ST9905-CS021213December 13, 2002
Mr. Ken Griffin, P.E.
City of Coppell
255 Parkway
Coppell, Texas 75019
RE:
Bethel Road
Hydraulic Design Basin G-1
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-l, 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 G-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
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1,328 cfs (additional 481 cfs). If the additional 481 cfs is not detained by the airport the system
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.
Table-l: Fully. Developed Discharges at Various Locations Along Stream G-1
No Detention on Airport Property
Location Drainage 100 Year 50 Year 25 Year 10 Year 5 Year
Area
Upstream of 464 2515 2133 1901 1550 1289
Bethel Rd.
Upstream of 406 2437 2062 1835 1492 1239
Freeport Pkwy.
Outfall from 325 2164 i834 '1634 1334
USPS
Upstream of 242 1328 1126 1006 828 694
Royal Lane
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 for the 25-year storm assuming no detention is provided on fully developed airport
property.
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.
G:~200XOO890-CoppellXHydraulic*XFrequem'y Analysi,s~Revised I Frequencey Analysis.doc