A seismic signature of river bedload transport during storm events

Abstract

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 (Zhuóshu) 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. Copyright 2011 by the American Geophysical Union.