Park West CC(1)-CS 870818 Federal Emergency Management Agency
Washington. D.C. 20.~72
The ltonorable Leu Duggan ~,:~' ~'~ ,,\x.':, Case $86-06-47R
Mayor of the City of Coppell .
p.o. 478
~is is in referenc~ to a letter dated ~y~2ff, 1986, and technical data
s~mitted by Mr. ~ Powell, P.E., fomer City Engineer for ~e City of Cop~ll,
·exas. In his letter, ~ich was fomardod to us by our Region VI office,
Mr. Powoll r~uested that ~e Federal Emergency Management Agency (F~) issue
a conditional Letter of Map Revision for a proposed channel modification and
bridge cons~uction project along Grapevine Cre~ be~een Coppell Road and
Interstate Highway 635. ~is proposed project involves reali~ment of the
Grapevine Cre~ channel including construction of a grass-lined channel wi~
the ~se ~d~ varying rrm 70 to 200 feet, consmction of a concrete ~op
s~ucture downstre~ of Interstate Highway 635, and construction of a bridge
at Freeport Parkway. ~echnical data s~mitted in sup~rt of ~is request
included a report entitled 'Conditional 5etter of ~ap Revision Request for
Grapevine Cre~ in Cop~ll, Texas, ~ prepared by Albert tl. Halff Associates,
Inc. ~is report contained a description of methodologies used and hydraulic
backwater models for existing and proposed conditions. Ad~tional te~nical
data were s~tted on several occasions at our r~uest; all r~red data to
process ~is r~uest were received by Mar~ 18, 1987.
During the course of review, we were informed that the channel modification
between Coppell Road and Freeport Parkway and the construction of the Freeport
Parkway bridge were completed. Mr. B. Anatole Falagan of Albert H. Halff
Associates, Inc., submitted hydraulic analyses, which reflected the completed
portion of the project and requested that FEMA review these analyses and issue
a Letter of Map Revision. In reviewing the analyses, we noted that excessive
velocities exist in the earthen channel between the downstream end of the
completed channel modifications and the Freeport Parkway bridge, which could
result in severe erosion and undercutting of both the bridge structure and the
modified channel during flooding events. FEMA will not perform a revision at
this time to the effective Flood Insurance Study (FIS), Flood Boundary and
Floodway Map (FBFM), and Flood Insurance Rate Map (FIRM) for the City of
Coppell to reflect the completed portion of the channel modification and
bridge construction project due to the unstable conditions created by the
partially completed project. 0~r regional office has been notified of the
stability problem with the completed portion of the project, and will be
contacting your community in an effort to resolve this problem.
We have reviewed the data submitted for the entire channel modification and
bridge construction project, and have determined that the project as a whole
meets the minimum floodplain management criteria of the National Flood In-
surance Program. The drop structure that is proposed between the completed
channel modification and the Freeport Parkway bridge would appear to create
stable channel conditions during flood events. If the entire project is
completed as proposed, a revision to the effective FIS, FBFM, and FII~M for the
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City of Coppell will be warranted. Base (~0~-year) Flood Elevations and
floodway boundaries for Grapevine Creek would be revised as depicted in the
aforementioned report. Please note that future revisions to the FIS, FBFM,
and FIRM or restudies of the flood hazards in this area could modify this de-
termination.
This determination is based on the 100-year flood discharges computed in the
effective FIS for the City of Coppell, and does not consider subsequent
changes in watershed characteristics that would tend to increase flood dis-
charges. The development of this project and other projects upstream could
result in increased flood discharges, which, in turn, could result in in-
creased 100-year flood elevations. Future restudies of your community's flood
hazards, which would take into account the cumulative effects of development
on flood discharges, could establish higher 100-year flood elevations in this
area.
This conditional Letter of Map Revision is based on minimum floodplain man-
agement criteria established under the National Flood Insurance Program. The
City of Coppell is responsible for approving all proposed floodplain develop-
ments, including this request, and for assuring that necessary permits re-
quired 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 construction 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 Program requirements.
It should be noted that National Flood Insurance Program regulation 44 CFR
60.3(b)(7) requires communities to "assure that the flood carrying capacity
within the altered or relocated portion of any watercourse is maintained."
This provision is incorporated into your community's existing floodplain
management regulations; consequently, upon completion of the project, re-
sponsibility for maintenance of the modified ~annel wil]. rest with your
community.
Upon completion of the entire project as proposed, your community may request
a revision to the effective FIS, FBFM, and FI~M. The revision request should
be submitted to our Region VI office and must include the data listed below:
"As-built" plans of the channel modification and bridge construction
project, certified by a registered engineer.
· A written description of the methodology used to determine hydrologic
and/or hydraulic parameters, if different from the effective FIS and
FIRM.
Revised water-surface profiles of the 40-, 50-, 100-, and 500-year
floods reflecting "as-built" conditions, including a zone determi-
nation.
as
The methodology and starting parameters for the revised profiles
should be consistent with the present effective FIS, i.e., same
discharges and hydraulic model, unless the parameters have been
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superseded by more current and. technically superior data 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 portions of the revised profiles
should coincide with the effective FIS profiles, i.e., hydraulic
calculations should be continued upstream and downstream of the
revised area until water-surface elevations coincide with those
in the effective FIS.
4e
Two floodway hydraulic backwater models. 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 project
and include any other channel modifications or encroachment that have
Occurred in the floodplain since the original floodway was delineated.
If, however, additional cross sections are used in the second model
to provide a more detailed analysis of the completed project and its
effects on flood hazards, an intermediate model incorporating the
additional cross sections should also be submitted. This model must
reflect floodplain conditions as they existed at the time that the
original floodway was delineated and, therefore, it is important that
any cross sections added to the original model describe those con-
ditions. In addition, any improvements to the original modeling
technique may be incorporated into this intermediate model. This
model will then become the new baseline model and will be used to
accurately measure the effects of the completed project.
The methodology and parameters for the revised floodway should
be consistent with the effective FIS, i.e., equal conveyance
reduotion to establish encroachment limits, unless changes as
specified in item 3a have been approved by FEMA.
Since only a portion of the floodway is being revised, it must
tie into the effective FIS floodway by duplicating the results
of the original baseline model at cross sections upstream and
downstream of the project.
The revised floodway must carry the waters of the 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 may not
exceed base flood elevations calculated in the new baseline
model by more than 1.0 foot. In all cases, the revised'floodway
elevations may not exceed revised base flood elevations by more
than 1.0 foot.
Se
Delineation of the 100- and 500-year flood boundaries, the 100-year
floodway boundary, and the locations and alignment of cross sections
and flow line used in the hydraulic model.
This information should be show~ on a map of suitable scale and
topographic definition to provide reasonable accuracy.
Ail items should be labeled for easy cross-referencing to
hydraulic model and summary data.
Source data and engineering documentation for the previously mentioned
items, as well as a bibliographic list of other sources of information
used.
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 support a request to revise an
effective PIS, FBFM and FIRM. Compliance with the criteria outlined in these
documents will expedite PEMA's review process, thus allowing the effective
FIS, FBFM, 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 and Technological Hazards Division of the Federal
Emergenoy Management Agency in Denton, Texas, at (817) 898-9127 or members of
our Headquarters staff in Washington, D.C., at (202) 646-2754.
Sincerely, ~
~L. M~atticks F
Chief, Risk Studies Division
Federal Insurance Administration
cc: Mr. B. Anatole Falagan Mr. Patrick Lee Acker, Albert H. Halff Associates, Inc.
MS. Shohre Daneshmand, Civil Engineer, City of Coppell