ABSTRACT The occurrence of high intensity storm rainfall in the western Mediterranean results in severe and often catastrophic flooding. In the Catalan coastal catchments alone, located in the northeaste of the Iberian peninsula, 296 zones have been estimated to be at risk from flooding. The change in catchment response to rainfall induced by urbanisation is one of the most dramatic of human impacts on the hydrological cycle. The basic effect of the increment of urban impervious areas on the rainfall–runoff processes is to increase storm runoff, thus increasing flood potential. Major trends of land use changes in the Mediterranean coastal streams show a marked progressive increment of the urbanised areas, basically related to tourist development of the coast. This study presents the characteristics of intense storm rainfall and flash flooding, which occur in the Ridaura catchment, located near the northeastern Spanish coast. The maximum daily rainfall values associated with significant runoff events are in the order of 200 mm over a 24 hours period and a recurrence interval of 7 to 10 years. The discharges resulting from the rainfall events are approximately 200 to 250 m3/s and produce flooding in the downstream summer resort of Platja d'Aro.