The contribution of grain sorting to the dynamics of the bedload active layer

Abstract

AbstractDuring the last 20 years, flume and field experiments have shown that grain sorting contributes to bed‐level fluctuations and bedload pulses. In this work, we propose a new analysis of these experimental data. From the flume data, we derive a model for gravel‐bed rivers where both local (bedform‐scale) slope and bedload are known to fluctuate through space and time, in the so‐called ‘bedload active layer’. The model uses standard concepts and empirical tools with reach‐averaged data for the hydraulics and sediment transport. It considers a maximum slope for local armouring equal to the mean bed slope (reach scale) affected by a coefficient which expresses the difference in mobility of the coarse fraction considered alone or in a mixture. The minimum local slope for bed erosion is the mean bed slope corrected by a coefficient that depends on the armour ratio Ar (ratio of the surface to the subsurface grain diameter) and the reach‐averaged transport rate. The model is compared with a compilation of scour–fill depths measured in the field. Results suggests that the slope fluctuations in 1D flume experiments are consistent with in‐channel bed‐level fluctuations associated with scour–fill processes in the active layer. The model also suggests that although the length scale of the maximum scour depth δ is on the order of the bed surface D90, it is well explained by the product between the mean bed slope S and the active channel width W, with δ ≈ 1.4SW. For the pulse intensity, we provide a justification for the simplified squared slope equation for solid concentration C = Qs/Q ∝ S2 (with Qs the solid discharge, Q the water discharge and S the slope), which has often been used in place of standard bedload equations for modelling highly concentrated bedload transport events in mountain streams.