St Andrews Est-SY040109REQUEST FOR A
CONDITIONAL LETTER OF MAP REVISION (CLOMR)
ON
DENTON CREEK
(OLD DENTON CREEK)
IN Tf~ CTTY OF COPPELL
DALLAS COUNTY, TEXAS
PROJECT: ST. ANDREWS ESTATES
Prepared for:
St. Andrews Land Company Ltd. -Holmes Builders, Inc.
1406 Halsey Way Suite 100
Carrollton, Texas 75007
Prepared bv:
Evans Engineering
1417 W. Main Street, Suite 1008
Carrollton, Texas 75225
October 27, 2003
Revised: January 9, 2004
REQUEST FOR CLOMR - ST. ANDREWS ESTATES
TABLE OF CONTENTS
SECTION I -- Narrative
A. General
B. Purposes of Study and Report
C. Site Description
D. Topography
E. Hydrology
F. FEMA Model
G. Hydraulics
H. Results and Conclusions
List of Figures
Figure 1 Location Map -City of Coppell (Source: Dallas Co. Appraisal District)
Figure 2 Vicinity Map (Source: Dallas Co. Appraisal District)
Figure 3 Map of Neighboring Developments
(Source: Dallas Co. Appraisal District)
Figure 4 Topographic Maps (Extracts of Carrollton
quadrangle, 1:20,000 & 1:10,000)
SECTION II -FEMA Application/Certification Forms
MT-1 Forms 1, 2 and 3 (Property Information Form, Elevation Form,
Revision Requester and Community Official Form
MT-2 Forms 1 and 2 (Overview & Concurrence Form,
Riverine Hydrology & Hydraulics Form)
APPENDICES -Technical Data
Appendix A -Preliminary Plat, "St. Andrews Estates," August 26, 2003
Appendix B -Grading Plan, "St. Andrews Estates," August 21, 2003
Appendix C - LOMR No. 97-06-503P, July 23, 1997
Appendix D -Topographic Map (1" = 200') --Cross-section stations and area of fill
Appendix E -Cross-Sections (HEC-RAS output)
Appendix F - Floodway Surface (HEC-RAS output)
Appendix G -Table of Water Surface Elevations, Existing and Proposed (HEC-RAS)
Appendix H -Extract of FIRM Panel 480170 0010 E, dated July 23, 1997
Extract of FIRM Panel 48113C 0155 J, dated August 23, 2001
(Showing proposed floodway/floodplain revision)
Appendix I -HEC-RAS Output (Report)
Appendix J -HEC-RAS Input Files (3.5" Disk)
Rev. Jan 9, 2004
REQUEST FOR A
CONDITIONAL LETTER OF MAP REVISION ~CLOMR~
OLD DENTON CREEK
PROJECP: "ST. ANDREWS ESTATES"
A. General
The subject development proposes filling a portion of the floodway of Old Denton Creek
and creating building lots, as shown by the attached plat (Appendix A.) The City of
Coppell requires a hydraulic study prior to filling any delineated floodway.
This study was undertaken to provide the necessary technical data for issuance of a
Conditional Letter of Map Revision (CLOMR) for floodway reclamation along Denton
Creek in the City of Coppell, Dallas County, Texas, as shown by the Grading Plan at
Appendix B.
The required fonns for this "Request for a Conditional Letter of Map Revision" are
included in Section II. The technical information is provided as Appendices.
B. Purposes of Studv and Report
The purpose of this study is a determination of the effects of filling the floodway of Old
Denton Creek. An objective of this report is to provide all necessary information for
obtaining a Conditional Letter of Map Revision (CLOMR) from the Federal Emergency
Management Agency.
C. Site Description
"St. Andrew's Estates" is a proposed residential subdivision that includes a cul-de-sac
street. The site touches on the southern banks of Old Denton Creek about 1000 feet
downstream of MacArthur Blvd. The ten-acre site was originally developed by the City
of Coppell as a service center (warehouse and shops) and was sold to Holmes Builders,
Inc. in 2002.
D. Topographv
A small tributary meanders through the subject property and enters the floodway of
Denton CreeklOld Denton Creek, creating a small appendage to the floodway and flood
plain. This is evident in the quadrangle maps shown at Figures 4a and 4b.
As documented by the attached LOMR, there is a split in the channel just upstream of
MacArthur Blvd where a portion of the flow in Denton Creek enters the old channel,
called "Old Denton Creek." The newer channel was created by a Corps of Engineers
project with the old channel left to help carry less frequent, larger flows.
E. Hvdrolocav
Denton Creek's contributory watershed area at a point just below its confluence with
Cottonwood Branch is 19.45 square miles with a peak discharge of 21,300 cfs for the
100-year storm event, according to the FIS. LOMR No. 97-06-503P documented a split
in flow at the divergence of Old Denton Creek, such that the 100-year flood peak
discharge in the latter channel is determined to be 4000 cfs.
Rev. Jan 9,2004
F. FEMA Model
The Flood Insurance Rate Map (FIRM) for this channel was reissued as Panel
No. 4811300155 J, effective date August 23, 2001. However, the delineation of
the floodway and flood plain in the area of the project do not correspond to
LOMR No. 97-06-503P, dated July 23, 1997. The important distinction is that the
FIRM delineates the entire width of the channel as well as the small tributary
described herein as floodway, in which case no fill would normally be allowed.
The LOMR, however, delineated only the center part of the channel as floodway
with the banks of the channel and the small tributary defined as flood plain (Zone
AE.) Since flood plains can usually be filled as long as the regulatory base flood
elevation (BFE) is not exceeded, it is important that the correct delineation be
used as a base for this report.
After consultation with the map specialists at PBS&J (FEMA's consultant for
Region VI) it was the consensus opinion that the referenced LOMR should be
used as the base for this study. PBS&J recommended including an
explanation to the effect that the FIRM dated August 23, 2001 is not
representative of the effective model (floodway and flood plain) and that approval
of subsequent LOMRs would return the delineated floodway to the previous
alignment.
G. Hydraulics (Appendices E, F, G, I and J)
The effective model was adopted in its entirety from LOMR No. 97-06-503P, July 23,
1997. The data for the Old Denton Creek hydraulic model was obtained from Ms.
Teresa Tumer of the Department of Engineering and Public Works in digital format. The
data was imported to HEC-RAS format and matched with the output produced in 1997.
PBS&J confirmed that the referenced LOMR is the effective model.
Three cross-sections were added to the input in order to model the floodway in the small
tributary appended to Old Denton Creek's channel. One of the new cross-sections (Sta
236+00) lines up with the small tributary and cross-sections were selected just upstream
and downstream of its confluence with Old Denton Creek.
Analyses of the hydraulic characteristics of channel were performed with the USACE's
HEC-RAS program (ver. 3.1.1.) By this model the probable water surface elevations
along the creek for the 100-year storm event were determined and compared for the
existing (pre-development) and proposed conditions. Refer to Appendix G in two pages.
Appendix F is an orthogonal view of the channel with the surface of the floodway
modeled.
H. Results and Conclusions
Comparing the flood elevations for the existing and proposed conditions, there is no
increase in 100-year flood elevation.
Based on the study output, the following conclusions can be made: The proposed
grading (Grading Plan, Appendix B) will not cause additional flooding at any location
along Old Denton Creek.
The CLOMR should be approved as shown by Appendix H
Rev. Jan 9, 2004