Sediment flows generate ground vibrations by exerting basal force
fluctuations on the riverbed, which motivates the use of seismology to
indirectly measure flow properties. Linking the force fluctuations and
properties of highly concentrated sediment flows, however, remains
particularly challenging due to complexities that arise from
grain-to-grain interactions. Here we conduct downscaled flume
experiments designed to investigate the influence of grain scale
processes on the generation of force fluctuations for stratified
sediment flows associated with significant grain sorting. We demonstrate
that, under such flow conditions, the amplitude of force fluctuations
decreases as the volumetric solid concentration increases. We suggest
that this dependency reflects the negative relationship between
volumetric solid concentration and particle agitation, which in turn
controls the amplitude of force fluctuations. We therefore advance that
volumetric solid should be incorporated in seismic models as a key
parameter describing the particle agitation of highly concentrated
sediment flows.