Reconstruction of a destructive debris-flow event via numerical modeling: The role of valley geometry on flow dynamics

Debris-flows are widespread natural phenomena characterized by high mobility (high velocity and long runout distance) and impact forces, which frequently cause human casualties and significant damage to infrastructure. To better understand the dynamics of such events, analyzing in particular the effect induced by the valley geometry on flow velocity, runout and mobilized volumes, in this work we reconstruct a real debris-flow event through numerical modeling. Specifically, we used a modified version of the BING model, a fluid-dynamic depth-integrated numerical model for debris flows, which has been properly modified to account for width changes along the valley. The studied event, which occurred in Scaletta Zanclea (Messina, north-eastern Sicily, Italy) on October 1, 2009, is exceptionally well constrained by field and topographic information. In this respect the flow velocity, estimated from two specific locations on the basis of field evidence, the distribution of erosional and depositional areas along the Racinazzo valley, based on the comparison of pre- and post-event digital elevation models (DEMs), and the runout distance were used as constraints to calibrate the model. Furthermore, we report a detailed description of the main event characteristics based on hydrological records and witness reports. The numerical modeling results are consistent with witness reports and the severe damage recorded in the Scaletta Marina village, and highlight the effect of the valley geometry on both the debris flow velocity and the erosion/deposition processes. The effect of changing valley width has been also quantified, resulting in accelerations of the debris in correspondence of the valley narrowing and stagnations at the plateaus. © 2015 John Wiley