AUTOGENIC ALLUVIAL GRADE ATTAINED WITH FALLING SEA LEVEL: A 2D EXPERIMENT

A river which conveys sediment without net deposition and net erosion through a segment of river is referred to as “graded” with respect to that segment. Recent renewal of debates as to the grade concept clarifies that (1) alluvial grade can be attained and maintained only during sea level fall, (2) the pattern of sea level fall to allow the attainment of grade is dependent primarily on basin-slope configuration, and (3) where averaged alluvial gradient (α) is equal to the pre-existing basal gradient (φ), a prograding but graded reach can be autogenically attained under conditions of any constant rate of relative sea level fall (Rslf) (Muto and Swenson, 2006). This latter notion of autogenic alluvial grade has been derived from one-dimensional moving-boundary modeling and corroborative one-dimensional flume experiments. As a discrete step from there, we here examine by means of flume experiments how two-dimensional alluvial grade appears in plan-view configuration. Each run of the experiments was conducted with φ being equal to α, constant rslf, constant sediment supply, constant upstream water discharge, and a wide (i.e. two-dimensional) flume so that the depositional system could grow free from lateral confinement. The results of the experiments imply as follows. First, two-dimensional grade of the alluvial system is physically possible, which can be judged from a standstill of upstream alluvial-basement transition and apparent stability of the active alluvial channels. Secondly, autocyclic lateral shifting of the active alluvial channels is suppressed in the graded system. Thirdly, instead, there develop seaward-protruding river channels (or channel-levee systems) which leave a main body of the alluvial system far behind. Runs of the experiments were also successful in causing autogenic bifurcation of seaward-protruding channels, which looked like ones in birdfoot deltas.