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 appl ied to two high-gradient, small catchments located in the Spanish Central P yrenees: the Aras basin, 18.6 km(2); the Montardit basin, 15.2 km(2). Palae ohydrological (palaeoflood) techniques, based on palaeostase indicators and the application of both Manning's equation and critical how 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 m(3) s(-1). Palaeoflood and historical hood analyses indicated a greater f requency of high magnitude events for both the Aras and Montardit basins th an suggested by the application of two different methodologies based on pre cipitation data (the rational and the unit hydrograph methods). The 1996 Ar a's basin flood was modelled using the unit hydrograph method, with most re liable results (217 m(3) s(-1)) obtained using a semidistributed basin mode l. For small, ungauged mountain basins, palaeoflood reconstruction provides an excellent tool to obtain past flood discharges on which to base the cal ibration of parameters (e.g. runoff threshold) used in rainfall-runoff meth ods. It is proposed that the combined results of long-term flood record ana lysis (based on palaeoflood and historical data) and of standard hydrometeo rological methods will aid in the assessment of flood risks for application to the design of emergency measures and floodplain management in ungauged mountain streams. (C) 2001 Elsevier Science B.V. All rights reserved.