Multiple knickpoints in an alluvial river generated by a single instantaneous drop in base level: experimental investigation
Abstract. Knickpoints often form in bedrock rivers in response to base level lowering. These knickpoints can migrate upstream without dissipating. In the case of alluvial rivers, an impulsive lowering of base level due to, for example, a fault associated with an earthquake or dam removal commonly produces smooth, upstream-progressing degradation. The knickpoint associated with suddenly lowered base level quickly dissipates. Here, however, we use experiments to demonstrate that, under conditions of Froude-supercritical flow over an alluvial bed, suddenly lowered base level can lead to the formation of upstream-migrating knickpoints that do not dissipate. The base level fall can generate a single knickpoint or multiple knickpoints. Multiple knickpoints take the form of cyclic steps (i.e., trains of upstream-migrating bedforms, each bounded by a hydraulic jump upstream and downstream). In our experiments, trains of knickpoints were transient, eventually migrating out of the alluvial reach as the bed evolved to a new equilibrium state regulated with lowered base level. Thus the allogenic perturbation of lowered base level can trigger the autogenic generation of multiple knickpoints, which are sustained until the alluvial reach recovers a graded state.