PA1502 - CS151215kimley-horn.com 12750 Merit Drive, Suite 1000, Dallas, TX 75251 972 770 1300
Via email
December 15, 2015
Mr. Michael Garza, P.E.
Assistant Director of Engineering
City of Coppell
265 Parkway Boulevard
Coppell, Texas 75019
RE: Grapevine Creek Trail
Grapevine Creek
Floodplain Study
Dear Mr. Garza:
Kimley-Horn and Associates, Inc. (Kimley-Horn) is in receipt of the Grapevine Creek Trail Drainage
Study prepared by Pacheco Koch (PK), dated September 21, 2015. Kimley-Horn has reviewed the
documents for adherence to the City of Coppell Floodplain Management Ordinance and National
Flood Insurance Program (NFIP) regulations. Kimley-Horn offers the following comments based on a
review of the above referenced study.
1. The City of Coppell Floodplain Management Ordinance requires conditional approvals be
obtained by the Federal Emergency Management Agency (FEMA) prior to issuing a
Floodplain Development Permit. The submittal provided by PK was not a Conditional Letter
of Map Revision (CLOMR); however, the project as presented does not require a CLOMR
per NFIP criteria (no increase in Base Flood Elevation as a result of the project). It is at the
City’s discretion whether or not to require a CLOMR submittal prior to issuing a Floodplain
Development Permit.
2. The proposed project will result in increases in 100-year fully-developed water surface
elevation within the adjacent railroad right-of-way. Approval of the water surface elevation
increases should be provided by the railroad, or the proposed grading should be revised to
eliminate the increases.
3. The proposed project will result in increases in erosive velocity, which is not allowed by the
City of Coppell Floodplain Management Ordinance. The proposed grading should be
revised to eliminate the increase in erosive channel velocity or the proposed gabion mattress
should be extended to encompass the entire limits of increased velocity.
4. The Preliminary Jurisdictional Determination (PJD) provided with the submittal indicates
approximately 40 linear feet of impact to the Ordinary High Water Mark associated with
Grapevine Creek. The workmaps included in the submittal appear to show impact in excess
of 40 feet. PK should verify that the proposed impacts are consistent with the PJD and that
no pre-construction notification under Nationwide Permit 14 is still applicable.
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kimley-horn.com 12750 Merit Drive, Suite 1000, Dallas, TX 75251 972 770 1300
5. The 100-year fully-developed peak flow used in the analysis is inconsistent with the peak
flow listed in the City-Wide Storm Water Management Study . Peak flows should be revised
to be consistent with the City-Wide study.
6. The 2-year fully-developed peak flow should be included in the analysis to check channel
velocity impact as required by the City of Coppell Floodplain Management Ordinance. The
2-year fully-developed peak flow can be obtained from the City-Wide Storm Water
Management Study.
7. The 100-year fully-developed starting water surface elevation at cross section 11588 is
inconsistent with the 100-year fully-developed flood profile for Grapevine Creek included in
the City-Wide Storm Water Management Study . The starting water surface elevation should
be revised to be consistent with the City-Wide study.
8. The proposed gabion mattress should be extended to encompass the entirety of the
proposed channel modification to mitigate potential erosion.
9. Construction plans for the proposed bridge should be provided to verify the deck information
included in the hydraulic model.
10. Construction plans and workmaps included in the study are not plotted to scale. These
drawings should be plotted to scale.
11. Expansion and contraction coefficients at the proposed bridge crossing should be adjusted
to 0.3 and 0.5, respectively.
12. The proposed bridge crossing will overtop in the modeled storm events. Ineffective flow
areas downstream of the proposed crossing, associated with the proposed crossing, should
be set to normal not permanent.
The Floodplain Study should be revised based on the above comments and resubmitted to the City of
Coppell for further review. The submittal should include a written response to each comment.
Kimley-Horn may offer additional comments as a result of the review of the revised submittal.
Please don’t hesitate to contact me at 972-776-1781 if you have any questions or comments
regarding this letter.
Sincerely,
Brad Pickering, P.E., CFM
Project Manager