where:

L = drain spacing, ft

K = hydraulic conductivity, ft/sec

D = distance between the water level in the drain and the impermeable layer, ft H = height of the water table above the water level in the drain midway between the two drains, ft Q = drain drainage rate per unit surface area, ft/sec

For a two-layered soil, Hooghoudt, as described by Wasseling (1973), developed a modified equation of

/8K1DH + 4K2H2\05

where K and K2 are the hydraulic conductivities above and below the drain, and d is the equivalent depth of the aquifer below the drain as illustrated in Figure 9.17.8. Using this equation involves either a trial-and-error procedure or the use of monographs which have been developed specifically for equivalent depth and drain spacing (U.S. EPA 1985b; Repa et al. 1982). Other equations for different subsurface configurations are available in Cohen and Miller (1983).

The diameter of the pipe can be calculated with the use of Manning's equation, assuming that the carrying capacity of the pipe is equal to the design seepage. The resulting equation (Luthin 1957) is d = 0.892(q • A)0 375 • A"0 1875 9.17(7)

where:

d = inside diameter of pipe, in A = drainage area, acres q = seepage coefficient, in/day I = hydraulic gradient d = inside diameter of pipe, in A = drainage area, acres q = seepage coefficient, in/day I = hydraulic gradient

<-l->rfj_ow Permeability

Layer

FIG. 9.17.8 Subsurface drain formulation. (Reprinted from U.S. Environmental Protection Agency, 1985, Leachate plume management, EPA/540/2-85/004, Washington, D.C.: U.S. EPA.)

<-l->rfj_ow Permeability

Layer

FIG. 9.17.8 Subsurface drain formulation. (Reprinted from U.S. Environmental Protection Agency, 1985, Leachate plume management, EPA/540/2-85/004, Washington, D.C.: U.S. EPA.)

The slope of the pipe should be selected so that the flow velocity in the drain is greater than the critical velocity of siltation of the soil that enters the drain (Soil Conservation Service 1973). When the velocity is less than 1.4 ft/sec, filter fabrics and silt traps or cleanouts should be installed around the pipe and along the subsurface drain.

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