The use of detention basins to reduce flash flood hazard in small and steep volcanic watersheds - a simulation from Madeira Island

Madeira, like many mountainous volcanic islands, is susceptible to flash floods. Throughout its history, about 40 large events resulted in more than 1200 victims and countless damages. Recently, urban areas expanded greatly, leading to a higher exposure of the population to flash floods. In order to analyse ways to reduce vulnerability and decrease hazard in the urbanised, flash flood prone watershed of the Machico River, the construction of detention basins (DB) is simulated. A hydrological and hydraulic model was performed to determine if they would be a viable option to protect downstream populations. Modelling suggests that two 12 m high outlet structures, and a storage capacity of 111 298 m(3) (DB1) and 121 095 m(3) (DB2), would reduce peak discharge by 72%, from a precipitation event with a return period of 100 years. Two identified sensitive Sections (S) had their fill rates reduced from 130.6% to 79.6% (S1) and from 128.6% to 33.4% (S2), thus preventing channel overflow. A concise economic analysis was made in terms of implementation and maintenance costs, as well as a SWOT analysis highlighting that DB should be regarded as a viable engineering solution to reduce vulnerability to flash floods hazards in volcanic islands with small, steep, and urbanised watersheds.