Triggering and propagation of exogenous sediment pulses in mountain channels: insights from flume experiments with seismic monitoring

Abstract

Abstract. In the upper part of mountain river catchments, large amounts of loose debris produced by mass-wasting processes can accumulate at the base of slopes and cliffs. Sudden destabilizations of these deposits are thought to trigger energetic sediment pulses that may travel in downstream rivers with little exchange with the local bed. The dynamics of these exogenous sediment pulses remain poorly known because direct field observations are lacking, and the processes that control their formation and propagation have rarely been explored. Here we carry out flume experiments with the aims of investigating (i) the role of sediment accumulation zones in the generation of sediment pulses, (ii) their propagation dynamics in low-order mountain channels, and (iii) the capability of seismic methods to unravel their physical properties. We use an original setup wherein we supply liquid and solid discharge to a low-slope storage zone acting like a natural sediment accumulation zone that is connected to a downstream 18 % steep channel equipped with geophones. We show that the ability of the self-formed deposit to generate sediment pulses is controlled by the fine fraction of the mixture. In particular, when coarse grains coexist with a high content of finer particles, the storage area experiences alternating phases of aggradation and erosion strongly impacted by grain sorting. The upstream processes also influence the composition of the sediment pulses, which are formed by a front made of the coarsest fraction of the sediment mixture, a body composed of a high concentration of sand corresponding to the peak of solid discharge, and a diluted tail that exhibits a wide grain size distribution. Seismic measurements reveal that the front dominates the overall seismic noise, but we observe a complex dependency between seismic power and sediment pulse transport characteristics, which questions the applicability of existing seismic theories in such a context. These findings challenge the classical approach for which the sediment budget of mountain catchments is merely reduced to an available volume, since not only hydrological but also granular conditions should be considered to predict the occurrence and propagation of such sediment pulses.

Brief communication: Seismological analysis of flood dynamics and hydrologically triggered earthquake swarms associated with Storm Alex

“…Similar peaks in seismic power generated by flood waves were observed during glacial lake outburst floods in the Himalayas by Cook et al (2018) and Maurer et al (2020). These peaks may also be associated with the passage of sediment pulses such as those experimentally investigated by Piantini et al (2021) in a torrential river setting. Such pulses can be generated by external sediment inputs to the river, triggered by the sudden destabilization of debris deposits at the base of slopes and cliffs.…”

Toward a general calibration of the Swiss plate geophone system for fractional bedload transport

“…It is further complicated by a sediment supply that varies in both space and time, due in part to cycles of building and breaking of an armoring layer at the riverbed (e.g. Church et al, 1998;Dhont and Ancey, 2018;Rickenmann, 2020;Piantini et al, 2021).…”

Solid Concentration as a Main Proxy for Basal Force Fluctuations Generated by Highly Concentrated Sediment Flows

“…Before force power computation, we apply a band‐bass Butterworth filter to the force signal in the frequency range $$100\mbox{--}2500$$ Hz to avoid dealing with strong plate resonances. We focus on this frequency range also because the contribution to seismic vibrations of impacts on the sidewall is particularly noticeable above $$2500$$ Hz (Supporting Information ), and that of water flow is significant below $$100$$ Hz (Piantini et al., 2021). In the selected frequency range, seismic waves are strongly attenuated with distance, such that the most contributing sources are located within about $$1$$ m around the sensors (Supporting Information ).…”

“…The bed and sidewalls of the flume are covered with sediments fixed with silicone. Every run consists in feeding the upstream storage area with constant liquid discharge [2417; 3525] ; see (Piantini et al, 2021) for overall scaling considerations). We use a bimodal grain size distribution typical of mountain rivers (John Wolcott, 1988;Sklar et al, 2017), with one mode corresponding to sand ( 𝐴𝐴 0.5 mm 𝐴𝐴 𝐴 𝐴𝐴 𝐴 2 mm) and the other to cobbles ( 𝐴𝐴 4 mm 𝐴𝐴 𝐴 𝐴𝐴 𝐴 8 mm), and with 𝐴𝐴 𝐴𝐴50 = 5 mm and 𝐴𝐴 𝐴𝐴84 = 9 mm, where D 50 and D 84 are the 50th and 84th percentile particle diameter, respectively.…”