ST9905-CS021025
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1
October 25, 2002
Mr. Ken Griffin, P.E.
City ofCoppell
255 Parkway
Coppell, Texas 75019
RE: Bethel Road
Hydraulic Design Basin G-I
Dear Mr. Griffin:
This letter is intended to supplement the drainage report prepared by TranSystems entitled
Regional Detention Analysis/Bethel Road Improvements dated May 30, 2002. The purpose of
the report was to update previous studies to include recent development of the Trade Point, The
Duke Freeport and the Koll developments and second to evaluate alternatives to alleviate
flooding of stream G-I, resulting from future development, specifically the development of the
airport property.
The hydrologic and hydraulic analysis presented in the above mentioned report provided for
offsite detention located within the airport property to limit post-development discharge to
existing conditions. TranSystems analyzed and presented options in the report, which addressed
the design of a storm water system that would contain the 100-year post-development storm.
However, this analysis provided for detention facilities located on the airport property. The
detention facilities (located within the airport) were sized to limit the 100- year post-development
discharge to the pre-developed conditions.
It is our understanding that the airport will not commit to the construction of detention facilities
that would limit the 100- year post-developed discharges to the current 100 year undeveloped
conditions. As a result, the city has asked TranSystems to evaluate the level of flood of
protection the city would be afforded assuming no detention in the upper reaches of stream 0-1.
The drainage area associated with the airport property is 242 acres and produces an existing 100-
year discharge of 847 cfs. When fully developed the un-detained discharge from the airport is
1,395 cfs (additional 548 cfs). If the additional 548 cfs is not detained by the airport the system
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designed by TranSystems will not contain the 100 year storm.
Currently, the selected alternate provides for the construction of an additional 3-10'X 5' box
culvert beginning at the outfall from the USPS open channel and extending 300' downstream of
Freeport Parkway, eventually outfalling to an improved open channel section. Our analysis
shows that if the airport does not limit discharges to match existing conditions the proposed
storm water system would no longer pass the 100 year discharge. The table below is provided to
summarize our analysis.
iJl'>l,<...
Upstream of
Bethel Rd.
Upstream of
Free ort Pk
Outfall from
USPS
Upstream of
Ro al Lane
Fully-Developed Discharges at Various Locations Along Stream G-l
No Detention on Air ort Pro e
Drainage 100 Year 50 Year 25 Year 10 Year 5 Year
Area
464 2761 2341 2065 1692 1408
4\b
D 5 ,,'-4
406 fC,L. 2244 1999 1634 1358
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Table-I:
Location
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DP-- 1) '21"_'''"~1 t:') '"
f)p I.., L.;~)'t
1975
*1764
*1447
1212
1189
1064
878
738
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Table I summarizes the discharges for a fully developed basin with no detention provided on the
airport property. The 4-10' X 5' box culverts (3-proposed and one existing) have a capacity of
1690 cfs before overtopping occurs and street flooding begins. This translates to a protection
level between a 10-year and a 25-year storm assuming no detention is provided on fully
developed airport property.
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This analysis has been provided to the city at the request of the City of Coppell. Please feel free
to give me a call if you have any questions regarding this analysis.
Sincerely,
TranSystems Corporation Consultants, Inc.
Paul J. Padilla, P.E.
Vice President
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