Coppell Ind 2-SY030701Detention Pond Study for
Tradepoint Business Park
Site Plan Submittal Package
Coppell, Texas
PRELIMINARY
(PENDING FINAL SITE LAYOUT)
Halff Associates, Inc.
July 2O03
AVO 21446
Prepared by Josh Logan, E.I.T.
Reviewed by Stephen Crawford, P.E.
Table of Contents
Introduction
Hydrologic Analysis
Required Detention Volume
Weir Outflow
Detention Pond Grading
List of Tables
Table 1. Maximum Flow Release
Table 2. Maximum Water Surface Elevation
Table 3. Actual Water Surface Elevation
Table Al. 100-yr Required Detention Volume
Table A2.
Table A3.
Table A4.
Table AS.
Table A6.
Table A7.
Table A8.
Table A9.
5-yr Required Detention Volume
2-yr Required Detention Volume
100-yr Storm Outflow
5-yr Storm Outflow
2-yr Storm Outflow
100-yr Storm Detention Volume
5-yr Storm Detention Volume
2-yr Storm Detention Volume
Page
1
1
2
3
4
2
3
4
Appx
Appx
Appx
Appx
Appx
Appx
Appx
Appx
Appx
Appx
List of Figures
Figure 1. Storm Drain Plan
Figure 2. Existing Outflow/Weir Structure
Figure 3. Maximum Allowable Water Surface Elevations
Figure 4. Detention Pond Plan
Figure 5. Actual Water Surface Elevations
Appx
Appx
Appx
Appx
Appx
Halff Associates, Inc. July 2003
Tradepoint Business Park
Stormwater Detention Pond Study
Introduction
The Tradepoint Business Park is a 98.84-acre site in Coppell, Texas (Dallas County) and zoned
PD-185 light industrial. A 500,000 sq. tt. warehouse presently exists in the southwest quadrant.
The current owner of the site (Hillwood Fund No. 1, L.P.) has requested that Halff Associates,
Inc. design utilities and prepare civil engineering plans for the proposed expansion of the
existing warehouse (Building 1) and the construction of an identical warehouse on the east side
of the site (Building 2). The design includes all water and sanitary sewer taps and services
required for the expansion and new building, grading of the site, and stormwater discharge.
An existing storm drain system currently collects the runoff and conveys it to an outfall channel
and detention basin. The detention basin releases excess runoff into the drainage system on the
adjacent property to the west (Duke-Weeks Realty Limited Partnership) per an easement
agreement (Special Warranty Deed, Vol. 2001026, Page 03950, Dallas County, Texas). A
horizontal weir with low flow orifices controls the outflow into the Duke drainage system. The
weir, detention pond, and storm drain system were constructed when the existing warehouse was
built (Figure 1). Halff Associates, Inc. has a copy of the original construction plans for the
drainage system, but no calculations as to how the pond was sized or the weir analysis are
included in these plans.
Hydrologic Analysis
The proposed additions to the site, and most notably Building 2, will require the existing outfall
channel to be filled in; thus the existing detention volume will be reduced. A hydrologic analysis
was necessary to determine the volume of runoff that will be created from a 100-year frequency
storm event so that the existing detention pond can be modified in order to detain the runoff.
The first step was to calculate the peak storm runoff. Per the City of Coppell Engineering
Design Standards, the peak storm runoff is calculated using the rational method, which states that
the highest flow rate occurs at the basin's time of concentration, to. The time of concentration for
the fully developed site is dependent upon the design of the stoma drain system. The longest total
time for any of the storm drain laterals will be used as the time of concentration. In this case, the
approximate time of concentration is 20 minutes.
The rational equation for flow is as follows.
Q = C*I*A
Where~
Q = flow rate in cubic feet per second (cfs);
C = unitless runoff coefficient that is based on the zoning district;
I = rainfall intensity in inches per hour; and
A = size of the drainage area, in acres;
AVO 21446 1
Halff Associates, Inc. July 2003
Since the site is zoned light industrial, a C factor of 0.9 is to be used according to the design
criteria. The intensity is calculated using the time of concentration per Technical Paper 40. For
atc of 20 minutes, the intensity is approximately 8.30 in/hr. Since the storm drain system
captures runoff for the entire site, the drainage area is the full 98.84 acres. Thus the rational
equation produces a peak flow rate of 738.3 cfs. The easement agreement for stormwater runoff
dictates the amount of flow that can be released from the site into the Duke property's drainage
system. Table 1 below shows the maximum outflow rates for various frequency storms as
dictated in the agreement.
Table 1. Maximum Flow Release
Event 2-yr 5-yr 100-yr
Q (cfs) 78 95 153
Since the peak discharge calculated above exceeds the maximum allowable discharge, detention
of the stormwater is necessary.
The pond will be designed such that the outflow from various frequency storms will not exceed
certain flow rates outlined in the easement agreement. The pond's attributes will be determined
by two controlling factors: 1) The volume of runoff created by the proposed development and 2)
The flow rate of runoff being released into the Duke drainage system. This release rate is
currently regulated by a weir structure with low flow orifices (Appendix, Fig. 2). This weir
structure will be used to regulate the flow for the proposed pond as well. Thus the pond will
need to be expanded in order to detain the volume of nmoffproduced from the 100-year storm.
Required Detention Volume
This volume is calculated using the Modified Rational Method. The maximum storage volume
is determined by deducting the volume of runoff released during the time of inflow from the total
inflow from storms of increasing durations. The inflow is determined by first calculating the
intensities for storms of various durations and then computing the flow rates using the Rational
Method. The inflow is thus calculated by the following equation:
Inflow (cfs) = Storm Duration (min)* corresponding peak discharge (cfs)* 60 sec/min.
The outflow volume is calculated by taking half of the respective inflow duration (the storm
duration + the time of concentration) multiplied by the maximum allowable flow:
Outflow (cfs) = 0.5*(t0 + storm duration)*max, outflow *60 sec/min.
The calculations (Appendix, Table Al) resulted in a required 100-year storage volume of
approximately 1,035,500 cubic feet, or 23.77 acre-feet. It is also necessary to ensure that the
detention pond design satisfies the release requirements for the five and two-year storm events.
Therefore it is necessary to repeat the volume calculations for each of those events so that the
pool elevations can be determined and the weir outflow analyzed for the regulating flow rates.
These calculations can be found in the Appendix (Tables A2, A3).
AVO 21446 2
HalffAssociates, Inc. July 2003
Weir Outflow
The outflow is currently regulated by the horizontal weir mentioned above and has two 1' x 3'
orifices at the bottom for low flow. The bottom of the existing pond has an elevation of 508 feet,
thus the bottom of the weir and the orifices have been placed at this elevation. The weir is 5'
high and 11 '9" wide with a top elevation of 513 feet.
The next step in sizing the pond is to determine the maximum water surface elevation of the 100-
year storm. Since the water surface elevation (head) at the weir determines the outflow, it is
necessary to determine the head on the weir that will produce the maximum allowable discharge,
which, as stated previously, is 153 cfs for the 100-year storm. The head at the weir also controls
the flow through the orifices:
Qo = Cd*A*(2*g*ho)1/2
Where: Qo = Flow through the orifice in cfs;
Ca = Entry loss coefficient = 0.60;
A = Orifice area = 2'(1 'x 3') = 6 fi2;
g = gravitational acceleration constant = 32.2 fi/s2;
ho = head on the orifice = water surface elev. - 508 fi.
To determine the flow going over the weir based on the water surface elevation, the following
equation for flow over a horizontal weir was used:
Qw = Cw*L*hw3/2
Where: Qw = Flow over the weir in cfs;
Cw = Weir coefficient = 3.2;
L = Horizontal length of the weir = 11.75 fi;
hw= Head on weir = water surface elev. - 513 ft.
Thus the total outflow from the weir structure is the sum of the orifice flow and the weir flow. A
rating curve spreadsheet was used to calculate the outflow by inserting the water surface
elevation and incrementally increasing it by one foot until the maximum allowable outflow was
achieved (Appendix, Table A4). The resulting maximum 100-year water surface elevation is
514.60 feet.
This calculation was repeated for the five and two-year storm frequencies that have maximum
outflows of 95 cfs and 78 cfs, respectively (Appendix, Tables A5, A6). The resulting maximum
water surface elevations for each storm event are shown below and in Figure 3 in the Appendix.
Table 2. Maximum Water Surface Elevation
Event 2-yr 5-yr 100-yr
Q (cfs) 78 95 153
Max, WSE 513.4 513.7 514.6
AVO 21446 3
HalffAssociates, Inc. July 2003
Detention Pond Grading
With the two controlling factors determined, the pond can be sized such that it is large enough to
detain a minimum 100-year volume of 1,035,500 fi3 while the water surface elevation cannot
exceed 514.60 feet. The bottom of the pond must be graded such that its minimum elevation is
508 feet at the outlet structure. The side slopes will remain at the 4H:IV minimums established
with the existing pond. The upper edge of the pond will be determined using this minimum
grade and the existing topography of the site. Figure 4 in the Appendix shows the grading plan
for the detention pond.
To calculate the volume of the pond, the average end area method was used with the proposed
contours. The volume is calculated by taking the average of the surface areas between two
contours and multiplying by the difference in elevation. Table A7 (Appendix) shows these
calculations and sums up the volumes cumulatively as the depth increases.
Table A7 shows that the cumulative volume at the 514 fi. contour is around 928,000 ft~, while
the cumulative volume at the 515 fi. contour is approximately 1,127,000 fi3. This shows that the
target volume of 1,035,500 will occur when the water surface elevation is somewhere between
the 514 and 515 foot elevations, so the surface areas of these two contours were averaged to
obtain the surface area of the 514.5 foot elevation. The cumulative volume of the pond when the
water surface is at 514.5 feet is around 1,040,000 fi3, which is greater than the target volume and
also occurs 0.1 feet lower than the maximum water surface elevation of 514.6 feet. This shows
that the detention pond size contains adequate storage and will not produce a greater outflow
than is required.
Similarly, the required detention volumes for both the five and two-year storm events were
checked with their respective maximum water surface elevations against the volume calculations
of the proposed detention pond (Appendix, Tables A8, A9). In each case, the elevation at which
the required detention volume was met was well below the maximum water surface elevation
(Appendix Fig. 5, Table 3 below).
Table 3. Actual Water Surface Elevations
Event 2-yr 5-yr 100-yr
Vlax. Q (cfs) 78 95 153
Vlax. WSE 513.4 513.7 514.6
~,ctual WSE 511.56 512.68 514.48
~,ctual Q 54.51 62.5 141.24
AVO 21446 4
APPENDIX
Detention Pond Study
Tradepoint Business Park
Site Plan Submittal Package
AVO 21446
Table Al. lO0-yr Required Detention Volume
Runoff Coefficient C
Drainage Area - A
Time of Concentration - tc
Maximum OUtT'Iow Rate - Q
acres
minutes
cfs
Inflow Inflow Outflow Outflow Storage
DurationiIntensity Depth Discharge Volume Duration Volume Volume
/minute.,inches/h inches Q=CiA Cu. Ft. (minutes) Cu. Ft. Cu. Ft.
5 11.6 0.97 1031.9 309,567 25 114,750 194,817
1(3 11.{ 1.93 1031.9 619,134 30 137,700 481,434
15 9.6(3 2.40 854.0 768,580 35 160,650 607,930
2(3 8.3(3 2.77 738.3 886,002 40 183,600 702,402
3(3 6.60 3.30 587.1 1,056,797 50 229,500 827,297
40 5.50 3.67 489.3 1,174,219 60 275,400 898,819
50 4.80 4.00 427.0 1,280,966 70 321,300 959,666
60 4.20 4.20 373.6 1,345,015 80 367,200 977,815
70 3.80 4.43~ 338.0 1,419,738 90 413,100 1,006,638
80 3.50 4.67 311.3 1,494,461 100 459,000 1,035,461
90 3.20 4.8(3 284.7 1,537,160 110 504,900 1,032,260
120 2.60 5.2(3 231.3 1,665,256 140 642,600 1,022,656
180 2.00 6.0(3 177.9 1,921,450 200 918,000 1,003,450
360 1.20 7.2(3 106.7 2,305,740 380 1,744,200 561,540
720 0.70 8.4(3 62.3 2,690,029 740 3,396,600 (706,571)
1440 0.40 9.55 35.4 3,058,307 1,460 6,701,400 (3,643,093
Required Storage Volume 1,035,461 cubic feet
23.77 acre-feet
Halff Associates, Inc.
Table A2. 5-yr Required Detention Volume
Runoff Coefficient C =
Drainage Area - A
Time of Concentration - tc =
Maximum Outflow Rate - Q
acres
minutes
Inflow Rate - Qin = 435.9 cfs
Inflow Inflow Outflow Outflow Storage
Duration Intensity Depth 3ischarg( Volume Duration Volume Volume
(minutes (inches/h~(inches Q=CiA Cu. Ft. (minutes) Cu. Ft. Cu. Ft.
10 6.90 1.15 613.8 368,278 30 70,200 298,078
15 5.70 1.43 507.0 456,344 35 81,900 374,444
20 4.90 1.63 435.9 523,061 40 93,600 429,461
30 3.90 1.9~ 346.9 624,471 50 117,000 507,471
40 3.30 2.20 293.6 704,532 60 140,400 564,132
50 2.80 2.33 249.1 747,230 70 163,800 583,430
6(3 2.50 2.50 222.4 800,604 80 187,200 613,404
7(3 2.2(3 2.57 195.7 821,953 90 210,600 611,353
8(} 2.0(} 2.67 177.9 853,978 100 234,000 619,978
90 1.9(} 2.85 169.0 912,689 110 257,400 655,289
120 1.50 3.00 133.4 960,725 140 327,600 633,125
180 1.10 3.30 97.9 1,056,797 200 468,000 588,797
360 0.70 4.20 62.3 1,345,015 380 889,200 455,815
720 0.40 4.80 35.6 1,537,160 740 1,731,600 (194,440)
1440 0.20 4.80 17.8 1,537,160 1,460, 3,416,400 (1,879,240)
Required Storage Volume 655,289 cubic feet
15.04 acre-feet
Halff Associates, Inc.
Table A3.2-yr Required Detention Volume
Runoff Coefficient C =
Drainage Area - A =
Time of Concentration - tc =
Maximum Outflow Rate - Q =
acres
minutes
cfs
Intensity (2-yr) = ~
Inflow Rate - Qin = 338.0 cfs
Inflow Inflow Outflow Outflow Storage
Duration Intensity Depth Discharge Volume Duration Volume Volume
minutes inches/h inches: Q=CiA Cu. Ft. minutes) Cu. Ft. Cu. Ft.
10 5.4(3 0.90 480.4 288,217 30 70,200 218,017
15 4.5(3 1.13 400.3 360,272 35 81,900 278,372
20 3.8(3 1.27 338.0 405,639 40 93,600 312,039
30 3.0(3 1.50 266.9 480,362 50 117,000 363,362
40 2.5(3 1.67 222.4 533,736 60 140,400 393,336
50 2.2(3 1.83 195.7 587,110 70 163,800 423,310
60 1.9~ 1.90 169.0 608,459 80 187,200 421,259
70 1.7(3 1.98 151.2 635,146 90 210,600 424,546
80 1.6(3 2.13 142.3 683,182 100 234,000 449,182
90 1.4(3 2.10 124.5 672,507 110 257,400 415,107
120 1.2(3 2.40 106.7 768,580 140 327,600 440,980
180 0.90 2.70 80.1 864,652 200 468,000 396,652
360 0.50 3.00 44.5 960,725 380 889,200 71,525
720 0.30 3.60 26.7 1,152,870 740 1,731,600 (578,730)
1440 0.20 4.80 17.8 1,537,160 1,460 3,416,400 (1,879,240
Required Storage Volume 449,182 cubic feet
10.31 acre-feet
Halff Associates, Inc.
TABLE A4. 100-yr Storm Outflow
OUTLET STRUCTURE:
Orifice Height
No. of Orifice
Orifice Bottom Elev.
Weir Length
Target Q =
ft
riflce Area =
Orifice C =
Weir C =
153 cfs Weir Elev =
ft2
ft
o [ o.oo L o
~0~ ~8.890 ~ 0~00 [ ~8.89
510 40~856 [~i0~ I 40186
51i 50.039 F0.0~I 5010~
51~ 57.7~ ! 0.00 [ 57.?~
~ 6~.60~ [ 0.00 [ 6~.60
5~
~ ' ~42 73
5~4.5
5~4.7S 75.058 [ 07.05
~ ?6.435 l 106.35 l~2.78
~6 ~713 [~95.38 ~277.0~
~17 8~.670 [300.80 ~8~.47
5i~ 91.358 [420.38 I 5~.7~
Halff Associates, Inc.
TABLE A5. 5-yr Storm Outflow
OUTLET STRUCTURE:
Orifice Height
No. of Orifice
Orifice Bottom Elev.
Weir Length
Target Q =
ft
ff
ft
95 cfs
)rifice Area =
Orifice C =
Weir C =
Weir Elev =
ft
Halff Associates, Inc.
TABLE A6.2-yr Storm Outflow
OUTLET STRUCTURE:
Orifice Height =
No. of Orifice =
Orifice Bottom Elev. =
Weir Length =
Target Q =
ft -ifice Area
Orifice C
ft
ft Weir C =
78 cfs Weir Elev
ft2
508 0 I 0.00 0
509 28.890
510 40.8~6 l 010(3 401~6
511 50.039 I 0.00 50.04
512 57.780 I 0,00 57.78
513 64.600I0.00 64.60
513.5 67.753 [ 13.29 81.05
~4 70.765 [ 37.60 '108.~
,5~ 76.435 I 106.35 1~Z78
5i6 81.713 I 195.38 277.09
i86.670 ~ 300.80 387.4~
51~ 91.358 ! 420.38 511.74
Halff Associates, Inc.
Table AT. 100-yr Storm Detention Volume
Elevation Surface Area Volume Cumulative Volume
5O8
509
510
511
512
513
514
10227.,"
125060.£
144189,6
162388.8
181132,4
200135.0
219451,5
67643.7,"
134624.8£
153289.2£
171760.60
190633.70
209793,25
67643.75
202268.55
355557.75
527318.35
717952.05
927745.3C
Total Volume = 1961702.20
Halff Associates, Inc.
Table A8. 5-yr Storm Detention Volume
Elevation Su~ace Area Volume Cumulative Volume
508 10227.5
509 125060.0 67643.75 67643.75
510 144189.~ 134624.80 202268.5~
511 162388.8 153289.20 355557.7~
512 181132.4 171760.60 527318.35
513 200135.0 63070.27 717952.0~
514 219451.5 209793.25 927745.30
515 239013.3 229232.40 1156977.70
516 258289.5 248651.40 1405629,10
517 277497.9 267893.70 1673522.80
518 298860.9 288179.40 1961702.2~
Total Volume = 1961702.20
Halff Associates, Inc.
Table A9. 2-yr Storm Detention Volume
Elevation Su~ace Area Volume Cumulative Volume
508 10227.5
509 125060,0 676~.75 67643.75
510 144189,6 134624,80 202268.55
511 162388.8 153289.20 355557,75
512 181132.4 77883.88 527318.35
5i3 200135.0 190633.70 717952.05
5i4 219451.5 209793.25 927745.30
515 239013.3 229232.40 1156977,70
516 258289.5 248651.40 i40~629.i6
517 277497.9 267893.70 1673522.80
518 298860.9 288179.40 1961702,20
Total Volume = 1961702.20
Halff Associates, Inc.
~,EIVNIINI'I]~d
CaCO~mH !I!
s~xo& 'lloddoD jo
i~ 9NI(rlIflfl XItVd SS~tlqlSFIfl J.31IOcI~t(IV~
Z
EL. 516'
-:i"~ ' ;"; ' ~ "; ': 8'
~' x 3' ~,'
Figure 2. Existing OufflowN~ir Struclum
MAX. IO0-YR
~,x. ~-~. ~s~ \
· ' -EL, 513 ~~'
Figure 3. Maximum Wator Suffam
TRADEPOINT BUSINESS PARK
Coppell, Texas
]II HILD,VOOO
A~IYNII~II:~EId
~I14 Ill
s~xo£ 'lloddoD ~o
9NIG'IIfI~ )I~VcI SSEINISfl~ Z_NIOdEt(IV~LL
ACTUAL 100-YR ~/SE
ACTUAL 5-YR WSE ~
Figure 5. Actual Water Surface Elevations
a~,~'~ TRADEPOINT BUSINESS PARK D~O..o.o
~'I~V"- Coppell, Texas DESIGN
Ill H~W.VO~ II! HILLWOGD