Assessment of dam failure flood and a natural, high-magnitude flood in a hyperarid region using paleoflood hydrology, Nahal Ashalim catchment, Dead Sea, Israel

1 A dam failure flood in 1993 with a peak discharge of 600-700 m3 s(-1) (volume of 0.5 x 10(6) m3) and a natural, rainfall-runoff flood with a peak discharge of 420 m3 s(-1) (volume of >2.0 x 10(6) m3) in 2004 in the Nahal Ashalim catchment (75 km2) in the hyperarid Dead Sea region were comparatively analyzed. The two floods, which caused serious damage to the Dead Sea Works industries, were analyzed for magnitude and frequency using regional analysis, extrapolation from the short systematic record, rainfall-runoff modeling, and paleoflood hydrology. The 7600-year paleoflood record was reconstructed at the Ashalim Cave at the upper Nahal Ashalim. Peak discharges in this high-gradient, mountainous wadi are highly sensitive to the roughness coefficient and the flow regime. The dam failure peak discharge is lower than the envelope curves of measured floods of the Dead Sea region and of paleofloods of the Negev and the Dead Sea deserts. The probability of the dam failure flood and the natural flood were estimated by various methods at >1% to >0.2% and 0.5%, respectively. Adding the peak discharge of the October 2004 flood increased the probability from >0.2% to 0.5-0.2%. The long paleoflood record encompasses different hydrological regimes related to climatic fluctuations, which are not relevant to the present hydrological regime; therefore the chosen period for frequency analysis was A.D. 1673-1996.