Slope angle ​

Steep slopes should alert you to possible slope instability, and if there is significant risk of instability you should not subject such sites to additional water from disposal systems.

On steep sites, it may be feasible to bench along a contour to make the site more level. But remember that the soil for disposal must be thick enough to give the minimum required depth on the upslope side of the trench.

An advantage of stable steeper slopes is the more rapid rate of rain run-off, so retained rain loadings are less than on gentler slopes.

Slope form ​

Landform types (convex spreading, straight simple, convex converging, concave spreading or concave converging) are useful descriptions of where surface flow and subsurface seepage might end up. The first adjective describes the slope in cross-section, and the second in plan.

The form you describe is the one which dominantly controls the direction of movement of surface runoff (and subsurface seepage) downslope from the disposal area, for a distance likely to be affected by any overflow from the disposal system.

Coupled with slope angle and the groundwater models in Part 2, you can estimate whether the subsurface seepage will present possible health risks if and when it emerges downslope.

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