Examining Bed Material Transport in a High Sand Content Channel

Bed material load plays a critical role in river morphology and ecology. This study aims to examine patterns of sediment transport in a poorly sorted sand-bed and evaluate whether high sand content impacts gravel transport rates. Bed material load samples were collected from a wide range of flows from three sites in the lower San Antonio River near Floresville, Runge, and Goliad, Texas, with sand content of bed sediment increasing from 56 to 86%. Samples were sieved from which total and fractional transport rates were calculated. Transport rates of sand and gravel fractions, partial sediment transport conditions, and similarity models were used to assess sediment transport patterns. The influence of sand content on gravel transport was evaluated using Shields numbers, ratios of supply and transport rates for gravel to reference sand as a function of bed sand content, and fractional transport rates as a function of transport stage. Peak fractional transport rates occur in sand sizes and decrease with increasing grain size. The degree of partial sediment transport increases with increasing grain size. Similarity analysis of dimensionless transport rates and transport stage yields a single trend at both the Floresville and Kenedy sites. The role of sand content in facilitating gravel transport is inconclusive and requires further research. Quantifying transport rates and patterns in a poorly sorted sand-bed river increases knowledge about the full range of sediment dynamics that occur in river channels, which can be used in river management to assess how sediment affects ecological stability.