HomeMy WebLinkAboutDrainage Analysis April 2010 (2)
THE PLAZAS AT ROCK PRAIRIE ROAD
PARKING IMPROVEMENTS
DRAINAGE ANALYSIS
April 2010
Prepared for:
VISTA EQUITIES GROUP
1117 ELDRIDGE PARKWAY
HOUSTON, TEXAS
Submitted to
By
ENGINEERS & CONSTRUCTORS
511 UNIVERSITY DRIVE, SUITE 204
COLLEGE STATION, TX 77840
OFFICE (979) 260-6963
FAX (979) 260-3564
CERTIFICATION
This abbreviated report for the drainage design of the parking lot expansion for
the Plazas at Rock Prairie Road was prepared under my supervision in
accordance with provisions of the Bryan/College Station Unified Drainage Design
Guidelines for the owners of the property. All licenses and permits required by any
and all state and federal regulatory agencies for t oposed drainage
improvements have been issued. '\t' ~... co .•... *"\$\ ... ..".. :... .....•
VE~i~·,·i3:·~ ....,..... ..... ········· ..····1···· ~. 7768' ,: fl: ~ \~ .l/j~ ••••~G/STE"fi: •••·~ \r~ ............•",G
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Veronica J.B. Morg , P.E., C.F.M.
Registered Professional Engineer
State ofTexas No. 77689
Parking Lot Expansion for
The Plazas @ Rock Prairie Road Drainage Analysis
INTRODUCTION
The purpose of this abbreviated drainage report is to present an analysis of the
necessary drainage infrastructure for the parking lot expansion for the Plazas @
Rock Prairie Road. The drainage infrastructure for this project consists of two
grate inlets and two segments of storm sewer pipe, all which discharge into an
existing storm sewer system and then to a detention pond previously designed
for the site.
The site is located at the northeast corner of Rock Prairie Road at SH6 in north
College Station as seen on Exhibit 1. This project will not add any new buildings
rather only additional parking for the shopping center. This parking was
contemplated with the original drainage analysis, so this minor report is simply
verifying that this parking lot design is in compliance with the original drainage
assumptions. In addition, this drainage analysis provides for the design
calculations for the two new grate inlets and storm sewer pipes that will connect
to the existing infrastructure.
GENERAL LOCATION & DESCRIPTION
The Plazas @ Rock Prairie Road is located in College Station, Texas on Lot 2-C
and 2, Block 1 of the Plazas at Rock Prairie Subdivision (See Appendix B). The site
is partially developed as a shopping center with the area at the corner of Rock
Prairie Road and SH6 currently undeveloped (Lot 2, Block 1).
DRAINAGE DESIGN CRITERIA
All drainage design is in accordance with the Bryan/College Station USDG.
• Rational Method will be used to design storm sewer pipe with the 10-year
storm event and a check on the 100-year storm overflow route.
• Manning’s equation and a 25% clogging factor was used for pipe sizing
calculations and can be found in Exhibit 5.
As stated previously, a drainage analysis was performed for this site during its
initial design. That original design did plan for the subject area to be constructed
as future parking. Appendix A is a complete copy of that original analysis,
performed by WLB Associates. In this drainage analysis, WLB designed a
detention facility to detain the post-developed runoff from the ultimate
developed 3.592 acre site.
PRIMARY DRAINAGE BASIN DESCRIPTION
The proposed project site is located within the Carters Creek Drainage Basin
(Foxfire Tributary). Exhibit 2 is a copy of a portion of the FEMA map panel #201
effective February 2000, which shows that there is no floodplain contained on
the site.
Plazas @ Rock Prairie Road 1
Drainage Report
STORMWATER RUNOFF ANALYSIS
Pre-Development Drainage Basin (Additional Parking Area Only)
The majority of Lot 2, Block 1 is currently vacant and consists of grass cover. This
area along with the flow from the existing parking lot currently flows toward an
open pipe inlet at the junction box at the southeast corner of the property. This
pipe system carries flow to the existing detention pond. A small graded swale
exists across Lot 2 to accommodate carrying this flow from the parking lot and
the vacant lot to the pipe. From visual observation during rain events, a portion
of the runoff that leaves the existing parking lot flows toward the storm sewer
under Rock Prairie Road at the SH6 frontage road. This should not be occurring
according to the original drainage report as this flow should be captured by the
swale and carried to the detention pond. This problem will be substantially
improved with this additional parking construction. It is important to note that all
building areas drain to the rear and are carried subsurface to the junction box at
the southeast corner of the property.
Post-Development Drainage Basin (Additional Parking Area Only)
The post-development patterns for this additional parking will be to capture the
runoff from the existing parking lot (DA1) and that from Drainage Area DA2 at
Grate Inlet (GT2) within the new parking lot. This flow will be carried by a 24-inch
RCP pipe on a .55% slope to Grate Inlet (GT4) and then in another 24” RCP pipe
on a .85% slope to the existing junction box (JB1). As seen on Exhibit 3, Grate
Inlet (GT4) will pick up flow from Drainage Areas DA3 and DA4. DA2, 3 and 4
were all assumed to have a C factor of .95 in this analysis for storm sewer and
grate inlet sizing.
Until a building footprint is designed for DA3, this total “C” is really unknown and
could vary from .8 to .9. The assumptions made herein are conservative for the
design of the grates and piping system and well within the assumptions of the
overall “C” of .85 in the original drainage report. A final check should be made
when the building is constructed to assure that the composite of Drainage Areas
2, 3 and 4 are no more than 85% impervious cover.
As seen on Figure Number 6 in the WLB Drainage Report in Appendix A, the
original design contemplated full buildout of Drainage Areas DA2, 3 and DA4.
Therefore this additional impervious cover which is being added with this project
has been accounted for in the original detention facility. On the WLB Figure 5C,
the area draining to this new parking lot is .53 acres, and from these new
calculations this area is .49 acres. The difference of .4 acres is likely the grassed
area just outside the parking lot along SH6 and Rock Prairie Road, which will
drain toward SH6 & Rock Prairie Road rather than toward the parking lot.
The size of the new grate inlets are shown on Exhibit 6 and are sized based upon
the 100-year storm event with 25% clogging and checked for ponding depth
and overflow. Exhibit 7 illustrates how high the water will pond over the grate
inlet in the 100-year event. In the case of GT2, the water surface elevation will
only reach elevation 308.72’, or .51 feet high and then will spill over the curb line
Plazas @ Rock Prairie Road 2
Drainage Report
of the parking lot. In the case of GT4, the water surface elevation will only reach
309.47’ or .56 feet high and then will spill over the curb line.
The hydraulic grade line (HGL) calculations for the storm sewer can be seen in
Exhibit 8.1 and 8.2. The downstream boundary condition for this HGL is assumed
as a 100-year water surface elevation of 306.00 within the pond. This assumption
is from the WLB Associates drainage report. As seen on Exhibit 8.2, the HGL for
the 100-year storm at GT4 is 307.85’ with a top of grate elevation of 308.91’
illustrating that the 100-year event is fully contained in the pipe system. The 100-
year HGL @ GT2 is 308.70’ with a top of grate elevation of 308.21’ and a top of
curb elevation of 308.77’. The 100-year storm event is now fully contained in the
parking lot with the HGL now below the top of curb and will be carried via storm
sewer to the detention pond. The maximum ponding elevation at this location is
.49’ well within the Bryan/College Station Unified Stormwater Design Guidelines.
CONCLUSION
The development of the additional parking for the Plazas @ Rock Prairie Road is
in compliance with the original assumptions provided in the original drainage
design. This additional impervious cover will create additional runoff but that has
been accounted for in the original detention pond design. The new grate inlets
and storm sewer pipe have been adequately sized to meet the Bryan/College
Station Unified Stormwater Design Guidelines.
Plazas @ Rock Prairie Road 3
Drainage Report
Plazas @ Rock Prairie Road 4
Drainage Report
ATTACHMENTS
EXHIBIT 1: Project Location
EXHIBIT 2: Firmette –FEMA Map Panel 142C (Effective July 2, 1992)
EXHIBIT 3: Plazas @ Rock Prairie Road Drainage Area Map
EXHIBIT 4: Rational Formula Drainage Area Calculations
EXHIBIT 5: Pipe Capacity Calculations
EXHIBIT 6: Inlet Capacity Calculations – 10-Year Storm & 100-Year Storm
Capacity
EXHIBIT 7: Inlet Ponding Depth Calculations – 100-Year Storm WSE’s
EXHIBIT 8.1: Plazas @ Rock Prairie Road HGL Calculations
EXHIBIT 8.2: Storm sewer HGL Data
APPENDIX A: WLB Associates, Inc. Drainage Report “Drainage Report for
Plazas at Rock Prairie State Highway 6 Access Road at Rock
Prairie Drive” dated March 28, 2006, Revised July 14, 2006.
APPENDIX B: Replat Lot 2 & 2-C Block 1 – Plazas at Rock Prairie
APPENDIX C: Grading & Erosion Control Plan
-
I
APPROXIMAn SCALE•600 0 600 FEET E'73 E3 E3
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TEXAS AND
INCORPORATED AREAS
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UNINCORPORATED AREAS 481196 0201 0
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MAP REVISED:
FEBRUARY " 2000
11W ;. an olIIcI. CClplI of a portion of the abole r""","",ed lood map. It _
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ELEVATION REFERENCE MARKS
REFERENCE elEVATION
MARK (FT. NGVD) DESCRIPTION OF LOCATION
RM 51 285.78 Railroad spike set 'In a power pole
focated on the south side of State
6 at Lick Creek.
RM 52 296.15 From the intersection of State Route 6
and Rock Prairie Road. east approximately
0.9 mile to the intersection of Rock
Prairie Road and Bird Pond Road; railroad
spike is in power pole located
approximately 34.0 feet east of Bird Pond
Road.
_f -~--
extracted ..Ing F~lT On-Une. lNa map doea nat ....eet changea
ubaequent to the date on the
For the I_eet pnlduct InIormllllon about NatIonal Rood l..urance
Program lood mapa check the FEMA Rood Map Store at -.mac.fems.gov---
EXHIBIT 2
EXHIBIT 4Rational Formula Drainage Area CalculationsPLAZA @ ROCK PRAIRIE ROAD DRAINAGE AREATOTAL AREAGRASSLANDPAVEMENTTOTAL CAOVERLAND FLOW LENGTHOVERLAND FLOW FALLGUTTER FLOW LENGTHGUTTER FLOW FALLVELOCITYCalc. TcUSE TcI2Q2I5Q5I10Q10I25Q25I50Q50I100Q100NO. AC. 0.45 0.95 ft. ft. ft. ft. ft/s min min In/Hr cfs In/Hr cfs In/Hr cfs In/Hr cfs In/Hr cfs In/Hr cfsDA1 0.94 0.06 0.88 0.86 1.0 1.0 1.0 1.0 10.4 0.0 10.0 6.33 5.5 7.7 6.6 8.6 7.5 9.9 8.5 11.1 9.6 12.5 10.8DA2 0.20 0.00 0.20 0.19 1.0 1.0 1.0 1.0 10.4 0.0 10.0 6.33 1.2 7.7 1.5 8.6 1.6 9.9 1.9 11.1 2.1 12.5 2.4DA3 0.20 0.00 0.20 0.19 1.0 1.0 1.0 1.0 10.4 0.0 10.0 6.33 1.2 7.7 1.5 8.6 1.6 9.9 1.9 11.1 2.1 12.5 2.4DA4 0.09 0.00 0.09 0.09 1.0 1.0 1.0 1.0 10.4 0.0 10.0 6.33 0.5 7.7 0.7 8.6 0.7 9.9 0.8 11.1 1.0 12.5 1.1EXHIBIT 4
EXHIBIT 5Pipe Capacity CalculationsPLAZA @ ROCK PRAIRIE ROADINLET NO.TO INLETTOTAL CATcDesign StormUnadjusted Design Flow Adjusted Design FlowNo. of PipesFlow Per PipeFriction SlopePipe SlopeSIZEVEL.CapacityLENGTHTravel TimeTc @ End##Ac. min yr cfs cfs # cfs % % " fps cfs ' min minGT-2 GT-4 1.1 10.0 100 13.2 16.5 1 16.5 0.53 0.55 24 5.4 16.8 159 0.50 10.50GT-4 JB1 1.3 10.5 100 16.3 20.4 1 20.4 0.81 0.85 24 6.7 20.9 9 0.02 10.52EXHIBIT 5
SUMP GRATE
10 Year Storm 100 Year Storm
GT2 GT2
Q = 9.10 cfs Q = 13.20 cfs
clog = 25% clog = 25%
Q clogged = 11.38 cfs Q clogged = 16.50 cfs
h = 0.50 ft h = 0.50 ft
Req. Open Area= 3.34 ft2 Req. Open Area= 4.84 ft2
Req. Open Area=480.60 in2 Req. Open Area=697.13 in2
SUMP GRATE
10 Year Storm 100 Year Storm
GT4 GT4
Q = 2.30 cfs Q = 3.50 cfs
clog = 25% clog = 25%
Q clogged = 2.88 cfs Q clogged = 4.38 cfs
h = 0.50 ft h = 0.50 ft
Req. Open Area= 0.84 ft2 Req. Open Area= 1.28 ft2
Req. Open Area=121.47 in2 Req. Open Area=184.85 in2
EXHIBIT 6
PLAZA @ ROCK PRAIRIE ROAD
PROPOSED INLET CAPACITY
SUMP CURB: SUMP GRATE:
Provide East Jordan Iron Works Frame and Grate Model EJIW V-5736 (38 1/2"x38 1/2")
693 sq in. min open area or approved equal.
Provide East Jordan Iron Works Frame and Grate Model EJIW V-5720 (19 3/4"x 19 3/4")
175 sq in. min open area or approved equal.
Q = 3.0*L*y^1.5 Q = 4.82*A*h^0.5
L= Q/(3.0*y^1.5) A = Q/(4.82*y^0.5)
100 Year Storm
GT2
Q = 13.20 cfs
clog = 25%
Q clogged = 16.50 cfs
Inlet Selected= V5736
Open Area Selected=4.81 ft2
T/G= 308.21 ft
h =0.51 ft
WSE= 308.72 ft
100 Year Storm
GT4
Q = 3.50 cfs
clog = 25%
Q clogged = 4.38 cfs
Inlet Selected= V5720
Open Area Selected=1.22 ft2
T/G= 308.91 ft
h =0.56 ft
WSE= 309.47 ft
EXHIBIT 7
PLAZA @ ROCK PRAIRIE ROAD
DEPTH OF PONDING
FL Pipe Upstream (ft)FL Pipe Downstream (ft)Length (ft)Slope (ft/ft)Diameter (in)Flow per Pipe (cfs)Friction Slope (ft/ft)Manning's Headloss (ft)Normal Depth (ft)HGL by Manning's Upstream (ft)HGL by Manning's Downstream (ft)HGL by Normal Depth Upstream (ft)HGL by Normal Depth Downstream (ft)HGL at Upstream Pipe (ft)HGL at Downstream Pipe (ft)Storm Sewer 1GT-2 / GT-4 303.97 303.09 159.50 0.0055 24 11.4 0.0025 0.4029 1.21 306.09 305.69 305.18 304.30 306.09 305.69GT-4 / JB1 302.99 302.91 9.35 0.0085 24 14.0 0.0038 0.0356 1.20 305.69 305.65 304.19 304.11 305.69 305.65JB-1 / CI-1 302.81 302.53 111.00 0.0025 24 17.4 0.0059 0.6539 2.00 305.65 305.00 304.81 304.53 305.65 305.00CI-1 / OUT1 302.39 302.00 78.00 0.0040 30 20.8 0.0026 0.1999 1.70 305.00 304.80 304.09 303.70 305.00 304.80FL Pipe Upstream (ft)FL Pipe Downstream (ft)Length (ft)Slope (ft/ft)Diameter (in)Flow per Pipe (cfs)Friction Slope (ft/ft)Manning's Headloss (ft)Normal Depth (ft)HGL by Manning's Upstream (ft)HGL by Manning's Downstream (ft)HGL by Normal Depth Upstream (ft)HGL by Normal Depth Downstream (ft)HGL at Upstream Pipe (ft)HGL at Downstream Pipe (ft)Storm Sewer 1GT-2 / GT-4 303.97 303.09 159.50 0.0055 24 11.4 0.0025 0.4029 1.21 306.79 306.39 305.18 304.30 306.79 306.39GT-4 / JB1 302.99 302.91 9.35 0.0085 24 14.0 0.0038 0.0356 1.20 306.39 306.35 304.19 304.11 306.39 306.35JB-1 / CI-1 302.81 302.53 111.00 0.0025 24 17.4 0.0059 0.6539 2.00 306.35 305.70 304.81 304.53 306.35 305.70CI-1 / OUT1 302.39 302.00 78.00 0.0040 30 20.8 0.0026 0.1999 1.70 305.70 305.50 304.09 303.70 305.70 305.50FL Pipe Upstream (ft)FL Pipe Downstream (ft)Length (ft)Slope (ft/ft)Diameter (in)Flow per Pipe (cfs)Friction Slope (ft/ft)Manning's Headloss (ft)Normal Depth (ft)HGL by Manning's Upstream (ft)HGL by Manning's Downstream (ft)HGL by Normal Depth Upstream (ft)HGL by Normal Depth Downstream (ft)HGL at Upstream Pipe (ft)HGL at Downstream Pipe (ft)Storm Sewer 1GT-2 / GT-4 303.97 303.09 159.50 0.0055 24 16.5 0.0053 0.8439 1.61 308.70 307.85 305.58 304.70 308.70 307.85GT-4 / JB1 302.99 302.91 9.35 0.0085 24 20.4 0.0081 0.0756 1.60 307.85 307.78 304.59 304.51 307.85 307.78JB-1 / CI-1 302.81 302.53 111.00 0.0025 24 25.2 0.0123 1.3645 2.00 307.78 306.41 304.81 304.53 307.78 306.41CI-1 / OUT1 302.39 302.00 78.00 0.0040 30 29.9 0.0053 0.4123 2.50 306.41 306.00 304.89 304.50 306.41 306.00Pipe SegmentPipe SegmentEXHIBIT 8.1PLAZAS @ ROCK PRAIRIE ROADHYDRAULIC GRADE LINE CALCULATIONS10 - YEAR HGL100 - YEAR HGL50 - YEAR HGLPipe SegmentEXHIBIT 8.1
'l,r/-u~ PR~Q~~~q?
WiHiam V. Wright
William M. Langford P.E.
WiHiam L. Blad< Jr. P.E.
t)\j-tq
July 14, 2006
City ofCollege Station
1101 Texas Avenue
College Station, Texas 77842
Attention: Carol Cotter
Re: Revised Drainage Report Submittal Plazas at Rock Prairie Retail Center at
Highway 6 and Rock Prairie Road, WLB Project No. 9500502
Dear Ms. Cotter,
The purpose of this letter is to submit the drainage report, which has been prepared for the
referenced commercial project, for review and approval. The report, entitled "Drainage Report
for Plazas at Rock Prairie State Highway 6 Access Road at Rock Prairie Drive" addresses
existing Pre-Developed Condition, Post-Developed Conditions, and design ofdetention facilities
as required by the Drainage Policy and Design Manual to offset development drainage impacts to
downstream. systems. This report has been revised to address City comments from our previous
submittal.
If you have any questions concerning this report, or require additional information, please do not
hesitate to call the undersigned at (713) 278-9300. Your help has been greatly appreciated.
Sincerely,
Draiange Report Trans Ltr rEV 01-14-06.doc
7915 Westglen Drive. Houston, Texas 11063 • Tel 113 278-9300 • paz 113 953-3615 • e-mail wlb@wlbeng.cQm
WLB ASSOCIATES, mc. Drainage Report Plazas at Rock Prairie
DRAINAGE REPORT
FOR
PLAZAS AT ROCK PRAIRIE
STATE HIGHWAY 6 ACCESS ROAD AT
ROCK PRAIRIE DRIVE
Prepared For:
Vista Equities Group
1117 Eldridge Parkway
Houston, Texas
Prepared By:
WLB Associates, Inc.
7915 Westglen
Houston, Texas 77063
March 28, 2006
(Rev. July 14,2006)
WLB ASSOCIATES, INC. Drainage Report Plazas at Rock Prairie
TABLE OF CONTENTS
PAGE NUMBER
1.0 GENERAL LOCATION AND DESCRIPTION }
2.0 DRAINGE BASINS AND SUB-BASINS }
3.0 DRAINAGE DESIGN CRITERIA 2
4.0 DRAINAGE FACILITY DESIGN 3
5.0 CONCLUSIONS 4
6.0 CERTIFICATION 5
LIST OF TABLES
TABLE} PRE & POST-DEVELOPMENT STORMWATER RUNOFF RATES
TABLE 2 DESIGN TRACT AND DETENTION DISCHARGE SUMMARY
TABLE 3 DETENTION POND SUMMARY
LIST OF FIGURES
FIGURE} GENERAL VICINITY MAP OF SUBJECT PROPERTY
AND SURROUNDING AREAS
FIGURE 2 SUBJECT PROPERTY AND ADJACENT PROPERTIES
FIGURE 3 GENERAL SITE DEVELOPMENT SCHEME
FIGURE 4 WATERSHED MAP OF SUBJECT PROPERTY AND
SURROUNDING AREA
FIGURE 5A EXISTING DRAINAGE CONDITIONS
FIGURE 5B PHASE} DRAINAGE CONDITIONS
FIGURE 5C ULTIMATE DRAINAGE CONDITIONS
WLB ASSOCIATES, INC. Drainage Report Plazas at Rock Prairie
LIST OF FIGURES
(continued)
FIGURE 6 TYPES OF PROPOSED SITE COVER MATERIALS
FIGURE 7A PRE-AND POST-DEVELOPMENT HYDROGRAPHS
FOR 5-YEAR STORM EVENT
FIGURE 7B PRE-AND POST-DEVELOPMENT HYDROGRAPHS
FOR la-YEAR STORM EVENT
FIGURE 7C PRE-AND POST-DEVELOPMENT HYDROGRAPHS
FOR 25-YEAR STORM EVENT
FIGURE 7D PRE-AND POST-DEVELOPMENT HYDROGRAPHS
FOR 50-YEAR STORM EVENT
FIGURE 7E PRE-AND POST-DEVELOPMENT HYDROGRAPHS
FOR lOa-YEAR STORM EVENT
FIGURE 8 DETENTION POND SCHEMATIC AND VOLUME
COMPUTATIONS
FIGURE 9 DETENTION POND VOLUME VERSUS DEPTH GRAPH
FIGURE 10 POND OUTLET DISCHARGE RATE VERSUS DEPTH GRAPH
FIGURE 11 SCHEMATIC EROSION CONTROL PLAN
11
WLB ASSOCIATES, INC. Drainage Report Plazas at Rock Prairie
1.0 GENERAL LOCATION AND DESCRIPTION
The subject tract (Plazas at Rock Prairie development) is located at the northeast
intersection comer of the northbound feeder road of State Highway 6 and Rock Prairie
Road in the Carter's Creek Watershed. The general location of the tract is shown on the
vicinity map included in this report as Figure 1, and the subject property and adjacent
tracts are shown in more detail in Figure 2. The subject tract consists of two parcels
comprising a total area of 3.592 acres. Parcel 1 consists 1.787 acres fronting Rock Prairie
Road on the south, and State Highway 6 along the west side of the parcel. The second
1.805 acre parcel consists of Lot 2-C, Block One, of the Replat of 8.062 AC out of the
Amending Plat of Cornerstone Commercial Subdivision Section One, which fronts State
Highway 6 north of Parcel 1. The subject tract is bordered on the north by a shared
private access drive to the Marriott Courtyard at Woodcreek Development, which is
located east of the Lot 2-C portion of the tract, and north of the Parcel 1 portion. Parcel 1
is bordered on the east by an undeveloped tract.
As can be seen from Figure No.2 the site is roughly "L" shaped and extends along the
SH6 and Rock Prairie Road frontage. Surface cover of Lot 2-C is comprised of grass,
with a very few scattered trees, and has a small portion of paved access driveway along
the north and east sides of the Lot which serve the Marriot development. Parcel 1 consists
of a stand of cedar trees with scattered shrubs and grass cover. A portion of the Lot 2-C
property currently drains via sheet flow to the roadside ditch alongside SH6. The
remainder of Lot 2-C and all of Parcel 1 appears to drain to Rock Prairie Road via sheet
flow and an onsite drainage swale located within Parcell, where it is collected by
drainage inlets constructed with the Rock Prairie Road storm sewer system.
The site is proposed to be developed as the Plazas at Rock Prairie Drive, an 18,661 SF
commercial retail building with parking facilities to be constructed in Phase 1, along with
two pad sites for future commercial retail construction. The General Site Development
Scheme is shown on Figure No.3. The building footprints and layouts shown for the
future pads are preliminary and subject to change in the future.
2.0 DRAINAGE BASINS AND SUB-BASINS
The 3.592 acre subject tract is shown by Figure No.4 to be within the Carters Creek
Drainage Basin near its' divide with the Lick Creek Drainage Basin. The lOa-year flood
plain/floodway is not on or immediately adjacent to the subject property, which is shown
to be in Zone X on Flood Insurance Rate Map Number 4804C 020lD effective date
February 9, 2000. On site detention will be utilized to limit post development discharges
from the tract to the adjacent lateral drainage systems of SH6 and Rock Prairie Road to
levels at, or below the pre-development discharge rates to avoid impact to these systems
due to development.
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WLB ASSOCIATES, INC. Drainage Report Plazas at Rock Prairie
3.0 DRAINAGE DESIGN CRITERIA
Analysis and design of storm drainage facilities for this project is in accordance with
design criteria contained within the publication "Drainage Policy and Design Standards
Storrnwater Management Plan for the City of College Station" hereinafter referred to as
the DPDS Manual.
A previous study entitled "Drainage Report For Courtyard Marriott At Woodcreek
Development Cornerstone Properties State Highway 6 Access Road At Woodcreek Drive
College Station, Texas" prepared by CSC Engineering & Environmental Consultants,
Inc. dated June 26, 2000 was utilized to design stormwater and detention facilities for the
Marriott development adjacent to a portion of the site. The detention facility, which
receives discharges from a 2.5 acre area of the Marriott development, discharges to the
SH6 Frontage Road ditch via a 12-inch concrete pipe sewer located within a 20-Ft Public
Utility Easement crossing Lot 2-C along its' southern boundary. The spillway for the
detention facility also discharges into a common access easement shared between the
Marriott Development and Lot 2-C. The existing topography indicates that spillway
discharges into the Access Easement area are collected within an existing swale which
crosses the Parcel 1 area south to Rock Prairie Road. At this point it is picked up by the
public storm sewer system via an opening in the back of a curb inlet which also drains the
road paving. Development plans will need to leave the 12-inch outfall discharge pipe to
SH6 in place, and maintain the emergency overflow from the pond for storms in excess
of 25-yr frequency.
The Rational Formula has been utilized to calculate stormwater discharges as the tract
area is less than 50-acres in accordance with DPDS manual requirements. Manning's
Formula has been utilized to analyze head losses and size storm sewer and drainage
swales, and the triangular hydrograph method has been used to size detention facilities.
Since detention is required for this development, the site storm sewer was designed to
convey 100-year frequency discharges for the site to the detention pond location without
exceeding allowable ponding elevations in parking lots or driveways as specified in
DPDS Manual, Item 3 Storage Depths, on Page 74.
4.0 DRAINAGE FACILITY DESIGN
The existing site is currently undeveloped with the exception of some minor amounts of
paving for a shared common access drive along the northerly and easterly sides of the Lot
2-C portion of the project area. Figure SA -Existing Drainage Conditions shows the pre
development drainage pattern for the property, which has a small drainage area
discharging to the SH 6 Frontage Road ditch, with the rest of the area discharging to
Rock Prairie Road. In order to comply with City of College Station requirements
concerning impact to downstream drainage systems, the proposed drainage system has
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WLB ASSOCIATES, INC. Drainage Report Plazas at Rock Prairie
been designed to limit Phase 1 and Ultimate Conditions post-development 5, 10, 25, 50,
and 100-year frequency discharges to at, or below the pre-development level discharges
determined for the property utilizing methods prescribed in the DPDS Manual.
The proposed Plazas at Rock Prairie Road project is to be constructed in phases. The first
phase of development is shown in Figure 5B -Phase 1 Drainage Conditions, which will
include an 18,861 SF retail building with parking and driveways to Rock Prairie Road.
Ultimate development conditions are shown on Figure 5C -Ultimate Drainage
Conditions, and assume development of the entire project as commercial retail. Proposed
Phase 1 Storm Sewers and Detention Facilities are being sized to provide capacity for the
ultimate drainage condition.
Drainage discharges for 5, 10, 25, 50 and 100-year frequencies were computed using the
Rational Method as required by the DPDS Manual. A Runoff Coefficient (C=0.30) for
existing conditions was determined from Table III-l for natural woodlands with land
slopes of 3.5 to 5.5 percent for the Parcell tract area, and also for the natural grassland
area of Lot 2-C which has slopes of around 1%. For ultimate developed conditions a
value of C=0.85 was used for the Commercial Reserve Areas, and C=0.70 was used for
the grassed detention reserve area which has a bottom area sloped steeply down to a
concrete pilot. The Time of Concentration for Existing Conditions was determined to be
less than 10 minutes using an average overland sheet flow velocity of 1.0 FPS per Table
III-2 of the DPDS Manual. Per manual requirements a minimum time of concentration of
Tc = 10 minutes was therefore utilized for both existing and developed conditions in
accordance with DPDS Manual Section III Paragraph B3c3. Rainfall intensities for a
Time of Concentration of 10-minutes were determined utilizing the DPDS Manual
Intensity Duration Frequency Curves of Figure III-1. A complete listing of the runoff
coefficients, intensity factors, and discharges computed for the Existing Pre-Developed,
Phase 1 and Ultimate Post-Developed Conditions are included in this report as Table 1
Pre & Post-Development Stormwater Runoff Rates. A summary of allowable rates of
discharge from the detention facility, design detention discharge rates, and total post
development tract discharge rates is included in Table 2: Design Tract and Detention
Discharge Summary Table.
Required detention capacity to offset downstream impacts due to development of the
subject property was determined by taking the difference in volume beneath the pre and
post-development hydrographs which were derived using the Triangular Approximation
method of the DPDS Manual. The 5, 10, 25, 50, and IOO-year hydrographs and required
detention volume computations are presented in Figures 7A thru 7E respectively. A
summary of the required allowable discharge rates, actual design detention discharge
rates, total tract post development discharge rates, and detention storage volume and
elevations is included in Table 3: Detention Pond Summary. Figure No.8 -Detention
Pond Schematic and Volume Computation shows the general detention pond layout and
lists storage volumes between the pond bottom elevation of 302 Ft (min. elevation of the
top of concrete pilot channel) and the emergency overflow weir elevation of 306 Ft. to be
approximately 27,918 CF. This is sufficient to provide for both the required 5-year thru
1OO-year frequency detention volumes, and also for the 10% excess volumes required by
- 3
WLB ASSOCIATES, INC. Drainage Report Plazas at Rock Prairie
the DPDS Manual to account for sedimentation. A graph of the proposed detention pond
volume versus depth is shown in Figure No.9.
Table 2 shows the total allowable pre-development existing condition discharges to the
public drainage systems in the right of ways of SH-6 and Rock Prairie Road. The table
also shows the determination of both the allowable and design detention pond discharge,
and the total post development design discharges from the tract to the public r-o-w for the
5, 10, 25, 50, and 100-year frequency storm events. The net allowable detention pond
discharges have been determined by subtracting the post-development discharges of the
areas draining directly to the public systems in the adjacent roads from the total pre
project discharges for each of the storm frequencies investigated. Final design of the
detention outfall limits pond discharges to a level equal to, or less than, the allowable
detention discharge rates.
Detention pond outflow is controlled by means of a 12-orifice installed in an 18-inch
outfall discharge pipe which connects to the Rock Prairie Road Storm Sewer Inlet located
at the southeasterly comer of the project area. Emergency and extreme event overflow
capacity is provided by a 15-ft wide concrete weir structure with a weir elevation 306
feet. This structure directs overflow to Rock Prairie Road. Figure No. 10 is a graph of
Pond Outlet Discharge Rate versus Pond Depth. The discharge rates shown indicate that
at elevation 306.0 the discharge through the 12-inch orifice from the detention pond is
7.80 CFS, which is slightly below the 25-year frequency target rate of 7.98 CFS. Since
the pond capacity below the emergency weir overflow elevation 306 exceeds the required
detention volume for the 100-year frequency design storm, and design outflows for each
of the storm frequencies investigated have been limited to the pre-development outflow
rate, or less, the detention facility will prevent negative impact to the downstream public
drainage systems for all frequencies examined. Above elevation 306 feet the emergency
concrete weir has been sized to convey the 100-year frequency developed condition
discharge of 30.07 cfs from the area which drains into the detention pond when the pond
elevation reaches 306.75 feet. At this elevation the weir velocity is approximately 2.7 fps,
and the storage surface provides 0.5 feet of freeboard below the minimum pond bank
elevation of 307.25 feet. Emergency and Extreme Event Discharges from the Marriott
Development detention pond discharge into the common access driveway behind the
Phase 1 Building Structure and are to be conveyed south through the development via
the driveway to Rock Prairie Road. Under normal conditions no flow is anticipated over
the concrete spillway for storm frequencies up to the 1DO-year event.
The detention pond is proposed to be a grassed lined facility with mInImUm 4-feet
horizontal to I-foot vertical rise side slopes, and a grassed maintenance berm on three
sides. The fourth side will consist of a paved driveway and future paved parking area.
The bottom of the pond is sloped at a minimum of 5% from the toe of the pond side
slopes to the proposed concrete pilot channels. The pond area can be readily accessed for
maintenance.
Maximum ponding elevations within the gutters of private onsite driveways, parking
areas, and at inlets are not expected to exceed 6-inches for the 100-year frequency event.
-4
WLB ASSOCIATES, INC. Drainage Report Plazas at Rock Prairie
This meets the requirements for maximum ponding depth specified in the DPDS Manual
Item 3-Storage Depths on page 74.
Erosion controls during site construction will consist primarily of a combination of silt
fences, hay bale barriers, and sedimentation traps. The locations of the proposed control
measures are shown on Figure 11.
5.0 CONCLUSIONS
The proposed development of the subject site will require provision of detention to offset
impact to downstream drainage systems due to construction of buildings, paved parking
and driveway areas, and associated storm drainage improvements. Onsite swales and
storm sewer have been designed to convey the majority of the development to a detention
facility to be located along the northeasterly boundary of the development. The storm
drainage and detention system have been designed in accordance with requirements of
the City of College Station DPDS Manual, and provide capacity to alleviate development
impact for the required 5, 10, 25, 50, and 100-year design storm frequencies. The
detention and drainage systems have been designed to provide adequate capacity for the
ultimate planned development of the site, and limit total ultimate post-development
discharges from the site to a level slightly below the existing condition discharge rates
computed for the pre-development conditions.
6.0 CERTIFICATION
"I hereby certify that this report for the drainage design of the Plazas at Rock Prairie
Road Development located at the northeasterly intersection corner of the northbound
Frontage Road of State Highway 6 and Rock Prairie Road was prepared by me (or under
my supervision) in accordance with the provisions of the City of College Station
Drainage Policy and Design Standards for the owners thereof'
William M. Langfi rd, PE
Registered Professional Engineer
State of Texas Registration No. 53170
- 5
TABLE 1 -PRE & POST DEVELOPMENT STORMWATER RUNOFF RATES
(CALCULATIONS USING RATIONAL METHOD)
RAINFALL INTENSITY I (inches/hour)
Per Figure III - 1 Drainage Policy and Design Standards Manual
at Time of Concentration T= 10 minutes
FreQuencyl 5-YR I 10-YR I 25-YR I 50-YR I 100-YR
Rate I I 7.68 I 8.36 I 9.86 I 11.15 I 11.64
rev.7-05-06
Q=CIA
EXISTING CONDTIONS (PRE DEVELOPMENT)
AREA AREA C 5-YR 10-YR 25-YR 50-YR 100-YR COMMENTS
No. (ACRES) CFS CFS CFS CFS CFS
0.51 0.3 1.18 1.28 1.71 1.78
3.21 0.3 7.40 8.05 10.74 11.21
3.72 0.30 8.57 9.33 12.44 1299
PHASE 1 CONDTIONS (POST PHASE 1 DEVELOPMENT)
AREA AREA C 5-YR 10-YR 25-YR 50-YR 100-YR COMMENTS
No. (ACRES) CFS CFS CFS CFS CFS
A1 0.17 0.75 0.98 1.07 1.26 1.42 1.48
A2 0.03 0.3 007 0.08 0.09 0.10 0.10
A1-2 0.2 0.68 1.05 1.14 1.35 152 1.59 AREA TO SH6 ROAD DITCH
B1 094 0.85 6.14 6.68 7.88 891 9.30
B2 0.51 0.50 1.96 2.13 2.51 2.84 2.97
B3 0.11 0.85 072 0.78 0.92 1.04 1.09
B4 0.19 0.85 1.24 1.35 1.59 1.80 1.88
B5 0.18 085 1.18 1.28 1.51 1.71 1.78
B6 0.67 0.70 3.60 3.92 4.62 5.23 5.46
B7 0.48 075 2.76 3.01 3.55 401 4.19
81-7 3.08 0.74 17.59 19.15 22.59 25.54 26.67 AREA TO DETENTION POND
C1 0.24 0.30 0.55 0.60 0.71 0.80 0.84
C2 0.08 0.85 0.52 0.57 0.67 0.76 0.79
C3 0.12 0.30 0.28 030 0.35 0.40 0.42
C1-3 0.44 0.40 1.35 1.47 1.74 196 205 AREA TO ROCK PRAIRIE ROAD
ULTIMATE CONDTIONS (POST FUTURE ULT. DEVELOPMENT)
AREA AREA C 5-YR 10-YR 25-YR 50-YR 100-YR COMMENTS
No. (ACRES) CFS CFS CFS CFS CFS
A1 0.17 0.75 0.98 1.07 1.26 1.42 1.48
A2 0.05 0.3 0.12 013 0.15 0.17 0.17
0.22 0.65 1.09 1.19 1.59 1.66
B1 094 0.85 6.14 6.68 7.88 8.91 9.30
B2 0.37 0.85 2.42 2.63 3.10 3.51 3.66
B3 0.16 0.85 1.04 1.14 1.34 1.52 1.58
B4 0.11 0.85 0.72 0.78 0.92 1.04 1.09
B5 019 0.85 1.24 1.35 1.59 1.80 1.88
B6 0.18 0.85 1.18 1.28 151 1.71 1.78
B7 0.48 0.85 3.13 3.41 4.02 4.55 4.75
B8 0.23 0.85 1.50 1.63 1.93 2.18 228
B9 0.43 0.75 2.48 270 3.18 3.60 375
309 0.84 19.84 21.60 28.81 30.07
0.2 03 0.46 0.50 0.59 0.67 0.70
0.08 0.85 0.52 0.57 0.67 0.76 0.79
0.12 0.3 0.28 0.30 0.35 0.40 0.42
0.4 0.41 1.26 1.37 1.83 1.91
TABLE 2: DESIGN TRACT AND DETENTION DISCHARGE SUMMARY TABLE
DESIGN STORM PRE-DEV EXIST. ULT.POST DEV. NET ALLOWABLE DESIGN TOTAL TRACT TOTAL TRACT PRE
FREQUENCY CONDITIONS CONDo TO R-O-W DETENTION POND DETENTION POND POST DEVELOPED DEV. ALLOWABLE
(YEARS) DISCHARGE (CFS) DISCHARGE (CFS) DISCHARGE (CFS) DISCHARGE (CFS) DISCHARGE (CFS) DISCHARGE (CFS)
5 8.57 2.35 6.22 6.22 8.57 8.57
10 9.33 2.56 6.77 6.77 9.33 9.33
25 11.01 3.03 7.98 6.82 9.85 11.01
50 12.44 3.42 9.02 7.32 10.74 12.44
100 12.99 3.57 9.42 7.8 11.37 12.99
Notes:
Net aillowable det.pond discharge rate =pre-dev existing tract total discharge -ultimate post dev discharge direct to public r-o-w facilities
Total tract post development discharge rate equals ult post developed discharge direct to public systems in r-o-w + design pond discharge
Project: Plazas at Rock Prairie
WLB Project No: 9500502
Time: 7/14/2006 14:57
TABLE 3: DETENTION POND SUMMARY
DESIGN StORM
FREQUENCY
(YEARS
ALLOWABLE DET.
DISCHARGE RATE
(CFS)
DESIGN DET POND
DISCHARGE RATE
(CFS)
VOLUME REQD. AT
DESIGN DISCHARGE
(CF)
VOLUME PROVIDED
AT DESIGN DISCH.
(CF)
DESIGN STORAGE
ELEVATION
(FEET)
5
10
25
50
100
6.22
6.77
7.98
9.02
9.42
6.22
6.77
6.82
7.32
7.8
12375
13473
16785
19341
20043
13484
17511
18854
22884
27918
304.48
304.96
305.1
305.5
306
Note: Where the design discharge rate is lower than the allowable discharge rate, the required volume has been computed using
the design discharge rate. This produces a higher required volume than would have been produced using the allowable rate of
discharge (ref exhibits 7C-7E).
Project: Plazas at Rock Prairie
WLB Project No: 9500502
7/14/2006 14:57
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GENERAL SITE
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LEGEND
BASIN REFERENCE NUMSER D
REGULATORY CHANNEl. REACHES I+-0 +t
PAIMARY DRAINAGE SYSTEM
SECONDARY DRAINAGE SYSTEM
BASIN BOUNDARY
SUB-BASIN ·BOUNDARY
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FIGURE NUMBER 4
WATERSHED MAP
OF SUBJECT PROPERTY
AND SURROUNDING AREA
PLAZAS AT
ROCK PRAIRIE
VISTA EQUITlES
GROUP
W'L B
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I=ot t="I t="I I I
PRo.£CT No: 9500502 I EXJSllNG DRAINAGE
W'L:g
A880C1A...... ZRC•
CONSULT1NG ENGINEERS
7915 lIlESTGlEN HOUSTON, TEXAS 77063
TEl. 713-278-9300 FAX 713-953-2515
VISTA EQUITIES
GROUP
PLAZAS AT
ROCK PRAIRIE
FIGURE NUMBER 5A
TOPO SOURCE:
SURVEY INFORMATlON SHOWN WAS REPRODUCED FRON A
TOPOGRAPHIC SURVEY DRAWING PREPARED BY HUGHES
SOUTHVl£ST SURVEYING CO•• DATED 6/10/05.
BENCHMARK:
EL£VATlONS SHOWN HEREON ARE BASED ON OTY Of COLLEGE
STATlON HORIZONTAL CONTROl MONUMENT NO. 130 IN HAO-27
TEXAS STATE PLANE COORDINATES CENTRAL ZONE (NGW 29).
FLOOD PLAIN:
BY GRAPHIC PLOTTING ONLY THIS PROPERTY IS IN ZONE ·X·
AS SHOWN ON THE FLOOD INSURANCE RATE MAP NUMBER
4804<:02010, EFFECTlVE DATE Of fEBRUARY 9, 2000.
THE 100-YEAR FLOOD PLAINjFLOODWAY IS NOT ON OR
1MMEDIATELY AOJACEN T TO THE PROPERTY.
SOURCE: FLOOD INSURANCE RATE MAP PANEL 4804<:02010
EfF. DATE: fEBRUARY 9. 2000
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PRo.£CT No: 11500502 I PHASE 1 DRAINAGE
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CONSUlTING ENQNEERS
7Q15 llESTGlEN HOUSTON, TEXAS 77063 m 713-278-9300 FAX 713-1153-2515
BENCHMARK:
El£VAllONS SHOWN HEREON ARE BASED ON OTY Of COLLEGE
STAllON HORIZONTAL CONTROl l.lONUl.lENT NO. 130 IN HAO-27
TEXAS STATE PLANE COORDINATES CENTRAL ZONE (NGI.tl 29).
PLAZAS AT
ROCK PRAIRIE
FIGURE NUMBER 5B
REV. 07/05/06
TOPO SOURCE:
SUR'vt:Y INFORl.lAllON SHOWN WAS REPRODUCED FROl.l A
TOPOGRAPHIC SUR\£Y DRAWING PREPARED BY HUGHES
SOUTHWEST SUR'vt:Y1NG CO., DATED 6/10/05.
FLOOD PLAIN:
BY GRAPHIC PLOT11NG ONLY THIS PROPERTY IS IN ZONE ·X·
AS SHOWN ON THE fLOOO INSURANCE RATE l.lAP NUl.lBER
480-4<:02010, EFFECll\£ DATE OF FEBRUARY 9. 2000.
THE 100-YEAR fLOOO PlAlN/fLOOOWAY IS NOT ON OR
1l.ll.lEDIATELY ADJACENT TO THE PROPERTY.
SOURCE: fLOOO INSURANCE RATE l.lAP PANEL 480-4<:02010 EFF'. DATE: FEBRUARY 9, 2000
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79t 5 lIIESTQ.£N HOUSTON. lEXAS 77063 Tn. 713-278-9300 FAX 713-1153-2515
PROJECT No; 11500502 I ULTIMAT£ DRAINAGE
FIGURE NUMBER 5C
VISTA EQUITIES
GROUP
ULTIMATE
DRAINAGE CONDITIONS
PLAZAS AT
ROCK PRAIRIE N
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'0 '"•BY GRAPHIC PLOTTING ONLY THIS PROPERTY IS IN ZONE ·X·Ioi c AS SHOVliN ON THE FlOOO INSURANCE RATE "'AP NUt.lBER
o ~ 4804C0201D, EFFECTI-..£ DATE Of FEBRUARY 9, 2000.
C) THE l00-YEAR FlOOO PlAIN/FLOOOWAY IS NOT ON ORZ ll< 1t.lt.lEDIATElY ADJACENT TO THE PROPERTY.
SOURCE: FlOOO INSURANCE RATE t.lAP PANEL 4804C0201D
C
~ EFF. DATE: FEBRUARY 9. 2000< :::I F ...J ~ TOPO SOURCE: ~ SUR-..£Y INFORt.lAl10N SHOVliN WAS REPROOUCED FROt.l A
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REV. 07/05/06
FIGURE NUMBER 6
TYPES OF PROPOSED
511£ COVER MA1£RIALS w PLAZAS AT
-.~...
ROCK PRAIRIE
~.m;;_' =:J=l VISTA EQUITIES
GROUP... ~-
_T .-IIB
A_OC1A..... :rBC.
........ 1111_1111_..
7S15 II:S1Q.EN HOUSTON, 1ElCAS 77083
TEL 713-278-1300 FAll 713-153-2515
PRO.£CT No: I!lClO!102-I SUB.£CT PROPERTY
;-----REQUIRED
EVENT =
Qpost = 19.84 cfs
~E-OEVELOP"ENT
POST-DEVELOPMENT
HYOROGRAPH
<Jl V ..... :J c 'E
o n
II
..Y n
35
30
en 25'+
()
o
w
C) 20 -r-I---'--------D:::
<t: ::r::
U
(/)
o 15
10
5
5 25
VOLUME OF DETENTION STORAGE FOR 5 YEAR
DIFFERENCE IN AREA = 12,258 C.F.
10 15 20 30
TIME, minutes
FIGURE NUMBER 7A
PRE-AND POST
DEVELOPMENT
HYDROGRAPH
FOR 5-YEAR
STORM EVENT
PLAZAS AT
ROCK PRAIRIE
VISTA EQUITlES
GROUP
Will D
A_OClA'I"II8.IBC.
ODNU.1IIICl IIlCIIGIlI
?lUI W[SllllDI HOUS1CIIl. 1EXAS 77083
m. 713-27&-1300 f'AX 713-lI»-~15
PftO.ECT II« lI5OOl102 I 5 YEAR EXHIBIT
) REQUIRED
EVENT =
I Qpost = 21.60 cfs
o
~
II u +'
POST-DEVELOPMENT
HYDROGRAPH
TIME, minutes
5
VOLUME OF DETENTION STORAGE FOR 10 YEAR
DIFFERENCE IN AREA = 13,347 C.F.
en (I)
+' :::J .~ FIGURE NUMBER 7BE
o t') PRE-AND POST
II DEVELOPMENT
..Y HYDROGRAPH
FOR 10-YEAR
STORM EVENT
t')
PLAZAS AT
ROCK PRAIRIE
VISTA EQUITIES
GROUP25 30 Will B A_OCIA..... IKe.
ClllIaIL1IlO IJlOIIIIRI
711 & WESTGI.EN HOUS1CIN. 1EXAS 77083
10. 713-2711-11300 rAle 71;)-11&3-2&1&
PIlO.I:CT N'" IlIOOl102 I 10 '/EAR EXHIBIT
35
30
(f)
'+u
o
W
l?
0:::: « ::r:: u
(f)
o
25
15
20
10
5
10 15 20
E
III V +J
:J .S
o n
II
..Y n
VOLUME
/
POST-DEVELOPMENT
HYDROGRAPH
I
I I I I I I I I I I I I I I I I I I I I I I I
PRE-DEVELOPMENT
HYDROGRAPH
DESIGN 25-YR DISCHARGE
HYDROGRAPH
en '+u
a
w
C)
Ct:: <t: ::r:: u
!J)
o
35
30
~ REQUIRED VOLUME OF DETENTION STORAGE FOR 25 YEAR
/ EVENT = DIFFERENCE IN AREA = 15,741 C.F. AT
/ ALLOWABLE DISCHARGE RATE 7.98 CFS.
Qpost = 25.47 cfs II
I
I FOR REDUCED DESIGN DISCHARGE RATE OF 6.82 CFS THE
25 REQUIRED INCREASES TO 16,785 C.F.
20
15
FIGURE NUMBER 7C
10
PRE-AND POST
DEVELOPMENT
HYDROGRAPH
FOR 25-YEAR
STORM EVENT5
PLAZAS AT
ROCK PRAIRIE
VISTA EQUITIES
GROUP5 10 15 20 25 WLS
A8800EA'I"BII. DlO.TIME, minutes CllIIaL1lIIO DGIGIII
191 & WES1lllDI HOUSTC»l, 1EXAS 7708J 1El 713-27IHI300 rAX 713-1&3-2&1&
PRO.ECT Hex 1!lOO!lCl2 I 2S YEAR EXHIBIT
30
35
30 Qpost = 28.81 cfs ~ REQUIRED VOLUME OF DETENTION STORAGE FOR 50 YEAR
EVENT = DIFFERENCE IN AREA = 17,811 C.F. AT
ALLOWABLE DISCHARGE RATE OF 9.02 CFS
FOR REDUCED DESIGN DISCHARGE RATE OF 7.32 CFS THE(f) 25'+VOLUME REQUIRED INCREASES TO 19,341 C.F. o
o
w
C) 200::::: «
I
U POST-DEVELOPMENT!J) HYDROGRAPH o 15
en Q)
+J
::J .s= E 10 o I")
II
..Y I")
5
PRE-DEVELOPMENT
HYDROGRAPH
DESIGN 50-YR DISCHARGE
HYDROGRAPH
5 10 15 20 25 30
TIME, minutes
FIGURE NUMBER 70
PRE-AND POST
DEVELOPMENT
HYDROGRAPH
FOR 50-YEAR
STORM EVENT
PLAZAS AT
ROCK PRAIRIE
VISTA EQUITIES
GROUP
Will :B
A ••OCZA..... IKe.CllIlIlIL1IlO _
7815 WESTlllDl HOUSTC»I. 1EXAS 77083 lU. 713-27&-1300 rAlC 713-153-2515
PROJECT N« 1!lOOllO2 I 50 YEAR EXHIBIT
35
10
Qpost == 30.07 cfs
30
~ REQUIRED VOLUME OF DETENTION STORAGE FOR 25 YEAR
/ EVENT == DIFFERENCE IN AREA == 18,585 C.F. AT
/ ALLOWABLE DISCHARGE RATE OF 9.42 CFS.
I I
I I FOR REDUCED DESIGN DISCHARGE RATE OF 7.80 CFS THE (f) I
'+-25 II VOLUME REQUIRED INCREASES TO 20,040 C.F.
U II
-;' I
a I I I I w II
0 I 20 I
0::: I
« ::r:: u POST-DEVELOPMENTIJ) HYDROGRAPH
0 15
CIJ V +J
:::l £ FIGURE, NUMBER 7EE
o ,...., PRE-'AND POST
II DEVELOPMENT ..Y ,...., HYDROGRAPH
FOR 100-YEAR STORM EVENT
PRE-DEVELOPMENT
HYDROGRAPH
DESIGN 100-YR DISCHARGE
PLAZAS AT ROCK PRAIRIE
5 10 15
HYDROGRAPH
20 25 30
VISTA EQUITIES
GROUP
TIME, minutes .'LB AaaOCIA a. IKC. ClIIlaL1lIlI ........ 7115 WESTGlIN HOUmlN, 'IEXAS 77083 m.. 713-2711-1300 r"", 713-153-2515
PRO.I:CT"oc I5OOl102 I 100 YEAR EXH1IllT
5
~ l!.! iii X x ~
VOLUME REPORT FOR PLAZAS
REPORTED: 7/7/2006
PRINTED: 7/10/2006
AT ROCK PRAIRIE DETENTION POND
BY: WDS .... 0:: <It: Q..
SITE NAME
DEl POND
SlRATUM NAME
FLAT ELEV 306 VS POND_07-07-06
SURFACE 1
flat elev 306
SURFACE 2 METHOD
pond_07-07-06Grid
A\o£RAGE (Cf)
27,918
WSEL
306 ><: g
It:
DET POND FLAT ELEV 306 VS POND_07-07-06 flat elev 306 pond_07-07-06Composite
I DE1ENllON POND SCHEMAllC
I-< DEl POND FLAT ELEV 305 VS POND_07-07-06 flat elev 305 pond_07-07-06Grid 654.8°1 661 17,847 305 I & VOLUME COMPUTAllONS
III -< N -<
DET POND FLAT ELEV 305 VS POND_07-07-06 flat elev 305 pond_07-07-06Composite 667.22
PLAZAS AT
~ '1 N o It'l o
DET POND
DEl POND
FLAT ELEV 304 VS POND_07-07-06
flAT ELEV 304 VS POND_07-07-06
flat elev 304
flat elev 304
pond_07-07-06Grid
pond_07-07-06Composite
344.86 1
355.67
350 9,450 304 ROCK PRAIRIE
VISTA EQUITIES
~ ~ III m ~ I o -< ~
6
DET POND
DET POND
DET POND
DEl POND
FLAT ELEV 303 VS POND_07-07-06
FLAT ELEV 303 VS POND_07-07-06
FLAT ELEV 302 VS POND_07-07-06
FLAT ELEV 302 VS POND_07-07-06
flat elev 303
flat elev 303
flat elev 302
flat elev 302
pond_07-07-06Grid
pond_07-07-06Composite
pond_07-07-06Grid
pond_07-07-06Composite
108.27 1
110.54
1. 26
12.97
109
2
2,943
54
303
302
GROUP
,..11 D
AIIIIOCZA...... IIIC.
~..71115 1ES1Q.EN. HCUSTON. 1EXAS 77083 Tn. 713-27&-13OlI F/4J/. 713-153-2515
PROoECT..... t5OO502 I F1GURE NO. 8
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0...:::::....-------------------------..1 o 1 2 3 5
(EL. 302) (EL. 303) (EL. 304) (El. 305) (EL. 307)
DEPTH OF STORAGE (FT.)
REV. 07 05 06
FIGURE NUMBER 9
DETENTION POND
VOLUME VERSUS
DEPTH GRAPH
PLAZAS AT
ROCK PRAIRIE
VlSTA EQUITIES
GROUP
"LDAIIIIOCZA..... IIIC.CDN!IUL1INIl _
7lI1~ WESTGlEN HOUSlON. TEXAS 77O&J m. 713-278-8JOO FAX 713-lI!lJ-~l~
PROJECT No: lISOOl102 PONO VOlUlolE EXHBlT
4
(El. 306)
60 .----------------------------------,
55
50
45
40
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(EL. 302) (El. 303) (EL. 304) (EL. 305) (EL. 306) (EL. 307)
ELEVAT10N
(FT)
FIGURE NUMBER 10
POND OUTLET
DISCHARGE RATE
VERSUS DEPTH GRAPH
PLAZAS AT
ROCK PRAIRIE
VISTA EQUITIES
GROUP
waLD
AROCZA"I'BII. DIe•
CIllNSIL1IIO ENGIIEERS
781li IESTIilDI HOUSTON. 1EXAS 77OS3 1El 713-27lI-tI3OO F'AX 713-Vli3-2!l1li
PRO.£CT No: lI:lOllOO2 IDISOIARGE RA1£ ElCItllIT
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SWPPP LEGEND
STAIllUZEO
CONSlRUCllON
Aca:SS~
REINFORCED
FABRlC FENCEx-8-x
IPS
D INL£T
PROTECllON BARRIER
REV. 07/05/06
AGURE NUMBER 11
SCHEMATIC EROSION
CONTROL PLAN
PLAZAS AT
ROCK PRAIRIE
VISTA EQUlllES
GROUP
'WI. •
..-oczA.....IIIC.
11115 101Q.EN t«IUS1ON, TEXAS 77'lIU~--Tn. 71~~ £AlLll:s-e53-25I.
PRO.£CT No: lI5OCI502 I sue..ECT
APPENDIX B