FS9901-CS 980403 8616 NORTHWEST PLAZA DRIVE
NEN
H alff Associates DALLAS, TEXAS 75225
(214) 739 -0094
FAX (214) 739 -0095
ENGINEERS • ARCHITECTS • SCIENTISTS
PLANNERS • SURVEYORS
April 3, 1998 QOA'i e
AVO 17091
Mr. Kenneth M. Griffin, P.E.
Assistant City Manager/ Director of Public Works
City of Coppell
255 Parkway Blvd.
Coppell, Texas 75019
Fax: 214-393-0931 or214- 304 -3570
Re: Review of a Conditional Letter of Map Revision Application prepared by MHE, for a
Portion of Denton Creek Upstream of DeForest Road, City of Coppell, Texas.
Dear Mr. Griffin:
We have completed the review of a Conditional Letter of Map Revision (CLOMR) application for a
portion of Denton Creek Upstream of DeForest Road in the City of Coppell, Texas, prepared by
Morrison Hydrology Engineering Inc. (MHE) in January 1998. This CLOMR application is
reviewed for compliance with the City of Coppell Floodplain Management Ordinance No. 94639
approved on March 8, 1994 and, in the process of doing so, for good hydraulic engineering practices
in general.
In addition to this letter report, you will find included in this package two (2) 11 "x17" sketches
(effective flow delineations for existing and proposed conditions) prepared for this project by Halff
Associates, Inc. The letter report discusses the main issues encountered in our review. The
following paragraphs address deficiencies in hydraulic modeling encountered in the models based
on both existing and ultimate landuse conditions (which should be serious enough for FEMA to ask
for revisions to the models before they will agree to issue a CLOMR), and deficiencies in the
submittal based on the City of Coppell Floodplain Management Ordinance.
1. The effective flow limits are not correctly represented at sections 18440, 19230 and
19330 in the proposed condition models. Because the right overbank at section 19170 is
completed raised out of the 100 -year flood plain to allow the proposed development to take
place, the upstream and downstream sections must be modified on the right overbank as well
to properly model ineffective flow areas. The right overbank at sections 18440, 19230 and
19330 should be rendered completely ineffective. The proposed pond, although useful for
valley storage considerations, is ineffective for water surface elevation calculations and to
offset or mitigate the proposed fill. Please refer to the two (2) attached sketches that depict
the existing and proposed flood plain effective areas. Note that the shaded areas on the
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sketches do not represent the true high ground, but ineffective and high ground areas as
perceived by the HEC -2 model. The heavy green line represents our interpretation of the
limits of ineffective flow upstream and downstream of the proposed fill. This problem
invalidates results based on water surface elevations, such as the no rise condition. This
appears to be the most serious deficiency in this submittal.
2. The hydraulic modeling does not correctly represent the proposed site improvements
downstream of inserted section 19170 for three reasons:
• The downstream portion of the site may be controlled by a backwater effect from the
Elm Fork of the Trinity River, not by Denton Creek. The Elm Fork section #105167
was not modified to model the proposed fill. Realistically, the Elm Fork sections are
so long at this location that modeling the proposed fill may have no effect on water
levels, if the section is effective in this area at all. The Denton Creek hydraulic model
begins a considerable distance upstream of the Elm Fork backwater and the
relationship between the two has not been established. This relationship is critical to
the analysis of flood impacts on the proposed project.
• At section 18440, the site improvements have no effect on water surface elevations
because the starting wsel is fixed at the beginning of the model.
• In addition to the ineffective flow limits problems mentioned in 1. above, the
conveyance is greatly overestimated on the right overbank between sections 18440 and
19170 in the proposed condition model. Section 18440 was realigned to model the full
width of the pond and 13 ft of its depth. The problem is that HEC -2 "sees" the lake
excavation further u/s than the lake exists, because the program averages the areas of
the upstream and downstream sections to determine water surface elevations at the
upstream section. The "true" effects of the project cannot be determined unless the
applicant adds cross sections downstream of 18440.
The combined effect is that the pond and fill are not represented properly in any model
downstream of section 19170. Halff Associates, Inc. recommends adding 1 or 2 sections
downstream of the site, starting with an identical section for both existing and proposed
models.
3. Other submittal deficiencies include:
• The added section 19170 is not perpendicular to the channel flow between bank
stations (which results in the channel area being overestimated by HEC -2) and a right
overbank reach of 500 ft seems unreasonable (estimated at about 200 ft).
• Although the MHE map legend indicates the 100 -year existing and proposed flood
plains (ultimate Q), it does not show either.
ENE Halff Associates mil.
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• The proposed floodway appears unrealistic between sections 19930 and 20980 on the
right overbank, as the floodway lies outside of effective flow limits.
• The CLOMR MT -2 form 4 page 6/6 is not included (water surface elevation check
table).
• Minimum fill elevations are not mentioned. Part of the site may be regulated by the
Elm Fork of the Trinity River flood elevations, which are also not discussed.
• Article 4, section C, also requests that all other necessary permits be obtained. That
might include a Corridor Development Certificate (CDC) and a Corps of Engineers 404
permit if applicable. Wetland considerations are not mentioned in the text of the
CLOMR.
• The valley storage calculations are not clear and may be oversimplified. Halff
Associates, Inc. recommends that these calculations be based on Digital Terrain Model
calculations or more traditional volumetric calculations utilizing regularly spaced cross
sections and the average end area method.
• Electronic files should be provided to facilitate the review.
Based on a review of the above referenced CLOMR application, we recommend that revisions be
made to the applicable hydraulic models and project, if necessary, before the CLOMR is forwarded
to FEMA for review, or before a flood plain development permit and a Corridor Development
Certificate (CDC) are considered.
Please call our offices at 214 - 346 -6220 or 214 - 346 -6226 if we can be of further help or if you have
any questions.
Sincerely,
HALFF ASSOCIATES, INC.
Walter E. Skipwith, P.E.
Vice - President
Enclosures.
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