Experimental study of the transport of mixed sand and gravel

We measured a wide range of transport rates for five different sand/gravel mixtures in a laboratory flume. Each mixture used the same gravel, and sand was added to produce mixtures containing 6, 15, 21, 27, and 34% sand. Control of other variables allows us to isolate the effect of bed sand content on transport. As sand content increases, gravel transport rates increase by orders of magnitude, even though the proportion of gravel in the bed decreases. The increase in gravel transport rate is most rapid over the range of bed sand content between 15 and 27%. The increase in transport rate is larger than predicted using standard scaling relations between transport rate and grain size, indicating that models of transport and sorting and predictions of stream channel response to sand inputs need to be revised to account for the influence of sand content. Bed surface grain size was measured at the end of each run. Surface grain size varied with sand content but showed little or no coarsening with flow strength and transport rate. This casts doubt on the idea that armor layers form at small flows and weaken or vanish with increasing flow and transport rate.