Bed load transport and boundary roughness changes as competing causes of hysteresis in the relationship between river discharge and seismic amplitude recorded near a steep mountain stream

Abstract

Hysteresis in the relationship between bed load transport and river stage is a well‐documented phenomenon with multiple known causes. Consequently, numerous studies have interpreted hysteresis in the relationship between seismic ground motion near rivers and some measure of flow strength (i.e., discharge or stage) as the signature of bed load transport. Here we test this hypothesis in the Erlenbach stream (Swiss Prealps) using a metric to quantitatively compare hysteresis in seismic data with hysteresis recorded by geophones attached beneath steel plates within the streambed, a well‐calibrated proxy for direct sediment transport measurements. We find that while both the geophones and seismometers demonstrate hysteresis, the magnitude and direction of hysteresis are not significantly correlated between these data, indicating that the seismic signal at this site is primarily reflecting hysteresis in processes other than sediment transport. Seismic hysteresis also does not correlate significantly with the magnitude of sediment transport recorded by the geophones, contrary to previous studies' assumptions. We suggest that hydrologic sources and changes in water turbulence, for instance due to evolving boundary conditions at the bed, rather than changes in sediment transport rates, may sometimes contribute to or even dominate the hysteresis observed in seismic amplitudes near steep mountain rivers.

Seismic signature of turbulence during the 2017 Oroville Dam spillway erosion crisis

“…Therefore, seismic monitoring may be a tool to quantify macroturbulent eddies and associated flow resistance in complex natural channels. The results of this study are consistent with those of Roth et al (2017), who suggested changes in channel morphology as a source for water turbulence-associated hysteresis in natural channels. This study also implies that the Gimbert et al (2014) model will under-20 predict seismic energy released in rivers with complex channels and macroturbulent eddies.…”

“…The theoretical scaling relationship between water-generated vertical component power ( ) and discharge ( ) for water turbulence alone with a simple channel geometry is ∝ 1.25 (Gimbert et al, 2014;Gimbert et al, 2016). Roth et al 2017, found a ∝ rough channel geometry with variable width, the flow depth is a function of discharge, channel shape, and flow resistance (Leopold et al, 1960). Therefore, determining the causes of the scaling exponent change is difficult.…”

“…Without a way to differentiate among these sources by direction and frequency, interpreting seismic observations will be limited. This challenge was highlighted by Roth et al (2016) and Roth et al (2017), who indicated that the turbulent signal from a waterfall downstream of their study river reach may have dominated the observed lowfrequency signals. Previous studies have attempted to locate the source of fluvial seismic energy by using arrays of seismometers, primarily by observing the variability in seismic amplitudes around the river section of interest (Burtin et al, 5 2011, andSchmandt et al, 2017).…”

“…This model, however, is based on assumptions of spatially uniform turbulence created by bed grain size; it ignores other sources of turbulence common in natural rivers and in engineered structures such as channel geometry variations. Recent work (Roth, et al, 2017) suggests that hysteresis between seismic power and discharge may also result from riverbed particle rearrangement, which leads to different turbulent characteristics within the flow. This fluvial seismic body of work suggests seismic monitoring may 30 be able to resolve changes in even in a dam spillway setting.…”

Particle transport mechanics and induced seismic noise in steep flume experiments with accelerometer‐embedded tracers

“…; Roth et al. ), previous theory and its application at a natural scale suggest that the bedload and turbulent flow sources may be distinguished by analyzing ground motion at various frequencies and river‐to‐station distances (Gimbert et al. ; Cook et al.…”