Field observations, rheological testing and numerical modelling of a debris-flow event

Debris flows generated from landslides are common processes and represent a severe hazard in mountain regions due to their high mobility and impact energy. We investigate the dynamics and the rheological properties of a 90 000 m3 debris-flow event triggered by a rapid regressive landslide with high water content. Field evidence revealed a maximum flow depth of 7-8 m, with an estimated peak discharge of 350-400 m3 s-1. Depositional evidence and grain-size distribution of the debris pose the debris flow in an intermediate condition between the fluid-mud and grain-flow behaviour. The debris-flow material has silt-clay content up to 15 per cent. The rheological behaviour of the finer matrix was directly assessed with the hall measuring system. The measurements, performed on two samples at 45-63 per cent in sediment concentration by volume, gave values of 3·5-577 Pa for the yield strength, and 0·6-27·9 Pa s for the viscosity. Based on field evidence, we have empirically estimated the yield strength and viscosity ranging between 4000 ± 200 Pa, and 108-134 Pa s, respectively. We used the Flo-2D code to replicate the debris-flow event. We applied the model with rheological properties estimated by means of direct measurements and back-analyses. The results of these models show that the rheological behaviour of a debris-flow mass containing coarse clasts can not be assessed solely on the contribution of the finer matrix and thus neglecting the effects of direct grain contacts. For debris flows composed by a significant number of coarse clasts a back-analysis estimation of the rheological parameters is necessary to replicate satisfactorily the depositional extent of debris flows. In these cases, the back-estimated coefficients do not adequately describe the rheological properties of the debris flow.