DR9502-CS 970625Ill
· ,, Halff Associates
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ENGINEERS · ARCHITECTS · SCIENTISTS
PLANNERS ° SURVEYORS
8616 NORTHWEST PLAZA DRIVE
DALLAS, TEXAS 75225
(214) 73~0094
FAX (214) 739~0095
June 25, 1997
AVO 15254
Mr. Kenneth M. Griffin, P.E.
Assistant City Manager/Director of Public Works
City of Coppell
255 Parkway Blvd.
Coppell, Texas 75019
Re:
Andrew Brown Park West and DeForest Road Service Center Conditional Letter of Map
Revision to the Federal Emergency Management Agency.
CLOMR 96-06-532R and LOMR 97-06-503P.
Dear Mr. Griffin:
You will find included herein information related to a Conditional Letter of Map Revision (CLOMR)
originally submitted to the Federal Emergency Management Agency (FEMA) on behalf of the City of
Coppell in August 1996. The CLOMR covered a proposed map correction at the DeForest Road
Service Center and proposed improvements to Andrew Brown Jr. Park West along Denton Creek. You
might remember that FEMA's response to the submittal was to agree to process the proposed map
correction at the service center but to request an additional in depth hydraulic analysis of the Denton
Creek / Cottonwood Creek confluence at the park site. FEMA decided to split the request into two
separate submittals, a CLOMR for Andrew Brown Jr. Park West and a LOMR for the DeForest Road
Service Center.
On Monday June 16, 1997, Halff Associates Inc. received from Ms. Laura Bittner, with Michael Baker
Jr., Inc., a short note describing the changes made to the effective models for Denton Creek and Old
Denton Creek in connection with the DeForest Road Service Center map correction. We are sending
you copies of all the documents which include a list of the changes made, a disk with 2 digital models (a
revised floodway model for Denton Creek and a revised floodway model for Old Denton Creek), a hard
copy output of each model and a 8.5xll color sketch of the revised floodplain and floodway
delineations. Our comments about the changes made by Ms. Bittner are as follows:
The study of the two channels as a split flow situation is appropriate. Two separate models
are necessary because HEC-2 cannot handle floodway encroachments on two separate
branches of the creek at once. We did not duplicate the split flow analysis or check the
discharges used by FEMA.
2. We welcome the effort made to improve the models' GR data per the latest topographic
information. The changes in effect slightly relocate the Old Denton Creek channel.
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· ,, Halff Associates
III
ENGINEERS · ARCHITECTS · SCIENTISTS
PLANNERS · SURVEYORS
Shifting the floodway south along the levee matches almost exactly the delineation proposed
in the CLOMR. However, trying to maintain the floodway widths generated by the HEC-2
model results in an excessive shift of the floodway and 100-year floodplain on the south
bank of Denton Creek upstream of McArthur Blvd. Also, the floodway widths were made
more consistent along Old Denton Creek but their delineation still does not quite match the
creek location as shown on the topographic map.
At cross-section 223+70, the reduction of the 100-year topwidth from 1500 feet to 530 feet is
an improvement, although we had delineated it less than 325 feet wide based on contours
shown on the topographic map.
We forwarded our comments to FEMA by telephone earlier today and do not anticipate any problems in
the issuance of a Letter of Map Revision (LOMR) at the DeForest Road Service Center.
Ms. Bittner's note did not address the Andrew Brown Jr. Park West CLOMR. You had opted not to
pursue FEMA's request for an in-depth hydraulic analysis of the site and the park improvements were
put on hold. Halff Associates Inc.'s original contract with the City of Coppell included a LOMR to be
submitted after construction of the park improvements. That task and budget ($4,600) still remain.
Please let us know your wishes concerning the remaining task and budget for this project.
Please call me at 214-739-0094 extension 220 at your earliest convenience to discuss the Andrew
Brown Jr. Park West budget issue or to comment on FEMA's review of the DeForest Road Service
Center LOMR.
Sincerely,
HALFF ASSOCIATES, INC.
Walter E. Skipwith, P.E.
Vice President
Attachments
cc Ms. Laura Bitmer, Michael Baker Jr., Inc.
files
file: avo\15254\word6Xlt062597.doc
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