Rainfall analysis in mountain streams affected by torrential floods on Madeira Island, Portugal

Abstract

Torrential floods are powerful and destructive events that result from a mix of debris and water moving rapidly down steep channels in mountainous areas. They are normally triggered by heavy rainfall. Predicting these flows is crucial, and analysing critical rainfall is essential for civil protection early warning systems. Empirical rainfall thresholds for Madeira Island were established using historical data on torrential floods and rainfall from 2009 to 2021. The analysis differentiates between antecedent rainfall, calculated with a power law for drainage over time, and event-specific rainfall, determined as the maximum 24 h precipitation during intense rainfall events. Our findings show that antecedent rainfall plays a significant role in triggering torrential floods on Madeira Island. Calibrated antecedent precipitation over 15 d (110 mm) and 30 d (130 mm), combined with a maximum rainfall of 250 mm in 24 h, exemplifies critical conditions. A strong correlation between maximum precipitation over 24 and 12 h was also found and can be described by a linear regression model, aiding in predicting critical 12 h rainfall maximums. Rainfall-runoff modelling also revealed preliminary findings on the relationship between catchment area, sub-daily precipitation peaks, and peak discharge. As far as we know, this is the first study to introduce combined rainfall thresholds for the occurrence of torrential floods in Madeira Island catchments.