Grain-size, sediment-transport regime, and channel slope in alluvial rivers

The general relationship between channel morphology and the grain size of sediment in the channel bcd is an important but poorly known aspect of alluvial rivers. An analysis of an equation for total sediment flux in the limits of suspension-, bedload-, and mixed-modes of transport indicates distinct, steady-state regimes of channel morphology. Such regimes are readily seen in published data for modern alluvial rivers by way of a conventional Shields plot or a plot of channel slope as a function of relative grain size d/h and the ratio w(s)/u*(/) where d and w(s) are, respectively, mean diameter and fall speed of bed sediments, and h and u*. are, respectively, mean depth and friction velocity of the flow. With slope and mode of transport in an alluvial river constrained by grain size and channel depth alone, estimates of discharge and sediment flux follow directly, introduction of the sediment flux relationship into conventional diffusion models for the evolution of an alluvial system provides nominal estimates of the response time for channel adjustment to some external changes. For some major modern rivers, this time of response along the entire length of channel is in the range 10(3)-10(5) yr, underscoring the potential for complicated, long-time interaction of large alluvial systems with, for example, climatic variability.