CF-Andrew Park 1-CS 861205 (2)I I
F P' ral -e anagement Agency
Mr. Dan M. Dowdey, P.E.
Threadgill-Dowdey & Associates/
16250 Dallas Parkway ~ ~---~ '!7'k%~y 7 j v
suite o0 0 ............
Dallas, Texas 75248 ~---- ...... ~,
Dear Mr. Dowdey: FFV~i~..._O~ ~.~. ~D~A/df_. ?_ ...... ,~,gI~/~Jri.~j
This is in response to your letter of August 21, 1986 concerning the Flood
Insurance Study (FIS) for the City of Cop~:~ll~ ~xas. In your letter, which.
was forwarded to us by our Region VI office, you requested that the Federal
Emergency Management Agency (FEMA) issue a Letter of Map Amendment (LOMA) to
reflect the effects of a completed floodplain modification project on the
south bank of Denton Creek from Denton Tap Road to Samuel Boulevard extended·
In support of this request, you submitted a topographic map, a copy of the
Flood Boundary and Floodway Map (FBFM) for the City of Coppell showing cross
section and development locations, and "as-built" cross-section plots.
Please be informed that a LOMA is issued only to remove the Special Flood
Hazard Area (SFHA) designation on an effective Flood Insurance Rate Map (FIRM)
from an area occupied by a particular structure that is above the Base (lO0-
~ear) Flood Elevation (BFE) but was inadvertently included in the SFHA.
Because this request is based on modifications to the Denton Creek floodplain,
which have changed the BFEs along Denton Creek, a revision to the FIS, FBFM,
and FIRM must be processed instead of a LO~A.
In order to process a revision, FEMA must receive written endorsement from the
community. This endorsement must include a statement that the community has
reviewed the floodplain modification project and found it to be in compliance
with all applicable local, State, and Federal requirements. Therefore, an
individual requesting a change to the FIS must do so through the community.
In addition, the revision request must be supported by the following technical
data, which should be submitted through the City of Coppell in order for us to
continue processing the request:
A written description of the methodology used to determine hydrologic
and/or hydraulic parameters, if different from the effective FIS and
FIRM.
HEC-2 model duplicating the effective HEC-2 baseline model repre-
senting the 100-year flood.
HEC-2 model of the 10-, 50-, 100-, and 500-year floods reflecting
"as-built" conditions, including a zone determination.
The methodology and starting parameters for the HEC-2 model
should be consistent with the present effective FIS, i.e., same
discharges and parameters, unless the parameters have been
superseded by more current and technically superior data and
analyses. (FEMA approval should be obtained before deviating
from the effective FIS parameters.)
b. Since only a portion of the existing HEC-2 model is being
revised, the upstream and downstream portions of the revised
models should coincide with the effective FIS models, i.e.,
hydraulic calculations should be continued upstream and down-
stream of the revised area until water-surface elevations
coincide with those in the effective FIS.
4. Two floodway hydraulic backwater models for the aforementioned
conditions. The first model should be a duplication of the original
floodway model used in the effective FIS. This is required to ensure
that the original data has been duplicated correctly. The second
model should incorporate the completed project reflecting "as-built"
conditions.
5. Plotted profiles of the 10-, 50-, 100-, and 500-year floods repre-
senting "as-built" conditions.
6. Delineation of the 100- and 500-year flood boundaries and the locations
and alignment of cross sections and flow line used in the hydraulic
model.
a. This information should be shown on a map of suitable scale and
topographic definition to provide reasonable accuracy.
b. All items should be labeled for easy cross-referencing to
hydraulic model and summary data.
Please forward all supportive data to our technical evaluation contractor,
Dewberry & Davis, 8401 Arlington Boulevard, Fairfax, virginia 22031, to the
attention of Mr. Fernando Pasquel. Processing of your request will be considered
inactive pending receipt of the required data.
We have enclosed documents entitled Conditions and Criteria for Map Revisions
and Conditions and Criteria for Floodway Revisions, which further describe the
nature and extent of the material needed to suppo~ a request to revise an
effective FIS, FBFM, and FIRM. Compliance with the criteria outlined in these
documents will expedite FEMA's review process, thus allowing the effective
FIS, FBFM, and FIRM to be revised as appropriate, in a e~mely 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) 898-5127, or members of
our ~eadquarters staff in Washington, D.C., at (202) 646-2754.
Sincerely,
A~L~ng Chief, Risk Studies Division
Federal Insurance Administration
Enclosures
cc: The Honorable Lou Duggan, Mayor of the City of Coppell
Mr. Ed Powell, Coppell City Engineer