Suspended sediment monitoring in alluvial gullies: a laboratory and
field evaluation of available measurement techniques
Abstract
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.