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Storm Water Drainage Study
Pi Beta Phi Renovations and Additions
College Station, Texas
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April 14, 2011
Mr. Alan Gibbs
City of College Station Planning & Development Services
1101 Texas Avenue
College Station, TX 77842
Re: Storm Water Drainage Study
Pi Beta Phi Renovations and Additions
College Station, Texas
Gessner Engineering Job No.: 11-0077
Dear Mr. Gibbs:
This report conveys the results of the storm water drainage study conducted by Gessner
Engineering for the proposed Pi Beta Phi Sorority House Renovations & Additions in
College Station, Texas. Gessner Engineering believes that all information contained in this
report is valid. Please contact us if you have any questions or if we can be of further
assistance.
Sincerely,
GESSNER ENGINEERING, LLC F7451
OF,T
,u{J . �PCF'Exq��l
Melissa P. Thomas, P.E.
j/ MELISSA P. THOMAS Gessner
✓U t �j .._ 98398 ,4
Kyle M. Zapalac, M. E.I.T. +�1.q�i:;!.'FNe���is��' Engineering
NAGessner Eng Data I1\11-0077 Pi Phi Sorority House\Civil\DOC\t l-0077 Drainage Report-rational.doc
2501 Ashford Drive
Suite 102
College Station, Texas 77940
P.O. Be. 10763, 77842
979.680.8840
Fax 979.680.8841
2204 S. Chappell Hill Street
Brenham, Texas 77833
979.836.6855
fax 979.836.6847
M
Executive Summary
This storm water drainage report is submitted to the City of College Station, Texas for
review on March 16, 2010 by Melissa P. Thornas, P.E. of Gessner Engineering, located at
2501 Ashford Drive, Suite 102, College Station, Texas 77840. This report is submitted on
behalf of the Pi Beta Phi Sorority House, at 1601 Munson Avenue, College Station, Texas
77840.
The proposed project consists of renovations and additions to the existing Pi Beta Phi
sorority house situated on an approximately 1.30 acre site. The site currently includes the
existing Pi Beta Phi Sorority House with accompanying concrete parking and sidewalks.
Proposed improvements include an approximately 2,544 square foot building addition to be
used as a house mother suite and library, two sidewalks as well as other house interior
renovations.
The site is located at the southeast corner of the intersection of Munson Avenue and
University Oaks at 1601 Munson Avenue in College Station, Texas. It is located in the
lower third of the Wolf Pen Creek watershed. The site is not located in the FEMA 100 year
flood plain, according to FIRM number 48041CO144 C.
The proposed site development provides for storm runoff to sheet flow off of the existing
parking and building areas south onto Munson Avenue.
Due to the size of development at this site, a Notice of Intent is not required for submittal to
the Texas Commission on Environmental Quality.
This drainage report includes discussion of hydraulic analysis with appendices including
drainage area maps and calculations.
Introduction
This storm water drainage report is intended to show the change in runoff from the existing
conditions to the proposed conditions for the Pi Beta Phi Sorority House.
Drainage Calculations for this site were prepared according to the Unified Stormwater
Design Guidelines utilizing the Rational Method. Proposed improvements include an
approximately 2,544 square foot building addition and two sidewalks from the new addition
to the existing parking area. One (1) existing sidewalk and one (1) concrete pad for
mechanical equipment was removed for the new building addition. Runoff Coefficient "c"
values from table C-3 of Appendix C of the Stormwater Design Guidelines were used
based on post -developed surface types as described above. Composite Runoff Coefficient
"c" values were calculated for the pre -developed and post -developed conditions. Pre -
developed flows were calculated based on the existing improvements on the subject tract.
The calculated pre -developed and post -developed flows include the one -hundred (100)
NAGessner Eng Data I I\I I-0077 Pi Phi Sorority House\Civi1\D0C\11-0077 Drainage Report-rational.doc
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year storms in accordance with the Bryan College Station Unified Stormwater Design
Guidelines.
Calculations
Calculations were performed according to the Unified Stormwater Design Guidelines
utilizing the Rational Method.
Time of Concentration
The time of concentration (travel time) for each drainage area was estimated by summing
the flow time for each segment of travel. For sheet flow, travel time was estimated by
Manning's Kinematic equation:
0.007 x (n7)0.8
1 - s 0.4
�i ` z
Tt -
travel time (hours)
n =
manning's roughness coefficient
L =
flow length (feet)
S =
slope (ft/ft)
P2 =
2-year, 24 hour rainfall (inches)
For shallow concentrated flow, the travel time was calculated from the flow velocity based
on the slope in the direction of flow. These velocities were taken from Table C-4 of the
Bryan College Station Unified Stormwater Design Guidelines.
The computed time of concentration is 2.07 minutes, therefore the minimum time of
concentration (Tc) of ten (10) minutes was used per Unified Stormwater Design Guidelines.
Peak Runoff Flow
Peak Runoff Flow from the site was determined based on the Rational Method. Runoff
Coefficient values (c) were determined from Table C-3, Appendix C of the Unified
Stormwater Design Guidelines, Composite Runoff Coefficient calculations for the pre and
post developed conditions are included in Tables One and Three. The peak pre -developed
flow calculation for the subject site is included in Table Two. The peak post -developed flow
calculation for the subject site is included in Table Four.
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Description
AREA (ac)
Runoff Coefficient "c"
Roof
0.20
0.97
Concrete/ Sidewalk
0.66
0.97
Grass
0.44
0.40
Composite Runoff Coefficient, "c":
0.78
Table One: Pre -Developed Composite Run-off Coefficient, "c" Calculation
Area
Drainage
Composite
Time of
Rainfall
100 Year Storm Maximum
Designation
Area (ac)
Runoff
Concentration
Intensity, ]too
Flow,
Coeff. "c"
Tc (min)
(Inlhr)
Q=cIA cfs
Al
1.30
0.78
10
11.64
11.74
Maximum Flow Routed From Site:
11.74
Table Two: Pre -Developed Flow Calculation
Description
AREA (ac)
Runoff Coefficient "c"
Roof
0.26
0.97
Concrete/ Sidewalk
0.67
0.97
Grass
0.38
0.40
Composite Runoff Coefficient, "c":
0.81
Table Three: Post -Developed Composite Run-off Coefficient, "c" Calculation
Area
Drainage
Composite
Time of
Rainfall
100 Year Storm Maximum
Designation
Area (ac)
Runoff
Concentration
Intensity, Itoo
Flow,
Coeff. "c"
Tc (min)
Inlhr)
Q=cIA (cfs)
Al
1.30
0.81
10
11.64
12.19
Maximum Flow Routed From Site:
12.19
Table Four: Post -Developed Flow Calculation
The resulting flow based on the increased runoff coefficients and impervious area at the
site is shown in Table Five below. The increase from the pre -developed flow is also
included.
Storm Event
Pre -Developed
Peak Flow (cfs)
Post -Developed Flow
(cfs)
Flow Increase (cfs)
100 year
1 11.74
1 12.19
1 0.45
Table Five: Pre -Developed and Post -Developed Site Outflows
Conclusion
Based on visual evidence, engineering drainage calculations and sound engineering
judgment, Gessner Engineering believes that the increase from the pre -developed flow to
the post -development flows is minimal for the one -hundred (100) year design storms for
this site.
Additionally, the minimal increase in flow of 0.45 cfs will not have a noticeable impact on
down stream conditions.
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I have conducted a topographic review and field investigation of the existing and proposed
flow patterns for stormwater runoff from Pi Beta Phi sorority house site to Wolf Pen Creek.
At build -out conditions allowable by zoning, restrictive covenant, or plat note, the
stormwater flows from the subject subdivision or site project will not cause any increase in
flooding conditions to the interior of existing building structures, including basement areas,
for storms of magnitude up through the 100-year event."
NAGessner Eng Data 1I\11-0077 Pi Phi Sorority House\Civil\DOC\l1-0077 Drainage Report-rational.doc
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