Windsor Estates-CS 990917Ill
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8616 NORTHWEST PLAZA DRIVE
DALLAS, TEXAS 75225
(214) 346 6200
FAX (214) 739-0095
September 17, 1999
Ave 17669
Mr. Kenneth M. Griffin
Floodplain/CDC Administrator
Coppell Departmcnt of Public Works
P.O. Box 478
Coppell, Texas 75019
RE: Corridor Development Certification application for Windsor Estates
Dear Mr. Griffin:
Halff Associates hits reviewed the comments made by the Corps of Engineers on thc above
referenced project and would like to make the following response:
This project is on Denton Crock ,a tributary of thc Elm Fork Trinity River. It falls into the
CDC Regulatory Zone only because The Elm Fork 100-year cvcnt will pond flood waters on
Denton Creek adjacent to this site. The 100 year flood plain in the area is defined by floods
originating on Denton Creek rather than the Ehn Fork. All Denton Creek backwater models
(FEMA and City of Coppell) are HEC-2 format. Therefore no HEC-RAS modeling was
performed. In addition, HEC-RAS could not be used due to its lack of a split flow by weir
option. HEC-2 must be used becanse of its ability to accurately determine split flow
discharges caused by Denton Creek flood waters overtopping the Dallas Gun Club road.
That portion of Denton Creek 100-year flood waters will combine with Elm Fork flood
waters, rather than rejoin Denton Creek.
Split flow discharges were established using thc revised existing conditions HEC 2 model.
These discharges were held for proposed conditions. This seems reasonable in light of the
accuracy of thc hydrologic estimates and allows for a more direct determination of proposed
prQiect hydraulic impacts.
3. HEC-2 models were used only for water surface elevalion comparisons.
The valley storage numbers provided by the Corps arc accurate based on model results alone
but ignore storage in areas which are not effective. If the non-effective flow areas
included in the valley storage calculations, the project reflects an actual gain in valley storage.
For example, on the right overbank between cross-seclions 6 and 9, them is a pond in the
non-effective flow area. During a 100-year flood event, 14.1 acre-feet of valley storage
occurs in this area that the hydraulic model does not account for. This is true for the area on
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MI'. Kenneth M. Griffin
Coppell Department of Public Works
September 17, 1999
Page 2
thc right overbank between cross-sections 14 and 20980. We calculate that there is
approximately 9.7 acre-ti of valley storage in this non-effective flow area that is not included
in the HEC-2 model. In all, there is approximately 23.8 acre-fi of valley storage that thc
HEC-2 model does not account for which more than offsels the 16.9 acre-fi loss mentioned
by Corps. Our valley storage calculations are based on standard embankment computation
techniques which provide mm-e accurate calculations of valley storage than those based on
HE(.? 2 or HEC RAS models in onr opinion.
5. As discnssed above, valley storage for the 100-year event is actually increased by on the
proposed project therefore exceeding CDC requirements lor 100 year flood valley storage.
6. As discussed in 4 above, there is no loss in valley storage for the 100-year flood event caused
by the proposed projecl when non effective flow areas are inc[nded.
A Standard Project Flood (SPF) discharge for this reach of Denton Creek could not be found
in existing documentation. Craig Lotion with thc Corps of Engineers stated in a conversation
in May 1999 that the old SPF (1978) for Denton Creek was 36500. The 500 year flood
discharge is 36200 and was substituted for thc SPF with thc approval of the City Engineer of
Coppell.
If you have any more questions, or comments, please contact mc at (214) 346-6220.
Sincerely,
HALFF ASSOCIATES, [NC.
Walter Skipwith
Vice President
cc: Kelly Jordan