Hydraulic Planning in Insular Urban Territories: The Case of Madeira Island—São João Stream, Funchal, Portugal

Abstract

This study’s primary goal was to conduct an analysis regarding the flood susceptibility of the main watercourse of the São João (Funchal) drainage basin. In addition, if proven necessary, we also aimed to suggest mitigation measures, such as sizing a detention basin and promoting adjustments of the riverbed’s roughness coefficient. This study also resorted to geomorphological data—obtained during the watershed characterization process—that were then utilized in the SIG ArcGIS software, in order to estimate the expected peak flow rate, considering a return period of 100 years using the Gumbel distribution. Finally, the Manning–Strickler equation was utilized to determine the river discharge point’s drainage capacity; the reason for that was to verify whether its drainage capacity was sufficient to drain the entire volume of rainwater associated with an extreme flood event. In summary, the results obtained by this study indicate that the drainage capacity of the river discharge point of the São João watershed (Funchal) is insufficient when considering an extreme flood event, for a return period of 100 years. Hence, it became necessary to explore the two aforementioned mitigation measures: first, regarding the detention basin, its sizing was calculated through both the Dutch method and the simplified triangular hydrograph method; second, aiming to increase the drainage capacity of the river discharge point, it is suggested that the roughness coefficient should also be modified.