Fractional Transport of Gravel in a Sand-dominated Channel

Size-selective bedload transport is well established in most gravel-bed channels. However, the degree to which these patterns of grain mobility extend to sand-gravel streambeds dominated by sand is less well documented. This study reports on fractional transport rates of bedload in the lower San Antonio River, Texas, where bed material may contain gravel contents from 0.1 to 57% with maximum grain sizes ranging from 2 to 128 mm. Although the irregular meandering river transports the vast majority of the annual sediment load in suspension, gravel-sized bedload has been documented for discharges that range from near baseflow to bankfull capacity. At the three study sites (Floresville, Kenedy, Goliad), transport observations collected using a Helley-Smith bedload sampler characterize fluxes along beds with different sand-gravel ratios, which is caused in part by differences in mud content. The transport rate of the entire gravel fraction is about two orders of magnitude less than that of the sand fraction at all three sites. For the individual gravel size fractions, relatively low flow shear stresses partially mobilize only some of the sizes present on the bed as expected. The range in transported sizes generally expands as flow stress increases. During the highest stresses at Floresville, coarser gravels are still underrepresented in the flux near bankfull capacity. In contrast, at Kenedy and Goliad, the proportion of bedload grain sizes relative to that found in the bed material approximates similarity for the sizes caught in the bedload sampler. Dimensionless shear stresses based on the median diameter of the gravel fraction suggest that equal transport mobility is probably reached at around 4 times that required for entrainment, which corresponds with results from previous work.