FS9901-CS030203· -, Halff Associates
ENGINEERS · ARCHITECTS · SCIENTISTS
PLANNERS · SURVEYORS
PBS&J, Inc.
12101 Indian Creek Court
Beltsville, Maryland 20705
Attn.: Scott Yi
Floodplain Engineer
Re:
Case No. 02-06-1090P
Community: City of CoppeH, Dallas County, Texas
Community No. 480170
8616 NORTHWEST PLAZA DRIVE
DALLAS, TEXAS 75225
(214) 346-6200
FAX (214) 739-0095
February 3, 2003
AVO 17669
Dear Mr. Yi:
Enclosed you will find effective and revised HEC-2 models, a water surface elevation
summary table and work maps for the above referenced case. The revisions are based on
comments contained in your email of January 6, 2003 (attached). The main task was to
extend the study (models and workmap) area upstream to the point at which the corrected
effective models converge within 0.5 feet of the effective models.
Through an extensive review of the study area, we have determined the effective models in
this area to be as follows:
Multiprofile Model (Section 18440 - 32725): This effective model was from the
LOMR for Cambridge Manor by Dowdey, Anderson & Associates, Inc. in March
29, 1995 (Case No. 95-06-019P). Halff Associates, Inc. renamed it as deforest.dat
for the CLOMR for this project (Case No. 00-06-276R). This model has been used
as the effective floodplain model throughout the CLOMR and LOMR stages of this
case.
2. The Effective Floodway Model is composed of four separate models in this area
originally from the Cambridge Manor's LOMR as follows:
o Downstream Model (Section 18440- 22370)
o Split Main Model (Section 22370 - 26830)
o Split old channel Model (Section 22370 - 25530)
o Upstream Model (Section 25530 - 56940)
These models were obtained from the FEMA Project Library. Errors in the effective
floodway model were found. Some of these errors were corrected during the preparation of
CLOMR Case # 93-06-091R as explained below:
DALLAS · FORTWORTH · HOUSTON · McALLEN · AUSTIN
TRANSPORTATION ° WATER RESOURCES · LAND DEVELOPMENT · MUNICIPAL · ENVIRONMENTAL · STRUCTURAL
MECHANICAL · ELECTRICAL · SURVEYING ° GEOGRAPHIC INFORMATION SYSTEMS
ARCHITECTURE · LANDSCAPE ARCHITECTURE ° PLANNING
Ill
Ill Halff Associates
ENGINEERS ° ARCHITECTS ° SCIENTISTS
PLANNERS ° SURVEYORS
"Hydraulic models were obtained from Dan M. Dowdy & Associates,
Consulting Engineers for the Lake Park Addition, "As Built" conditions,
These models are as approved by FEMA (through Dewberry & Davis) and
published in the preliminary FIS dated December 18, 1991. However, it was
noted during the preparation of this park study that the floodway HEC-2
model's provided did not compute the water surface elevations on file with
FEMA. Further investigation found that Section 26830 had been corrected
by Dewberry & Davis, in-house, with verbal approval from Dowdy &
Associates. This correction is incorporated into the Duplicated Effective
Models included in Appendix D and Appendix E. A copy of the Preliminary
FIS letter from FEMA with copies of this portion of the mapping is included
in Appendix B of this submittal."
The referenced "Appendix B" from CLOMR Case # 93-06-091R is attached to this letter.
As a result of this and other corrections (described below), the corrected effective floodway
model was based on the following:
1. Section 22370 and downstream reaches were based on the effective floodway model
(4rstfdwy.dat) with project updates
2. All upstream sections; the effective multi-profile model was used as a base file and
ET cards were added based on the above-mentioned effective floodway models
3. The split flow area was updated to be more consistent with current base map.
The effective floodway model did not appear to truly reflect split flow. Therefore, the post
project floodway model includes both channels as one section through this area. The
developed high ground between the split channels is above the base flood elevation and
therefore not in the floodway. As a result, the post project floodway model now ties into the
effective floodway model within 0.5 feet at section 23800. Although both floodplain and
floodway models extend to section 32725, the workmap was not extended upstream of
section 26830.
The final results have been summarized in Table 1, which lists the base flood water surface
elevation from the effective model, corrected effective model and revised (post-project)
model. The effective model results are based on the four different models as described
previously. Hard copy HEC2 results for the effective models are attached.
The multi-profile effective model results are from the model, forest.dat. Downstream
floodway results (section 18440 - 22370) are from the model, 4rstfdwy.dat. Split floodway
information (section 22370 - 25530) was based on the split main model, denmfw2.dat.
Upstream floodway information (section 25770 - 26830) was based on the upstream model,
dcbfl2.dat
mil
-- Halff Associates
ENGINEERS ° ARCHITECTS · SCIENTISTS
PLANNERS ° SURVEYORS
In summary, the following revisions to Case # 02-06-1090P have been made:
1. The Corrected Effective and Post Project HEC-2 floodway models have been
extended to cross section 268 +30. The corrected effective and post project models
now agree with the effective models within 0.5 feet at section 23800.
2. The water surface elevation summary table has been extended to cross section 26830
to reflect those updates.
3. The work map was extended to include cross section 26830.
If you have any further questions on this revision, please contact Walter Skipwith at 214-
346-6220 or Allen Xu at 214-217-6657. Thank you very much for your consideration of
this request.
Very truly yours,
HALFF ASSOCIATES, INC.
Walter Skipwith, P.E.
Vice President
cc: Kenneth M. Griffin, P.E., Director of Engineer and Public Works, City of Coppell
Kelly Jordan, Coppell Optimum Assets
Enclosures
File: i:\17000s\17669Xdoc'669LOMR_RESP3.d~c
APPENDIX B
From
CLOMR Case # 93-06-091R
Federal Emergency Management Agency
Washington, D.C. 20472
I cF~TIFIED I,~IL
RETOP~ RECEIF~ REQUESTED
DEC 18 1991
(lOO)
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The Houorable Mark Wolfe
Mayor of the City of Co~pell
P.O. Box 478
Coppell, Texas 75019
RE: Flood Insurance Study for the City of Coppell, Dallas and D~nton Counties, TX
Dear Mayor wolfe:
please find enclosed two preliminar~ copies of the revised areas of the Flood
Insurance S~udy 'fo~ ~he above referenced communi~y' We have comp!aced our
technical evaluation of the available data'and have incorporated the
chan~es into the enclosed copies. A description of the change with
corresponding affected per:iOn of the Flood Insurance Study ts attached for your
information and review,
If you have any comments that might Justify modifying our results, please use one
copy of the enclosed map(s) to Identify the area of concern and return
within 30 calendar days from the da~e of this letter. If you are in agreement
with our findings, please respond with a letter stating that you have reviewed
the accompanying material. If we do ~ot receive any comments within 30 days, we
will continue processing.
~n the near future, we will publish a notice of proposed Base (1CO-year) Flood
Elevation (BFE) determinations in your local newspaper- This notice will
i~tttate a 90-day period durtng which your community may submit appeals or
comments On the proposed BFEs reflected in the revised Flood Insurance $~udyfor
your community.. Following the 90-day appeal period and the resolution of any
appeals, we will finalize the proposed BFEs and establish an effective date for
the revised ~tudy,
If yOU have any questions, please ¢ontac~ the Chief, Natural and Technological
Hazards DlvisioD of the Federal Emergency Management Agency in Denton, Texas
at (817) 898-5127.
Sincerely,
- Villi&mR. Locke
Chief, Risk Studies Division
Federal Insurance Administration
Enclosures
cc: State Coordinating Agency
Mr. Ronald W. Horrison, P.E., Senior Hydrologist
Mr~ Err A. Meyer, Nathan D. Meier Consulting Engineers, Ioc.
MrS! William Al Anderson, P.E., Dan M. Dowdey & AsSociates
FEMA Regional Chief. Region VI
Ms. [[. bohre Daneshmm~d~'Act~ng City Enginee[
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RussellSR. Doyle, t'.E., City Engineer,
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~ity q~ C_._~.P.9~!l, Dallas and Denton Counties, Texaf
The Flood Insurance Study (FI$) and Flood Insurance RaCe Map (FIRM) for the
City of Coppell have been roviaed to reflect updated hydrologic and hydraulic
analysts for Elm Fork of Trinity River and updated hydraulic analyses for
Grapevine Creek, Denton Creek, and CottoTlwood Branch. These analyses were
prepared for the City of Coppell by Klmley-HorlI and Associates, Inc., and
Morrison Hydrology/En§lneering, Inc. Additional updated hydraulic analyses
for Denton Creek from th~ diuergenca of Old Denton Creek to Denton Tap Road
were prepared by Dan M. Dowdey & Associates, to reflect, the completed Lake
Park Addition.
In addition, detailed flood hazard information was prepared for Tributary G-1
of Grapevine Creek. The hydrologic and hydraulic analyses for Tributary G-1
of Grapevine Creek ware prepared by Goodwin and Marshall, Inc.
Tile following is a summary of the chan~es to the FI~M and FIS Report:
~IRM 9u~hqr eSQ,?O, panel 0010
The 100- and 500-year floodplain boundaries, Base (100-year) Flood Elevations
(BFEs), and the Federal Emergency Management Agency (F!MA)-dcsi~nated floodway
for Elm Fork of Trinity River, Denton Creek, and Cottonwood Branch have been
modified for their entire length within the community, and Grapevine Creek has
been modified from its confluence with Elm Fork of Trinity River to approxl-
ma=ely 125 feet upstream of Southwestern Boulevard. In addition, a
designated floodway has been added for the Old Denton Creek channel jus~ south
of the levee on Denton Greek. Corporate limits have been revised in
northwest portion of the city to reflect recent deannexaticn~.
For Tributary G-1 of Grapevine Creek, the 500-year floodplain was changed to
lOO-year floodpla£n, and BFEs and a FEMA-desi~nated floodway were added from
~he confluence with Grapevine Creek to approximately 0.6 mile upstream of
Bethel School Road. An approximate 100-year (Zone A) floodplain has been
determined for the remaining port/on of Tributary G-1 of Grapevine Creek.
lIS
The ~I$ Report. including Tables 1 and 2 (Sun~ary of Discharges and Floodway
Data) and Flood Profiles 01P-05P. O?P-09P, and ]IP-laP have been revised to
reflect ~h~ aforementioned updated flood hazard information. In addition.
flood profiles 06I' and 10P have been added t. 0 reflect the new de,ail'ed flood
information for Tributary G-1 of Grapevine Creek and Old Denton Creek,
respectively.
NATIONAL FLOOD INSURANCE
FIRM
FLOOD INSURANCE RATE MAP
and
~TREET INDEX ..
CIT~ OF '. ' '
COPPELL,"
TEXAS
DALLAS AND DENTON COUNTIES
ONLY PANEL PRINTED
DEC 1 8 ~
COMMUNITY-PANEL HUMBER
480170 0010 E
MAP REVISED:
l~ederal Emersency Management Asency
.t
FOL'D. '
D£WBI:::RRY & DAVi'$
TRIM.
i~OLO
IE
FOLD
ZONE X ZONE X
INDREW BROWN
oMMi~NITY. PARK
ZON~
'ZOxNE X
M 196
THiS AREA PROTECTED FROM THE 100-YEAR FLOOD
BY LEVEE. DIKE, OR OTHER 6TRUCTURES SUBJECT TO
FAILURE OR oVERTOPPiNG DURING LARGER FLOODS
CounTY ~,OUNDARY
Table I
FEDERAL EMERGENCY MANAGEMENT AGENCY
WATER SURFACE ELEVATION CHECK
.~ommunity Name lFIooding Source lProject Name/identifier
City of Coppell I Denton Creek I w/ndsor Estates & DeForest Court
REVISED (POST-PROJECT)
LOCATION ~DUPLICATED MODEl. ~CORRECTED EFFECTIVE
~ forest see notes' ~ rvexd4st rvxfdwy2 7669remk 7669fwn ,
~ ......... NCWSEL~ FCWSEL~ SURC.3 ~::~-::: :~.:;-.~.--' NCWSEL~ FCWSEL2 SURC.3 NCWSEL~ FCWSEL2 SURC.3 ~CWSEL :CWSEL SURC.3
· 446.56 447.56 1 .~0 446.56 447.56 1 .O
447.77 448.16 0.4 447.'~"'-~- ~448.16 04
~~ T 449.07 o.t
449.92 ~ ~ ~ 449.94 0 .'~"--~'~
450,43 ~ ~- --~0.43 450.42 0.0
~~--'-~- 450.6--'--9- 450.7'----;-- 0.0
o.s__o,w~,,E,._~__,,_ ~~ ~~ 450.8"---~ 450.70 o.t
~ ~ ~-- .- 450.87 ~ o,t
453.12 1.0 ~ 452 .-"~.-.-.-.-.-~ ~--~'~ -- '- 452.8~ 452.90 --0.1
[ ~ ~i 452.95 452.96 - 452.91 452.98 0.1
~-- - ~~ 453.00 453.0f - 45~.99 453.05 o.f
~ 452.23 453,23 1~0 .........
........... ;I 453.08 453.09 0.0 453.06 453.13
453.11 453.12 -- 453.1'1 453.17 0.1
r Estate~ and D®Fora ~ '~---~3.13 -- 453.13 4------53.19 0.1
- ~ 0.0 4'--s3.14 ~ --
J S °f DeF°fes' C°ur~ ~ 453'26 1.0 ~ -- ~3.--"~-- 0.0 453.17
~- 453.27 1.0 ~-
~ 452.40453.37 1.0 ~ 453.29__ 0.0 453.2.___L ~ ~ __
~ 452.404~3.~ t.o ~ 453~7 453.2~ ___0.0 453.2____!_7 ~ "--o.t -- ~ -
452.40 453.36 1.0 ~ 453.28 453.29 0.0 453.27 453.35 0.1
~ ~ 452.64 453.57 0.9 I 453.49 453,50 0.0 453.48 453.56 0.1
MODEL CONVERGE HERE [~_.,,, ........ 453.56 453.57 0.0 ~,~ 453.85 453.86 0.0 453.85 453.92 0.1
I 453.92 454.02 0.1 ,,,,"~ . 454.20 454,20 0.0 454.19 454.26 0.1
454.09 454.23 0.0 454.16 454.22 0.1
454.35 454.35 454.16~__'0.1.~_.__ 454.2~4 454.22- 0.0
454.73 454,73 --~54,62 0.0 '--'454.63 454.6.~__L6 0.0
455.38 455.28 ~ -- 0.1 455.28 --455.43 0.2
I~OMMENTS:
Effective floodway model results are from the three different models: Section 18400-22370 from 4rstfdwy.dat, Section 23800-25530 from denthfw2.dat, Section 25770*26830 from dcbfl2.dat
1-100-year (natural) Water Surface Elevation 2-Encroachment (floodway) Water Surface Elevation 3-Surcharge Value
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Sheet I of I
7669heCasum. XLSLOMR Halff Associates, Inc. Z/10/031:29 PM