SJ

Impermeable layer

FIGURE 3.7 Definition sketch for calculation of drain spacing.

require a surface-water discharge permit during the period of high groundwater but will function as a nondischarging system for the balance of the year.

The drainage design consists of selecting the depth and spacing for placement of the drain pipes or tiles. In the typical case, drains may be at a depth of 1 to 3 m (3 to 10 ft) and spaced 60 m (200 ft) or more apart. In sandy soils, the spacing may approach 150 m (500 ft). The closer spacings provide better water control, but the costs increase significantly.

The Hooghoudt method (Luthin, 1973) is the most commonly used method for calculating drain spacing. The procedure assumes that the soil is homogeneous, that the drains are spaced evenly apart, that Darcy's law is applicable, that the hydraulic gradient at any point is equal to the slope of the water table above that point, and that a barrier of some type underlies the drain. Figure 3.7 defines the necessary parameters for drain design, and Equation 3.16 can be used for design:

where

Horizontal permeability of the soil (ft/d; m/d). Height of groundwater mound above the drains (ft; m). Annual wastewater loading rate expressed as a daily rate (ft/d; m/d). Average annual precipitation expressed as a daily rate (ft/d; m/d). Distance from drain to barrier (ft; m).

The position of the top of the mound between the drains is established by design or regulatory requirements for a particular project. SAT systems, for example, require a few meters of unsaturated soil above the mound in order to maintain the design infiltration rate; SR systems also require an unsaturated zone to provide desirable conditions for the surface vegetation. See Chapter 8 for further detail. Procedures and criteria for more complex drainage situations can be found in USDI (1978) and Van Schifgaarde (1974).

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