DR9305-CS 930616Graham Assoc., Inc.
CONSULTING ENGINEERS & PLANNERS
June 16, 1993
Mr. Ken Griffin
City of Coppell
255 Parkway Blvd.
Coppell, Texas 75019
Re; Proposed Grapevine Springs Park - FEMA Submittal
Dear Mr. Griffin;
This letter is being written in responce to your concerns raised
at yesterday's meeting with Jeff Williams, of our office, Jack
Hedge and T.R. Kennedy of Dallas County.
The as-built HEC-2 modeling performed by Albert Halff Assoc.
dated October, 1991 (which was furnished to us by the City last
month) starts at cross section 27620. This is upstream of Bethel
Road. Photo copies of this modeling have been included in our
report, and the modeler's comments at section 27620 read "start-
ing WSEL from Kimley Horn FIS update May 1990". Also, for the
area near Bethel Road, the water surfaces on the printed profile
in the FIS appear to agree with the Kimley Horn modeling. There-
fore, we conclude that the Kimley Horn model is valid, at least
for the small portion we are using, from section 27070 to 27620.
Our vertical control for our field surveys came from FEMA RM's,
which are on NGVD 1929. We can not vouch for, (nor should we be
expected to be responsible for), the vertical datum used in the
Albert Halff Assoc. as-built modeling, the Kimley Horn modeling,
or the City's own topographic maps. Presumably, these are all
on NGVD 1929, since the City has approved and/or adopted them.
The "topographic work map" refered to on FEMA form 5, page 1,
item 1, uses the City's aerial topographic maps (dated 2-29-92)
as its base map. We have amended this form to explain the vert-
ical datum in more detail.
We would prefer to characterize our anticipated hydraulic cond-
itions as "proposed", not as "revised or post-project" since the
work has not actually been completed. Calling something "post-
project" implies that it has already been built. We feel that
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June 16, 1993
Mr. Ken Griffin
City of Coppell
page two
our responses on FEMA form 4, page 2 are correct. We have con-
firmed this with the staff at FEMAts Denton office. According to
the FEMA staff, these forms will soon be updated, and hopefully
this part of this form will be re-written.
The revised existing 100 year water surface is 1.56 feet higher
than the effective at section 30130. This difference is due to
the more accurate modeling of the existing hydraulic conditions.
There are several factors that contribute to the improved
accuracy :
1. The addition of 16 field surveyed sections into a stream
reach of 2500 feet which, in the effective FIS, contains
no hydraulic sections, field surveyed or otherwise.
2. Accurate site topographic maps, with 2~ contours. The
effective FIS floodplain maps were obviously drawn with-
out the benefit of these maps.
3. The experience gained from personally having made
several site visits over a period of the last 4 years,
including visits after the flood of 5-17-89.
4. Revisions to both the channel and overbank roughness
coefficients to accurately represent the actual field
conditions.
5. Hydraulic modeling of the dam, channel lake and eroded
by-pass channel~ features overlooked in the effective
FIS modeling.
Let me take~ this opportunity to state for the record (again)
that the 1.56 feet rise in the 100 year water eurface is not due
to work proposed by Dallas County, nor is it due to work already
performed by Dallas County or by others. The rise is soley due
to a more accurate modeling of the existing conditions.
As compared to our revieed existing conditions, there is indeed a
rise of 0.01 feet in ~he 100 year wa~er surface between secti0ns
5.0 and 5.3 on the Main Channel. The actual rise of 0.01 feet does
not include differences in water surface of 0.02 feet at section
5.21 since there is a suppressed water surface at this section.
-The rise in water surface is accompanied by a decrease in th6 en-
ergy grade line of 0.03 feet over this same reach. The rise is
minimal and is wholely contained on the applicantts property.
June 16, 1993
Mr. Ken Griffin
City of Coppell
page three
FEMA form 4, page 5 item 7 and FEMA form 1, page 2 item lB have
been amended to acknowledge this rise. Also, the water surface
elevation check table has had an additional page added to show
the water surfaces along the Main Channel flow path, (previously,
only the By-Pass Channel was reported).
It is our understanding that FEMA requires only one copy of the
report. The Denton FEMA Office does not retain copies of
hydraulic reports for CLOHR; only forwards them to Washington.
Please insert the enclosed revised pages into your reports be-
fore sending it to FEMA.
I believe that this covers all your points of concern.
do not hesitate to call if you have any questions.
Please
Sincerely;
Neal Chisholm, P.E.
Graham Associates
enclosures
cc; Jack Hedge, P.E. - Dallas County