1987 Student Professional Paper: Undergraduate Division: Influence of Stratigraphy and Structure on Knickpoint Erosion

Abstract

A knickpoint, an abrupt change in the slope of a channel, commonly forms in a stream channel where a resistant bed overlies a more erodible unit. The mechanisms of knickpoint erosion are traditionally explained by a cantilever-toppling process in which erosion of the lower strata removes the support from the capping rock leading to failure. It is hypothesized that the structural characteristics, bedding thickness and fracture spacing, influence the mechanisms of knickpoint erosion. It is suggested that fluid boundary shear acting on the stream bed erodes thin, discontinuous caprock units and the turbulent flow conditions can produce uplift pressures and excess pore pressures which enhance the erosion of the caprock. The hypotheses were evaluated using a laboratory air flume and styrofoam particles and slabs. Fluid boundary shear rapidly eroded thin, discontinuous caprock units independently of the nature of the lower unit. Elevated pore pressures within the caprock and the lower unit contributed to erosion by reducing the sliding resistance between beds. Thinly bedded units with widely spaced fractures achieved an equilibrium form and eventually failed by the cantilever-toppling mechanism. As slab mass (via bedding thickness and fracture spacing) increases, the knickpoint erosion process trends toward a cantilever-toppling mechanism which is controlled by fracture spacing. The flume study indicates that the fracture spacing is the most significant control on knickpoint erosion.