HomeMy WebLinkAboutDrainage Report DRAINAGE STUDY
FOR THE PROPOSED
STERLING HEIGHTS APARTMENTS
B&A#11533
LOCATED IN:
College Station,Texas
First Submittal: 4/15/2015
Prepared By:
Sam J. Vernon, P.E.
OF
Bleyl &Associates
1722 Broadmoor Ste. 210 ,�.,,.��ww,
Bryan,Texas ;"p'st,.......
OF•TF'r,9S'�1
Phone:979.268.1125 , 5 : I�' '.• ''• •, * (I
Fax:979.260.3849 $ *1...,.i.,
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0....SAM JVEFiNON
1��:,. 9353 ...:•. s
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( ‘‘`\/ON`Ae
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This report for the drainage design of Sterling Heights was prepared under my
supervision in accordance with provisions of the Bryan/College Station Unified Drainage
Design Guidelines for tip owners of the property. All licenses and permits required by
any and all st)te and federal regulatory agencies for the proposed drainage
improvem nts ave e 'ssued."
Sam J. Ver on, P. .
State of Texas No. 99353
GENERAL LOCATION AND DESCRIPTION
Scope of Drainage Plan
The purpose of this drainage letter is to analyze the hydrological effects of site development for Sterling
Heights.
This project is located on 218 Sterling Street within the Wolf Pen Creek Watershed. This drainage study
analyzes flows from the drainage area immediately surrounding the proposed development.
Description of Property
The site of the proposed Sterling Heights is currently undeveloped grass land with a few trees. The site
will be developed into approximately 1 acre of medium density residential housing. The surrounding
drainage areas are comprised of approximately 17 acres of single family and multi-family residential
housing.
DRAINAGE DESIGN CRITERIA
Hydrologic Criteria
The following design criterion was used:
1. The rational method was used for determining rainfall storm events and runoff.
2. The equations for calculating rainfall intensities were taken from BCS Unified Stormwater Design
Guidelines:Table C-1.
3. Runoff coefficients were taken from BCS Unified Stormwater Design Guidelines: Tables C-1 & 2.
Existing Drainage Characteristics
There are no existing storm drainage features on the proposed site or along Sterling Street. A single
outfall, consisting of a 20' inlet in sump and 4 grate inlets located at the north eastern end of Richard
Street, receives all existing flows from the site.
Proposed Drainage Characteristics
The proposed site will consist of one building comprising 24 units and a total area of 28,861 ft2. The site
will also include 52 parking spaces and 1,028 ft2 of parking islands. All storm water will be carried above
ground by overland flow throughout the parking lot and will be conveyed into Sterling Street. The water
will flow northeast down Sterling Street for approximately 430' until the intersection with Crest Street. It
will then flow northwest along Crest Street for approximately 250' until its intersection with Richard
Street. It then flows approximately 90' northeast before it enters the existing grate and curb inlets at the
end of Richard Street. These inlets will receive flows undetained. A picture of the existing inlet structure
is attached in Exhibit 4.
Drainage System Analysis
The total area contributing to the flows at the outfall consist of two drainage areas, DA-1 and DA-2.
Exhibit 1 shows the overall drainage map.
DA-1 is approximately 15.64 acres and was analyzed using pavement, grass and residential areas. The
proposed site is located within DA-1. For pre-developed conditions, the proposed site was analyzed as
grass, and the overall drainage area had a weighted runoff coefficient of C=0.75. For post-developed
conditions, the proposed site was analyzed as pavement and grass areas, and the overall drainage area
had a weighted runoff coefficient of C=0.78.
DA-2 is approximately 2.09 acres and was analyzed using pavement and residential areas. It was
assumed that due to a valley gutter at the intersection of Manuel and Crest Streets that approximately
half of the flow from DA-2 would contribute to the total flow at the outfall point. For both pre and post-
developed conditions, the overall drainage area had a weighted runoff coefficient of C=0.77. Exhibit 2
shows the rational method calculations. Flow differences and percentage calculations for each storm
event can be found in Exhibit 3.
A capacity analysis was done for the existing 20' sump inlet and the four existing grate inlets. Equation 1
was used to determine the capacity of the sump inlet.
Q = 3.0Ly1.5 [Equation 1]
The depth from the gutter to the top of the inlet was measured in the field and was found to be 14".
The total capacity of the inlet was determined to be 75.61 cfs. This calculated capacity was reduced by
ten percent per BCS guidelines to yield an effective capacity of 68.05 cfs.
A capacity analysis was also done for the four existing grate inlets. Equation 2 was used to determine
the capacity for each grate inlet.
Q = 4.82A0°.5 [Equation 2]
The same depth of water on the inlet was approximated to be 12". Each grate inlet had 60 clear
openings, and the size of each clear opening was measured to be 7.25" x 1.375". The total capacity of
the four inlets was determined to be 80.35 cfs. This calculated capacity was reduced by twenty five
percent per BCS guidelines to yield an effective capacity of 60.26 cfs for all four inlets.
Inlet capacity calculations can be found in Exhibit 4. Table 1 shows the existing capacity for the sump
and grate inlets compared to the anticipated flows for each storm event.
Table 1: Contributing Flows to the Inlets and Excess Capacity
Storm Event Pre- Post- Inlet Capacity Post-Development
Development Development (cfs) Excess Capacity
(cfs) (cfs) (cfs)
2 Year 44.35 45.98 128.31 82.33
5 Year 58.84 61.01 128.31 67.30
10 Year 69.06 71.60 128.31 56.71
25 Year 80.97 83.95 128.31 44.36
50 Year 94.25 97.71 128.31 30.60
100 Year 100.10 103.77 128.31 24.54
Note: The above table summarizes flows based on the equation (DA-1) +(DA-2)/2.This is to account for
the bypass flow of DA-2 as discussed above.
From COCS GIS information an existing 42" pipe at a slope of 1.07%was found to convey flow from the
analyzed inlets. Using manning's full flow analysis the pipe was found to have a capacity of 104.07cfs
with a velocity of 10.82 ft/sec.This full flow capacity is just above the estimated developed 100 year
flow of 103.77cfs anticipated to be received by the inlets.The existing 42" pipe has a flowline of 272.67'.
Under full flow conditions the headwater of the pipe would be at 276.17'. The elevation of the grate
inlets is located at 278.5'. This allows adequate room for the grate inlets to receive flow without having
tailwater effects from the existing 42" pipe. The existing 42" pipe calculations and COCS GIS information
are attached in Exhibit 5.
CONCLUSIONS
From Table 1 it can be seen that the inlets will have sufficient capacity to receive post-developed flows
from the Sterling Heights development. For the post-development analysis, the inlets will have an excess
capacity during the 100 year storm event of 24.5 cfs.
REFERENCES
1. BCS Unified Stormwater Design Guidelines, Effective August, 2012.
EXHIBITS
• Exhibit 1-Overall Drainage Map
• Exhibit 2 - Rational Method Calculations
• Exhibit 3—Flow Difference Calculations
• Exhibit 4—Inlet Capacity Calculations
• Exhibit 5—42" Pipe Calculations
EXHIBIT 1 : OVERALL DRAINAGE MAP
Bleyl &Associates
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REVIEW ONLY
THIS SET OF PLANS WAS PREPARED
UNDER THE DIRECTION OF SAM J.
VERNON,P .,SEA.No.993530N
LI' 3/30/2015.THIS DOCUMENT IS RELEASED
_ FOR THE PURPOSE OF INTERIM REVIEW
ONLY AND NOT TO BE USED FOR
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CONSTRUCTION.
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EXHIBIT 4: INLET CAPACITY CALCULATIONS
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EXHIBIT 5: 42" PIPE CALCULATIONS
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Worksheet for 42" Pipe
Project Description
Friction Method Manning Formula
Solve For Full Flow Capacity
Input Data
Roughness Coefficient 0.013
Channel Slope 0.01070 ft/ft
Normal Depth 3.50 ft
Diameter 3.50 ft
Discharge 104.07 ft3/s
Results
Discharge 104.07 ft3/s
Normal Depth 3.50 ft
Flow Area 9.62 ft'
Wetted Perimeter 11.00 ft
Hydraulic Radius 0.88 ft
Top Width 0.00 ft
Critical Depth 3.11 ft
Percent Full 100.0 %
Critical Slope 0.00952 fUft
Velocity 10.82 ft/s
Velocity Head 1.82 ft
Specific Ene-gy 5.32 ft
Froude Number 0.00
Maximum Discharge 111.94 ft3/s
Discharge Full 104.07 ft3/s
Slope Full 0.01070 ft/ft
Flow Type SubCritical
GVF Input Data
Downstream Depth 0.00 ft
Length 0.00 ft
Number Of Steps 0
GVF Output Data
Upstream Depth 0.00 ft
Profile Description
Profile Headloss 0.00 ft
Average End Depth Over Rise 0.00 %
Bentley Systems,Inc. Haestad Methods SoB@idtecEitlerMaster V8i(SELECTseries 1) [08.11.01.03]
4/15/2015 8:09:17 AM 27 Siemons Company Drive Suite 200 W Watertown,CT 06795 USA +1-203-755-1666 Page 1 of 2
Worksheet for 42" Pipe
GVF Output Data
Normal Depth Over Rise 100.00 %
Downstream Velocity Infinity ft/s
Upstream Velocity Infinity ft/s
Normal Depth 3.50 ft
Critical Depth 3.11 ft
Channel Slope 0.01070 ft/ft
Critical Slope 0.00952 ft/ft
Bentley Systems,Inc. Haestad Methods SoBSidtecEhlrarMaster V8i(SELECTseries 1) [08.11.01.03]
4/15/2015 8:09:17 AM 27 Siemons Company Drive Suite 200 W Watertown,CT 06795 USA +1-203-755-1666 Page 2 of 2