The anatomy of effective discharge: the dynamics of coarse sediment transport revealed using continuous bedload monitoring in a gravel‐bed river during a very wet year

Abstract

Indirect, passive approaches for monitoring coarse bedload transport could allow cheaper, safer, higher‐resolution, longer‐term data that revolutionises bedload understanding and informs river management. Here, insights provided by seismic impact plates in a downstream reach of a flashy gravel‐bed river (River Avon, Devon, UK) are explored in the context of plate performance. Monitoring of a centrally‐situated plate (IP1) during an extremely wet 12‐month period demonstrated that impacts were related to discharge as a measure of transport potential (R2 = 0.38) but that factors other than transport limitations are important. Analysis of discrete flow events revealed consistent rising‐limb and falling‐limb impact spikes biased toward the latter for larger events. Such patterns may result from disruption of the upstream armour layer (rising limb) and supply enhancements related to both upstream mass bank failures and/or flood routing of non‐local sediment sources (falling limb). Installation of additional impact plates indicated that plate IP1 was indeed dominantly related to instantaneous discharge, that a three‐plate lateral array somewhat better explained impact variability (R2 = 0.49), and that the bedload track shifts laterally with discharge. Aggregating event‐total IP1 impacts against volumetric discharge further increases explanation as intra‐event and stochastic bedload factors are subsumed but left 26% unexplained variance related to the unsampled bedload mass, inter‐event supply differences, and attributes of plate performance. Annualising the data created an impact‐based 'effective discharge’ for this extremely wet year that was closer to morphological bar‐full in magnitude than bankfull, but the preceding results imply this outcome is related as much to supply limitations as transport limitations. Overall, passive approaches offer a liberating prospect for bedload monitoring, capable of producing insights only achievable through high resolution, extended time periods. Such results could potentially inform threshold conditions and geomorphological effectiveness of flows for future river management strategies. Copyright © 2015 John Wiley & Sons, Ltd.

Self‐adjustment of stream bed roughness and flow velocity in a steep mountain channel

“…These bankfull stages result in very high (estimated) bankfull flows of up to $25 m 3 /s (Table 1). In contrast, the streambedforming effective discharge estimated from magnitude-frequency analysis of the bed load transport intensities [e.g., Andrews, 1980;Downs et al, 2015;Soar and Thorne, 2013;Wolman and Miller, 1960] amounts to 4 m 3 s 21 (Figure 9).…”

Bed Load Variability and Morphology of Gravel Bed Rivers Subject to Unsteady Flow: A Laboratory Investigation

“…Bed load transport in gravel bed rivers is a complex phenomenon characterized by high spatial and temporal variability (e.g., Downs et al, ; Garcia et al, ; Habersack et al, ; Pitlick, ). Such variability derives from a variety of mechanisms acting at different scales (Singh et al, ), ranging from the grain scale to the reach scale.…”

Variability of Bed Load Transport During Six Summers of Continuous Measurements in Two Austrian Mountain Streams (Fischbach and Ruetz)

“…Averaging measurements of bed load transport over increasing time periods has been shown to clearly increase the correlation between a measure of transport and a measure of water discharge (Downs et al, ; Lenzi et al, ; Recking et al, ; Rickenmann, ; Rickenmann & McArdell, ). This study demonstrated a substantial increase in the correlation coefficient R between log( Q b ) and log( Q ) for aggregation times of about 1–2 h (Figure ), which integrates over typical short term fluctuations of 15–35 min of bed load transport rates in natural gravel bed streams (Habersack et al, ; Hilldale, ; Reid & Frostick, ; see also section 4.2).…”

“…Alternative measuring techniques are indirect methods such as acoustic bed load monitoring in gravel bed streams. The interest in these methods has grown considerably in the last decade or so (Gray et al, ; Rickenmann, ), and an increasing number of related, process‐oriented studies have been published (Aigner et al, ; Beylich & Laute, ; Downs et al, ; Habersack et al, ; Kreisler et al, ; Magirl et al, ; Mao et al, ; Mizuyama et al, ; Raven et al, ; Reid et al, ; Rickenmann, ; Rickenmann & McArdell, ; Schneider et al, ; Turowski et al, ; Uchida et al, ; Vatne et al, ). These indirect techniques have the important advantage of providing continuous measurements in time.…”