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Section 8 - Wastewater Storage Pond Design

Use this section if your operation is a CAFO and requires storage of  stormwater runoff. This will help you calculate, roughly, the necessary storage volume. For a more accurate analysis, contact your local NRCS office.

Step 1. Calculate runoff volume

A) Calculate the total area that contributes to stormwater runoff and compute a weighted curve number (CN).

NOTE: Keep your runoff producing area to a minimum by diverting clean water from outside contributing drainage areas, roofs, and covered lots. This will minimize the amount of runoff you have to control.

Storm Runoff Producing Areas
Source Area (sq. ft.) CN Product (CN x Area)
Roof or covered lots   98  
Paved open lots   98  
Unpaved open lots   91  
Contributing drainage area      
Totals      


 Weighted CN = Total Product ÷ Total Area = __________

B) Determine the 25-year, 24-hour rainfall value for your location by using a NOAA Atlas map. You can obtain the map from your local NRCS office. You may have to interpolate between lines on the map to determine the rainfall value for your operation.

C) Determine the 25-year 24-hour runoff value using the weighted curve number value calculated above. Use the following table . You may have to interpolate between numbers to determine runoff value.

 Runoff Values in Inches for the 25 Year, 24 Hour Storm Event for Various Curve Numbers 
  Curve Number
Rainfall 40 45 50 55 60 65 70 75 80 85 90 95
1.0 0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.1  0.2 0.3 0.6 
1.2 0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.1  0.2  0.3 0.5 0.7 
1.4 0.0  0.0  0.0  0.0  0.0  0.0  0.1  0.1  0.2  0.4 0.6 0.9 
1.6 0.0  0.0  0.0  0.0  0.0  0.1  0.1  0.2  0.3  0.5 0.8 1.1 
1.8 0.0  0.0  0.0  0.0  0.0  0.1  0.2  0.3  0.4  0.7 0.9 1.3 
2.0 0.0  0.0  0.0  0.0  0.1  0.1  0.2  0.4  0.6  0.8 1.1 1.5 
2.5 0.0  0.0  0.0  0.1  0.2  0.3  0.5  0.7  0.9  1.2 1.5 2.0 
3.0 0.0  0.0  0.1  0.2  0.3  0.5  0.7  1.0  1.3  1.6 2.0 2.5 
3.5 0.0  0.1  0.2  0.4  0.5  0.8  1.0  1.3  1.6  2.0 2.5 2.9 
4.0 0.1  0.2  0.3  0.5  0.8  1.0  1.3  1.7  2.0  2.5 2.9 3.4 
4.5 0.1  0.3  0.5  0.7  1.0  1.3  1.7  2.1  2.5  2.9 3.4 3.9 
5.0 0.2  0.4  0.7  1.0  1.3  1.7  2.0  2.5  2.9  3.4 3.9 4.4 
6.0 0.5  0.8  1.1  1.5  1.9  2.4  2.8  3.3  3.8  4.3 4.9 5.4 
7.0 0.8  1.2  1.7  2.1  2.6  3.1  3.6  4.2  4.7  5.3 5.8 6.4 
8.0 1.3  1.7  2.3  2.8  3.3  3.9  4.5  5.0  5.6  6.2 6.8 7.4 
9.0 1.7  2.3  2.9  3.5  4.1  4.7  5.3  6.0  6.6  7.2 7.8 8.4 
10.0 2.2  2.9  3.6  4.2  4.9  5.6  6.2  6.9  7.5  8.2 8.8 9.4 
11.0 2.8  3.5  4.3  5.0  5.7  6.4  7.1  7.8  8.5  9.1 9.8 10.4 
12.0 3.4  4.2  5.0  5.8  6.6  7.3  8.1  8.8  9.5  10.1 10.8 11.4 
13.0 4.0  4.9  5.8  6.6  7.4  8.2  9.0  9.7  10.4  11.1 11.8 12.4 
14.0 4.7  5.6  6.6  7.4  8.3  9.1  9.9  10.7  11.4  12.1 12.8 13.4 
15.0 5.3 6.4 7.4 8.3 9.2 10.0 10.9 11.6 12.4 13.1 13.7 14.4

Reference: EFM 2, Table 2-2.

D) Compute required runoff storage volume.

Multiply drainage area from Part A by the 25-year, 24-hour runoff value from Part C.
 
_______________ x ________________  x 12 inches/ft =  _________________
drainage area
(sq. ft.) 		runoff value
(inches)  		  runoff storage volume
(cubic ft.)

 

E) Compute runoff volume from normal precipitation.  Reference Appendix A for runoff values.

 
_______________ x ________________  x 12 inches =  _________________
drainage area
(sq. ft.) 		Avg. Normal Runoff
(inches)  		1 foot runoff volume
(cubic ft.)

Step 2. Calculate total storage volume

A) Add process wastewater volume and clean water volume to runoff volume.

Source Volume 
(cubic ft.)
Runoff volume (Normal Precipitation)  
Runoff volume (25-yr., 24-hr. Precipitation)  
Process wastewater volume  
Clean water volume  
Total water volume  

B) Calculate volume of manure and other solids that enter the pond.

Waste produced daily (cubic feet)/1,000 lb. cow = 1.30
 
Source Volume
(cubic ft.)
Manure  
Other solids, bedding, etc.  
Total solid volume  

C) Calculate total storage volume.

_______________ x ________________ = _________________
total water volume
(cu. ft.)		 total solid volume 
(cu. ft.)  		total storage volume
(cu. ft.)

Step 3. Size the pond

 Make sure the volume is big enough to hold the total storage volume calculated in Step 2C.

Sideslopes:
 Clay liners need 4:1 side slopes (Z = 4).
PVC or PE liners need 3:1 side slopes (Z = 3).
Natural earth excavation side slopes are from 2:1 to 3:1 (Z = 2 or 3).

For a rectangular pond, use the following equation to calculate volume:
 V = [(4 x Z2 x d3) / 3] + (Z x BL x d2) + (Z x BW x d2) + (BW x BL x d)

V = volume
BL = bottom length
Z = sideslope ratio
BW = bottom width
d = depth

Start out by guessing how big your pond should be, plug those numbers in, and compute the storage volume. Compare your answer with the required storage volume from Step 2c.

Z =
d =
BL =
BW =
V = __________

Adjust the size and try again.

Z =
d =
BL =
BW =
V = __________

You're getting close! Try again.

Z =
d =
BL =
BW =
V = __________

Step 4. Adjust pond depth

Adjust pond depth
Adjustment Depth
(ft.)
Depth (ft) from Step 3  
Depth of normal precipitation less evaporation for the storage period (use table from Step 1E for normal precipitation and the table in Appendix B for evaporation)  
Depth of 25-year, 24-hour storm (from Step 1B)  
Freeboard (1 ft minimum) 1 
Dead storage (0.5 ft.) 0.5
Final depth (total)  

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