The timing and magnitude of channel bed erosion by three debris flows was measured in 2008 at the Illgraben catchment, Switzerland, using a scour sensor which consisted of a vertical array of erodible sensor elements. During the largest debris flow, sediment was entrained progressively and stepwise at the flow head within 20 s after front arrival, and onset of erosion started before maximum values for flow height and normal and shear stress, measured nearby, were reached. Erosion in one of the two smaller debris flows also occurred at the head of the flow, but the magnitude of erosion was at the detection limit for the sensor. For the other small debris flow, we were not able to determine the timing of erosion due to the presence of a sediment layer covering the sensors. Measurements of pressure fluctuations along the channel sidewall, which are produced by interparticle collisions within the flow, indicate that the entrainment of sediment is coincident with the largest mean and fluctuating pressures, suggesting that interparticle collisions may drive the erosion process at the front of debris flows. After erosion at the head of the debris flows, sediment was deposited on top of the erosion sensor columns, indicating that the bed was reworked to a larger depth than directly visible at the surface after the event. Observations from elsewhere along the channel support our measurements of the magnitude of net debris flow erosion and indicate that significant erosion can be expected on debris fans when the flow is confined to a channel. Debris flow entrainment and subsequent bulking of the flow influence the flow dynamics and therefore should be considered in debris flow models and hazard assessment.