The Dynamics of Suspended Sediment in a Typical Alpine Alluvial River Reach: Insight From a Seasonal Survey

Abstract

The transport of suspended sediment is associated with important social, economic, and environmental issues. It is still unclear, however, how suspended sediments eroded on hillslopes are transferred downstream through the river system. In this study, we aimed to investigate this process by applying a sediment budget approach to a typical 3.5-km-long Alpine braided reach. Using high-frequency suspended load measurements combined with Monte Carlo simulations for uncertainty propagation, we observed a significant buffering behavior of the reach studied. Thirty-two of the 48 events observed during the 2-month campaign showed significant differences between upstream and downstream mass transported as suspension, despite the reach studied was short compared to the upstream drainage area (130 km 2 ). These differences at the event scale varied widely within an envelope comprised between a net erosion equivalent to 74% of upstream suspended mass and a net deposition equivalent to 71%. Budgets were found to be controlled at a nearly instantaneous time scale by the liquid discharges and the suspended sediment concentrations in an opposite way: for low upstream concentrations, net erosion increased when the discharges increased, while above a certain concentration, net deposition increased when the concentrations increased. Moreover, coarse particles mobility in the reach (characterized via bedload transport measurements) appeared to have a strong influence on the availability of suspended particles as both quantities evolved concomitantly through time. These observations have important implications for our understanding and modeling of the transfer of suspended particles in gravel bedded streams.

Field Application and Validation of a Seismic Bedload Transport Model

“…At 30 and 50 Hz, a clear change in scaling power occurs as discharge exceeds a value (11.9 m 3 /s) that corresponds to the modeled threshold of full particle mobility (Figure 8b, 8c) and can be related to pavement breakup in the braided reach as observed here by Misset et al (2019). For discharges higher than this critical value, the power‐law trend is steeper with an exponent of 4.4 at 30 Hz and 5.3 for 50 Hz.…”

“…We studied a braided river reach approximately 600 m long and up to 90 m wide (Figure 1) that lies at an elevation of just above 1,000 m a.s.l., near the village of Villar‐Loubière. The riverbed has a gentle gradient of ~1% and is largely composed of gravel at the surface and includes finer material in the subsurface (Misset et al, 2019). The particle‐size distribution of the riverbed in the braided reach was determined by Misset et al (2020) and is shown in Figure 2.…”

Using Continuous Turbidity and Seismic Measurements to Unravel Sediment Provenance and Interaction Between Suspended and Bedload Transport in an Alpine Catchment

“…The observed suspension‐gravel bed interactions are consistent with several flume and scarce field results. Previous studies show that fine particles infiltrate in the gravel matrix even for very low Rouse numbers and stay in the subsurface until the surface coarse bed particles are mobilized (Diplas, 1994; Frostick et al., 1984; Krishnappan & Engel, 2006), so that a relationship between suspended load and bedload is observed in gravel bedded streams (Meunier et al., 2006; Misset, Recking, Legout, Valsangkar, et al., 2019; Misset, Recking, Navratil, et al., 2019; Park & Hunt, 2017; Turowski et al., 2010). Also, in addition to the strong relationship between turbidity and seismicity, a weak relationship during the summer periods, when most high turbidity peaks occur, provides an indication of the origin of suspended sediment ( bed related vs. hillslope related ).…”

“…The finest fraction of fine sediments stored in the bed matrix (obtained using a resuspension technique detailed in Misset, Recking, Legout, Valsangkar, et al. 2019) was found to have a GSD range similar to surface deposits and sampled suspended particles.…”

Evaluation of the deposit dynamics on a gravel bar after different hydrologic events

“…It remains however very difficult to estimate the total amount of fine sediment stock on gravel bars since these sediments are easily washed out and can also infiltrate into the gravel matrix. Indeed, it often represents a nonnegligible part of the sediment flux passing through the system (Misset et al, 2019). Moreover, very different dynamics can be observed from one class of sediments to another, i.e.…”