A seismic signature of river bedload transport during storm events

GEOPHYSICAL RESEARCH LETTERS, VOL. 38, L13407, doi:10.1029/2011GL047759, 2011 A seismic signature of river bedload transport during storm events Leslie Hsu, 1,2 Noah J. Finnegan, 1 and Emily E. Brodsky 1 Received 12 April 2011; revised 3 June 2011; accepted 3 June 2011; published 14 July 2011. [ 1 ] Seasonal patterns in high frequency seismic waves near rivers can record energy transmitted to the river bed from particle impacts during bedload transport. Here we show that single storm events in a river can also be observed seismically. We analyzed the high frequency seismic noise in a reach of the Cho‐Shui (Zhuoshui ˇ ) River in central Taiwan and made detailed observations during individual storm events. Discharge, derived from a water level gage 4.25 km from the seismometer, is highly variable due to typhoons. We found a correlation between seismic amplitude and discharge that differs on the rising and falling limbs of three storms. During each storm, for a given discharge the amplitude of seismic waves are on average two times greater on the rising limb of the storm than on the falling limb, in both aggradational and erosional events. Clockwise hysteresis in both aggradational and erosional events implies that water turbulence, alone, is not the source of the seismic waves. If seismic wave amplitude correlates linearly with the flux of bedload, this implies a roughly two‐fold decrease in transport efficiency over the time‐scale of days during individual storms. The observed change in transport efficiency can plausibly be explained by the disturbance of bed armor during storms and subsequent reformation during the waning stages. This data highlights the potential for fluvial seismology to reveal the dynamics of bedload transport. Citation: Hsu, L., N. J. Finnegan, and E. E. Brodsky (2011), A seismic signature of river bedload transport during storm events, Geophys. Res. Lett., 38, L13407, doi:10.1029/2011GL047759. 1. Introduction [ 2 ] Gravel and cobble transport by rivers governs channel change, and is therefore of great importance to geomor- phologists and river engineers. The most commonly used approaches to estimating bedload transport rates are empir- ically calibrated relationships based on flume experiments [e.g., Meyer‐Peter and Muller, 1948; Wilcock and Crowe, 2003]. However, it is difficult to assess how effectively such relationships predict transport rates during extreme events in large rivers because quantitative measures of bed- load transport are labor intensive and, at high flows, often dangerous to obtain. For logistical reasons, most bedload studies have been carried out in small mountain streams. [ 3 ] Particle impacts on the river bed transfer momentum, which in turn generates elastic (seismic) waves. Therefore, seismology can potentially constrain bedload transport rates in rivers. High frequency (>1 Hz) seismic waves have been Department of Earth and Planetary Sciences, University of California, Santa Cruz, California, USA. Now at Lamont-Doherty Earth Observatory, Palisades, New York, USA. Copyright 2011 by the American Geophysical Union. 0094‐8276/11/2011GL047759 used to study earth surface processes such as ocean waves [Adams et al., 2002], landslides [Favreau et al., 2010], rockfalls [Deparis et al., 2008], debris flows [LaHusen, 2005; Suwa et al., 2003; Burtin et al., 2009], and snow avalanches [Vilajosana et al., 2007; Cole et al., 2009]. In all of these cases, remote monitoring is possible because geo- phones or seismometers capture ground vibrations caused by the surface process. A few studies have used seismo- logical data to study fluvial processes [e.g., Govi et al., 1993], and recent work showed that high frequency seis- mic waves may provide a way to monitor bedload sediment transport in inaccessible rivers in the Himalaya. Burtin et al. [2008] showed that seismic wave amplitude along the Trisuli River in Nepal correlated well with hydrological processes such as daily precipitation and river discharge cycles. In addition, over the year of available data, the seismic ampli- tude was greater for a given river stage at the start of the Indian Monsoon than at the end. This was interpreted as evidence for a bedload source for seismic waves, as opposed to water turbulence, because significant hysteresis is not expected in the relationship between stage and turbulence, whereas hysteresis is common in sediment rating curves. However, in addition to sediment supply, many other factors (e.g., bed armoring, channel geometry) might influence the relationship between river stage and the amplitude of ground vibrations. [ 4 ] In this paper we move closer to a direct connection between bedload transport and seismic wave amplitude by studying one site on the Cho‐Shui River in Taiwan that has experienced repeated, short‐lived, and quantitatively‐ documented typhoons. This dataset combines nearby water level, channel cross‐section, discharge, and seismic ampli- tude measurements from existing infrastructure. A major goal of our work is to determine if a consistent, time‐varying relationship between the amplitude of ground vibrations and river flux holds during multiple events. By examining several events with different durations and intensities, we show that during each storm there is a clear distinction between the seismic amplitudes generated for a given water discharge as the water rises compared to when it falls. Whether the event is aggradational or erosional, there is a greater amplitude of seismic noise at the beginning of the storm in all three typhoon events. This key observation combined with a contrasting observation for smaller subevents and simple hydraulic arguments suggests that bedload transport controls the seismic wave generation. Furthermore, the observed hysteresis is consistent with an increase in the threshold stress for sediment entrainment during each event caused by packing and/or coarsening of the alluvial bed. 2. Site and Methods [ 5 ] The study site on the Cho‐Shui River in Taiwan was selected because it has highly variable water discharge, high L13407 1 of 6