Westhaven-CS140129 ;p; HALFF
January 29, 2014
AVO 28875 W003
Mr. Kenneth M. Griffin, P.E.
Director of Engineering and Public Works
City of Coppell
PO Box 9478
Coppell, Texas 75019
Re: Review of Floodplain Study Submittal by Kimley-Horn and Associates, Inc. — Letter of
Map Revision (LOMR) for Westhaven Residential Development—Denton Creek
Dear Mr. Griffin,
We have completed a review of the Letter of Map Revision (LOMR)for Westhaven Residential
Development — Denton Creek prepared by Kimley-Horn and Associates, Inc., dated December
16, 2013, and received on January 16, 2014. This review was authorized by the City of Coppell
dated January 21, 2014, and received by Halff Associates, Inc. on January 21, 2014. This
general review consists of checking the submittal package for completeness and compliance with
the City of Coppell floodplain ordinances, regulations, and policies.
The following comments are a result of this review:
1. The as-built project seems to have no significant impact on water surface elevations on
Denton Creek for the fully developed watershed condition.
2. The as-built project seems to have no significant impact on the 2-year or 100-year
velocities on Denton Creek for the fully developed watershed condition.
3. Valley storage loss on Denton Creek due to the as-built project does not exceed the
maximum allowable of 15%. Valley storage reductions for the standard project flood
event was not determined since the as-built project modifications have no significant
impact on the standard project flood backwater elevation from the Elm Fork of the Trinity
River.
4. Be advised that our copy of the LOMR submittal contained two (2) copies of the hydraulic
model output for the FEMA post-project floodway condition. A copy of the FEMA pre-
project floodway output was not included. Please correct the omission and resubmit.
5. Be advised that the water surface elevations shown in Table 3 for both "Pre-Project" and
"Post- Project" conditions do not match the hydraulic model output for each condition
found in Appendix A. Please revise and resubmit.
HALFF ASSOCIATES,INC.
1201 NORTH BOWSER ROAD TEL (214)346-6200 WWW.HALFF.COM
RICHARDSON,TX 75081-2275 FAX (214)739-0095
;p; HALFF
6. The as-built project seems to have no significant impact on water surface elevations on
Denton Creek for existing conditions (FEMA).
7. The as-built project does not create any surcharges greater than 1.0 feet or less than 0.0
feet.
8. Be advised that an explanation may need to be included as part of this LOMR submission
which explains why floodway topwidths are greater than the 100-year floodplain
topwidths at cross-sections 34950 and 35253. Typically, floodway topwidths are less than
or equal to the 100-year un-encroached floodplain topwidths.
9. The applicant has indicated on MT-2 Form 3 Sections B and C that sediment transport was
not considered as part of this project; however, an explanation was not provided. Please
correct the omission and resubmit.
Based on this information, Conditional Approval is granted.
This review was performed for the benefit of the City of Coppell. This is not a detailed
engineering review of any construction plans, and Halff Associates, Inc. assumes no liability for
the adequacy of the planning, design or construction of the project.
Do not hesitate to call me at 214-346-6245 if you have further questions or comments
concerning this review. Please include the reference "AVO 28875"on all correspondence.
Sincerely,
HALFF ASSOCIATES, INC.
Russell P. Erskine, P.E., CFM
Project Manager
cc: Kyle A. Dickey, P.E.
Kimley-Horn and Associates, Inc.