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The Honorable Lou Duggan `('.;:; �� Case f UG- 06 -47R Q 4
Mayor of the City of C:oppcll
P.O. Box 478 O� a
Coppell, Texas ILE COP
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Deal: Mayor � �
This is in reference to a letter dated 11ay,-29, 1986, and technical data
submitted by Mr. Ed Powell, P.L•'., former City Engineer for the City of Coppell,
Texas. In his letter, which was forwarded to us by our Region VI office,
4G Mr. Powell requested that the Federal Emergency Management Agency (PIMA) issue
t a conditional Letter of Map Revision for a proposed channel modification and
abridge construction project along Grapevine Creek between Coppell Road and
• '`n way 635. This -proposed project involves realignment of the
Grapevine Crack channel including construction of a grass -lined channel with
_1he..Uasdth varying from 70 to 200 feet, construction of a concrete drop
struc•tur own s tream of Interstato Highway 635, and construction of a bridge
a �Tcjaport Parkw Technical data submitted in support of this request
includ °cT " "'a r' " ° °c�ior entitled "Conditional 'Letter of Map Revision Request for
Grapevine Creek in Coppell, Texas," prepared by Alutrt 11. Halff Associates,
Inc. This report contained a description of methodologies used and hydraulic
backwater models for existing and oscd conditions. Additional technical
data were submitted on several ccasion at our request; all required data to
process this request were received by Marsh 18, 1987.
"During the course of review, we were informed that the channel modification
beL Coppell Road and Freeport Parkway and the construction of the Freeport
.Parkway bridge were completed. Mr. B. Anatole I'alagan of Albert H. llalff
Associates, Inc., submitted hydraulic analyses, which reflected the completed
portion of the project and, requested that FEMA review these analyses a uc
ett, of Map Revision. In reviewing the analyses, we noted that cessive
; ve -L l ocities exist in the earthen channel between the downstream end of
comp - ed channel modifications and the Freeport Pa bri d�ce, which could
u �„resu lt ia_ severe erosion and undercutting of both thn bridge str ucture an - ie
difiecl channel during flooding events. PEMA will not perform a�revislonn
this time - th - a ~ effective Flood Insurance Study (PIS) , flood Boundary and
"" Floodway Map (FD F14), and Flood Insurance Rate Map (T•lm) for the City of
Coppell to reflect the comp; d portion of the channel modification and
bridge cons truction projcc due to the unstable conditibfis created by the
partially completed project en notified of the
a y1�"`"proSlem wiii e complete ion of the project, and will be
contacting your community in an effort :o resolve this problem.
- .L •.• :.
We have reviewed the data submitted for the entire channel modification and
bridge construction project, and have determined that the project as a whole
Bets 'tie minimum floodpla:in management criteria Of the National flood In-
�urancc Program. The drop structure that is proposed between the co mpleted
c hannel modification and the Freeport Parkway bridge would ear to area
nnel co,nditions'during flood events. if the entire _ 1s
! a• , ,)l¢.tcd - --as -prCpos0d, a revision to the effective PIS, 1B1M, and FIRM for the
ppell will be warranted. Base (100 -year) Flood Elevations
. ..oundaries for Grapevine Creek would be revised as depicted
aforementioned report. Please note that future revisions to the FIS,
and FIRM or restudies of the flood hazards in this area could modify
- tenon.
and
in the
FBFM,
this de-
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 increa oo dis-
c bArqe p. The development of this project and other projects upstream could
result in increased flood discharges, which, in turn, could result in in-
creased -- IOD2.7.ygar� 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.
,T is conditional Letter of Map Rev. � s 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, a nd for ass that necessary permits re-
quutred by Federal or State law have been received. State and community
officials, based on knowlgdge 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 channel will rest with your
community.
Upon completion of the entire proje as promo =ed, your community may request
a revision to the effective FIS, FBFM, and FIRM. The revision request should
be submitted to our Region VI office and must include the data listed below:
1. "As- built" plans of the channel modification and bridge construction
project, certified by a registered engineer.
2. A written description of the methodology used to determine hydrologic
and /or hydraulic parameters, if different from the effective FIS and
FIRM.
3. Revised water - surface profiles of the 10 -, 50 -, 100 -, and 500 -year
floods reflecting "as- built" conditions, including a zone determi-
nation.
a. 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
/'`
superseded by more current and technically superior data and
znalyses. (FEMn approval should be obtained before deviating
from the effective FIS parameters.)
b. 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. �-
4.
Two floodway hydraulic backwater models. The first should be a
duplication of the original baseline model used in the effective FIS.
This is require(J- sure that the original data has been duplicated
correctly. Th s:eco:ndmodel should incorporate the completed project
and include any d � ci 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 •L
original floodway was delineated and, therefore, it is important that
any cross sections added to the original model describe those con-
di In addition, any improvements to the original modeling
''technique ay be incorporated into this intermediate model. This
w� ll then become the new baseline model and will be used to
accurately measure the effects of the completed projecto
d
a. 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 changer, as
specified in item 3a have been approved by FEMA.
b. 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.
c. 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"f
elevations may not exceed revise base flood elevations by more
than 1.0 foot.
5. 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.
n
nformation should be shown on a map of suitable scale and
vu,pu��Laphic definition to provide reasonable accuracy.
b. All items should be labeled for easy cross - referencing to
hydraulic model and summary data.
6. 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 Ploodway 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 FEMA's review process, thus allowing the effective
PIS, 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
Emergency Management Agency in Denton, Texas, at (817) 898 -9127 or members of
our Headquarters staff in Washington, D.C., at (202) 646 -2754.
Sincerely,
&ChL Matticks
Chief, Risk Studies Division
Federal Insurance Administration
Enclosures
cc: Mr. B. Anatole Falagan
Mr. Patrick Lee Acker, Albert II. IIalff Associates, Inc.
Ms. Shohre Daneshmand, Civil Engineer, City of Coppell
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