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... Halff Associates
ENGINEERS • ARCHIFECTS • SCIENTISTS
PLANNERS• SURVEYORS
September 17, 1998
AVID 16521
Mr. Kenneth M. Griffin, P. E.
Assistant City Manager/ Director of Public Works
City of Coppell
255 Parkway Blvd.
Coppell, Texas 75019
Re: Hydraulics Study of the Revised A uaq tic an
Letter of Map Revision (LOMR) Submittal.
Andrew Brown Jr. Park (East).
Dear Mr. Griffin:
8616 NORTHWEST PLAZA DRIVE
DALLAS, TEXAS 75225
(214) 739 -0094
FAX (214) 739 -0095
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At our Tuesday September 1 ", 1998 meeting, you requested Halff Associates, Inc. to analyze the
impact of the revised recreation center location on flood levels in the Andrew Brown Jr. Park
(East). In addition, you asked us to contact FEMA, regarding LOMR submittal procedures.
As shown on the a ttache d wat er s urfac e e levatio n c omparis on table (100 -year Flood Discharges,
Existing Landuse Conditions), the proposed reclamation causes a maximum 0.05 foot rise in flood
levels within the park itself (section 29831) when compared against existing (pre - development)
conditions. This rise is 0.02 foot higher than the one quoted in the 1992 Conditional Letter of Map
Revision ( CLOMR). Also, a residual 0.01 foot rise appears as far as section 32725, located
upstream of Denton Tap Road, within the western part of Andrew Brown Jr. Park. Such a rise may
be unacceptable since it may impact the Cottonwood Branch hydraulics and prompt the Federal
E mergency Management Agency (FEMA) to request a complicated re- analysis of that stream.
Rises of about 0.01 foot also occur at Denton Tap Road.
I have also included three exhibits of the proposed Recreation Center site. Exhibit A compares the
revised grading (revised to accommodate the Recreation Center) with the original grading that was
reflected in the 1992 CLOMR. Exhibit B shows proposed changes to the recreation center which
compensate for the rises caused by the Recreation Center plan revisions. Exhibit C shows the
proposed Recreation Center against the currently effective (FIS) 100 -year floodplain.
As you know, the revised Recreation Center site encroaches further into the existing floodplain than
did the reclamation that was approved in the CLOMR (see exhibit A). To eliminate the rises that
this encroachment causes, ear offsetting excavation must occur q t he encroachment must be %K
reduced. Exhibit B shows a mitigation scheme that, although it causes a 0.07 foot rise at section
29831 (the Recreation Center location) based on existing landuse discharges, confines those rises
within Andrew Brown Jr. Park East itself. However, these adjustments may be unacceptable from
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;p; Halff Associates
ENGINEERS • ARCHITECTS • SCIENTISTS
PLANNERS • SURVEYORS
the Architect's point of view, since it modifies the fire l ane layou and r educes the turligLQmlLall1l�
e�nd to fir I This scheme qLsQ r equires enlarging the ex isting o`nd west of the Recreation
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Center.
We are forwarding a copy of this letter to the Architect, Brinkley Sargent Architects, for their
review, approval or suggested changes. If the mitigation shown in Exhibit B is unacceptabl the
Architect and the City may wish to consid additional rnitieation to the East of the proposed peoh�l><y
Recreation >tlte1 _.tq,__Qf&tA11c- la ld�xel rises at and nRstream of Denton tap rQ3d. In the g oa h
meantime, we will suspend further floodplain analysis until we receive further direction from the
City.
Walter Skipwith attempted to contact Alan Johnson of FEMA headquarters concerning LOMR
procedures. The call was transferred to Laura Bittner of Michael Baker, Engineers. Ms. Bittner
says that it is acceptable and not uncommon to postpone final LOMR submittals while projects
such as parks or landfills are constructed in phases over many years. Based on this conversation, it
appears that the LOMR application can be postponed until all proposed improvements in Andrew
Brown Jr. Park (East) are constructed. A LOMR may be obtained for a portion of a site for which a
CLOMR has been granted, but FEMA would rather receive a LOMR request addressing the entire
site improvements (of course, any proposed changes should be tested and documented to satisfy
the City of Coppell Floodplain Ordinance). As shown on Exhibit C, the proposed Recreation
Center building (an insurable structure) is out of the effective FIS 100 -year floodplain and
floodway.
Please do not hesitate to call 214- 739 -0094 (ext. 226) if you have any further questions or
comments regarding hydraulic modeling at the Recreation Center site at Andrew Brown Jr. Park
(East) or the FEMA LOMR submittal procedures.
Sincerely,
HALFF ASSOCIATES, INC.
Sabine Borgnet - Harris, EIT
enclosures
cc Brad Reid, Park Planning and Landscape Manager, City of Coppell
Karen D. Vaughn, Brinkley Sargent Architects
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