RESPONSE OF A CHANNEL WITH ALTERNATE BARS TO A DECREASE IN SUPPLY OF MIXED-SIZE BED-LOAD - A FLUME EXPERIMENT

The response of a channel with a topography and modeled bed material size typical of gravel bed rivers to reductions in sediment supply was investigated in a laboratory flume filled and fed with a sand-gravel mixture. After a series of quasi-stationary altemate bars were formed under equilibrium sediment transport, feed rate was reduced in two steps to one third and one tenth the initial rate as discharge was held constant. The primary response following both reductions was an increase in bed surface particle size as a corridor of intensive bed load transport contracted and local transport rates decreased. After the first feed rate reduction, the channel incised by twice the mean water depth, on average, and caused distal bar surfaces to emerge as terracelike features. Bar roughness decreased, and mean boundary shear stress exerted on bed surface particles increased. Little incision occurred after the second feed rate reduction.