Experimental study of knickpoint and longitudinal profile evolution in cohesive, homogeneous material
A series of experiments were conducted in homogeneous bedrock to study knickpoint and longitudinal-profile evolution. Knickpoints are created by successive drops in base level, which simulate intermittent uplift. Channel morphology and flow characteristics vary systematically along a knickpoint reach. As the knickpoint lip is approached, width decreases, but depth, velocity, and bottom shear stress increase. As a result, knickpoints are rapidly destroyed through the process of (1) knickpoint replacement and (2) subsequent knickpoint inclination. Knickpoint replacement consists of two morphologically distinct, erosional reaches: the knickpoint face below the knickpoint lip, which decreases in slope, and an incising reach above the knickpoint lip, which increases in slope. Knickpoint replacement is a significant process in the evolution of knickpoints in homogeneous bedrock. Within experimental limits, this study supports the notion that knickpoints do not undergo parallel head-ward retreat in homogeneous bedrock, given intermittent uplift, except in those possible cases of extensive and pervasive bedrock jointing. Only the erosional sediment-transport discontinuity above the knickpoint is developed in cohesive sediments. Below the knickpoint, a steep, bedrock channel slope, low sediment load, and rapid decrease in grain size of sediment load preclude development of the aggradational sediment-transport discontinuity. Knickpoint evolution can be characterized by three general models: parallel retreat, replacement, and inclination. These models are based on the following criteria within the knickpoint reach: (1) relationship between bottom shear stress, τ 0 , and critical shear stress needed to initiate motion, τ c ; (2) nature of the bed-load transport discontinuities; and (3) spatial variability of bedrock resistance to fluvial erosion.