Billingsley-CS040607 =1„ Kimley -Horn
and Associates, Inc.
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June 7, 2004
•
Suite 1800
12700 Park Central Drive
Dallas, Texas
Mr. Kenneth M. Griffin, P.E. 75251
Director of Engineering and Public Works
City of Coppell
255 Parkway Boulevard
Coppell, TX 75019
Re: Review of CLOMR Request on Grapevine Creek
158.6 Acre Tract
KHA No. 063124007
Dear Ken:
We have completed our review of the referenced Conditional Letter of Map
Revision (CLOMR) request for the proposed subdivision. The following
comments are based on the requirements outlined in the City of Coppell Drainage
Manual and Federal Emergency Management Agency (FEMA) criteria.
Comments are in no particular order and should be addressed by number. KHA
recommends that:
1. The hydrology used in this report is based on reports that were performed
for the City of Coppell in 1990 and 1991. The flows found in these reports
do not match the flows from the Dallas County Flood Insurance Study
(FIS). Therefore, this submittal includes a revision to the FIS hydrology of
Grapevine Creek. Excerpts from the reports used, as well as modeling from
those reports, should be included with this submittal to provide backup for
the revised flows. MT -2 Form 2 should be corrected to reflect that the
FEMA hydrology is being revised.
2. Specify the vertical datum (NGVD29, NAVD88, etc.) of the survey in the
report or on the floodplain workmaps.
3. The existing and proposed floodplain linetypes appear to be reversed on the
legend for the ultimate condition floodplain delineation map. Revise the
linetypes so the legend and the delineations match each other.
4. There are instances where the floodplain width delineated on the Grapevine
Creek — FIS workmap does not match the width given in the proposed
condition HEC -2 modeling. For example, the HEC -2 model states that the
proposed 100 -year floodplain width at cross section 13339 is 934 -feet, but
the workmap shows a floodplain width of approximately 1380 -feet. Check
the floodplain widths and revise the modeling and the workmaps to be
consistent with one another.
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TEL 972 770 1300
FAX 972 239 3820
❑ _ ❑ Kimley -Horn Mr. Kenneth M. Griffin, P.E. June 7, 2004, Page 2
and Associates, Inc.
5. There are instances where the floodplain width delineated on the Grapevine
Creek — 100 -Year Ultimate Condition workmap does not match the width
given in the proposed condition HEC -2 modeling. For example, the HEC -2
model states that the proposed ultimate condition 100 -year floodplain width
at cross section 13339 is 1085 -feet, but the workmap shows a floodplain
width of approximately 2200 -feet. Check the floodplain widths and revise
the modeling and the workmaps to be consistent with one another.
6. Based on elevations, cross section 13829 is in a critical depth condition
during the FIS 100 -year event. Revise the model to remove the critical
condition, or provide an explanation of why the critical condition should
remain.
7. Revise the model so that the revised cross sections contain the modeled
flows. For example, 100 -year ultimate condition floodplain elevation for
cross section 13339 is 452.01, but the right overbank in the cross section
extends to an elevation of 450.
The above comments should be addressed, and the CLOMR request should be
formally submitted for a second review.
If you have any questions or comments, please do not hesitate to call.
Sincerely,
KIMLEY -HORN AND ASSOCIATES, INC.
�,
Dan F. Grant, P. .
Project Manager
cc: Allen L. Xu, P.E., C.F.M — Halff Associates
G: \HYDRO\ PROJECTS \63124007\LtrSBill01.doc