Multi-year, high resolution data on coarse bedload transport: implications for river science

Passive, indirect monitoring of river bedload offers the prospect of obtaining high-resolution measurements of coarse sediment transport over multi-year time periods at relatively low cost and without the dangers inherent to active approaches. Based on experiences derived from a five-year programme of monitoring using seismic impact plates, we offer several observations on the extent to which such measurements do indeed offer a ‘revolutionary’ concept in fluvial sedimentology, as suggested by Gray et al. (2010). First, near-continuous data permits detailed examination of time-dependent behaviors in coarse sediment transport related to instantaneous fluctuations, event-based hysteresis, inter-event variability and the extent to which such factors are dominated by energetic controls (Downs et al. 2016). Second, high resolution particle counts facilitate a new and complementary class of data-driven bedload assessment. Based on bedload entrainment theory, monitored bedload counts and a probabilistic optimization of individual particle sizes in transport, the result is an uncertainty-bound range of bedload estimates without the inherent overestimation associated with traditional bedload formulae (Soar & Downs 2017). Extending high resolution data collection to multiple years with very variable hydrology, we observe that effective discharge at our site occurred consistently at ‘bar-building’ flows significantly below bankfull, emphasizing the importance of reach-scale bedload availability and transport on the falling limb of the hydrograph. Partly in consequence, bedload rating curves are not stationary and demonstrate a two-phase rating relationship. Capacity-related controls were dominant only in flows above bankfull while the dominant below-bankfull element consists of a discernable ‘bulge’ that is maximized in wet years and appears to reflect sediment supply variations (Downs & Soar 2018). Exploring these controls further, we develop a sensitivity-style index for sediment supply that highlights the role of annual wetness, reach-scale bed conditioning and antecedent hydrology on time dependent behavior in coarse sediment transport (Downs & Soar, submitted). Overall, high resolution passive monitoring suggests that bedload dynamics may be highly dependent on a site’s ‘hydrogeoclimatic’ context and position in the watershed, and that ‘partial data’ perspectives obtained from active bedload monitoring may have focused on the influence of capacity-related controls to the detriment of factors related to coarse sediment supply.