ST9902-SY170424Fort Worth5237 N. Riverside Drive, Suite 100Fort Worth, Texas 76137817.336.5773
Dallas17304 Preston Road, Suite 1340Dallas, Texas 75252214.461.9867 Denton1517 Centre Place Drive, Suite 320Denton, Texas 76205940.383.4177
Registered by the Texas Board of Professional Engineers, Firm No. F-230 Registered by the Texas Board of Professional Surveyors, Firm No. 100116-00
engineers
surveyors
landscape architects
www.tnpinc.com
April 24, 2017
Kate E. Ploetzner, P.E., CFM
Kimley-Horn
12750 Merit Drive, Suite 100
Dallas, Texas 75251
RE: Sandy Lake and Freeport Parkway at Cottonwood Branch – Kimley-Horn Review
2nd Review Comment Response
TNP# CPL 00243-002
Dear Ms. Ploetzner,
This document serves as a response to Kimley-Horn’s Review comments made via email on November 21,
2016, as they pertain to the Sandy Lake and Freeport LOMR. Reproductions of Kimley-Horn’s written
markup comments, as well as our response to these comment, are provided below. Revised copies of all
reports, maps, and digital models impacted by these comments are enclosed with this letter.
***
Previous Comments from October 19, 2016 Comment Letter
Comment 2: Mapping should not be based on the preliminary maps for Dallas County. FEMA will only
accept topographic information, mapping, and hydraulic modeling that is based on current FEMA effective
information. Revise the regulatory floodplain and floodway delineations to tie-in to FEMA effective
floodplain and floodway delineations. Update MT-2 Form 1, the topographic workmaps and Annotated
FIRM accordingly to be based on current FEMA effective information.
The November 2016 report continues to reference Preliminary DFIRM mapping.
The report has been updated to reference the FEMA effective information, not the
preliminary DFIRM mapping.
Comment 8: Floodway encroachments in the provided hydraulic models should be revised such that they are
not located inside of the channel bank stations.
The floodway encroachment at pre-project cross section 12868 is within the banks.
The encroachment stations for the cross section 12868 has been adjusted, so the floodway
stations are located outside of the channel bank stations.
Comments 9: Discrepancies were found when comparing the mapped floodplain and floodway widths to
the hydraulic modeling output widths. Revise the modeling and/or mapping to remove this inconsistency.
There are still discrepancies. Some examples include the post-project floodway limits of cross sections
9890 and 12567, post-project 100-year (FIS discharges) floodplain limits of cross sections 12868
and12707, and post-project 100-year ultimate floodplain limits of cross sections 12957, 12868, and
12539.
The cross sections have been adjusted to remove the inconsistency.
Comment 10: Mapped floodplains on the topographic workmaps are not consistent with topography in
some locations; especially near the Sandy Lake Road crossing. For example, a portion of the Sandy Lake
bridge is mapped as being overtopped in the 100-year event per both floodplain workmaps. Additionally,
mapped floodplain elevations are not consistent with the elevations shown in the hydraulic modeling output
at some locations. Revise the mapping to remove these inconsistencies.
There are locations where mapped 100-year water surface elevations on the topographic workmaps
do not match HEC-RAS output. Some examples include cross section 9890 in post-project 100-year
conditions (FIS discharges and ultimate discharges), cross sections 12539 and 12420 in post-project
100-year conditions (FIS discharges) and cross sections 12957 and 12868 in post-project 100-year
conditions (ultimate discharges).Design plans need to be signed and sealed by a professional engineer
for our record.
The cross sections have been adjusted to match the ground survey. As-built ground survey
and topo points (shown in the workmap) serve as the basis for all the cross sections near
the bridge. This data is supplemental with 1-ft LIDAR topo as needed. The floodplain and
floodway delineations have been updated accordingly.
Comment 15: The water surface elevation summary tables provided in the report are inconsistent with the
workmaps. Revise the report accordingly to remove this inconsistency.
There are inconsistencies between the HEC-RAS output to the tables on the workmaps. For example, the
pre-project 100-year water surface elevation (using FIS flows) for cross section 10130 is 483.67 per
HEC-RAS output; however, it is 485.37 per the provided topographic workmap.
The water surface elevation summary table has been updated to reflect the HEC-RAS
output.
Comment 16: There are no cross section labels on the “Topographic Workmap – Freeport Existing/FIS
discharges”. Revise the workmap to include cross section labeling. Additionally, the North Arrow is incorrect
on the Freeport Parkway topographic workmaps. Revise the workmaps to remove this error.
The label for cross section 12567 is missing from the above referenced workmap.
The label for cross section 12567 has been added to the Topographic Workmap-Freeport
Existing/FIS Discharges.
Comment 17: Geometry shown in the post-project hydraulic model cross section 9590 is not consistent with
the topographic workmap. Revise the modeling accordingly.
Geometry is inconsistent between the hydraulic modeling and the topographic workmap, particularly
along the right overbank of the cross section.
Field revisions during construction of Sandy Lake were necessary for both the gabions and
grading. The revised as-built plan is provided with this response to comments. The
hydraulic model has been updated to reflect the revisions made to the Sandy Lake bridge
at cross section 9590.
Comment 18: Gabion wall and proposed grading contour information shown on the “Topographic
Workmap – Freeport Existing/FIS Discharges” and the “Topographic Workmap – Freeport Ultimate
Conditions Discharges” at cross sections 12868, 12707, 12567, 12539 and 12420 do not match the
information shown on the provided channel grading plan sheets from the “Reconstruction Plans for Freeport
Parkway” (Teague, Nall & Perkins, September 2015). Provide as-built information for the channel
improvements near Freeport Parkway that matches the as-built hydraulic modeling and floodplain
workmaps. Revise the modeling and workmaps as necessary.
There are inconsistences when comparing the topographic information shown on the provided
topographic workmaps to the grading shown on the “Reconstruction Plans for Freeport Parkway”
sheets, particularly on the left overbank of cross section 12868, southwest of the proposed gabion
wall.
The hydraulic model has been adjusted to reflect the as-built grading in the overbanks of
the cross sections listed above.
Comment 20: Provide the effective water surface elevation values in MT-2 Form 2, Section B.1.
Effective water surface elevation values are blank in the MT-2 Form. This information is required by
FEMA.
There is no effective water surface elevation for the given cross sections.
New Comments
1. Floodway limits on the Annotated FIRM do not match those on the provided topographic workmaps.
Revise the mapping to remove this inconsistency.
The Annotated FIRM map has been adjusted to remove the inconsistency.
2. There are inconsistencies in the post-project modeling regarding Manning’s “n” values for gabion
mattress and walls. For example, Manning’s “n” values of both 0.025 and 0.06 were assigned to
gabion surfaces at post-project cross section 12567 and a value of 0.035 was assigned to gabion
surfaces at post-project cross section 12707. Provide justifications for the varying Manning’s “n”
values or revise the modeling to remove this inconsistency.
The Manning’s n-value has been revised.
Thank you for your help in this matter. Please do not hesitate to contact me with any questions.
Sincerely,
tnp
teague nall & perkins
Niraj A. Acharya P.E., CFM
Cc:
Letter of Map Revision
Cottonwood Branch
at
Sandy Lake Rd. and Freeport Parkway
Prepared For:
City of Coppell
265 E. Parkway Blvd.
Coppell, TX 75019
Prepared By:
5237 N. Riverside Drive, Suite 100
Fort Worth, TX 76102
Cottonwood Branch LOMR – Bridge Improvements (Sandy Lake Rd & Freeport Pkwy)
Registered by the Texas Board of Professional Engineers, Firm No. F-230 Registered by the Texas Board of Professional Surveyors, Firm No. 100116-00
Table of Contents
Background .......................................................................................................................................................... 1
Project Location ................................................................................................................................................... 1
Effective Data ..................................................................................................................................................... 1
Hydraulic Modeling ........................................................................................................................................... 1
Model Descriptions ........................................................................................................................................ 2
Modeling Parameters ................................................................................................................................... 3
Ultimate Conditions Considerations ................................................................................................................ 5
Conclusion ............................................................................................................................................................ 6
Appendices
APPENDIX A – EXHIBITS
o Exhibit 1: Location Map
o Exhibit 2A: Topographic Workmap - Sandy Lake Rd. - FIS Discharges
o Exhibit 2B: Topographic Workmap - Sandy Lake Rd. - Ultimate Discharges
o Exhibit 3A: Topographic Workmap - Freeport Pkwy. - FIS Discharges
o Exhibit 3B: Topographic Workmap - Freeport Pkwy. - Ultimate Discharges
o Exhibit 4: Annotated FIRM
o Exhibit 5: Final Construction Plans – Sandy Lake Rd.
o Exhibit 6: Final Construction Plans – Freeport Pkwy.
APPENDIX B – HYDROLOGIC DATA
o Flood Insurance Rate Map Panels
o FIS Peak Discharge Summary
o Flood Profiles (Cottonwood Branch and Denton Creek)
o Floodway Data
APPENDIX C – HYDRAULIC DATA
o HEC-RAS Standard Summaries
APPENDIX D – FEMA FORMS
o Payment Information Form
o MT-2 Form 1 – Overview and Concurrence
o MT-2 Form 2 – Riverine Hydrology & Hydraulics
o MT-2 Form 3 – Riverine Structures
APPENDIX E – COMPACT DISK
Cottonwood Branch LOMR – Bridge Improvements (Sandy Lake Rd. & Freeport Pkwy.)
1
Background
Teague Nall and Perkins, Inc. (TNP) was contracted by the City of Coppell to design bridge improvements
at Sandy Lake Rd. and Freeport Parkway, along Cottonwood Branch. The bridge improvements consist
reconstruction of existing bridges at both crossings to be both wider and taller. Construction for both
projects has recently been completed. This report and attached data document the project and provide
support for a Letter of Map Revision (LOMR) request from FEMA.
Project Location
The project is located along Cottonwood Branch at Sandy Lake Rd. and Freeport Parkway. Cottonwood
Branch crosses Freeport Parkway approximately 2,200 feet south of the intersection of Freeport
Parkway and Sandy Lake Road. The creek then flows approximately 2,900 feet downstream (towards
the northeast) before crossing Sandy Lake Road. The creek ultimately combines with the main stem of
Denton Creek, approximately 1.5 miles downstream of Sandy Lake Road. Exhibit 1 – “Location Map”
- depicts the stream alignment and location. Cottonwood Branch is a FEMA studied stream (Zone AE,
with a Floodway) as shown on Flood Insurance Rate Map (FIRM) 48113C0010L.
Effective Data
Flows published in the effective Flood Insurance Study for Dallas County were used for existing conditions.
These flows, which include the 10-, 50-, 100-, and 500-year peak discharges. The bridges are designed to
convey the 100-year ultimate flows, which were obtained from the “City-Wide Storm Water Management
Study for the City of Coppell (Halff Associates; Jan 1991).” Peak flows are summarized in Table 1. Based
on coordination with the City, the bridge at Sandy Lake Road is designed for 2-feet of freeboard under FIS
discharges and 1-foot of freeboard under Ultimate discharge. Based on further discussions with the City, the
bridge at Freeport Parkway has been designed with less than one foot of freeboard in Ultimate conditions.
Table 1: Cottonwood Branch Flow Summary
Hydraulic Modeling
The following paragraphs summarize the hydraulic models that were obtained and created for this
project. Requests were made to FEMA in order to obtain the effective floodplain and floodway models for
Cottonwood Branch. This request resulted in a digital HEC-2 floodway model for the stream. FEMA was
unable to locate any floodplain models as part of this data request. FEMA also provided a hardcopy of the
letter of determination that was distributed after acceptance of the LOMR for the Parkway Boulevard
improvements (LOMR Case No. 94-06-307P). Unfortunately, it does not appear that the digital models
provided by FEMA match the revised flood profiles provided with the letter of determination. In fact, the
provided digital model does not include stream crossings at Parkway Boulevard or Denton Tap Road.
Model summaries are provided in the PDF format in Appendix D (compact disk).
Cross Section
FEMA Ultimate
Comments 10-yr 50-yr 100-yr 500-yr 100-yr
(cfs) (cfs) (cfs) (cfs) (cfs)
12957 3,600 5,000 5,700 7,100 8,250 At Freeport Pkwy.
10130 4,200 5,900 6,700 8,600 8,250 At Sandy Lake Rd.