Quantifying debris-flow characteristics remains a key challenge in natural hazard research. Recent studies suggest that seismic signals can reveal debris-flow properties. However, the accuracy and applicability of theoretical models linking bulk flow properties to the seismic signature of a debris flow remain elusive. By extending a previously proposed model of random single-particle impact forces to multi-particle force chains acting on the channel bed, we describe the generation of high frequency seismic signals, and thereby estimate the bulk flow dynamics of six debris-flow events at Illgraben, Switzerland. The theoretically predicted basal force fluctuations agree with in-situ measurements at a basal force plate and seismic signals recorded adjacent to the channel. According to our model, both random single-particle impacts and random impacts caused by multi-particle force chains control basal force fluctuations. The relative contributions of single particles and of multi-particle force chains may vary significantly for different events and different positions within events. The relative contribution of multi-particle force chains is expected to be larger in the flow front, where particle concentrations are higher. The ratio between single-particle and multi-particle contributions appears to control the non-linear relation between flow depth and the magnitude of the high frequency seismic signals. Our results suggest that fluctuating basal forces are strongly controlled by particle size and flow depth, and open new perspectives for the understanding of bulk flow characteristics, such as flow depth and weight, and of the high frequency seismic signals generated by debris flows.