Magnolia Park-CS 861211
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Federal Emergency Management
Washington, D.C. 20472
The Honorable Lou Duggan
Mayor of the City of Coppell
P.o. Box 478
Coppell, Texas 75019
December 11. 1986
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Case #86-06-13R
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Dear Hayor Duggan:
This is in reference to a letter, dated February 25, 1986, and t
submitted by Hr. Ed Powell, P.E., City Engineer for the City of
his letter, which was forwarded to us by our Region VI office,
requested that the Federal Emergency Management Agency (FEMA) is
ditional Letter of Hap Revision (LOHR) for a proposed swale cons
floodplain fill project along Denton Creek immediately upstream
Road in Coppell, Texas.
chnical data
oppell. In
. Powell
ue a con-
ction and
f Denton Tap
The technical data subm! tted wi th Hr. Powell's letter were prepa
Davis & Associates, Inc., and included HEC-2 hydraulic backwater
models of Denton Creek for both existing and proposed conditions location and
topographic maps that include cross-section locations and base (lOO-year)
flood and floodway boundaries, and plotted cross sections. Addi ional data
were submitted on July 9, 1986 by Mr. Mark W. Roberts, P.E., of rockette-
Davis-Drake, Inc., (formerly Jack R. Davis & Associates, Inc.), ich included
revised HEC-2 hydraulic backwater computer models and a revised pographic
map delineating the base (100-year) flood and floodway boundaries. The
revised models and revised map tie the proposed swale and fill pr ject into
another proposed project, currently under construction, located i ediately
downs tream in an area known as the Gateway Development in the Ci t of Lewisville,
Texas. A conditional LOHR was issued to the City of Lewisville 0 September 3,
1985 for the Gateway Development project.
We reviewed the data submitted by Mr. Powell and Hr. Roberts and ave de- ~
termined that the proposed swale construction and fill project me ts the
minimum floodplain management criteria set forth by the National lood In-
surance Program (NFIP) regulations. If the project were complet as proposed,
a revision to the effective Flood Insurance Study (PIS), Plood Bo dary and
Floodway Map (FBFH) I and Flood Insurance Rate Hap (FIRM) I for you community
would be warranted. This revision would show the revised flood a floodway
boundaries and Base (1 Oo-year) Flood Elevations (BFEs) as comput in the
revised HEC-2 models submitted on July 9, 1986, by Hr. Roberts. lease note
that future revisions to the FIS, FBFK, and PIRM for the City of oppell or
restudies of the flood hazards in this area could modify this det rmination.
Also note.~at_thi8 determination is .ade '1-_t;h.,.tbe ass\J.1q'p~_on .tha the down-
stream Ga~ewaL-Developau;nt..erojectis constructed as proposed. I addition,
we--have a conce;n-~th-~- proPosed -;tream ';elocity ;1:~roas-sect on 32305.
Due to the high veloci ty in this portion of Denton Creek, channel protection
measures must be made prior to FEMA issuing a revision to the effective FIS,
FBFM, and FIRM. You should require that this concern ~ resolved rior to
approving any construction within the currently effective floodway or place-
ment of structures in the floodway fringe at an elevation that is low the
effective BFE.
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The determination made in this letter is based on the 100-year
charges computed in the effective FIS for your community, and d
sider subsequent changes that would tend to increase flood disc arges.
development of this project and other projects upstream could result in
increased flood discharges, which, in turn, could result in inc ased 100-year
flood elevations. Future restudies of the flood hazards in this area, which
would take into accowlt the cumulative effects of development on flood dis-
charges, could establish higher 10o-year flood elevations in thi
This conditional LOHR is based on minimum floodplain management
established under the NFIP. Your community is responsible for a proving all
proposed floodplain developments, including this request, and fo assuring
that necessary permits required by Federal or State law have bee received.
State and community officials, based on knowledge of local condi ions and in
the interest of safety, may set higher standards for constructio or may limit
development in floodplain areas. If the State of Texas or the C ty of Coppell
has adopted more restrictive or comprehensive floodplain managem nt criteria,
these criteria take precedence over the minimum NFIP requirement.
NFIP regulation 44 CFR 60.3(b)(7) requires communities participa ing in the
NFIP to -assure that the flood carrying capacity within the alte ed or re-
located portion of any watercourse is maintained.- Without pro r mainte-
nance, such as the regular clearing of a channelized stream, cha el modifi-
cation projects will, in time, fail to function as designed, the eby recreating
the flood hazard that they were intended to mitigate. Therefore, before FEMA
will revise the FIS, FBFM, and FIRM to reflect the effects of the completed
project, we require a written statement that your community will phold its
responsibili ty for assuring that the channel is maintained in ord r to preserve
its design function.
Upon completion of the proposed swale construction and fill proje
community may request a revision to the effective FIS, PBFM, and
revision request must include the data listed below. Several of
have already been submitted by Mr. Roberts, but may need to be re
submitted to incorporate any changes resulting from the aforement
and any design changes that may take place during construction.
t, your
IRH. The
ese i terns
ised and re-
oned concern
1. Written endorsement of the revision request; a written s atement
concerning maintenance of the project, as described abov , and
documentation demonstrating that the above-mentioned con ern has been
resolved.
2. -As-built- plans of the swale construction and fill proj ct, certified
by a registered engineer.
3. HEC-2 hydraulic models of the 10-, 50-, 100-, and 500-year floods
and the floodway model representing "as-built" conditions with the
aforementioned concern resolved.
3
4.
Delineation of the 100- and SOo-year flood boundaries,
boundary, and the locations and alignment of cross sect
flowline used in the hydraulic model representing -as-b
di tions .
e floodway
ons and
It- con-
5. Revised water-surface profiles of the 10-, 50-, 100-, a 500-year
floods reflecting -as-built- conditions, including a zo e determi-
nation.
We have enclosed documents, entitled Conditions and Criteria for
and Conditions and Criteria for Floodway Revisions, which fur the
nature and extent of the material needed to support a request to
effective FIS, FBFM, and FIRM. Compliance with the criteria out
documents will expedite FEHA's review process, thus allowing the
FIS, FBFM, and FIRM for your community to be revised as appropri
timely manner.
Should you have any questions regarding this matter, please do no
contact the Chief, Natural and Technological Hazards Division of
Emergency Hanagement Agency in Denton, Texas, at (817) 898-5127 0
our Headquarters staff in Washington, D.C., at (202) 646-2754.
hesi tate to
e Federal
members of
Sincerely,
~b
Acting Chief, Risk Studies Di vi ion
Federal Insurance Administratio
Enclosures
cc: Hr. Ed Powell, P.E.
Hr. Mark ft. Roberts, P.E.
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