Multiple knickpoints in an alluvial river generated by a single instantaneous drop in base level: experimental investigation

Abstract

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, upstreamprogressing 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, an instantaneous drop in 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, that is, 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 base-level fall can trigger the autogenic generation of multiple knickpoints which are sustained until the alluvial reach recovers a graded state.

Planform evolution of deltas with graded alluvial topsets: Insights from three‐dimensional tank experiments, geometric considerations and field applications

“…If the downstream end of the depositional system is fixed, alluvial grade can be attained forcibly with a stationary base level (‘forced grade’; Fig. C), just as for cases of a non‐deltaic alluvial river that is dammed up with a downstream weir (Postma et al ., ; Cantelli & Muto, ; Bijkerk et al ., ) and for a deltaic alluvial river that is perched on a ‘shelf edge’ at which the feeder flows simply dump the entire supplied sediment into very deep water (Parker, ; Kim et al ., ). The attainment of alluvial grade in this downstream‐fixed setting can be explained in terms of the effect of basin water depth, as discussed below.…”

“…In the F series runs, however, the basinward end of the ‘shelf’ was aligned with a submerged overfall so that the delta could not prograde beyond this line (Figs C and ). This distinct drop in the basement functioned as a type of downstream weir against which the alluvial topset river readily attained grade (Cantelli & Muto, ). Four runs were conducted under slightly different conditions (Table ).…”

Upstream migrating knickpoints and related sedimentary processes in a submarine canyon from a rare 20-year morphobathymetric time-lapse (Capbreton submarine canyon, Bay of Biscay, France)

“…The generation of knickpoints and their migration may be related to turbiditic events, and in fact analogic experiments showed that one turbidity current can generate knickpoints (Toniolo and Cantelli, 2007). Previous laboratory experiments on non-indurated sediments (simulating river alluvial beds) demonstrated that, under Froude-supercritical flow conditions, an instantaneous drop in base level can lead to the formation of upstream-migrating knickpoints (Cantelli and Muto, 2014). A single base-level fall can generate a single knickpoint, or multiple knickpoints that lead to a new equilibrium profile.…”

Contributions to sequence stratigraphy from analogue and numerical experiments

“…it is a moving boundary), the feeder alluvial river has no chance to attain grade with stationary base level (Muto et al in press). Numerical models have shown that alluvial grade can be realized by three mechanisms (Table 1): (1) autogenic grade attained by equilibrium response to constant sea-level fall in a moving-boundary setting (Muto & Swenson 2006); (2) allogenic grade attained by non-equilibrium response to decelerating sea-level fall in a moving-boundary setting (Muto & Swenson 2005a); (3) forced grade attained by equilibrium response to stationary sea-level in a downstream-fixed boundary setting (Muto & Swenson 2005b;Postma et al 2006;Cantelli & Muto 2014). These three processes of attaining grade are also affected by geomorphological conditions, particularly alluvial slope (α) and subaqueous basin slope (φ): α = φ for autogenic grade, α < φ for allogenic grade, and a very large value of φ for forced grade (Muto et al 2012).…”