .~ W. R. Cullen, P.E.
Project Engineer
REVIEWED FOR
c o~~PLtAN..C~
AUG 3 O 200J~·
COLLEGE STATION
ENGINEERING
DRAINAGE REPORT FOR
PRIVETT DENTAL DEVELOPMENT
COLLEGE STATION, TEXAS FILE
Prepared for
City of College Station
P.O. Box 1000
College Station, Texas 77805
Prepared by
CSC Engi neering & Environmental Consultants, Inc.
3407 Tabor Road
College Station, Texas 77808
January 3, 2001
csc Drainage Report for Proposed Privett Dental Development
TABLE OF CONTENTS
Page
GENERAL DESCRIPTION OF SITE AND PROPOSED DEVELOPMENT ...................................... .
PRE-DEVELOPMENT DRAINAGE PATTERNS AND MAJOR DRAINAGE BASINS .................... 2
SCOPE OF REPORT AND DRAINAGE DESIGN CRITERIA............................................................. 2
STORMW ATER RUNOFF COMPUTATIONS..................................................................................... 3
USE OF THE RATIONAL FORMULA........................................................................................ 3
RUNOFF COEFFICIENTS (C) ......... ............................................................................................ 3
TIME OF CONCENTRATION (Tc}.............................................................................................. 5
RAJ.NF ALL INTENSITY (1).......................................................................................................... 5
AREA OF SITE CA) ....................................................................................................................... 6
STORMWATER RUNOFF QUANTITIES................................................................................... 6
STORMWA TER DETENTION COMPUTATIONS .............................................................................. 7
REQUIRED MINIMUM DETENTION STORAGE VOLUME ................................................... 7
DETENTION STORAGE AREA.................................................................................................. 7
STORMWATER ROUTING COMPUTA TlONS ................................................................................... 8
METH.ODO LOGY......................................................................................................................... 8
ROUTING COM PUT ATJONS AND CONCLUSIONS................................................................ 8
CERTIFICATION ............................................................................................................ :....................... 9
REFERENCES ......................................................................................................................................... 10
Table l .
Table 2.
Table 3.
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
LIST OF TABLES
Summary of Runoff Coefficient ("C") Values for Post-Development Conditions............... 4
Computed Rainfall Intensity Values for Referenced Storm Return Period .......................... 6
Calculation of Pre-and Post-Development Stormwater Runoff Rates Using the
Rational Formula.................................................................................................................. 6
LIST OF FIGURES
General Topography in Region of Privett Dental Development
Existing Topography and Proposed Site Development
Bee Creek Drainage Basin
Pre-and Post-Development Hydrographs for 25-Year Storm Event
Austin Colony Site Plan
Cumulative Outflow (Discharge) versus Depth of Storage
Inflow and Outflow Hydrographs Illustrating Routing for 25-Year Storm Event
Inflow and Outflow Hydrographs Illustrating Routing for 100-Year Storm Event
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csc Drainage Report for Proposed Privett Dental Development
GENERAL DESCRIPTION OF SITE AND PROPOSED DEVELOPMENT
The site, which is the subject of this drainage report, is located at the northwestern corner of the
intersection of Rio Grande Boulevard and Rock Prairie Road in College Station, Texas. The subject
property, depicted in Figure 1, is situated on a 1.94-acre tract of contiguous property that can be described
as undeveloped commercial property. Existing surface vegetative cover at the site consists of isolated
native trees and grasses. The surface topography across the site generally slopes toward the east. Adjacent
properties can be described as fully developed with residential land usage to the north and east, and
recreational land use to the south. The adjoining property to the west supports an assisted living
(retirement) facility with an adjacent church and associated parking.
It is our understanding that a new dental clinic known as the Privett Dental Clinic is planned for
development at the referenced location. We anticipate that the development will eventually consist of
approximately three separate structures over the approximately 1.94-acre site. However, at the present
time only one building will be designed and constructed as part of the initial phase of the project. The
presently planned building will essentially be a single-story structure, although a raised central portion of
the building that will be approximately two stories in height may also be constructed. The building will
have plan or "footprint" dimensions of approximately 49 by 80 ft or approximately 3,920 ft2 of area.
Support paving, either reinforced concrete or hot-mix asphaltic concrete (HMAC) will be installed to the
west of the Privett Dental Clinic building and will include an entry drive onto Rock Prairie Road.
Although the Privett Dental Clinic will be the only structure constructed during the initial phase
of the development, the owner had expressed interest in the design of applicable detention facility for
fully developed conditions. The following structures and respective covered areas (roof or slab,
whichever is larger) are planned for the proposed development (fully developed):
• Structures -dental office (3,765 ft2) and two future office buildings (4,025 ft2, each)
• Pavement and sidewalks -dental office sidewalk (1 , 113 ft2), dental office pavement (8,915
ft2), future office building pavement (18,926 ft2).
The total impervious cover associated with the proposed development is approximately 40,770 ft2
(.93 acres). It should be noted that the grass cover and associated slopes for greenspace and landscaped
areas proposed to be developed on the site are similar in character and grade to the existing "pre-
development" conditions. As a result, these landscaped areas were not considered as adding to the
quantity or rate of stormwater runoff associated with post-development conditions.
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csc Drainage Report for Proposed Privett Dental Development
PRE-DEVELOPMENT DRAINAGE PATTERNS AND MAJOR DRAINAGE
BASINS
The 1.94-acre site is located in the Bee Creek subbasin of the Carter Creek Drainage basin as
illustrated on Figure 2. Topographically, the surface elevation decreases by approximately 3 ft for a
distance of 420 ft across the site, from El 317 msl near the northwest corner to El 314 msl on the
southeast corner of the property. This easterly drainage pattern is consistent with a slope of approximately
0.7%.
There are no distinct drainage channels across the site. Stormwater runoff from the site is
predominantly by sheet flow to Tributary "A" of Bee Creek. The 100-year floodplain is not present within
the site boundaries.
SCOPE OF REPORT AND DRAINAGE DESIGN CRITERIA
This report addresses the need for detention of drainage from the subject site following the
construction of the planned Privett Dental Development. The site and proposed development were
evaluated in accordance with the criteria outlined in the Drainage Policy and Design Standards Manual
(DPDSM) of the City of College Station, Texas. The DPDSM is part of the Stormwater Management Plan
for the City of College Station. It should be noted that detention requirements for the planned
development were evaluated based on runoff characteristics for the larger drainage subbasin in which the
subject site is located. This approach was deemed appropriate due to the current drainage patterns in the
area of the subject site as depicted on Figure 2. As can be seen from a review of Figure 2, there is an
existing drainage feature that currently crosses a portion of the proposed project site. The drainage feature
consists of an existing curb inlet, a reinforced concrete pipe (RCP) outlet from the curb inlet, and an open
ditch along Rio Grande Boulevard into which the RCP outlet pipe flows. The curb inlet is situated on the
northern side of Rock Prairie Road adjacent to the site and services the northern portion of Rock Prairie
Road to approximately Westchester Avenue to the west and a strip of developed land adjacent to the
northern side of Rock Prairie Road. The RCP is 24 inches in diameter and flows northeastward from the
curb inlet box before making a 90° turn to the northwest to discharge into the open ditch along Rio Grande
Boulevard. The open ditch is grass lined along the boundary of the subject site but becomes concrete lined
to the north of the site. The flow in the ditch is to the northwest and Tributary "A" of Bee Creek.
As a result, the "site area" for the analysis discussed herein was considered to be the entire
contributory drainage based on the previously described drainage feature that traverses the site. The
contributory drainage consists of both off-site and on-site areas that total approximately 5.93 acres. This
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csc Drainage Report for Proposed Privett Dental Development
report also discusses specific drainage control structures related to the detention of stormwater runoff
from the proposed development.
STORMWATERRUNOFFCOMPUTATIONS
USE OF THE RATIONAL FORMULA
The Rational Formula was used to compute the total volume of stormwater runoff generated from
the fully developed site and to assess the quantity of stormwater which was required to be detained to
"offset" the increased runoff associated with the new development. Use of the Rational Formula is
reasonable for this project since the contributory area of runoff is less than 50 acres, an area sometimes
referenced in the literature as an upper limit for use of the Rational Formula. In addition, the subject site
is located within a Secondary Drainage System and not within a Primary Drainage System. The Rational
Formula is not recommended for use within a Primary Drainage System. Therefore, the Rational Formula
was used to determine the peak discharge for both pre-and post-development conditions.
The general equation for the Rational Formula is well known:
Q =C IA
where:
Q =discharge of stormwater in units of cubic feet per second (cfs)
C = coefficient that represents the average runoff characteristics of the land cover within the
drainage area of interest, i.e., the runoff coefficient, which is dimensionless
I = rainfall intensity in units of inches per hour (in/hr), and
A = area of the site that contributes to the stormwater runoff in units of acres
The values for each of these components used to compute the stormwater runoff at the subject site are
discussed in the following sections.
RUNOFF COEFFICIENTS (C)
The runoff coefficients or "C" values were computed for both pre-and post-development
conditions for the entire 5.93-acre drainage basin area. Pre-development conditions were assumed to
represent the entire drainage basin area including the 1.94-acre subject site in its present condition, i.e., as
an undeveloped grass and tree covered area. The post development conditions were assumed to be the
same for the drainage basin with the exception that the 1.94-acre lot was developed by the addition of
buildings, paved parking and drive areas, and landscaped areas. The values of the runoff coefficients used
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csc Drainage Report for Proposed Privett Dental Development
for the pre-and post-development conditions were the values presented in Table III-1 of the DPDSM for
) different types of surface covers and land uses.
Pre-Development "C". The coefficient for the pre-development condition for the entire 5.93-
acre drainage basin was a constant value based upon existing and proposed land use. A land use
description corresponding to "Medium Density Residential (Average Slope 0-1 % )" as presented in the
previously referenced Table III-1 was used in the analysis. The range in C values for the described land
use was listed as 0.55 to 0.65 in Table III-I . Therefore, an average C value of .55 was used as being
representative of the entire drainage basin for pre-development conditions.
Post-Development "C". The post-development "C" values were also determined from the
referenced table. Values of runoff coefficients for the areas of the drainage basin outside of the subject
site area were determined based upon the land use category described as "Medium Density Residential"
in Table III-I as previously described. Values of runoff coefficients representative of post-development
conditions for the subject site were based upon specific types of land cover associated with the
development. Two types of land cover were used to represent the proposed development. One type of
cover was intended to represent the planned buildings and paved areas, which are areas referenced as
"Impervious Areas (Pavements, Rooftops, etc.)" in Table III-I and have a recommended C value of 0.90.
The second type of cover was assumed to be landscaped areas with finished slopes in the range of l to
3 .5%. Table III-I recommends a range in C values of 0.40 to .070 for landscaped areas with comparable
slopes. A value of 0.55 was chosen for the analysis since the finished grade slopes will be closer to l %
than to 3 .5%.
An average or weighted C value was then computed for the entire drainage basin, considering the
changed surface covers produced by ~evelopment of the subject site. The calculations for the average or
weighted C value are presented in Table 1.
Table 1. Summary of Runoff Coefficient ("C") Values for Post-Development Conditions
Area of Extended
Coverage, Typical "C" Multiplication of "C"
Type of Ground Cover (square feet) Values Values x Area
Off-site Area, Medium Residential Area 175,982 0.55 96,790
On-site Area, Concrete sidewalks, buildings & pavement 40,769 0.90 36,692
for Privett site
On-site Area, Landscaped Areas 43,560 0.55 23,958
Summation 260,311 157,440
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Therefore, the average "C" value associated with the post-development conditions can be
/ .) determined by dividing the overall basin area (260,311 ft:2) into the summation of extended multiplication
of the individual areas times the respective "C" values (157,4400 fr):
Average "C" value= 157 ,440 ft2 I 260,311 ft2 = 0.60
Thus, the weighted "C" value across the site for post-development conditions was determined to be
approximately 0.60 .
TIME OF CONCENTRATION (tc)
The time of concentration at a site is used to determine the intensity of the rainfall event used for
computing stormwater flows and required detention volumes. The time of concentration is defined as "the
time required for the runoff to be established and flow from the most remote part of the drainage area to
the point under design."
The time of concentration for the subject site was calculated based upon the elevation difference
and the flow distance from near the northwest portion of the subbasin, to the recessed inlet on Rock
Prairie Road, to a point of discharge in the existing open ditch on the northeast corner of the subject
property as illustrated in Figure 3. The slope or grade of the site for pre-development conditions was
L...) determined from the general topographic maps of the City and the topographic survey of the site. The
slope for post-development conditions was determined from the proposed site grading plan, which will be
consistent with the existing site topography with the exception of localized grading near the proposed
structures. Using the surface travel distance of approximately 1,600 ft and a weighted flow velocity
computed from the various methods of conveyance along the flow path (Figure 3), a time of concentration
of 10 minutes was determined for pre-development flow. The post-development time of concentration
was computed in a similar manner to using a flow distance of 1,600 ft. The time of concentration for post-
development conditions was also computed to be 10 minutes using a weighted flow velocity of 2.7 ft/sec
(Figure 3). Based on DPDSM criteria, a minimum time of concentration of 10 minutes is required for the
design of drainage system components, including detention facilities.
RAINFALL INTENSITY (I)
The rainfall intensity values were computed for the 10-minute time of concentration previously
discussed using the intensity-duration-frequency curves developed by the Texas Department of
Transportation. The computed intensities calculated for storm events with "return periods" of 5, 10, 25,
50, and I 00 years are indicated in Table 2.
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Table 2. Computed Rainfall Intensity Values for Referenced Storm Return Period
Storm Return Period
(years)
AREA OF SITE (A)
5
10
25
50
100
Rainfall Intensity
(inches/hour)
7.7
8.6
9.9
11.l
12.3
As previously discussed, the site proposed for development encompasses approximately 1.94
acres. However, due to the previously discussed pre-existing drainage conditions, detention requirements
were evaluated based on a subbasin area of 5.93 acres. Accordingly, computations associated with the
evaluation of stormwater runoff were developed using an area of 5.93 acres.
STORMW ATER RUNOFF QUANTITIES
Stormwater runoff rates were calculated using the Rational Formula. Runoff rates were calculated
for both pre-and post-development conditions for the 5 .93-acre site and are presented in Table 3.
Table 3. Calculation of Pre-and Post-Development Stormwater Runoff Rates Using the Rational
Formula
Storm Site Areaore Site Area005t Intensity pre lntensity00,1 Q 1pre Qpost Q2
diff
Event (acres) (acres) Cpre Cpost (in/hr) (in/hr) (cfs) (cfs) (cfs)
5 5.93 5.93 .55 .60 7.7 7.7 25 .3 27.8 2.5
IO 5.93 5.93 .55 .60 8.6 8.6 28.4 31.2 2.8
25 5.93 5.93 .55 .60 9.9 9.9 32.4 35.6 3.2
50 5.93 5.93 .55 .60 11.l 11.1 36.7 40.3 3.6
100 5.93 5.93 .55 .60 12.3 12.3 40.5 44.5 4.0
Notes:
I. Permitted rate of outflow from detention basin.
2. Difference in flows that will require detention.
The calculated pre-development flows presented in Table 3 for the 25-year storm event represent
the maximum permissible discharge flow rates from the detention facility. The difference in the pre-and
post development flows for the 25-year storm event listed in Table 3 (Qdiff = 3.2 cfs) represents the
increase in flow rate from the development that must be detained.
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STORMWATER DETENTION COMPUTATIONS
REQUIRED MINIMUM DETENTION STORAGE VOLUME
The required storage volume of the detention basin was calculated such that the peak discharge of
the ultimate development hydrographs for the 25-year design storm was limited to a discharge less than a
defined target discharge. The target discharge was defined by the DPDSM to be the peak discharge of the
pre-development hydrograph for the 25-year storm event. As previously mentioned, detention
requirements have been computed for "fully developed" conditions even though the site development will
occur in a phased approach.
The required detention storage volume was determined as the difference in area between the pre-
and post-development hydrographs, which are depicted on Figure 4. The Triangular Approximation
Method was used to determine the hydrographs. The hydrographs were constructed by assuming that the
peak discharge, as calculated from the Rational Formula, occurs at a time equal to the time of
concentration and that one-third of the flow volume occurs before the peak discharge is reached and two-
thirds occur following the peak discharge. The Triangular Approximation Method of developing
hydrographs is generally considered to be acceptable for analysis of secondary drainage systems with an
area of less than 50 acres, which is applicable to the drainage basin addressed in this report.
By comparing the two hydrographs in Figure 4, it is apparent that the differenc~ m areas
attributable to increased flows from the proposed development is relatively small. As previously
discussed, both the pre-and post-development flows were developed for the entire 5.93 acres of
contributory drainage area. The difference in area between the two hydrographs, or the required minimum
volume of the detention storage area for the 25-year storm event; was calculated to be approximately
2,900 ft3 . A l 0% increase in the required volume as specified in the DPDSM resulted in a design
detention vo 1 ume of 3, l 90 ft3.
DETENTION STORAGE AREA
Given the relatively low storage volume required for detention and existing site conditions, the
owner has elected to take advantage of the natural topographic features and construct a detention area
within the existing drainage area at the site. The location of the proposed stormwater detention is
presented in Figure 5. The average depth of the drainage basin is approximately 2.0 ft from El 310 to El
312 msl. The volume available for detention is approximately 3235 ft3, which is approximately 2.0 %
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csc Drainage Report for Proposed Privett Dental Development
larger than the required volume of 3, 190 ft3. An outlet structure will be constructed on the north end of
{ ) \ the drainage basin to control discharge from the detention facility.
STORMWATERROUTINGCOMPUTATIONS
METHODOLOGY
The detention basin was analyzed for flow routing through the areas under different storm events.
The purpose of the routing analysis was to simulate the performance of the detention basin in the form of
inflow and outflow hydrographs.
The storage-routing analysis was performed based upon the Puls Method. The Puls Method is a
procedure for graphically solving the continuity equation for storage reservoirs using the characteristic
height-storage and height-discharge curves. As previously discussed, the equivalent of the height-storage
curve was developed graphically from the grading plan and the invert elevation of the outlet structure. A
height-discharge or discharge versus depth of storage curve was also developed for the outlet structure.
The curve for the area of interest is illustrated in Figure 6 -Cumulative Outflow (Discharge) versus
Depth of Storage.
ROUTING COMPUTATIONS AND CONCLUSIONS
Routing analysis was performed for two storm events: the 25-year event, which represents the
design storm; and the 100-year event as specified in the DPDSM. Inflow and outflow hydrographs
illustrating routing for 25-year and 100-year storm events are presented graphically on Figures 7 and 8,
respectively. As can be seen from Figure 7, the maximum outflow from the detention basin is
approximately 14 % of the maximum pre-development rate of 32.4 cfs. Therefore, the detention basin has
the capacity to store the excess volume of stormwater associated with the planned development of the site
and discharge the stored water at a rate that is "equal to or less than the peak discharges of the pre-
development hydrographs for the design [25-year] storm" as specified in the DPDSM. In addition, the
results of the routing analysis for the 100-year event as illustrated in Figure 8 indicate that the higher
flows associated with this event will flow over the top of the spillway and not adversely impact the
integrity of the basin.
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CERTIFICATION
"I hereby certify that this report for drainage design of the stormwater detention basin at the 1.94-acre
Privett Dental Development located at the intersection of Rock Prairie Road and Rio Grande Drive in
College Station, Texas, was prepared under my supervision in accordance with the provisions of the City
of College Station DPDSM for the owners thereof."
W.R. Cullen, P.E.
Registered Professional Engineer
State of Texas P.E. Number 65215
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csc Drainage Report for Proposed Privett Dental Development
REFERENCES
Chow, Ven T., Maidment, David R. and Mays, Larry W. 1988. Applied Hydrology. McGraw-Hill Book
Company. New York, NY.
City of College Station, Texas. October 1992. Drainage Design Guideline Manual.
Davis, Victor D., and Sorensen, Kenneth E. 1969. Handbook of Applied Hydraulics. McGraw-Hill Book
Company. New York, NY.
Mason, John M. and Rhombrerg, Edward L. 1980. On-Site Detention. Prepared for Texas Engineering
Extension Service, Texas A&M University. College Station, TX. Publication No. PWP: 03355-01.
McCuen, Richard H. 1982. A Guide to Hydrologic Analysis Using SCS Methods. Prentice-Hall, Inc.
Englewood Cliffs, NJ.
United States Department of Agriculture. January 1975. Urban Hydrology for Small Watersheds.
Technical Release No. 55 . Engineering Division, Soils Conservation Service, U.S. Department of
Agriculture.
Wanielista, Martin P. 1978. Stormwater Management Quantity and Quality. Ann Arbor Science. Ann
Arbor, MI.
Westaway, C.R. and Loomis, A.W. 1979. Cameron Hydraulic Data. (161h Edition). Ingersoll-Rand.
Woodcliff Lake, NJ.
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FIGURES
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