Summit P1-CS 951020Graham Associates, Inc.
CONSULTING ENGINEERS & PLANNERS
October 20, 1995
Mr. Ken Griffin
City Engineer
City of Coppell
255 Parkway Blvd.
Coppell, Texas 75019
Re; Proposed East Tract - Baptist Foundation Tract
Coppe!l, Texas
Dear Mr. Griffin;
At your request we have investigated the hydrologJ~c impacts of
the proposed East Tract, Springs of Coppell project. The
project is consistant with the recommendations concerning
loss of floodplain storage contained in the Storm Water Mana-
gement Plan by Albert H. Halff and Associates.
The scope, extent and features of the proposed project has
changed since our August 31st letter. As previously stated,
tn.e proposed project does not raise the 100 year f].ood levelz
(using ~he FEMA discharge) upstream of the project.
After closely examining the 1991 Al.bert Ha]fl reFort ('A City-
Wide Storm Water Management Plan ~or the City of Coppel, Texas')
we learned that the 2 year and 100 year ultimate discharges for
Grapevine Creek were NOT calculated by Halff, but were from an
earlier report by Kimley Horn and Associates. (see page 1-10
parag. 3 & 5 and page II-12 footnote #2). We obtained a hardcopy
listing of the HEC-1 model for Grapevine Creek from the Kimley
Horn report 'Master Drainage Plan, Phase I', dated 1990.
We established this model on our computer, re-ran the Kimley Horn
data and the table below shows a comparison between our results,
the original Kimley Horn output and the Halff report.
I KIMLEY HO~
LOCATION I AREA I 2 YR I 100 YRI 2 YR I 100 YR
Southwestern Blvd.
As you can see, we are able to duplicate the original Kimley Horn
results quite closely (and that the Halff figures are somewhat
different).
Summit Office Park
1300 Summit Ave., Suite 712
Ft. Worth, Texas 76102
(817) 332-5756
FAX (817) 336-69O9
Centerpoint Two
616 Six Flags DHve, Suite 400
Arlington, Texas 76011
(817) 649-8530 · Metro (817) 640-8535
FAX (817) 63,3-5240
NafionsBonk
705 W. Ave. B, Suite 201
Garland, Texas 75040
(214) 272-4655
FAX (214) 272-4655
October 20, 1995
Mr. Ken Griffin
page two
We then re-modeled the Grapevine Creek watershed using more acc-
urate discharge / storage information derived from our on-site
topographic data and hydraulic model. The rest of the model was
un-altered. We also determined the discharge / storage inform-
ation for the proposed condition and re-ran the HEC-1 modeling,
again changing only the flood routing data.
I KIMLEY HORN I REVISED EXIST. I PROPOSED
LOCATION I AREA I 2 YR I 100 YRI 2 YR I 100 YRI 2 YR I 100 YR
Southwestern I 6.27 I 3841 I 11645 I 3839 I 11645 I 3839 I 11645
As you can see, both the revised existing and proposed condition
discharges are lower than those in the original KimleY Horn report.
This is to be expected, since both these conditions have more on-
site floodplain storage than in the Kimley Horn model. The pro-
posed condition does reduce the floodplain storage approx. 3.8'6%,
however there is no hydrologic effect at Southwestern Rd.
A comparison between the flood routing data used in the original
Kimley Horn modeling, our revised existing and proposed condition
flood routing data is shown on the enclosed graph. The proposed
condition increases the 100 year floodplain storage 15.88% as com-
pared to the original Kimley Horn model.
According to the Halff report, the Corps of Engineers recommends
a maximum of 15% reduction in floodplain storage at the 100 year
flood for minor tributaries having less than 10 square miles of
drainage area (page II-8 parag. 4). Our proposed plan D2 is in
keeping with this recommendation.
Also enclosed is a 100 year water surface comparison table using
the various hydraulic and hydrologic conditions.
Do not hesitate to call if you have any questions.
Sincerely, /'~ ~_
Jeff Williams, P.E.
Vice President
Graham AssoCiates Inc.
cc Bruce Heller
BAPTIST FOUNDATION PROJECT -
GRAPEVINE CREEK
CITY OF COPPELL, TE~
100 YEAR ULTIMATE DISC
EAST TRACT
AS
HARGES
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
1991 AHA REPORT
DISCHARGI WATER
cfs ISURFACE
11250 I 495.36
11250 I 495.45
BETHEL
11250 495.56
11250 495.68
not modeled'as
split flow
5.14
5.2
5.21
5.24
5.3
69
75
81
91
111
6.0
30130
END OF
I
11250 I 502.60 I
REVISED EXIS
DISCHARG
cfs
10846
10846
10846
10846
10846
10846
10846
10846
10846
10846
10846
10846
TING
W~. TER
SU~ .FACE
4c~5.00*
4c~5.11
ROAD
4c~
4c.
4~
4~.
4c
4c.
4.~
4c.
5(
5(
start of m~
2646I 5
264615
264615
woo.
264615(
264615(
264615(
woo(
2646I
26461
I
start of
8200
8200
8200
8200 51
8200 51
FLOW
5.24
5.34
.6.19
.6.44
~6.61
17.71
,9.17
,9.52
~0.09
~1.51
10-17-95
PROPOSED COND. D2
DISCHARG
cfs
10960
10960
WATER
SURFACE
495.08*
495.18
10960
10960
10960
10960
10960
10960
10960
10960
10960
10960
495.31
495.41
496.24
496.48
496.65
497.76
499.18
499.63
500.17
501.65
10960 I 502.10
10960 I 503.34
10846 I 5~)1.15 I
10846 I 5~)3.96 I
SPLIT
Lin channel flow split
~1.74 I 4170 I 501.91
~1.75 I 4170 I 501.92
~1.76 I 4170 I 501.96
[en footbridge
)1.76 I 4170 I 501.96
)1.76 I 4170 I 501.96
)1.74 I 4170 I 501.91
[en footbridge
591-67 I 4170 I 501.74
5~1.66 I 4170 I 501.71
bypass
581.45
5~)1.55
5~)1.59
)1.85
)2.05
channel flow splitl
6790 501.58
6790 501.88
6790 501.92
6790 502.09
6790 502.23
PAGE 2
GRAPEVINE CREEK
CITY OF COPPELL, TEXAS
100 YEAR ULTIMATE DISCHARGES
10-17-95
SECTION
NUMBER/
LOCATION
30189
30201
30250
31310
31480
31500
31550
31750
32740
1991 AHA REPORT
DISCHARG I WATER
cfs I SURFACE
11250 I 502.51
RAILROAD
11250I 503.43I
112501504.59I
112501 506.74I
11700I 506.841
REVISED EXISTING
DISCHARGI WATER
cfs I SURFACE
10846 I 503.79
10846 I 504.32
10846 I 505.26
10846 I 506.79
11645 I 506.84
SOUTHWESTERN BLVD.
11700I 507.04 I 11645 I 507.04 I
11700I 508.22 I 11645 I 508-20 I
117oo I 508.48 I 11645 I 508.46 I
11700I 508.97I 116451 508.95I
PROPOSED COND. D2
DISCHARGI WATER
cfs ISURFACE
10960 I 503.20
10960 I 503.86
10960 I 504.88
10960 I 506.64
11645 I 506.74
11645 I 506.94
11645 I 508.09
11645 I 508.37
11645 I 508.87
* STARTING WATER SURFACE CALCULATED BY HEC-2 USING SECTION AREA
AND ESTIMATED SLOPE OF ENERGY GRADE LINE.
NOTE ;
1991 AHA REPORT IS 'A CITY-WIDESTORM WATER MANAGEMENT
PLAN FOR THE CITY OF COPPELL, TEXAS' BY ALBERT H. HALFF
ASSOCIATES, INC. DATED JANUARY 1991. THE HALFF MODEL
HAS NO CROSS SECTIONS BETWEEN SECTIONS 27620 AND 30130.
ALSO, THE ULTIM~TE DISCHARGES IN THE HALFF REPORT ARE
SUPPOSEDLY FOR ~ODELING BY KIMLEY-HORN AND ASSOC. (1990)
HOWEVER THEY DO NOT AGREE EXACTLY.
THE REVISED EXISTING MODELING REFLECTS AS-BUILT CONDITIONS
IN THE DALLAS COUNTY PARK PROJECT. THE REVISED EXISTING
HYDROLOGY INCLUDES A MORE ACCURATE ACCOUNTING OF ON-SITE
FLOODPLAIN STORAGE THAT INCLUDES THE STORAGE IN THE 'POND'.
THE ONLY CHANGE BETWEEN OUR ULTIMATE CONDITION HYDROLOGIC
MODELING AND KIMLEY-HORN'S IS IN THE ON-SITE FLOODPLAIN
STORAGE ROUTING.
THE PROPOSED CONDITION MODELING REPRESENTS PLAN D2 AND
DOES NOT COUNT FLOODPLAIN STORAGE ON THE LOTS OR STREETS
IN THE DEVELOPMENT. IT DOES COUNT NEARLY ALL THE STORAGE
IN THE 'POND' SINCE THIS PLAN DOES NOT PROPOSE FILLING
THIS AREA.
IGraham Associates, Inc.
~TING ENGINEERS & PLANNERS
By g"' ~" //~"'?~ ~
Data //~ -'//7-- 9,~ Sheet of