Hydrodynamic characterization of past flash-flood events and their associated hazards from dendrogeomorphological evidence in Caldera de Taburiente National Park (Canary Islands, Spain)

Abstract. Las Angustias river is an ungauged stream located in the Caldera de Taburiente National Park (La Palma, Canary Islands, Spain), where frequent intense flash-flood events occur, sometimes with fatal consequences (4 deaths, November 2001) and considerable financial implications (over 700 000 euros in recent years). The aim of this research is to analyse the flood hazard at the Playa de Taburiente, one of the most popular sites in this protected area, with more than 60 000 visitors per year. The use of classic data sources and hydrological or hydraulic modelling methods for flood hazard analysis has important limitations in this area because of incomplete precipitation and flow data information and low representative values of the statistical time series, which may lead to under- or over-estimated results. Alternative or complementary data sources and methods including palaeohydrological techniques can therefore be used here for flood hazard analysis. A detailed dendrogeomorphological study of the river system was carried out using Canarian pine trees located on the stream bed and river bank with external evidence of flash-flood damage, including scars and exposed roots. The preliminary results identify and date at least eight winter flood events between 1962–1963 and 2011–12. In spite of the uncertainties deriving from the incomplete precipitation data and the mobile alluvial riverbed, the models provide an estimate of past flood discharge magnitudes. E.g. for the 1997 flood event a 1235 m³ s^-1 flood minimizes the RMSE over the disturbed tree sample; furthermore, this flow value clearly exceeds the return period considered and means a distinct behavioral change in this gorge, from a braided channel with emerged bars to a single channel occupying the whole river bed. These numerical results and maps could improve flood hazard and risk analysis and should be useful for the national park land use management and visitors planning.