The ‘revolutionary’ potential of passive bedload monitoring for river science and management

Field research into river bedload has relied largely on direct measurements using expensive sediment trapping facilities or labor intensive, potentially dangerous and time-limited active sampling. Passive, indirect measurements of particles passing an acoustic or impact sensor offer the prospect of high-resolution measurements over extended time periods from low cost, non-intrusive, portable devices. Based on recent exploratory experiences (Downs et al., 2016; Soar and Downs, 2017; Downs and Soar, 2018; Downs and Soar in prep), we offer four observations on the extent to which such surrogate measurements do indeed offer the ‘revolutionary’ concept in fluvial sedimentology suggested by Gray et al. (2010a). The first emphasizes the degree that near-continuous data permits fresh examination of time-dependent behaviors such as instantaneous fluctuations, event-based hysteresis, selective path transport and multi-annual trends. Second, high resolution particle counts facilitate data-driven bedload estimation based on bedload entrainment theory, monitored bedload counts and a probabilistic optimization of individual particle sizes in transport. The resulting model, BLIP, provides an uncertainty-bound, indicative range of estimates, not prone to the overestimation associated with traditional bedload formulae. Continued monitoring at high resolution over multiple years with very varied flow types permits the observation that effective discharge at our site occurred consistently at sub-barfull flows indicating the importance of bedload availability along the sediment thalweg combined with network-scale replenishment during wet periods. Related, the multi-year data highlighted that discharge-bedload rating curves are not stationary and revealed the existence of a two-phase rating relationship with a consistent below-bankfull ‘bulge’ in rate, maximized in wet years, and appearing to relate to variations in sediment supply. Capacity-related controls were dominant only in flows above bankfull. We conclude that high resolution perspectives on coarse bedload transport emphasize the potential importance of water year type and sediment supply limitations on rates of bedload transport, and that a sensitivity-style sediment supply index could produce a valuable indication of time dependent behavior in coarse sediment transport. High resolution data from passive monitoring focuses attention on a site’s ‘hydrogeoclimatic’ context and position in the watershed as factors determining coarse sediment dynamics and suggests that previous ‘partial data’ perspectives on bedload transport may have placed over-emphasis on rising limb sediment entrainment, under-emphasis on falling limb dynamics and focused on the influence of capacity controls to the detriment of supply-related controls.