Deforest Court-CS020906mil
III Halff Associates
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8016 NORTHWEST PLAZA DRIVE
DALLY, S, TEXAS 75225
(214} 346-6200
FAX ,(214) 73~0095
PBS&J, Inc.
12101 Indian Creek Court
Beltsville, Maxyland 20705
Attn.: Joseph A. Martinelaza, Jr.
Program Manager, Region VI
Re:
Case No. 02-06-1090P
Community: C~ty of Coppell, Dallas County, Texas
Community No. 480170
September 6, 2002
Ave 17669
Dear Mr. Maninenza:
Enclosed, you will find additional information provided in response to comments received in
a letter from your office (July 29, 2002).
!, Please provide the duplicate effective model that includes the entire revision area.
Th~ duplicated effective models have been included in this package (Multi-plan
model, forest.dat and floodway model, 4rstfwy.dat).
Subparagraph 65.6(aX2) 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 (lO0-year
or base flood) and those developed previously for area~ not affected by the revision~
Our review indicates that the corrected effective and proposed hydraulic analysis
along Denton Creek da 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 l~ydraulic analysis for Denton Creek that tie into
the effective hydraulic analysis within O. J foot, orwithin O. O feet, if practical.
Th~ corrected effective and the Post-Project models have been extended per the
CLOMR and a revised table, printout and electronic files provided.
3. CorrectedEffective
A review of the corrected effective model reveals left and right encroachment
stations outMde of the 1% annual chance floodplain at Cross Sections 0, 1, 1.1, 2, 3,
4, 5, 9, 10, 11, 21635, and 21645. Please redefine the encroachment statlone. In
addition, if this affects the top widths for these cross sections, please resubmit a new
topographic work rnap;
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il
ll-' Halff Associates
ENGINEERS · ARCHITECTS · SCIENTISTS
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8616 NORTHWEST PLAZA DRIVE
DALLAS, TEXAS 75225
(214) 34~6200
FAX (214} 73~-0095
We have revised the encroachment stations at 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-year
floodplain and have not been changed. A comparison table is provided. The
workmap is unaffected.
The discharsea at Cross Sections 7, 9, 10, 11, 12, and 13 decrease in the downstream
direction. Normally, the diacharses should be increasing in the downstream
direction. Please verify and revise. In addition, please provide clarification to the
split flow analysis and it$ impact, on the Elm Fork of Trinity River
The discharges 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 model/mapping was compared with the detailed
Denton Creek model/mapping prepared for this project's CLOMIL As far 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 F_,Lm Fork are
not caused or impacted by the Windsor Estates/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 pm/des, 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 CLO/vlR 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/DeFox'est Corot
developer.
The floodway discharge is not equal to the natural discharge at Cross 3ection 10.
Please revise
This is a typo (discharge at Section 10 should he 11630.5 cfs instead of 11630.4 cfs).
The problem has been corrected (see attached model).
lost Pro]ect
A review of the post project model reveals left and right encroachment stations
outside of the 1% annual chance floodplain at Cross Sectioas O, 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-year
floodplain and have not been changed. A comtm'ison table is provided. The
workmap is unaffected.
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Halff Associates
ENGINEERS · ARCHITECTS · SCIENTISTS
PLANNERS · SURVEYORS
8616 NORTHWEST PLAZ~ DRIVE
DALLAS, TEXAS 75225
(214)
FAX (214) 739-(X)g5
The discharges at Cross Section~ 7, 9, 10, II, 12, and 13 decrease in the downstream
direction. Normally, the discharges should be increasing tn the downatrcam
directior~ Please verify and revi~e. In nddition, please provide clarification to the
split flow analysis and its impacts on the Elm Fork of Trinity River.
Sea response to 3b,
The floodway dischorge is not equal to the natural discharge at Cross Section 11.
Please revise.
This is a typo (Discharge at Section 11 should be 12776 cfs instead of 11276 cfa).
The problem has been corrected (sea 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 FIRM have not changed due to the
revision~ made in thi~ response.
The final results are listed in the attached summary table for your review, ff you
have any further questions on this revision, please contact me immediately. In
addition, please provide a schedule for tho 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.
Walter E. Skipwith, P~.
Vice-President
Enclosures
cc: Mt. Kenneth M. Griffin, P.E., Director of Engineer and Public Works, City of Coppell
Fib:: [:X17000s\17669k~:Va69LOMR._RESP.doc
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