Freeport NIP(3)-CS 860310CERTIFIED MAIL
RETURN PdDCEIPT REQUESTED
The Honorable Lou Duggan
Mayor of the City of Coppell
P.O. Box 478
Coppell, Texas 75019
Federal Emergency Management Agency
Washington, D.C. 20472
March 10, 1986
Dear Mayor Duggan:
This is in reference to a letter dated December 9, 1985, from Mr. J. E.
Powell, P.E., City Engineer, to the Federal Emergency Management Agency
(FEMA). In his letter, Mr. Powell requested r~nat F~A issue a conditional
Letter of Map Revision (LOMR) for a proposed channeliza~ion project on a
portion of Cottonwood Branch, located west of State Road. With his letter,
Mr. Powell forwarded a flood plain stu-2y prepared by Caffey and Morrison, Inc.,
entitled Study Revision of Freeport North Addi=ion Along Cottonwood Branch,
dated November 1985.
Based on our review of the submitted da~a, we believe t_hat the channelization
project, if completed as proposed in the aforementioned flood plain study,
will be sufficient to reduce the. 100-year water-surface elevations, Special
Flood Hazard Area, and regulatory floodway from existing conditions within the
Cottonwood Branch flood plain in the ~_cinity of the proposed project. The
potential hydrologic impacts associated with ~-he development of this area or
areas upstream have not been considered in e_h!s de%ermination. This conditional
LOMR is issued with respect to minim%%m standards of the National Flood Insurance
Program (NFIP). Because your communit_v_ may e~--~orce more restrictive flood
plain management criteria, final approval of -.he project rests with your
community.
The basis of this conditional LOMR is a proposed ch~annel modification project.
It should be noted that NFIP regulatiom 44 CF~r. 60.3(b)(7) requires that
communities "assure that the flood carrying capacity within the altered or
relocated portion of any.watercourse is maint~%ined." This provision is
incorporated into your community's existing flood plain management regulations.
Consequently, upon completion of this ?roposed project, the ultimate responsi-
bility for' maintenance of the channel ~ill res= with your =ommunity.
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Upon-completion of the proposed channe!iza~ion,_ your oommuni~ may request a
revision to the effective Flood Insurance Stu-2y (FIS) and Flood .Insurance Rate
Map (FIRM). 'The revision request must include the data listed, below:
Documentation_which indicates commun:=y endorsement of the.revision
request.
0
WAs-built" plans of the channelization, certified by a registered
engi neet.
A written description of the methodology used to determine hydrologic
and/or hydraulic ~arameters, if different fro~ the effective FIS and
Revised water-surface profiles of the 10-, 50-, 100-, and 500--year
floods reflecting "as-built" conditions, including a zone determina-
tion.
The methodology and starting parameters for the revised profiles
should be consistent with the ~resent effective FIS, i.e., same
discharges and hydraulic model, unless the parameters have been
superseded by more current and technically superior dat~. and
analyses. (FEMA approval should be obtained before deviating
from the effective FIS parameters.)
Since only a portion of the existing profiles is being revised,
the upstream and downstream ~ortions of the revised profiles
should coincide exactly with the effective FIS profiles, i.e.,
hydraulic calculations should be continued for a great enough
distance upstream and downstream of the revise~ area until
water-surface elevations coincide with those in the effective
FIS.
Two floodway hydraulic backwater models should be submitted. The
first should be a duplication of the original baseline 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 channelization and include any other channel modifica-
tions or encroachment that may have occurred in the flood plain since
the original floodway was delineate~. If, however, additional cross
sections are use~ in the second model ~o ~ovide a more detailed
analysis of the completed channel mo~ification project an~ its
effects on flood hazards, an intermediate model should also be
submitted. This model must reflect flood plain conditions as they
existed at the time that the original floodway was delineated and,
therefore, it is important that any croes sections a~ded to the
original model describe those conditions. In a~dition, any improve-
ments to the original modeling technique may be incorporated into
this intermediate model. This model will then become the new baseline
model an~ will be used to accurately measure the effects of the
completed channel modification project.
The methodology and parameters for the revised floodway should
be consistent with the effective FIS, i.e., equal conveyance
reduction to establish encroachment limits, unless changes as
specified in ite~ 4a have been approved by FEMA.
be
Since only a portion of the floodway is being revised, it must
tie into the effective FIS floodway exactly by duplicating the
results of the original baseline model at cross sections upstream
and downstream of the completed project.
The revised floodway must carry the waters of the proposed base
(100-year) flood without increasing the water-surface elevations
of that flood by more than 1.0 foot over the original baseline
model at any point. If additional cross sections have been
incorporated, then revised floodway elevations also ma~ not
exceed base flood elevations calculated in the new baseline
model by more than 1.0 foot.
-Delineation of the 100- and 500-year flood boundaries, the 100-year
~floodway boundary, and the location and alignment of =ross sections
land flow line used in the hydraulic model.
a. This information should be shown on a map of s~itable scale and
topographic definition to provide reasonable accuracy.
be
All items should be labeled for easy cross-referencing to hy-
draulic model and summary data.
Source data and engineering documentation for the previously men-
tioned items, as well asa bibliographic list of other sources of
information used.
Upon receipt and review by FEMA of items 1-7 above, and verification that the
design of the ahannelizationproject conforms to sound engineeringpractice
and meets all applicable NFIP standards, FEMAwill take action to incorporate
the effects of the completed project into the effective FIS and FIRM for your
community.
We have enclosed documents, entitled Conditionsand Criteria for MaWR'vlsi°ns
and Conditions and Criteria for Floodwa~ Revisions, which further describe the
nature and extent of the material needed to support a request to revise an
effective FIS and FIRM. Compliance with the criteria outlined in these
documents will expedite F~iA's review process, thus allowing the effective FIS
and FIRM for your 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 an~ Technological Hazards Division of the Federal
Emergency Management Agency in Denton, Texas, at (817) 387-5813 or members of
our Headquarters staff in Washington, D.Co, at (202) 646-2754.
Acting Chief, Risk Studies Division
Federal Insurance A~ministration
Enclosures
Mr. J. E. Powell, P.E., City Emgineer
City of Cop~ell, w/enclosures
Mr, Ronald W. Morrison, P.E., Vice-President,
Caffey an~ Morrison, Inc., w/enclosures