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Date: 0411412020
Ca/cs by:JTW
Civil Engineering Department
200 Broad Street Boise, Idaho 83702 (208)343-4635
Seepage Bed 1
Determine the retention volume required.
Stormwater Runoff 000-year, 1-hour storm event)
Runoff Coefficients:
Table 1 C = 0.9000 Heavy Commercial
Developed Drainage Area:
Table 2 1A= 0.5324 ac Heavy Commercial
1 B = 0.5869 ac Heavy Commercial
Peak Discharge:
Peak discharge for the 100 year, 10 minute event(2.58 in/hr)
Q1Apeak = C*I*1A
where: Q1peak = peak discharge rate to the sand and grease trap (cfs)
C = ratio of rainfall to runoff coefficient
(See Table 1 above)
I = 2.58 in/hr(10 minute time of concentration min.)
1A= drainage area size in acres
(See Table 2 above)
Q 1 Apeak = 1.236 cfs
Q1 Bpeak — C*1*1 B
where: Q1Bpeak = peak discharge rate to the sand and grease trap (cfs)
C = ratio of rainfall to runoff coefficient
(See Table 1 above)
I = 2.58 in/hr(10 minute time of concentration min.)
1 B = drainage area size in acres
(See Table 2 above)
Q2peak = 1.362 cfs
Peak Velocity:
Peak Velocity through a 1000 gallon sand and grease trap for the peak discharge event for Al.
V 1 Amax= Q 1 Apeak/Asand/grease
where: Vmax= velocity through sand and grease traps (ft/s)
Q1Apeak = peak discharge rate to the sand and grease traps (cfs)
Asand/grease = cross-sectional area between baffles (20" by 50.5", 7.01 ft`)
V 1 Amax= 0.18 ft/s
since Vmax is less than 0.5 fps, ok
Civil Engineering Department
200 Broad Street Boise, Idaho 83702 (208)343-4635
Peak Velocity through a 1000 gallon sand and grease trap for the peak discharge event for A2.
V1 Bmax = Q1 Bpeak/Asand/grease
where: V1 Bmax= velocity through sand and grease traps (ft/s)
Q1Bpeak = peak discharge rate to the sand and grease traps (cfs)
Asand/grease = cross-sectional area between baffles (20" by 50.5", 7.01 ft`)
V1 Bmax = 0.19 ft/s
since Vmax is less than 0.5 fps, ok
Required Storage Volume:
Depth Considerations:
Determine the dimensions required to retain the required volume, final depth must agree with geotech
recommendations.
Volume generated by the 100 year, 1 hour event(0.96 in/hr)
Vloo = C*I*A*t*1.25
where: V100 = volume to the seepage bed +25% for sedimentation (cf)
C = ratio of rainfall to runoff coefficient (See Table 1 above)
I = 0.96 in/hr
A= drainage area size in acres (See Table 2 above)
t= 3600 seconds/hr
V100 = 4352 cf(increase 25%for sedimentation)
Anticipated infiltration rate of pit run is 8 in/hr(0.667 ft/hr).
i = 880 infiltration during event (cf)
Vreq = V1 ON
Vreq = 3472 cf
Pipe Length: 66 ft
18 dia. Pipe Area: 1.767 sf/ft
Pipe Volume: 116 cf
Rock Length: 66 ft
Rock Width: 20 ft
Rock Depth: 6.5 ft
Rock Volume: 8580 cf
Rock Volume excluding Pipe: 8464 cf
Void Volume: 3385 cf 40% of volume
Storage Volume: 3501 cf
Since storage volume is greater than Vreq the volume provided is adequate.
Civil Engineering Department
200 Broad Street Boise, Idaho 83702 (208)343-4635
Percolation:
Anticipated infiltration rate of pit run is 8 in/hr(0.667 ft/hr).
If free draining soil is encountered on entire bed bottom, determine how long it takes to drain design storm.
T = V/(P *A)
where: T = time to percolate design storm (hr)
P = percolation rate 8 in/hr(0.667 ft/hr)
A= effective surface area at bottom of seepage bed = 1320 sf
V= 3472 cf
T = 4.50 hr
Seepage bed will drain the design storm in 4.5 hr.
If free draining soil isn't encountered for entire bed bottom, determine minimum window size required.
A= V/(T* P)
where: T = time to percolate design storm (24 hr)
P = percolation rate 8 in/hr(0.667 ft/hr)
V= 3472 cf
A= Area of drainage window
A= 216 sf
Minimum drain window size is 216 sf.
Civil Engineering Department
200 Broad Street Boise, Idaho 83702 (208)343-4635
Seepage Bed 2
Determine the retention volume required.
Stormwater Runoff 000-year, 1-hour storm event)
Runoff Coefficients:
Table 1 C = 0.9000 Heavy Commercial
Developed Drainage Area:
Table 2 2A= 1.3380 ac Heavy Commercial
213 = 1.1530 ac Heavy Commercial
Peak Discharge:
Peak discharge for the 100 year, 10 minute event(2.58 in/hr)
Q2Apeak — C*1*2A
where: Q2Apeak = peak discharge rate to the sand and grease trap (cfs)
C = ratio of rainfall to runoff coefficient
(See Table 1 above)
I = 2.58 in/hr(10 minute time of concentration min.)
2A= drainage area size in acres
(See Table 2 above)
Q2Apeak = 3.106 cfs
Q213peak — C*1*213
where: Q213peak = peak discharge rate to the sand and grease trap (cfs)
C = ratio of rainfall to runoff coefficient
(See Table 1 above)
I = 2.58 in/hr(10 minute time of concentration min.)
2B drainage area size in acres
(See Table 2 above)
Q2Bpeak = 2.677 cfs
Peak Velocity:
Peak Velocity through a 1000 gallon sand and grease trap for the peak discharge event for Al.
V2Amax= Q2Apeak/Asand/grease
where: Vmax= velocity through sand and grease traps (ft/s)
Q2Apeak = peak discharge rate to the sand and grease traps (cfs)
Asand/grease = cross-sectional area between baffles (20" by 50.5", 7.01 ft`)
V2Amax= 0.44 ft/s
since Vmax is less than 0.5 fps, ok
Civil Engineering Department
200 Broad Street Boise, Idaho 83702 (208)343-4635
Peak Velocity through a 1000 gallon sand and grease trap for the peak discharge event for A2.
V2Bmax = Q213peak/Asand/grease
where: V2Bmax= velocity through sand and grease traps (ft/s)
Q26peak = peak discharge rate to the sand and grease traps (cfs)
Asand/grease = cross-sectional area between baffles (20" by 50.5", 7.01 ft`)
V2Bmax = 0.38 ft/s
since Vmax is less than 0.5 fps, ok
Required Storage Volume:
Depth Considerations:
Determine the dimensions required to retain the required volume, final depth must agree with geotech
recommendations.
Volume generated by the 100 year, 1 hour event(0.96 in/hr)
Vloo = C*I*A*t*1.25
where: V100 = volume to the seepage bed +25% for sedimentation (cf)
C = ratio of rainfall to runoff coefficient (See Table 1 above)
I = 0.96 in/hr
A= drainage area size in acres (See Table 2 above)
t= 3600 seconds/hr
V100 = 9685 cf(increase 25%for sedimentation)
Anticipated infiltration rate of pit run is 8 in/hr(0.667 ft/hr).
i = 1951 infiltration during event (cf)
Vreq = V100-I
Vreq = 7735 cf
Pipe Length: 133 ft
18 dia. Pipe Area: 1.767 sf/ft
Pipe Volume: 235 cf
Rock Length: 133 ft
Rock Width: 22 ft
Rock Depth: 6.5 ft
Rock Volume: 19019 cf
Rock Volume excluding Pipe: 18784 cf
Void Volume: 7513 cf 40% of volume
Storage Volume: 7748 cf
Since storage volume is greater than Vreq the volume provided is adequate.
Civil Engineering Department
200 Broad Street Boise, Idaho 83702 (208)343-4635
Percolation:
Anticipated infiltration rate of pit run is 8 in/hr(0.667 ft/hr).
If free draining soil is encountered on entire bed bottom, determine how long it takes to drain design storm.
T = V/(P *A)
where: T = time to percolate design storm (hr)
P = percolation rate 8 in/hr(0.667 ft/hr)
A= effective surface area at bottom of seepage bed = 2926 sf
V= 7735 cf
T = 4.53 hr
Seepage bed will drain the design storm in 4.53 hr.
If free draining soil isn't encountered for entire bed bottom, determine minimum window size required.
A= V/(T* P)
where: T = time to percolate design storm (24 hr)
P = percolation rate 8 in/hr(0.667 ft/hr)
V= 7735 cf
A= Area of drainage window
A= 483 sf
Minimum drain window size is 483 sf.
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