Windsor Estates-CS020906;;; Halff Associates
ENGINEERS ARCHITECTS SCIENTISTS
PLANNERSSURVEYORS
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PBS&J, Inc.
12101 Indian Creek Court
Beltsville, Maryland 20705
Attn.: Joseph A. Martinenza, Jr.
Program Manager, Region VI
Re: Case No. 02-06-1090P
Community: City of Coppell, Dallas County, Texas
Community No. 480170
Dear Mr. Martinenza:
8616 NORTHWEST PL4ZA DRIVE
DALLAS, TEXAS 75225
(214) 346-6200
FAX (214) 739-0095
September 6, 2002
AVO 17669
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Enclosed, you will find additional information provided in response [o comments received in
a letter from your office (July 29, 2002).
1. Please provide the duplicate effective model that includes the entire revision area
The duplicated effective models have been included in this package (Multi-plan
model, foresLdat and floodway model, 4rstfwy.dat).
2. Subparagraph 65.6(a)(2) of the National Flood Insurance Program regulations
states that to avoid discontinuities between revised and unrevised flood data,
hydraulic analysis must have a logical transition between revised elevations of the
flood having a 1 % chance of being equaled or exceeded in any given year (1 DD-year
or base flood) and those developed previously for areas not affected by the revision.
Our review indicates that the corrected effective and proposed hydraulic analysis
along Denton Creek do not tie into the effective hydraulic analysis at the upstream
and downstream ends of the proposed revised reach. Please provide revised
corrected effective and proposed hydraulic analysis for Denton Creek that tie into
the effective hydraulic analysis within 0.5 foot, or within 0.0 feet, if practical.
The corrected effective and the PosbProjec[ models have been extended per the
CLOMR and a revised table, printout and electronic files provided.
3. Corrected Effective
a. A review of the corrected effective model reveals left and right encroachment
stations outside of the 1 %a annual chance floodplain at Cross Sections 0, 1, 1.1, 2, 3,
4, S, 9, 10, 11, 21635, and 21645. Please redefine the encroachment. stations. In
addition, if this affects the top widths for these cross sections, please resubmit a new
topographic work map:
DALLAS FORT WORTH HOUSTON McALLEN
TRANSPORTATION WATER RESOURCES LAND DEVELOPMENT MUNICIPAL ENVIRONMENTAL STRUCTURAL
MECHANICAL • ELECTRICAL • SURVEYING GEOGRAPHIC INFORMATION SYSTEMS
ARCHITECTURE LANDSCAPE ARCHITECTURE PLANNING
~~~
;
Halff Associates 8616 NORTHWEST PLAZA DRIVE
DALLAS, TEXAS 75225
(214)346-6200
.
. FAX (214) 739-0095
ENGINEERS ARCHITECTS SCIENTISTS
PLANNERSSURVEYORS
We have revised [he encroachment stations at 21635, and 21645, consistent with the
previous approved CLOMR model. Encroachments for the other referenced sections
aze set at either the bank station or very near to the location of the 100-yeaz
floodplain and have not been changed. A comparison table is provided. The
workmap is unaffected.
b. The discharges at Cross Sections 7, 9, 10, 11, 12, and 13 decrease in the downstream
direction. Normally, the discharges should be increasing in the downstream
direction. Please verify and revise. In addition, please provide clarification to the
split flow analysis and its impacts on the Elm Fork of Trinity River
The dischazges in this area are a result of a split flow analysis described in the
LOMR Request text (pages 4 and 5). The need for the split flow became evident
when the new Elm Fork hydraulics modeUmapping was compared with the detailed
Denton Creek modeUmapping prepared for [his project's CLOMR. As faz as the
discharges go, the Denton Creek models are identical to those in the CLOMR, which
was approved on March 15, 2000. Please note that any changes to the Elm Fork aze
not caused or impacted by the Windsor Esta[es/DeForest Court project. Furthermore,
the Elm Fork/Denton Creek confluence problems (need for split flow analysis, lack
of a logical Elm Fork backwater intersection with Denton Creek profiles, problems
with floodway) were brought to the attention of FEMA's MCC during the review
period for the current Dallas County FIRM's in the form of the CLOMR submittal
and subsequent phone conversations. The need for revisions to the Elm-Fork models
and FIRM should not be the responsibility of the Windsor Estates/DeForest Court
developer.
a The floodway discharge is not equal to the natural discharge at Cross Section 10.
Please revise
This is a typo (discharge at Section 10 should be 11630.5 cfs instead of 11630.4 cfs).
The problem has been corrected (see attached model).
4. Post Project
a. A review of the post project model reveals left and right encroachment stations
outside of the 1% annual chance floodplain at Cross Sections 0, 1, 1.1, 2, 3, 4, 5, 6,
7, 11, 12, 13, 21635, and 21645. Please redefine the encroachment stations.
We have revised the encroachment stations at 11, 21635, and 21645, consistent with
the previous approved CLOMR model. Encroachments for the other referenced
sections are set at either the bank station or very near to the location of the 100-yeaz
floodplain and have not been changed. A comparison table is provided. The
workmap is unaffected.
DALLAS FORT WORTH HOUSTON McALLEN
TRANSPORTATION WATER RESOURCES LAND DEVELOPMENT MUNICIPAL ENVIRONMENTAL STRUCTURAL
MECHANICAL ELECTRICAL SURVEYING GEOGRAPHIC INFORMATION SYSTEMS
ARCHITECTURE LANDSCAPE ARCHITECTURE PLANNING
;;; Halff Associates
ENGINEERS ARCHITECTS SCIENTISTS
PLANNERSSURVEYORS
8616 NORTHWEST PLAZA DRIVE
DALLAS, TEXAS 75225
(214)346-6200
FAX (214) 739-0095
b. The discharges at Cross Sections 7, 9, 10, 11, 12, and 13 decrease in the downstream
direction. Normally, the discharges should be increasing in the downstream
direction Please verify and revise. !n addition, please provide clarification to the
split flow analysis and its impacts an the Elm Fork of Trinity River.
See response to 36.
c. The floodway discharge is not equal to the natural discharge at Crass Section 11.
Please revise.
This is a typo (Dischazge at Section 11 should be 12776 cfs instead of 11276 cfs).
The problem has been corrected (see attached model)
5. Please reflect the comments on item 4 to topographic work map and annotated Flood
Insurance Rate Map (FIRM) and submit revised copies of the topographic work map and
annotated FIRM
The topographic work map and annotated FIl2M have not changed due to the
revisions made in this response.
The final results aze listed in the attached summary table for your review. If you
have any further questions on this revision, please contact me immediately. In
addition, please provide a schedule for the completion of this review. I can be
reached at 214-346-6220. Thank you for your consideration of this request.
Very truly yours,
HALFF ASSOCIATES, INC.
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Walter E. Skipwith, P.E.
Vice-President
Enclosures
cc: Mr. Kenneth M. Griffin, P.E., Duector of Engineer and Public Works, City of Coppell
Pile: I:\17000s\! 7669Vioc\669IAMR_RESP.dce
DALLAS FORT WORTH HOUSTON McALLEN
TRANSPORTATION WATER RESOURCES LAND DEVELOPMENT MUNICIPAL ENVIRONMENTAL STRUCTURAL
MECHANICAL ELECTRICAL SURVEYING GEOGRAPHIC INFORMATION SYSTEMS
ARCHITECTURE LANDSCAPE ARCHITECTURE PLANNING