PA1501B-CS160203Dunaway No. 000810.001
January 29, 2015
Mr. Michael Garza, P.E.
Assistant Director of Engineering
City of Coppell
265 Parkway Boulevard
Coppell, Texas 75019
Reference: Andrew Brown Park – Denton Creek Floodplain Study
Dear Mr. Garza:
We have received review comments for the Andrew Brown Park – Denton Creek Floodplain
Study from Kimley-Horn and Associates on September 23, 2015. Below are our responses to
the comments that were provided:
1. Discussion regarding USACE 404 permitting requirements should be added to the
report. If one is required, a 404 permit for the proposed improvements should be
obtained prior to issuance of the Floodplain Development permit.
a. A 404 permit was already obtained from the USACE. Acceptance letter is
included in Appendix D.
2. Discussion regarding water rights permitting should be added to the report. If one is
required, a water rights permit for the proposed lake improvements should be obtained
prior to issuance of the Floodplain Development permit.
a. Discussion added to the report; a water rights permit is not required.
3. Evidence of compliance with the Endangered Species Act should be provided. The
applicant should refer to FEMA Procedure Memorandum 64 for additional information.
An official species list should be obtained from the U.S. Fish and Wildlife Service
Information for Planning and Conservation (IPaC) online system.
a. As part of the 404 permit approval process it was determined that this project
will not affect any species listed as threatened or endangered as indicated in
the 404 permit approval letter.
4. The proposed improvements are located within the FEMA effective floodway and will
increase FEMA regulatory floodplain elevation by more than 0.00’. As such, a
Conditional Letter of Map Revision is required from FEMA. Dunaway should submit the
CLOMR to the City of Coppell for review and approval prior to submittal to FEMA.
a. Please find the CLOMR attached to this letter.
5. The proposed water surface elevation increases appear to impact Denton County
Levee Improvement District (LID) No. 1 facilities based on the FEMA effective map.
Dunaway should coordinate with the LID to confirm the increases are acceptable, or
that they do not impact LID facilities.
Andrew Brown Park – Denton Creek Floodplain Study
01/29/2016
Page 2
a. We have coordinated with the Levee Improvement District and obtained
existing top of levee and freeboard data. It appears that in the vicinity of the
cross sections where there is a maximum of 0.22-ft rise in BFE, the levees were
constructed to provide more than 7-feet of freeboard over the 100-year BFE;
therefore the minor increases due to the project should not adversely impact
the levees.
6. Backwater from the proposed water surface elevation increases may impact properties
other than the park in the vicinity of cross section 27442. Dunaway should confirm
whether the backwater will impact offsite properties, and make adjustments to the
design if necessary.
a. Due to the river station equation, the cross section that was previously identified
as 27442 is now 28132. The increase in 100-year BFE at this cross section is
only 0.08-ft and still contained within the existing tributary banks. Based on
available contours, the houses next to the tributary appear to be at least 3.28-
ft higher than the 100-year BFE; therefore no adverse impact is expected on
these offsite properties.
7. The workmaps should be extended to show the entire subject reach. Several cross
sections are cut off with the current workmap limits.
a. Workmaps have been revised as suggested.
8. Floodplain mapping should show a logical tie-in to the FEMA effective floodplain
boundary. It is unclear if the revised existing and proposed floodplains tie into the FEMA
effective floodplain at the upstream and downstream extents of the subject reach.
Floodplain mapping should be adjusted as necessary.
a. Floodplain mapping has been revised to better tie-in to the FEMA effective
floodplain boundaries.
9. Dunaway should provide a velocity comparison table using the 2-year, fully-developed
flows to verify the proposed improvements do not result in erosive conditions.
a. A velocity comparison table is included in Appendix B. The channel, left
overbank, and right overbank velocities do not appear to be erosive.
10. The starting water surface elevation used in the HEC-RAS model does not match the
Flood Insurance Profile and City-Wide Storm Water Management Study. Based on the
location of FEMA Cross Section “C”, it appears the starting water surface elevation
should be determined at approximately station 26220. The starting water surface
elevation should be revised.
a. In the FIS profile, there is a station equation at this cross section, however since
the effective model is a HEC-2 model, it is not clear if the model actually reflects
the station equation. Nevertheless, to be conservative, the downstream
boundary known water surface elevation for this study model has been
determined from the FIS profile as requested.
11. The City and FEMA effective model input and output should be provided in their original
HEC-2 format. Dunaway should also include a comparison that shows the difference
Andrew Brown Park - Denton Creek Floodplain Study
un9t2016
Page 3
between the effedive HEC-2 models and the converted HEC-RAS models. Any
significant differences between the HEG2 and HEGRAS results should be
investigated and explained.
a. The original HEC-2 models are now included in the submittal ffable 1). There
are no significant differences between he HEC-2 and the duplicate effective
HEC-RAS models.
12. The models do not appear to tie into the FIS and City fully-developed profiles wihin 0.5
feet at the upstream end of the subject reach. The models should be extended to a
point where the ti+'in is within 0.5 feet.
a. One additional cross section was added to the model; now the BFEs tie within
0.5 feet.
13. The fully-developed flows used in the study do not match the City-Wide Storm Water
Management Stndy. The flows should be revised to match üre study.
a. The fully-developed flows were taken from Table ll-6 Comparison of
Discharges, from the CityWide Storm Water Management Study prepared by
Halff Associates, dated January 1991.
14. A conected effec'tive hydraulic workmap should be provkled for the fullydeveloped
condition flows. o,a. The conected effective hydraulic wokmap is provided in Appendix A - Figure
3. This figure includes the fully-developed floodplain delineation.
15. The existing and proposed lakes are modeled as ineffective flow areas with a top
elevation of 453, but the normal pool elevation appear to be approximately 448 based
on cross section data. Howwas the ineffective flow area elevation determined?
a. lt is correct that the normal pool elevation appears to be at 448, however there
is a berm just downstream of the lake with a top elevation of 453; thus until the
watersurface elevation atthe lake reaches 453, there is no effec{iveflow.
Sincerely, ,
DUNAWAY ASSOCIATES, L.P,
a Texas limited partnership
Ørk?
Dr. Cuneyt Erbatur, P.E., CFM, LEED AP
Project Manager
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Letterl_27_
Enclosure
cq Brad Pickering, P.E., CFM
Kimley-Horn and Associates
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