Bed Texture and Topographic Response to Increased Sediment Supply: The Sand Bed Case

Abstract

When water and sediment supply to a river change, the short-term channel response will be a combination of adjustments in bed grain size (texture) and the accumulation or evacuation of sediment in the channel (topography). This response is well documented for gravel bed rivers, but little attention has been given to sand bed rivers, the focus of this paper. If the channel response is predominantly textural, subsequent changes in channel geometry may be minor. If the channel response is predominately aggradation or degradation, long-term changes in channel dimension, planform, and slope may occur. Here, we use a mixed-size morphodynamic model to explore the interaction between the textural and topographic responses to an increase in sediment supply in sand bed rivers. First, we consider how the steady-state transport condition varies as a function of sediment supply rate and grain size. We then evaluate the path to steady-state using numerical morphodynamic experiments. We find that bed aggradation in response to an increase in sediment supply can be reduced, eliminated, or even reversed depending on the sediment supply grain size. The path to the new steady-state condition involves two adjustment phases: the first is textural-dominated, and the second is topography-dominated. Under the common condition of an increased supply of finer sediment, rapid textural adjustments can produce a transport capacity that is a large fraction of the new supply rate long before the system has reached complete equilibrium. Under conditions of supply coarsening, the initial textural adjustment is less dominant, and aggradation is immediate.