Seaward protrusion of a graded channel-lobe system: 2D model experiments

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 in a deltaic system having moving boundaries, (1) alluvial grade can be attained and maintained only during sea level fall, and (2) there exist two different types of grade: that is, “allogenic grade” attained by nonequilibrium response to unsteady sea level fall and “autogenic grade” attained by equilibrium response to steady sea level fall. This new view of alluvial grade has been derived largely from 1D physical experiments. As a discrete step from there, we are seeking a channel(-lobe) pattern peculiar to graded alluvial rivers, by means of 2D experiments. The experiments consisted of two different series of runs: one was of a fixed-boundary system with stationary sea level, the other was of a moving-boundary system with constant fall of sea level. The fixed-boundary series, in which deltas' seaward progradation was limited by drowned weir, has clarified that a graded alluvial system holds only a single, quasi-straight channel that barely migrates laterally but is fixed to a particular axial position of the delta. A fixed channel was produced in the moving-boundary series too, but in the latter the channel-lobe system showed striking seaward protrusion. The present experimental study implies that alluvial grade can function to suppress autocyclicity of active feeder channels.