The impact of rainstorms on floods in ephemeral channels in southeast Spain

This paper presents and discusses data on rainfall, stage and estimated dis charge for a large flood occurring in two catchments in southeast Spain in September 1997. Rainfall and stage were recorded using automatic logging eq uipment and discharge was estimated using measurements of channel cross-sec tions and water depth estimated from trash lines. Total precipitation in th e Rambla de Torrealvilla was 50 mm in 2 days with maximum rainfall intensit ies of 80 mm h(-1) Total rainfall in the Rambla de Nogalte was 195 mm in 3 days, with maximum intensities of 200 mm h(-1). In the Torrealvilla, this r ainfall produced three flood peaks with maximum stage approaching 2.5 m. In the Nogalte, there was only one flood peak, which was 0.5 m deep. Estimate d discharge varied widely throughout both catchments with maxima of 120 m(3 ) s(-1) in the Torrealvilla and 60 m(3) s(-1) in the Nogalte. Maximum disch arges occurred at times of high rainfall intensity, but intensity alone did not explain why some tributaries had very small discharges. Variations in discharge in the ephemeral channels were due to combinations of lithology, morphology and land use. The predominantly marl catchment of the Torrealvil la had a lower threshold rainfall intensity than the schists of the Nogalte . Within each catchment sub-basins characterised by steep, gorge like terra in and sub-basins where agriculture had been abandoned both resulted in hig her flood discharge. The contributing areas for the September storms were u p to two thirds of tributary catchment areas, Comparison of rainfall data r ecords shows that the September flood was the fifth largest on record and h ad a recurrence interval of 7 years. The largest (1973) flood, which is kno wn to have caused substantial damage and a number of deaths, was only a 30- year event. The floods on the Torrealvilla destroyed at least two check dam s and evidence suggests that these had little effect on reducing the impact of the floods. (C) 1999 Elsevier Science B.V. All rights reserved.