Grapevine Creek-CS 920604 (2) ,~o,.,()C-~. t .... iNC. June 4, 1992
City of Coppell
P.O. Box 478
CoppeLt, TX 75019
Atto: Ken Griffin, P.E., City Engineer
Flood Plain Administrator
Re: Grapevine Creek LOMR, Southwestern Boulevard to LH635,
FEMA Case No. 92-06-006P
Dear Ken:
As technical consultant for the above referenced project Albert H. Halff Associates, Inc.
is addressing the following comments from FEMA as provided by their February 3, 1992 data
request (copy attached).
1) Evidence of compliance with NFIP Regulation Pan 65.7(b)(1) requiring
public notification of a floodway change.
Public Notice was published in the February 21, 1992 issue of the
Citizens' Advocate, Vol. VIII, No. 7, the official newspaper of the
City of Coppell. A copy o{' this notification along with the
required certification from the ,newspaper is attached.
2) Evidence of compliance with NFIP Regulation 65.6(a)(12) requiring
documentation on how the flood-carrying capacity of the channelized
portion of Grapevine Creek is to be maintained.
- This issue was discussed with Mr. Matt ,Miller, FEMA, in
September 1991, prior to submission of this request for a Final
Letter of Map Revision. He stated that the city is ultimately
responsible for the maintenance of relocated channels. NFLP
Regulation 44CFR 60.3(b)(7) requkes communities to "assure that
the flood carrying capacity within the altered or relocated portion
of any watercourse is maintained." Ordinances which allow the
city to enter upon private property and perform necessary
maintenance are provided routinely in many cases. The City of
Coppell's Erosion and Sedimentation Control and Nuisance Control
Ordinances provide for abatement by the city. A letter to this
effect, from Mr. Steven G. Goram, Director of Public Works, along
with the applicable ordinances is included with this submittal.
TPAr'; SPORT,.X"CN WATER PESCL:PCES
SL~R',,'E r :rlG ,GLOBAL POS
~-' :,C SC ~,;:E
ALBERT H. HALFF ASSOCIATES, INC.
ENGINEERS ' SCIENTISTS ' SURYEYORS
City of Coppell
June 4, 1992
Page 2
3) Technical assessment which addresses the probable stability of the site
during the occurrence of the 100-year flood, especially in the locations
where channel velocities are greater than 5 feet per second.
Albert H. Halff Associates, Inc. feels that this comment has been
addressed extensively during the 10 year period since this channel
project and Freeport Parkway were constructed. This project was
built in 1981 without submission to the Federal Emergency
Management Agency for a Conditional Letter of Map Revision.
Since Halff Associates became the engineer for this property
(1985), the existing channelization and bridge have been submitted
to FEMA at least 4 times. The active conditional letter (Case No.
86-06-47R) specifically states that "the drop structure that is
proposed between the completed channel modification and the
Freeport Parkway bridge would appear to create stable channel
conditions during flood events" .(copy included with this package).
Although the structure was m6ved downsu:eam because of the
131635 right-of-way there was no increase in velocity over those
shown in the hydraulic modeli.ng for the conditional letter (see
Table 1). :
In addition, copies of the boring logs for two locations along
Grapevine Creek are included for your information. These logs
indicate the soils are high to medium PI silty clays. The City of
Coppell's criteria is a maximum velocity of 8 fps for unlined
channel sections such as this.
We feel that this 'data provides all of the information necessary for issuance of a Final
Letter of Map Revision. Please feel free to call if you have any questions or comments.
Yours very, truly,
ALBERT H. HALFF ASSOCIATES, INC.
n Hansen, P.E.
Attachments