Stable Knickpoints Formed in Cohesive Sediment

In rivers and streams, vertical step-changes in bed surface elevation or knickpoints have been historically interpreted as unstable landforms. Geomorphic studies in Mississippi have shown that knickpoints commonly occur along the Yalobusha River system where cohesive clay units are exposed at the bed surface. Detailed examination of these units show that erodibility coefficients can vary widely, suggesting that some knickpoints may be non-migrating and act as natural grade-control structures. An experimental program was designed to construct a non-migrating headcut in an experimental flume and to examine the effects of backwater height on overfall erodibility. A clay loam was incrementally packed into a flume, and a knickpoint 0.6-m high was subjected to flows ranging from 0.2 to 2.5 m³/s. Although some bed erosion was observed, the knickpoint remained fixed in position. Highest rates of erosion were observed in the area of overfall impingement when flow discharge was high and backwater level was low. Increasing the height of the backwater decreased the erosivity of the overfall nappe. A jet-test device was used to define the critical shear stress and erodibility coefficient of the cohesive bed. These values when combined with predictive relations for stress at an overfall and an excess shear stress model for rates of erosion were comparable to those observed. It is postulated herein that along the Yalobusha River knickpoints become destabilized at low flow stages when the overfall impinges the bed, thus causing scour, tension cracks, and cantilever mass failure. At higher flow stages, the knickpoints become drowned and the overfall nappe floats. The relation between overfall erosivity and backwater height is important for predicting knickpoint migration in cohesive material.