Characterization and modeling of lahars in the Prêcheur river, Martinique.

The Rivière du Prêcheur is located to the West of Montagne Pelée, in the Northern part of Martinique island (French West IndiesLesser Antilles). For several decades, it has been the scene of numerous lahars directly threatening the Prêcheur village at the mouth of the river. In recent years, several major lahar events have been correlated to massive destabilizations of the Samperre cliff, 9 km upstream from the sea, creating a reservoir of loose materials at the bottom of the cliff. In 2010, a lahar overflowed a bridge and inundated part of the Prêcheur village. A new large destabilization of the Samperre cliff started on 02/01/2018, involving more than 4 × 10⁶ m³ of material. In the following days, intense rainfalls triggered several lahars that reached the Prêcheur village but stayed in the river bed. Since then, lahars and collapses have continued to occur, even though their frequency decreases with time and their intensity is smaller compared to the onset of the crisis. For instance, one single lahar overflowed the river bed on the 22/02/2018 without significant impact on the infrastructures. A field mission in Februray 2019 was carried out. It enabled to characterize the granulometry of the lahar deposits and the geomorphology of the river bed and banks. The deposits of the debris flows and hyperconcentrated flows are mainly composed of sand, gravel and boulders, suggesting predominantly frictional flows with limited viscosity. Four scenarios were identified as potentially hazardous for the Prêcheur village: 1) a new large destabilization of the Samperre cliff, 2) the remobilization of the material at the base of the Samperre cliff, 3) the remobilization of previous lahar deposits in the river bed, and 4) the failure of natural dams in the upper part of the river. These three scenarios were modeled with 2 shallow-water numerical models, SHALTOP and r.avaflow, and the results are compared to geomorphological observations as well as measured flow velocities.