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PONDS USER MANUAL
9-3
9.3
EXAMPLE #9 - DESIGN OF UNDERDRAIN POND
(contained in data file EXAMPLE9.DAT)
Assume that the pond described in example #4 is to be designed as an underdrain system
and an outfall is available to allow gravity flow of the subsurface drainage system. The
following are the model input parameters.
Pond & Stormwater Volume Parameters
Treatment Volume, PAV (cubic feet):
340,000 ft³
Area at Top of Pond, ATOP (square feet):
100,000 ft²
Vertical Distance From Natural Ground or Finished Grade to Pond Bottom, d (ft)
5 ft
Aquifer Parameters
Hydraulic Conductivity of Soil, K (ft/day):
Depth to Base of Aquifer From Pond Bottom, B (ft):
10 ft/day
7 feet
Drain Parameters
Drain diameter, D (inch)
Thickness of gravel envelope above pipe, t (inch)
Depth of soil cover on underdrain, H (ft)
6 inches
3 inches
2 feet
Design Criteria
Recovery Time, T (day):
Factor of Safety, FS (dimensionless):
Minimum distance between water table mound and pond bottom, R (ft)
3 days
2
0.5 feet
Background Seepage
Ground water baseflows into underdrained pond, qb (gpm)
20.22 gpm
Parameters to Compute Minimum Diameters of Lateral Drain and Main Drain
Manning's "n" Value For Lateral Drain Pipe (dimensionless)
Slope of Lateral Drains, I1 (%)
Maximum Length of Single Lateral Drain, l (ft)
Manning's "n" Value For Main Drain Pipe (dimensionless)
Slope of Main Drain, I2 (%)
Percent of Total Length of Laterals Connected To Main Drain, P (%)
0.015
0.45%
200 feet
0.016
0.50%
25%
As noted on the output in Exhibit 9, the required lateral drain spacing is approximately
20 feet and a total lateral drain length of 5118 feet is required. A lateral drain diameter of
6 inches is adequate, but an 8-inch diameter main drain pipe is needed. The outfall will
have to accomodate at least 2.67 cfs during recovery of the treatment volume.
August 7, 1995