ST8201-CS 880304Federal Emergency Management Agency
Washington, D.C. 20472
The Honorable Lou Duggan ~ ~ j~
P.O. Box 478
Coppell, Texas 75019 i,i/'~!J ' 8 J98
Dear Mayor Duggan:
This is in reference to a letterI - _ I
Mr. H. Wayne Ginn, P.E., City Engineer for the City of Coppell, ~hich was
forwarded to us by our Region VI office, in which he requested that the
Federal E~ergency Management Agency (F~24A) review a floodplain study for the
proposed bridge replacement project at Denton Tap Road over Grapevine Creek
and the proposed floodplain reclamation project on Grapevine Creek jest
upstream of Denton Tap Road. This letter addresses the proposed bridge
replacement project while the upstream reclamation project is addressed in
a separate letter. With his-letter, Mr. Ginn submitted a floodplain study
entitled Floodplain Hydraulics Study, Grapevine Creek, Coppell, Tp~, dated
September 22, 1987, prepared by PAWA-Winkelmann & Associates, Incorporated
(now Kimley-Horn and Associates, Incorporated), which included the following
items: a description of the ~ethodologies used~ HEC-2 hydraulic ~odels of
'Grapevine Creek for existing and proposed conditions~ floodplain end floodway
maps~ plotted cross sections~ a cro~s-sect'ion location ~api and plotted water-
sttrface profiles. We also received a letter dated January 72, 1988, and HEC-2
models dated January 8, 1988, submitted by Mr. Carl V. Anderson, P.E., of
Kimley-'Morn and Associates, Incorporated. This information addressed our
concerns as discussed in our January 8, 1988 phone conversation with
Mr. Anderson.
After reviewing the submitted data, we have determined that the proposed
bridge replacement project meets the minimum floodplain ~anage~ent criteria of
the National Flood Insurance Program (NFIP). If the project is completed as
proposed, a revision to the Flood Insurance Study (FRS), Flood Insurance Rate
Map (FIRM), and Flood Boundary and Floodway Map (FBFM) for the City of Coppell
would be warranted. This revision would show a decrease in the Base (100-
year) Flood Elevations (BFEs}, the regulatory floodway width, and the 100-year
floodplain of Grapevine Creek in the vicinity of Denton Tap Road. Future
revisions to the FIS, FIRM, and FBFM for the City of Coppell or future restudies
of the flood hazards in these areas could modify this deter~instion.
This dete~mination is based on the 100-year flood discharges c~eputed in the
effective FIS for your co~munity, a~d does not cons/der subsequent changes in
watershed characteristics that would tend to increase flood discharges. The
development of this project and other projects upstream could result in
increased flood discharges, which, in turn, could result in increased 100-year
flood elevations. Future restudies of your community.s flood hazards, which
would take into account the cumulative effects of development on flood dischargest
could es~ablish higher ]00-year flood elevations in this area.
Please note the requiresmnts for floodway revisions as out, tined in Part 65
of the NFIP regulations, Section 65.7 (copy enclosed). The proposed floodway
represents a decrease in width at several crues sections with respect to the
current regulatory floodway in the City of Coppell. When a floodway change is
proposed, a copy of a public notice distributed by the community stating the
community's intent to revise the floodway or a statement by the community that
it has notified all affected property owners and affected adjacent jurisdictions
must be submitted to FEMA. This requirement will have to be addressed when
requesting a map revision to reflect the effects of the completed project.
This conditional Letter of Map Revision is based on minimum floodplain man-
agement criteria established under ~,he NFIP. Your community is res~onsible
for approving all proposed floodplain developments, including this request,
a~ for assuring that necessary permits required by Federal or State law have
been received. State and community officials, based on knowledge of local
conditions and in the interest of safety, may set higher standards for con-
struction or may limit development in floodplain areas. If the State of Texas
Or the City of Coppell has adopted more restrictive or comprehensive floodplain
management criteria, these criteria take precedence over the minimum NFIP
r equi reme nts.
NFIP regulation 44 CFR 60.3(b)(?) requires communities ~articipating in the
ProgrAm to 'assure that the flood carrying capacity within the altered or
relocated portion of any watercourse is maintained.' Without proper mainte-
nance, such as the regular clearing of a channelized stream, channel modifi-
cation projects will, in time, fail to function as designed, thereby recreating
the flood hazard that they were intended to mitigate. Therefore, upon com-
pletion of the project, your community must uphold its responsibility for
assuring that the modified channel £~ maintaieed in order to preserve its
design function.
Upon completion of the proposed bridge replacement project, your community may
request a revision to the effective FIS, FIRM, and FBFN. T~e revision request
must include supporting data as outlined in the enclosed documents, Conditions
and Criteria for Ma~ Revisions and Conditions and Criteria for Floodway Revi-
sions and must address the requirements of NFIP regulation 65.7 as outlined
earlier. Compliance with the criteria outlined in these documents will
expedite F/~4A's review process, thus allowing the FIS, FIRM, and FBFM for ~our
community to be revised as appropriate, in a timely manner.
Should you have any questions regarding this matter, please do not hesitate to
contact the Chief, Natural and Technological Hazards Division of the Federal
Emergency Management Agency in Denton, Texas, at (817) 898-9127 or members of
our Headqunrters staff in Washington, D.C., at (202) 646-2754.
Sincerely,
les Division
Federal Insurancm Administration
Enclosures
cos Mr. Carl V. Anderson,
Mr. N. Wayne Ginn, P.E.
Mr. John C. Karlsruher, p.E., Glnn, Incorporated