Estates DC-CS 851230CERTIFIED MAIL
RETURN RECEIPT REQUESTED
The Honorable Lou Duggan
Mayor of the City of Coppell
136 Glenwood Drive
Coppell, Texas 75019
Federal Emergency Management Age-- cy
Washington, D.C. 20472
Dear Mayor Duggan:
This is in reference to a letter, dated September 23' 1985, and technical data
submitted by Mr. H. Wayne Ginn, P.E., City Engineer for the City of Coppell,
Texas. In his letter, Mr. Ginn requested, on behalf of Nathan D. Maier
Consulting Engineers, Incorporated, that the Federal 'Emergency Management
Agency (FEMA) issue a conditional Letter of Map Revision (LOMR) for a proposed
flood plain modification project. This project involves placement of fill in
the Elm Fork of Trinity River flood plain and realignment of the regulatory
floodway on the west bank of this river. The proposed project is to be
located in an area known as the Keas Construction Tract along Denton Creek in
Coppell, Texas.
The submitted technical data, prepared by Nathan D. Maier Consulting Engineers,
Incorporated, included a report entitled Floodplain Reclamation Study, Keas
Construction Tract, Co~ell and Carrollton, Texas, dated July 1985. This
report included existing and proposed LRD-1 backwater models of the Elm Fork
of Trinity River, cross-section plots, water-surface profiles, and a map
referred to as Plate 2 delineating existing and proposed flood plain and
floodway boundaries. In addition, an LRD-1 backwater model incorporating the
proposed fill was submitted by Mr. Err A. Meyer, P.E., of Nathan D. Maier
Consulting Engineers, Incorporated, with a letter of transmittal dated October 30,
1985.
Based on our review of the technical data submitted, we believe that the
regulatory floodway along this reach of the Elm Fork of Trinity River will be
reduced as shown on Plate 2. Also, the proposed fill on the Keas Construction
Tract will not raise the Base (100-year) Flood Elevations along the Elm Fork
of Trinity River, provided the construction is completed as proposed. The
potential hydrologic impacts associated with the development of this area or
areas upstream have not been considered in this determination. This con-
ditional LOMR is issued with respect to minimum standards of the National
Flood Insurance Program (NFIP). Because your community may enforce more
restrictive flood plain management criteria, final approval of the project
rests with your community.
During the course of our review, we identified the following concerns with the
submitted technical report:
o
The LRD-1 backwater models also incorporate fill and an overflow
swale project downstream from this project. A conditional LOMRdated
May 16, 1985, has already been issued for the fill and swale project,
and construction is currently in progress. The LRD-1 backwater model
submitted for the fill and swale project had t_he cross sections
oriented left to right looking upstream, while the LRD-1 backwater
model for t_his project has the cross sections oriented left to right
looking downstream. When your community requests a Flood Insurance
Study (FIS) and Flood Insurance Rate Map (FIRM) revision for these
projects, a hydraulic model representative of "as-built" conditions
should have a consistent orientation of the cross sections.
When your community requests an FIS and FIRM revision for these
projects, any fill which has been placed in the flood plain should be
shown on the "G" cards of the LRD-1 backwater models.
These concerns must be resolved prior to F~A issuing a revision to the
effective FIS and FIRM for the City of Coppell to reflect the effects of the
completed projects.
Upon completion of the proposed fill project, your community may request a
revision to the effective FIS and FIRM. The revision request must include the
data listed below:
Documentation which indicates community endorsement of the revision
request.
"As-built" plans of the fill 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.
e
Revised water-surface profiles of the 10-, 50-, 100-, and 500-year
floods reflecting "as-buil~" conditions, including a zone determi-
nation.
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
analyses. (FEMA approval should be obtained before deviating
from the effective FIS parameters.)
Since only a portion of the existing profiles is revised, the
upstream and downstream portions of the revised profiles should
coincide with the effective FIS profiles, i.e., hydraulic
calculations should be continued for a great enough distance
upstream and downstream of the revised 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 fill project and include any other channel modifi-
cations or encroachment that may have occurred in the flood plain
since the original floodway was delineated. If, however, additional
cross sections are used in the second model to provide a more de-
tailed analysis of the completed project and 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 cross sections added to the original model describe those
conditions. 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 fill 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 item 4a 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 fill 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 may 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 cross sections
and flow line used in the hydraulic model.
ae
This information should be shown on a map of suitable scale and
topographic definition to provide reasonable accuracy.
Ail 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 as a bibliographic list of other sources of
information used.
Upon receipt and review by FEMA of items 1-7 above, satisfactory resolution of
the aforementioned concerns, and verification that the design of the fill
project conforms to sound engineering practice and meets all applicable NFIP
standards, FEMA will take action to incorporate the effects of the completed
project into the effective FIS and FIRM for your community.
4
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 FIS and FIRM. Compliance with the criteria outlined in these
documents will expedite FEMA's review process, thus allowing the effective FIS
and FIRM for your community to be revised as 'appropriate, in a timely manner.
In the future, please submit all FIS and FIRM revision requests directly to
the Chief, Natural & Technological Hazards Division, Federal Emergency Manage-
ment Agency, Federal Regional Center, 800 North Loop 288, Denton, Texas 76201,
for required FEMA Region VI review and concurrence. When revision requests
are mailed directly to our Headquarters office by a community, we forward them
to our appropriate regional office for review. Unnecessary mail delays will
be eliminated if all future requests are submitted directly to our Region VI
office.
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) 387-5811 or members of
our Headquarters staff in Washington, D.C., at (202) 646-2754.
~Acting Chief, Risk Studies Division
Federal Insurance Administration
Enclosures
cc: Mr. H. Wayne Ginn, P.E., City Engineer for the City of Coppell w
Mr. Dennis L. Johnson, P.E., Nathan D. Maier Consulting Engineers,
Incorporated