Simplified simulation of rock avalanches and subsequent debris flows with a single thin-layer model: Application to the Prêcheur river (Martinique, Lesser Antilles)

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High discharge debris flows in mountainous and volcanic areas are major threats to populations and infrastructures. Modeling such events is challenging because the associated processes are complex, and because we often lack data to constrain rheological parameters. In this work, we show how extensive field data can help model a rock avalanche, and the subsequent remobilization of the deposits as a high discharge debris flow, with a single one-phase thin-layer numerical code, SHALTOP, and up to two rheological parameters. With the Prêcheur river catchment (Martinique, Lesser Antilles) as a case study, we use geological and geomorphological data, topographic surveys, seismic recordings and granulometric analyses to define realistic simulation scenarios and determine the main characteristics of documented events for model calibration. Then, we model a possible 1.9×106 m3 rock avalanche. The resulting deposits are remobilized instantaneously as a high discharge debris flow. We show that, for a given unstable volume, successive collapses allow to better reproduce the dynamics of the rock avalanche, but do not change the geometry of the final deposits, and thus the initial conditions of the subsequent debris flow simulation. The location of the debris flow initiation has also little influence on simulation results. However, progressive remobilization of materials slows down the debris flow and limits overflows, in comparison to an instantaneous release. Nevertheless, high discharge debris flows are well reproduced with an instantaneous initiation. Besides, the range of travel times measured for other significant debris flows in the Prêcheur river is consistent with our simulation results.