Grenada - Flash Flood Hazard - CHARIM

The Caribbean islands are frequently plagued by floods as a result of heavy rainfall during tropical storms and hurricanes. These floods are termed “flash floods”, from their rapid onset and relatively short duration, and are directly caused by runoff produced during a rainfall event. The islands mostly consist of a central mountain range, with small catchments ranging from the centre part of the island to the sea. These catchments can be anything from 5 to 50 km2 in size. Hydrologically speaking, each island is made up of up to 50 larger catchments, with various types of land cover and soils, determining the hydrological behaviour. In tranquil conditions, the rivers have a low baseflow level, fed by local groundwater bodies constrained to the valleys. During a tropical storm, the soils on the slopes quickly saturate and literally overflow, or the rainfall intensity can be so high that the infiltration capacity of the soil is not sufficient. Hence severe overland flow and erosion may take place, leading to flooding along the river channels. The water level can rise from 0.5 m to more than 4 m at given locations, within 2 hours’ time (sometimes much less) from the start of the rainfall. Since many valleys are inhabited, especially near the coastline, these flash floods can cause great damage and casualties. The shape and condition of the river channel has a large influence of the flood behaviour: small and narrow channels quickly overflow, or channels that have a decreased size because of sediment may overflow much more quickly