Billingsley-CS130322 NNE HALFF
March 22,2013
AVO 29119
Aswartha Balla, CFM
LOMC Clearinghouse
Attention: LOMR Manager
847 South Pickett Street
Alexandria,VA 22304-4605
Re: Case No. 13-06-1839P — Request for a Letter of Map Revision (LOMR) for the
MacArthur Center site on Grapevine Creek in the Cities of Coppell, Dallas, and Irving,
Texas.
Dear Mr.Balla,
Halff Associates has reviewed the comments made by the Federal Emergency Management
Agency(FEMA) involving Case No. 13-06-1839P and has the following responses:
1. Cross sections 13339 and 13620 have been extended to contain water surface elevations.
2. Topwidths
a. Floodway
i. The differences in top widths at cross-sections 12402, 12820, and
13339are due to islands in the floodway that are not reflected in the
floodway mapping.
ii. Mapping has been altered to reflect the computed floodway width at
cross-section 15113
b. Floodplain
i. The difference in top width at cross-section 13339 is due to high points
along the cross section that subtract from the total top width in the
floodplain.
ii. Mapping has been altered at cross-section 15113 to reflect the computed
floodplain width. However, the left floodplain and floodway stations fall
within the channel, and so have been mapped at the left channel bank
station.
3. In preparing the original Conditional Letter of Map Revision (CLOMR) (Case No.: 04-
06-A148R, approved January 2006) for the 158+ acre tract just west of the southwest
corner of MacArthur Boulevard and Beltline Road, Halff Associates, Inc. requested the
effective model for Grapevine Creek from the FEMA Project Library through PBSJ. The
contact person for the FEMA Project Library at the time, Mr. Venkat Venkatraj,
determined that a digital or hard copy of the effective model was not available and
suggested that the previous study in the area performed by Halff Associates, Inc. in 1991
should be used as the effective model for Grapevine Creek. Therefore, the 1991 model
that has been the basis for further studies along Grapevine Creek in this area may not
necessarily reproduce the floodplains and floodway shown on the effective mapping.
HALFF ASSOCIATES,'NC.
1201 NORTH.300,3ER ROAD -- ,. 5200 N A ,,.r,H.l r r.CON
R ICHARLSOSL 1X 75081-2275 FAF Ci_-4?739-0095
U.. HALFF
The CLOMR pre-project (existing conditions) model included updated modeling of the
then-recently constructed Beltline Road Bridge. That model was used as the existing
conditions model for the Hemingway Court LOMR (Case No.: 06-06-BE95P, approved
May 2007). The approved Hemingway Court Post-Project HEC-2 model included the
Beltline Road Bridge improvements, but the mapping from the downstream face of
Beltline to a point downstream of Moore Road was not altered to reflect the water surface
elevations computed as part of the Hemingway Court LOMR. The Hemingway Court
LOMR was approved by FEMA in May 2007 without comments on the mapping
irregularities. Therefore, the FEMA-approved LOMR model became the effective model
even though the effective mapping was not updated to reflect the elevations on the
FEMA-approved model profile. The floodplain and floodway widths shown as effective
mapping at Beltline Road and the St. Louis Southwestern Railroad in Figure 5 of this
LOMR application are not based on the elevations from the effective model.
In order to tie into the effective mapping and fix the mapping error at Beltline Road and
upstream, we have updated the models from cross section 15630 through cross section
16480 with data pulled from topography based on the 2009 TNRIS LiDAR data. Once
approved, these changes will allow the effective mapping to reflect the profile computed
by the effective model in the reach from cross section 11301 through cross section 17600.
4. Revised files are on the CD/DVD accompanying this submittal.
If you have any additional comments or questions concerning this submittal, please feel free to
contact me at(214) 346-6245.
Sincerely,
HALFF ASSOCIATES,INC.
t i
Russell P. Erskine,P.E., CFM
Project Manager
cc: Kenneth M. Griffin,P.E.
City of Coppell
Steve Parker,P.E.
City of Dallas
Garry Fennell,P.E.
City of Irving
Kenneth Mabry
Billingsley Development Corporation
Attachments