Cottonwood Est-SY 191009kimley-horn.com 13455 Noel Road, Two Galleria Office Tower, Suite 700, Dallas, TX 75240 972 770 1300
October 9, 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
September 22, 2019 and received by Kimley-Horn on October 2, 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:Prepare pre-project and post-project HEC-RAS models using 100-year ultimate condition
flows from the City’s Storm Water Management Study by Halff Associates, Inc. dated 1991 and confirm
the proposed improvements adhere to the City of Coppell Floodplain Management Ordinance.
Comment 2:Cross section 8584 is shown on the hydraulic workmap but is not included in the provided
HEC-RAS models. Cross section 8621 is included in the HEC-RAS models but not shown on the
hydraulic workmap. Revise the modeling and mapping to correct this inconsistency and update the
report appropriately.
Comment 3:Provide workmaps that show the 100-year existing and post-project floodplain
delineations using FEMA FIS flows and City fully developed flows, respectively, in addition to cross
section locations and existing condition topography.
Comment 4:Provide workmaps that show proposed topography, cross section locations and 100-year
proposed and post-project floodplain delineations using FEMA FIS flows and City fully developed flows,
respectively, in addition to cross section locations and proposed condition topography.
Comment 5:Pre-project cross sections 8823 to 8651 include a blocked obstruction within the onsite
existing pond at elevation 469.46, which is assumed to be the pond’s normal water surface elevation
and outfall elevation; however, per the topography shown on the hydraulic workmap provided, the
outfall elevation seems higher than 469.46. Provide supporting documentation and justification
regarding the assumed normal water surface elevation of 469.46 and confirm the HEC-RAS modeling
is consistent with the pond’s normal water surface elevation.
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kimley-horn.com 13455 Noel Road, Two Galleria Office Tower, Suite 700, Dallas, TX 75240 972 770 1300
Comment 6:There are locations where post-project HEC-RAS geometry seem inconsistent with
proposed grading. Verify geometry in the HEC-RAS modeling is consistent with the proposed grading,
including but not limited to cross sections 8823 and 8651.
Comment 7:An impoundment is proposed near cross section 8783. It is unknown where the cross
sections are in relation to proposed grading. Confirm that proposed condition HEC-RAS cross sections
are placed at the toe and top of banks of this proposed impoundment to accurately model the hydraulic
effects of the impoundment. Provide detailed design calculations to the City of Coppell to show that
erosion will not occur due to this proposed impoundment.
Comment 8:The report stated that Manning’s “n” values are 0.04 at the inlet and outlet of the existing
and proposed pond due to the increased friction at these locations. The pre-project and post-project
HEC-RAS model doesn’t include an “n” value of 0.04 at cross section 8621. Revise the modeling and
report to correct this inconsistency.
Comment 9:A right overbank Manning’s “n” value of 0.06 seems high for the areas that include short
grasses. Revise the pre- and post-project modeling to show a lower, more appropriate Manning’s “n”
value or provide justification that 0.06 is representative of this area.
Comment 10:The Manning’s “n” value along the normal water surface of the existing and proposed
pond is set at 0.06. Reduce the Manning’s “n” value to be consistent with the normal water surface.
We recommend using a value of 0.02.
Comment 11:Per the Post-Project HEC-RAS modeling, the normal water surface elevation of the
proposed pond is assumed to be 469.50; however, the elevation near the outfall of the pond (cross
section 8621) shows an elevation of 469.78, which is higher than the normal water surface of the pond.
Per the proposed grading exhibit, the normal water surface elevation seems higher than 469.50. Revise
the modeling and grading to show positive drainage at the outfall of the pond and provide more detail
regarding the proposed pond’s outfall structure and elevation on the proposed hydraulic workmap. Also,
confirm the normal water surface elevation is consistent at all HEC-RAS cross sections containing the
proposed pond.
Comment 12:Due to the pond outfalling directly onto a neighboring property, show that the pond
peak outflow is lower in post-project conditions than compared to pre-project conditions and erosive
conditions will not be created at the proposed pond outfall.
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kimley-horn.com 13455 Noel Road, Two Galleria Office Tower, Suite 700, Dallas, TX 75240 972 770 1300
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