The Prêcheur river catchment belongs to the Montagne Pelée volcano massif located in the north of Martinique island, in the Lesser Antilles arc. In June 19–20, 2010 two high-discharge lahars overflowed and partially damaged the Prêcheur bridge, interrupting the road traffic for several days, and inundating part of the Abymes quarter. These non-eruptive, rain-triggered lahars were the consequence of the mobilization of loose volcanic material accumulated in the river bed by a massive landslide of the Samperre cliff that occurred on May 11, 2010.
In order to improve lahar risk assessment for the Prêcheur river, we review the existing knowledge on lahars and floods for the period 1851–2011. We improved the database mainly on the basis of geophysical (seismic and acoustic) data for the most recent period (1980-present). We show that: (1) the largest event ever observed in Prêcheur is the eruption-related lahar of May 8, 1902 at 06:00 UTC, (2) high-discharge, non-eruptive lahars able to partially destroy or submerge the bridge occur every 10.3 years in average since 1950, (3) geophysical records show that there are more low magnitude lahars than previously recognized because witness accounts report the larger events and therefore filter out the smaller ones; by contrast geophysical instruments are able to detect both high and low magnitude events, (4) the recent paroxysms of 2009–2010 belong to a landslide-lahar crisis which started in 2004; the onset of the landslide-lahar crisis was unnoticed by the population due to the low magnitude of the events.
The results of this compilation stress the importance of the presence of geophysical instrumentation sufficiently close to the river for monitoring small scale landslides and lahars. Indeed, these low magnitude events are likely to be unnoticed by the local population and therefore likely to be absent from the collective memory and local newspapers. However, they can inform about the imminence of more significant events, as observed in 1980 and 2010.