Coppell Industrial-CS001114 (2)Graham A iates, Inc.
CONSULTING ENGINEERS & PLANNERS
Centerpoint Two Suite 400/616 Six Flags Drive
Arlington, Texas 76011 - Metro 640-8535
November 14, 2000
Mr. Mike Martin, P.E.
City of Coppell - Engineering
P.O. Box 478
Coppcll, Texas 75019
Re: Coppell Industrial Addition - Tradcpoint Ph~.se 1
Proposed by Champion Partners
Dear Mr. Martin,
Please consider this letter as our technical hydrologic calculations for the proposed
distribution facility on Bethel Road north of the USPS facility. Due to the present
drainage inadequacies on the downstream (east) property (oxvned by Duke Weeks,
Realty), we are proposing a detention pond with a pumped discharge.
The City's stated criteria is to have the developed condition 100-year flood discharge no
larger than 150 cfs, based on the existing condition (undeveloped) discharge. Also, the
Cny has stated that the proposed conditions discharges can not exceeded the existing
for the, 2 mid 5-year storms. As discussed previously with the City, we propose to empty
the detention pond by pumping the water out and discharging it along our eastern
property line thru a multiple outlet header pipe being approximately 400 feet long. Our
Oetention pond analysis is based on the rational method (Q=CIA) and the. City of Dallas's
detention calculation procedure.
Figure 1 shows generalized inflow and outflow hydrographs for a detention pond with a
pun'.ped outlet. Note; the pump has a constant discharge, regardless of the water level in
the pond, unlike a pond with a gravity outlet which has a variable discharge with respect
to tl,e pond's water level. The difference between the 2 hydrographs represents the
storage volmne. In our analysis, the pumps were not turned on until the discharge
coming into the pond exceeded the pump discharge rate.
The volume between the 2 hydrographs can be e'~sily calculated if we consider it to be a
trapezoid and a triangle.
S = ((Qi - Qo) x ((Td - Tc) + (Td - Tc + 2 x (Tc - To)) / 2) + (Qo x To / 2)) x 60
S = ((Qi - Qo) x ((Td -- Tc + Td - Tc + 2xTc - 2xTo)) / 2) + (Qo x To / 2)) x 60
S = ((Qi - Qo) x ((2xTd - Tc - Tc + 2xTc - 2xTo)) / 2) + (Qo x To / 2)) x 60
S = ((Qi - Qo) x ((2xTd - 2xTc + 2xTc - 2xTo)) / 2) + (Qo x To / 2)) x 60
S = ((Qi - Qo) x ((2xTd - 2xTo)) / 2) + (Qo x To / 2)) x 60
S = ((Qi - Qo) x (Td - To) + (Qo x To / 2)) x 60
Where;
S is the required storage in cubic feet
Qi is the maximum discharge in to the detention, in cfs
Qo is the pumped discharge out of the detention, in cfs
Td is the storm duration, in min.
Tc is the time to the peak inlet discharge, in min.
To is the time the pumps come on, in min.
Only Tc and Qo are known, however To and Qi can be calculated for any given Td.
To = Tc x Qo / Qi and Qi = C x I x A
Where A is the drainage area in acres, C is the weighted runoff coefficient and I is the
rainfall rate in inches per hour, Remember, I is a function of the storm duration, Td.
Td has to be greater than Tc and can be as long as 24 hours (1440 min.). However, ifTd
is long enough, Qi will be less then Qo, therefore no storage xvould be required. Also, a
very short Td does not generate a very large runoff volume and would require a very
small storage volume. Clearly, there is a Td that results in maximum required storage.
Determining this Td is a trial and error calculation. (The City of Dallas uses a trial and
error calculation in their methodology.)
According to the City's drainage manual, rainfall is to be based on TP-40. We have
provided copies of pages from TP-40 for the 2, 5 and 100-year rainfall for the ½, 1, 2, 3
and 6 hour stom~ durations. As you can see, reading rainfall amounts from these pages is
somewhat subjective, since the relatively few iso-rainfall lines do not exactly pass thru
Coppell. Table 1 shows our interpretation of the rainfall amounts and rates using the TP-
40 maps. Using this data, we performed a linear regression analysis of Log p and Log(Td
+ D), and determined the optimum valve of D so that the regression correlation
coefficient (r) is as close as possible to 1.0, (r = 1.0 or -1.0 would be a perfect
correlation). We determined rainfall equations in the form;
p = B / ((Td + D)^ E),
which is the same form used by TxDOT in their Hydraulic Manual. Table 1 also shows
the coefficients B, D and E, that we derived for each equation, as well as the p values
calculated by our equations. As you can see, the calculated values are quite close to the
rainfall rates from TP-40.
For our project, the existing condition 100-year discharge is 150 cfs (by agreement) and
A is 98.9 acres (see sht. C16) and C is 0.3 for the existing condition, therefore;
Q=CxlxA or I=Q/C/A or I=150/.3/98.9=5.056
Since I in the rational equation is the same as p in the rainfall equation;
I = p = 99.5 / (Tc + 6.5) ^ 0.7537 or Tc = (99.5 / p)" (1 / 0.7537)) - 6.5
Where p = 5.056 in./hr., Tc = 45.6 min., for the existing condition. Using this same
Tc, and our rainfall equations for the 2 and 5-year storms, we calculate the existing 2 and
5-year discharges to be 67.8 and 88.8 cfs, respectively.
For phase 1, we propose to construct an "L" shaped on-site detention pond as shown on
the grading plan (see sht. C13). This 'pond' will be normally wet, having a normal pool
of 506.0, which will be accomplished by pumping. We have included the elevation
verses storage graph for our proposed Phase 1 detention pond as Figure 3. No flood
storage volume was counted below elevation 506.0, even though the actual bottom of the
pond is elevation 504.0. The north-south portion of the phase 1 pond, along the eastern
property line, will remain in the ultimate design, however the outfall channel part,
parallel to Bethel Road, will be replaced in the ultimate design.
The proposed phase 1 pumps will consist of 2-14 cfs pumps, however only 1 pump will
be allowed to operate at a time. (The other 14 cfs pump being a spare.) Therefore, for
our storage volume calculation, Qo = 14. The proposed outlet discharge is less than the
existing 2, 5 and 100-year discharges. Also, we considered the proposed phase 1 time of
concentration to be 10 min., therefore Tc = 10. For the proposed phase 1 condition, the
weighted C was calculated using the following;
Developed area = 30.51 ac ~ C = 0.90
Pond area= 4.4 ac~C=l.00
Bethel ditch area = 1.28 ac @ C = 0.70
Undeveloped area = 62.71 ac ~ C = 0.30
Total area = 98.9 ac
Weighted C -- (30.51 x .9 + 4.4 x 1.0 + 1.28 x .7 + 62.71 x .3) / 98.9
C = 0.5214
The discharge into the pond, Qi, is defined as;
Qi = C x A x I or Qi = .5214 x 98.9 x I
The 100-year rainfall intensity, I, comes from our rainfall equation;
I = 99.5 / (6.5 + Td) ^ .7537
Substituting this into the above equation we get;
Qi = .5214 x 98.9 x 99.5 / (6.5 + Td) ^ .7537
Also, the time at which the pump tums on, To, is defined as;
To -- Tc x Qo / Qi or To = 10 x 14 / Qi
The required storage volume, S, is defined as;
S = ((Qi - Qo) x (Td - To) + (Qo x To / 2)) x 60
where Qo = 14 and Qi and To are functions of Td.
We tried several valves of Td such that S is maximized. The results of these trials are
shown below.
trial Td
in min.
180.
360.
410.
415.
420.
425.
500.
600.
calc. Qi
in cfs
99.72
59.93
54.42
53.94
53.46
52.99
46.96
41.00
calc. To
in min.
1.40
2.34
2.57
2.60
2.62
2.64
2.98
3.41
calc. S
in cf
919,164
986,622
989,179
989,367
989,289
989,178
984,158
967,908
maximum
We have enclosed a graph showing the inflow and outflow hydrographs for a storm of
duration 415 minutes (see Figure 2). Similar calculations for the 2 and 5-year storms,
also with a pumped discharge of 14 cfs, result in durations of 130 and 167 minutes and
maximum storage volumes of 314,695 and 452,457 cf, respectively. Using the phase 1
elevation verses storage graph, we have water levels in the detention pond of 511.2, 512.4
and 516.3 for the 2, 5 and 100-year storms (see Figure 3).
Please feel free to call myself or Chuck Stark with any questions you may have.
Sincerely,
Neal Chisholm P.E.
Graham Associates, Inc.
cc; Kerry Borden - Champion Partners
David Meinhardt - Meinhardt & Quintang
Table 1
Champion - Tradepoint Project
Rainfall Information
Duration
in min.
30.
60.
120.
180.
360.
2-year storm
amouml rate II amoum
(TP40) [ in./hr. II (TP40)
1.50
1.90
2.25
2.45
2.95
correlation I
coefficient[ .999860
B [ 50.6
D [ 6.0
E I 0.7856
II 5-year storm [ll00-year storm[
rate ]l amount l rate I
in./hr. II (TP40) I in./hr-I
3.00 [I 1.95 3.90 II
1.90 II 2.50 2.50 II
1.12511 3.00 1.50 II
0.81711 3.25 1.08311
0.492[I 3.90 0.65 I[
II II
[I .999872
Il 74.9
II 8.5
[I 0.8069
3.25 16.50
4.20 [4.20
5.20 12.60
5.70 11.90
6.90 11.15
........... I ........
II .999902
II 99.5
[I 6.5
II 0.7537
Duration I 2-year storm II 5-year storm II100-year storm
in min. I calculated rate II calculated rate II calculated rate
I in./hr. II in./hr. [I in./hr.
30. I 3.031 II 3.937 II 6.612
60. I 1.882 I1 2.473 II 4.207
120.I 1.133 [I 1.489 II 2.591
180.I 0.834 II 1.093 Il 1.934
360.[ 0.490 II 0.636 It 1.162
0
0
EXAMPLE
Time, in min.
Inflow ---- OutfloTM
5O
o 40
o 30
2O
10
0 200 400 600
Tradepoint - Phase 1
8O0
Time, in min.
1000 1200 1400
[,--~.-Inflow --m--Outflow
1600
TRADEPOINT phase 1
517
515
z 513
O
X
511
5O9
507
5O5
200000
400000
600000 800000 1000000
STORAGE VOLUME (c0
1200000
1400000
1600000
t-a-- Series2 i