Rainfall-induced lahars in the Belham Valley, Montserrat, West Indies

Rain falling on loose volcanic debris over the Soufrière Hills Volcano, Montserrat generates hazardous floods in the Belham Valley. These rainfall-induced lahars vary greatly in discharge and sediment concentration in space and time. They differ from examples documented on other volcanoes in that: (1) the eruption has been continuing since July 1995, generating repeated pulses of excess sediment; (2) rainfall is the only significant trigger; (3) the system is small, with short distance to the sea and relatively low altitude at the catchment top. Repeat mapping and comparison with pre-eruption data demonstrate significant geomorphological change, with c. 120 m shoreline progradation and c. 0.4 m a−1 mean aggradation rate in the middle to lower valley. The nature of the hazard and area of risk have changed as the valley has aggraded, the channel widened and the runoff efficiency increased (as a result of rilling and vegetation removal). Lahars in the Belham Valley correlate with days when >10 mm rain fell in 24 h, with more events triggered in the late rainy season. The flows are mainly Newtonian but one non-Newtonian flow event has been demonstrated and is described in detail (20 March 2000). This flow is explained by direct volcanic ash input to the runoff.