Combined palaeoflood and rainfall–runoff assessment of mountain floods (Spanish Pyrenees)

This paper reports a comparative study of the use of palaeohydrological and hydrological techniques to estimate magnitude and frequency of floods applied to two high-gradient, small catchments located in the Spanish Central Pyrenees: the Arás basin, 18.6 km2; the Montardit basin, 15.2 km2. Palaeohydrological (palaeoflood) techniques, based on palaeostage indicators and the application of both Manningʹs equation and critical flow equation, were used to estimate discharge values corresponding to seven cross-sections of the Montardit stream. This method yielded discharges ranging from 7 to 112 m3 s−1. Palaeoflood and historical flood analyses indicated a greater frequency of high magnitude events for both the Arás and Montardit basins than suggested by the application of two different methodologies based on precipitation data (the rational and the unit hydrograph methods). The 1996 Arás basin flood was modelled using the unit hydrograph method, with most reliable results (217 m3 s−1) obtained using a semidistributed basin model. For small, ungauged mountain basins, palaeoflood reconstruction provides an excellent tool to obtain past flood discharges on which to base the calibration of parameters (e.g. runoff threshold) used in rainfall–runoff methods. It is proposed that the combined results of long-term flood record analysis (based on palaeoflood and historical data) and of standard hydrometeorological methods will aid in the assessment of flood risks for application to the design of emergency measures and floodplain management in ungauged mountain streams.