Fractional transport rates in a poorly sorted sand-bed river

Abstract

Size-selective transport is well-established in most gravel-bed channels but how grain mobility evolves in sand-gravel streambeds dominated by sand is less well-known. This study reports on fractional transport rates in the lower San Antonio River, Texas, where bed material contains gravel content that ranges from 0.1 to 65%. A Helley-Smith bedload sampler was deployed at three locations (Floresville, Kenedy, Goliad) at discharges that span from near baseflow to bankfull capacity. Relatively low discharges partially mobilize only some of the sizes present on the bed but the range in transported sizes generally expands as flow magnitude increases. Associated changes in the median diameter of sediment fluxes are relatively subdued as discharge increases over two orders of magnitude. Partial mobility dominates the sediment transport regime at all three sites. During the highest flow discharges at Floresville, coarser gravels are still underrepresented in the flux near bankfull capacity. In contrast, the proportion of bedload grain sizes relative to that found in the bed material approaches similarity conditions for the sizes caught in the bedload sampler at Kenedy and Goliad. Conservatively computed transport stages suggest that equal mobility conditions for poorly sorted sand mixtures can require higher flow stress conditions than reported for gravel-bed channels. Results have consequences for the quality of aquatic habitat, prediction of streambed stability, and formulation of morphodynamic models.