Morphodynamics of downstream fining in rivers with unimodal sand-gravel feed

Rivers often show a downstream decrease in both the bed slope (long profile upward concavity) and bed surface characteristic grain size (downstream fining). Downstream fining is gradual, except at the gravel-sand transition, where bed grain size changes over short downstream distances, often only a few channel widths. A gravel-sand transition can be expected to arise when the sediment in transport is bimodal, with sand and gravel modes but a paucity of 1-10 mm gravel range. However, it has also been suggested that gravel-sand transitions may be generated autogenically, as decreasing bed slope in the downstream direction drives sand abruptly out of suspension and buries gravel which might be well under the threshold of motion. To study the problem of autogenic gravel-sand transitions, we propose a river morphodynamic model which erases any explicit distinction between gravel and sand transport. Such a distinction between gravel and sand is generally implemented in previous models of sediment transport and river morphodynamics. A unimodal mixture of sand and gravel is used in the present simulation to exclude the effect of bimodality of sediment on the gravel-sand transition. Upward concavity in the profile is forced through basin subsidence. We consider a river channel from a mountain valley to a subsiding foreland basin by implementing a spatially varying width of floodplain where sediment can deposit. Simulation results show that downstream fining is manifested throughout the entire reach, with an abrupt grain size transition occurring immediately downstream of where the floodplain width increases dramatically and gravel deposits as the river comes into the foreland basin.