Figure 1

Nicholas Doriean

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Gully erosion is a significant source of fine suspended sediment (<63µm) and associated nutrient pollution to freshwater and marine waterways. Researchers, government agencies, and monitoring groups are currently using monitoring methods designed for streams and rivers (e.g., autosamplers, rising stage samplers, and turbidity loggers) to evaluate suspended sediment in gullies. This is potentially problematic because gullies have unique hydrological and operational challenges that differ to those of streams and rivers. Here we present a laboratory and field-based assessment of the performance of common suspended sediment monitoring techniques applied to gullies. We also evaluate a recently-described method; the pumped active suspended sediment (PASS) sampler, which has been modified for monitoring suspended sediment in gully systems. Discrete autosampling provided data at high temporal resolution, but had considerable uncertainty associated with the poor collection efficiency (25 ± 10%) of heavier sediment particles (i.e., sand). Rising stage sampling, while robust and cost-effective, suffered from large amounts of condensation under field conditions (25-35% of sampler volume), thereby diluting sample concentrations and introducing additional measurement uncertainty. The turbidity logger exhibited low uncertainty (< 10%) when calibrated with suspended sediment concentration data from physically collected samples, however, this calibration approach needs to be performed on a site-specific basis to overcome the error associated with the impact of different particle size distributions on the turbidity measurement. The modified PASS sampler proved to be a reliable and representative measurement method for gully sediment water quality, however, the time-integrated nature of the method limits its temporal resolution compared to the other monitoring methods. We recommend monitoring suspended sediment in alluvial gully systems using a combination of complementary techniques (e.g., PASS and RS samplers) to account for the limitations associated with individual methods.