ST9401-SY 960212Morrison Hydrology/Engineering, Inc.
210 Arnold · Arlington, Texas 76010 · Metro (817) 461-0321
February 12, 1996
Mr. Ulys Lane
Weir and Associates, Inc.
4300 Beltway Place
Suite 130
Arlington, Texas 76018
Re: Denton Creek at Denton Tap Road Coppell, Texas
Dear Mr. Lane:
As we discussed the model through this area is very complicated and does not
lend itself to evaluation with a one dimensional model like HEC2. However,
there are ways to develop a more realistic model by analyzing the separate
flow paths leaving the Denton Tap Road bridge.
In order to accomplish this analysis, we assumed three separate flow paths
occur at about cross section location 30810. Using the city topo and
interpolating channel flowlines, we developed a cross section at this
location. We called the three flows paths:
Denton Creek Diversion. This flow path confined flow to the Denton Creek
diversion channel.
Original Denton Creek channel. This flow path was confined to the
original Denton Creek channel.
Right channel. This flow path generally refers to flow in the right
overbank area created by flow from Cottonwood Creek.
The result of a rating curve for these three paths is shown in Figure 1,
Figure 2, and Figure 3 attached. Using the total ultimate condition flow of
26830 cfs and equal flood elevations, it was determined that the following
discharges occur in each of the flow paths:
Denton Creek Diversion
Original Denton Creek Channel
Right Channel
18100 cfs
7080 cfs
1650 cfs
Using these discharge values as tailwater conditions and assuming that the
Denton Creek ultimate flow continues through the bridge from upstream, it is
possible to determine flood levels in a way that is more realistic than the
FEMA model.
Page '2-
The results of this model may be seen in the Table 1 below:
TABLE 3
SUMMARY TABLE FOR PROPOSED
CONDITION BASED ON THE MOST
REALISTIC PREDICTION OF THE
100 YEAR FLOOD ELEVATIONS
SECT. 100 YR. Q AVG.
NO. ELEV. CFS VEL.
26830 450.48 26830 12.42
28080 454.42 26830 7.98
28870 455.82 26830 4.87
29610 456.23 26830 5.07
30570 456.78 26830 5.86
30810 455.26 18100 14.19
31230 458.75 18100 7.68
32150 460.19 16681 8.94
32200 460.59 16681 7.34
32323 460.84 16681 6.53
32378 460.36 16681 9.43
32725 462.15 16681 4.14
33145 462.47 16681 3°83
33470 462.74 16681 4.5
34260 463.48 16681 3.17
34850 463.4 16681 8.83
34950 464.68 16681 4.42
Figure 4 is a floodplain delineation of this model. We have also attached the
computer model.
This model shows the following flood elevations at Denton Tap Road:
Downstream face
Upstream face
460.59'
460.84'
We hope this provides some additional clarification of this very complicated
model.
Sincerely,
MORRISON HYDROLO(IY/ENfllNEERINfl, INC.
Ronald W. Morrison, P.E.
Senior Hydrologist
(133_~) NDIIVA3']3
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