Seepage steps in the New Forest, Hampshire, England

Abstract

Seepage steps are erosional scarps produced on hillslopes by the removal of material on the downslope side by concentrated seepage. In plan, the scarps form arcuate lines approximately following the contour. In profile, four elements can be distinguished: a convex slope above the step, an almost vertical face, a convex debris slope below the face, and often a concave slope beyond the debris slope. In the New Forest, Hampshire, England, the seepage is caused by concentration of water above the junction between a permeable sandstone and an underlying impermeable clay. The seepage face is composed partly of superficial material and partly of the permeable rock, the contact between the two strata being obscured by the debris and found in one case to be 8 meters below the level of the base of the step. The water table was always found to be <1 meter below the level of the base of the step. It is concluded that the position of the step is determined directly by the level of the water table and indirectly by the clay‐sandstone junction. The height of the step varies considerably: the mean maximum height for 54 valley sides is 2.16 meters; the absolute maximum height recorded is 6 meters. Erosion of the face is active at some points; at other points the feature appears to be relict. It is usually most active near the heads of valleys.