Bethel Rd Est 2-CS010703Kimley-Horn
end Associates, Inc.
July 3, 2001
Mr. Ken Griffin, P.E.
Director of Engineering and Public Works
City of Coppell
P.O. Box 478
Coppell, TX 75019
Tributary G-1 of Grapevine Creek
Coppell, Texas
·
Suite 1800
1270~ Park Central D~ive
Dallas, Texas
75251
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972.304.3673
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Dear Ken:
Kimley-Hom and Associates, ~c. (KHA) has performed a review of the
Floodplain Analysis Report for a Conditional Letter of Map Revision for a
portion of Tributary G-1 of Grapevine Creek, prepared by JD Landes, Inc. (JDL),
dated June 2001.
The submitted report includes data to apply for a Federal Emergency
Management Agency (FEMA) Conditional Letter of Map Revision (CLOMR).
KHA reviewed the document for adherence to FEMA criteria and has also noted
instances where City criteria may not have been met with respect to the City
Floodplain Management Ordinance or Subdivision Ordinance. KHA
recommends that JDL address the following items:
The Effective Flood Insurance Rate Map (FIRM) showing the project
impact to the FEMA floodplain/floodway is not very readable in KtIA's
report copy. Please provide readable copies of the exhibit. It is
important to note here that JDL may be interpreting the FIRM
incorrectly. There is not a 500-year floodplain associated with Tributary
G-1 on the FIRM. The workmaps included in the submittal call out the
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Kimley-Horn
and Associates, Inc.
City o f Coppell, July 3, 2001, Page 2
100-year and 500-year floodplain and 100-year floodway, but the
modeling does not correspond to the floodplains delineated on the
workmaps. JDL should provide clarification and/or and explanation. It
is also unclear what hydrology the floodplain delineations are based on.
A clarification of the linework on the workmaps is needed.
The FEMA floodway should be shown on any workmaps showing the
FEMA floodplain.
Digital models have not been included with the CLOMR submittal. It
would be helpful in review if these models could be provided. FEMA
typically will request digital copies of the models used.
On Table 6A, slight increases (ranging from 0.01 to 0.02 feet) are
occurring at 6 cross sections through the subject roach. These increases
appear to be a result of the proposed fill. Article 4, Section C.5.a of the
Coppell Floodplain Management Ordinance states that the proposed
development "will not increase the water surface elevations of the design
base flood (fully-urbanized watershed)." The design or modeling should
be revised accordingly.
Table 2 in the report calls out a "software anomaly" to account for the
increase in water surface elevation between the current effective and
duplicate effective models. JDL should reference the specific source that
they believe is the source of this anomaly and why the reviewer should
discount it. It is likely that the FEMA reviewing agency will ask for the
same information.
The production of the corrected effective model for the tributary shows
that three homes on the north side of the tributary are now slightly in the
"500-year floodplain" (or 100-year floodplain - to be determined by the
design engineer under Comment 1) by overlay of the floodplain
boundary. It is advised that the finished floor elevation of these
structures be obtained and compared to the proper floodplain elevation to
evaluate the potential flood hazard to these s~ructures. Note that homes
may not be located in the 100-year floodplain (FEMA or City fully
developed) by elevation or by overlay of the FIRM.
A comparison of Table 4 to Table 6 shows negative surcharges in the
100-year floodway as a result of the proposed development. It is
unlikely FEMA will accept these floodway modeling results while they
include negative surcharges. It may be possible to adjust these values by
attempting to optimize the floodway as outlined in Comment No. 8.
Kimley-Horn
and Associates, inc.
10.
11.
12.
13.
14.
It is noted that the floodway modeling does not appear to have
encroachments made from the floodplain to the floodway, attempting to
optimize the floodway elevation 1'0" above the floodplain elevation. A
comparison table is needed between the lO0-year floodplain and
floodway elevations in the duplicate effective, existing (corrected
effective), and proposed improved conditions to properly evaluate this
effect. Once the floodway is set, no rise will be allowed by FEMA in the
LOMR process. It is advisable to attempt to optimize the floodway using
the encroachment model.
It appears by evaluating the FIRM that this project will require
adjustment of the FEMA floodway. There is not proper modeling in the
submitted report that supports such an adjustment. Efforts to address
comment No. 8 above should assist in addressing this comment.
An encroachment model is not required for fully developed watershed
conditions in existing or proposed hydraulic conditions.
It is recommended that fully developed models be clearly separated from
FEMA flow models for submittal as a CLOIVIR. It is acceptable to
submit them to FEMA, but their reviewer will ignore the fully developed
models in the submittal.
Please have noted in the report and on the workmaps what cross
section(s) the water surface elevations are taken from to compare to the
finished floor elevations of the proposed lots. Any interpolation
performed should be explained in the report.
In producing a corrected effective model, it is advisable for the design
engineer to reevaluate the Marming's "n" values in the tributary and its
overbanks during a field visit. The "n" values remain consistent from the
duplicate effective to the corrected effective and finally the proposed
condition model. While the adjustment of these values should not affect
the proposed encroachment, they may increase the base flood elevations
through the reach of the project and affect the fill and wall design. Any
adjustment is based on the design engineer's judgment with proper
backup and research.
It appears that the proposed condition - ultimate discharge HEC-2 model
is missing from the Appendix. It should be included for comparison and
review.
Kimley-Hom
and Associates, Inc.
City of Coppell, July 3, 2001, Page 4
15.
Forms required by FEMA to process a CLOMR are not included in the
submittal. They must be reviewed and signed before submitting the
CLOMR request to FEMA. Downloadable forms may be found at
http://www, fema. gov/mit/t sd/DL MT-2.htm.
16.
It is advisable that the City has the design engineer evaluate the proposed
development for impact to valley storage. Coppell's Floodplain
Management Ordinance requires a separate analysis for this condition
through the project reach.
The CLOMR report is currently not complete for submittal to FEMA. The
comments listed should ha addressed before a submittal is made. Additional
comments to the ones listed above may be generated as a result of the reply to the
above comments.
All responsibility for the adequacy of the design plans and models shall remain
with the design engineer. In reviewing these plans and models, KHA must rely
upon the adequacy of the work of the design engineer.
With recent changes with the FEMA consultant reviewer, KHA would be happy
to provide the City of Coppell with details of the updated process. There have
also been recent developments with respect to the fully developed floodplain and
FIRM mapping that KHA has become aware of. Let me know if you need to
discuss this information.
Please feel free to contact me if you have any questions or concerns, or need
clarification of the comments.
Sincerely,
Kimley-Horn and Associates, Inc.
Project Manager
cc: Justin D. Landes - JD Landes, Inc.
1107 East 1't Street
Fort Worth, TX 76102
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