During the last 20 years, numerous sediment transport flume experiments have shown that grain sorting generates bed level fluctuations and bedload pulses. In this work, we first propose a new analysis of these experimental data. From this analysis, we derive a model for gravel bed rivers where both local slope and bedload are known to largely fluctuate through space and time, in the socalled 'bedload active layer'. The model considers a maximum slope for local armoring equals to the mean bed slope affected by a coefficient which expresses the difference of mobility of the coarse fraction considered alone or in a mixture, and a minimum local slope for bed erosion equals to the mean bed slope corrected by a coefficient that depends on the armor ratio Ar (ratio of the surface to the sub-surface grains diameter) and the reach-averaged transport rate. The model is then compared with a compilation of scour-fill depths measured in the field. Results suggest that the slope fluctuations in 1D flume experiments are consistent with in-channel bed-level fluctuations associated to scour-fill processes in the active layer, with a maximum scour depth which is slope dependent. For the pulse intensity, we provide a justification to the simplified squared slope equation used to compute solid concentration C = Qs/Q proportionnal to 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.