Summit P1-CS 950831Graham Associates, Inc.
CONSULTING ENGINEERS & PLANNERS
August 31, 1995
Mr. Ken Griffin
City Engineer
City of Coppell
255 Parkway Blvd.
Coppell, Texas 75019
Re; Proposed East Tract - Baptist Foundation Tract
Coppell, Texas
Dear Mr. Griffin;
Enclosed for your review and preliminary approval are drawings,
maps and hydraulic calculations (HEC-2 models) for the proposed
East Tract, Springs of Coppell project. As per the City'G
requirements, the 2 and 100 year ultimate and 100 year FEMA
discharges were modeled. The 10, 50 and 500 year FEMA discharges
will be modeled at a later date for the FEMA submittal.
The proposed project does not encroach upon the floodway, as
delineated in the Dallas County project, and it is proposed
that the existing off-channel 'pond' will not be filled. To ~%1~1
offset the mino~ losses in conveyance caused by the proposed/_~k~~
fi].l in the East overbank, a bench will be exc~vated~in the ~--~
West overbank between the bypass channel and the Railroad as
shown on the enclosed plan. The proposed fill will be at
least 2 feet above the proposed 100 year water surface using
the FEMA discharge. This is at a higher elevation than 1 foot
above the proposed 100 year water surface using the ultimate
discharge.
As shown on the Table 1, the proposed project does not raise
the 100 year flood levels (using the FEMA discharge) upstream
of the project. We have met with Dallas County about this
project and they have given tentative verbal approval and will
provide final written approval after the City's review.
It is our understanding that the currently effective hydrauli(~
models on Grapevine Creek have not changed since the Dallas
County project was sent to FEMA, except that the starting water
surfaces for the 10 and 50 year profiles were corrected by FEMA
as noted in their letter of November 11, 1993.
We have made post construction field surveys of the Dallas Co.
project and have included this information in our hydraulic
analysis of Grapevine Creek. This modeling is being used as
the 'existing conditions' for the proposed East Tract-Springs
of Coppell project.
S~t Office Po~ Cen~r~int Two Nofio~nk
1300 ~rnmit A~., Sui~ 712 616 ~x F~ Drive. ~i~ 400 7~ W. Ave. B, ~i~ ~1
Ft. Worth, ~x~ 761~ Aflin~on, Texas 76011 ~dond, ~x~ 75040
(817) ~2~7~ ~17) ~9~ * Meffo (817) 640-8535 ~14) 272~
F~ ~17) 3366909 F~ ~17) 6~52~ F~ ~ 14) 272-4655
August 31, 1995
Mr. Ken Griffin
page two
The proposed project will change the flow split between the
main.and bypass channels. This change is as follows;
.~ I existing cond. II proposed cond.
I main I bypassll main I bypassl
2 year ultimate Q I 1225 I 2525 I I 1550 I 2200
100 year ultimate Q I 2695 I 8555 I I 4250 I 7000
100 year FEMA's Q I 2600 I 7900 I I 4000 I 6500
This will also change channel velocities as shown on Table 1
for the 100 year FEMA discharge. The net result will be a
lowering of velocities~in the bypass channel (about 1 fps
lower) and a slight rise of velocities in the main channel
(from 1 to 3 fps higher). Note that the main channel does
have a high degree of erosion protection with the stone and
concrete walls lining the channel.
We estimated the floodplain storage that would be lost due to
this proposed project and have summarized our findings on
Table 2. According to volumes calculated by the HEC-2 pro-
gram, the proposed project would cause the loss of approx.
12 acre feet of valley storage between Bethel Road and the
Railroad at the 100 year ultimated discharge. However, the
proposed on-site storage would be approx. 42 acre feet MORE
than used by Albert H. Halff in their 1991 report. There-
fore, we believe that the ultimate discharges, as calculated
by Halff, will not be changed by this project.
Do not hesitate to call if you have any questions.
Sincerely,
i]'
Neal Chisholm, P.E.
Graham Associates
SECTION
NUMBER/
LOCATION
27570
27620
1.0
2.0
2.5
3.0
3.3
3.6
4.0
4.5
5.0
5.1
5.11
5.14
5.2
5.21
5.24
5.3
69
75
81
91
111
6.0
30130
30189
30201
30250
31310
31480
31500
31550
31750
32740
34045
34330
34870
35450
35570
TABLE 1
GRAPEVINE CREEK
100 YEAR FEMA DISCHARGE
8-29-95
EXISTING COND.(AS-BUILT) PROPOSED COND. (D2) I
LF FWAYI WATER I CHAN LF FILL WATER CHAN I RISE
STA. ISURFACE I VEL. .STA. SURFACE VEL. I
BETHAL ROAD
2078 494.92 9.2 no fill 494°92 9.2 0.00
1992 495.61 8.4 2035 495.61 8.4 0.00
1390 496.24 7.5 1460 496.24 7.5 0.00
1291 496.46 9.7 1370 496.46 9.7 0.00
1300 496.61 10.7 1365 496.61 10.7 0.00
1280 497.62 10.3 1315 497.62 10.3 0.00
1200 499.01 6.8 1250 498.96 7.1 -0.05
1282 499.36 8.4 1360 499.40 8.4 0.04
1400 499.91 8.5 1455 499.92 8.5 0.01
1520 501.31 4.0 1545 501.36 4.4 0.05
START OF FLOW SPLIT,
Qmain = 2600 cfs I Qmain = 4000 cfs
I 501.54 I 1.6 I I 501.61 I 2.4 0.07
I 501.54 I 2.0 I I 501.63 I 3.1 0.09
I 501.56 I 1.9 I I 501.66I 2.9 0.10
FOOT BRIDGE
I 501.56 I 1.9 I 501.66 I 2.9 I 0.10
I 501.56 1 2.0 I 501.66 I 3.0 I 0.10
I501-54 I 3.2 I 501-61 I 4.8 I 0.07
FOOT BRIDGE
I 501.47 I 4.5 I 501.45 I 7.0 I-0.02
I 501.46 I 5.1 I 501.42 I 7.9 I-0.04
Qbypass = 7900 cfs Qbypass = 6500 cfs I
1154 I 501.25 I 6.3 1170 501.29 I 6.3 I 0.04
1172 I 501.36 I 6.3 1210 501.59 I 5.2 I 0.23
1158 I 501.40 I 6.6 1210 501.63 I 5.4 I 0.23
1173 I 501.66 I 5.6 1230 501.79 I 4.6 I 0.13
1183 I 501.85 I 4.5 1290 501.93 I 3.6 I 0.08
END OF FLOW SPLIT
1205 I 500.97 t 13.1 I 1260 501.81 I 10.6 I 0.84
2222 I 503.73 t 8-4. I 2330 503.04 I 9.2 I-0.69
2245 I 503.58 I 9.2 I no fill 502.93 I 9.8 I-0.65
RAILROAD
2245 I 504.10 I 10.I I no fitl I 503.56 I 9.2 I-0.56
2222 I 504.99 I 7.7 I no fill I 504.52 I 7.7 I-0.47
1920 I 506.57 I 7.1 I no fill I 506.35 I 7.1 I-0.22
SOUTHWESTERN BLVD.
2700 506.74 8.1 no fill 506.58 8.1 -0.16
2700 506.90 8.1 no fill 506.74 8.1 -0.16
2500 507.81 6.2 no fill 507.63 6.2 -0.18
1268 508.07 3.5 no fill 507.92 3.5 -0.15
1294 508.51 3.8 no fill 508.37 3.8 -0.14
1351 509.31 4.1 no fill 509.20 4.1 -0.11
1334 509.48 5.1 no fill 509.39 5.2 -0.09
1330 510.40 5.7 no fill 510.34 5.7 -0.06
1302 511.60 5.8 no fill 511.56 5.8 -0.04
1302 511.86 5.6 no fil,1 511.83 5.6 -0.03
TABLE 2
GRAPEVINE CREEK
100 YEAR ULTIMATE DISCHARGE
8-30-95
SECTION
NUMBER/
LOCATION
~27400
27540
27570
27620
1.0
2.0
2.5
3.0
3.3
3.6
4.0
4.5
5.0
5.1
5.11
5.14
5.2
5.21
5.24
5.3
69
75
81
91
111
6.0
30130
30189
1991 ~ REPORT
VALLEY
STORAGE
0.00
8.06
WATER
SURFACE
495.36
495.45
B E T H A L
9.45 495.56
11.87 495.68
not modeled as
split flow
DALLAS CO PROJECT
VALLEY
STORAGE
0.00
8.
ROAD
WATER
SURFACE
495.36
495.45
12.
22.
28.
30.
33.
40.
48.
52.
88.
start
495.56
495168
496.37
496.61
496.76
497.92
499.43
499.78
500.36
501.78
of
Qmain =2695 cfs
14.
18.
20.
21.
24.
31.
31.
34.
END OF
97.19 I 502-60 I 151.
98.94 I 502-51 I 153.
RAILROAD
502.01
502.02
502.03
502.03
502.03
502.01
501.94
501.94
PROPOSED COND. D2
VALLEY I WATER
STORAGE-ISURFACE
0.00 I 495.36
8. I 495.45
12.
22.
28.
30.
33.
39.
47.
50.
82.
495.56
495.68
496.37
496.61
496.76
497.92
499.36
499.82
500.36
501.85
flow split
Qmain =4250 cfs
13.
16.
19.
20.
23.
30.
30.
32.
502.11
502.13
502.16
502.17
502.16
502.11
501.94
501.92
Qbypass=8555 cfs
11. I 501.72
16. I 501.82
18. I 501.86
20. I 502.12
22. I 502.31
FLOW SPLIT
144. I 501.41. I 132.
I 504.23 I 139.
I 504.04 I 141.
Qbypass=7000 cfs
8. I 501.77
12. I 502.08
14. I 502.12
16. I 502.29
18. I 502.43
I 502-301
1503.53I
1503.37I
note; Valley storage is in acre-feet above section 27400.
Ultimate 100 year discharge is 11250 cfs as calculated by
Albert H. Halff Assoc. for the City df Coppell in 1991.