Cottonwood Est-SY 191125kimley-horn.com 13455 Noel Road, Two Galleria Office Tower, Suite 700, Dallas, TX 75240 972 770 1300
November 25, 2019
Sent via email
Mr. Kumar Gali, P.E., CFM, D.WRE
Assistant Director of Public Works
City of Coppell
255 Parkway
Coppell, TX 75019
RE:Comments for 125 Cottonwood Drive Lot Improvements
Dear Mr. Gali:
Kimley-Horn and Associates, Inc. (Kimley-Horn) is in receipt of the floodplain study submittal for the
proposed improvements at 125 Cottonwood Drive, prepared by O’Brien Engineering, Inc. (OEI) dated
November 11, 2019. Kimley-Horn has reviewed the documents for adherence to the City of Coppell
Floodplain Management Ordinance and National Flood Insurance Program regulations. Kimley-Horn
offers the following comments based on a review of the above documents:
Comment 1:The previous comment letter requested the pre-project and post-project HEC-RAS
models to be prepared using 100-year ultimate condition flows from the City’s Storm Water
Management Study by Halff Associates, Inc. dated 1991 (1991 Study). Flows in the provided HEC-
RAS model are inconsistent with the 1991 Study. Per the Cottonwood Branch Plate 5-14 in Appendix
F of the 1991 Study, we assume the flows should be consistent with what is shown below. If you believe
otherwise, provide justification.
Cross
Section
Locations
2-year
Ultimate
Flow (cfs)
100-year
Ultimate
Flow (cfs)
10130 to 9590 2,750 8,250
9590 to 8000 2,910 8,710
8000 to 6710 2,990 8,390
Once the HEC-RAS models are revised, confirm the proposed improvements adhere to the City of
Coppell Floodplain Management Ordinance.
Comment 2:There is a Manning’s “n” value of 0.65 on the left overbank of cross section 8793, which
is incorrect. Revise the HEC-RAS modeling to remove this error.
The previous comment letter stated that a right overbank Manning’s “n” value of 0.06 seems high for
the areas that include short grasses and to revise the pre- and post-project modeling to show a lower,
more appropriate Manning’s “n” value for short grasses or provide justification that 0.06 is
representative of this area. OEI’s response to this comment was that Manning’s “n” values were revised
within the right overbank to maintain continuity of flow through the right overbank. Though we don’t
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kimley-horn.com 13455 Noel Road, Two Galleria Office Tower, Suite 700, Dallas, TX 75240 972 770 1300
agree with a Manning’s “n” value of 0.07 or 0.085 to be used for short grasses, we understand the
methodology of using these Manning’s “n” values for this particular study and we accept this approach.
Further modifications to the modeling and the report as a result of addressing these comments may
result in additional comments not noted in this letter. The City should reserve the right to make
additional comments to subsequent submittals. Should you have any questions regarding these
comments, please do not hesitate to contact me at (972) 770-3024 or kate.ploetzner@kimley-horn.com.
Sincerely,
Kate E. Ploetzner, P.E., CFM
Engineer