Effectiveness of exceptional rainstorms on a small Mediterranean basin

Abstract

A comprehensive investigation of rainstorms and their consequent impacts on landscape evolution is geomorphologically important, but only scant information may be available on exceptional events, because parameters on synoptic conditions, rainstorm, landforms and hydrology for such events may be incomparable with previous knowledge. We studied an exceptional storm on April 2, 2006, in the Ramot Menashe region, Israel. Our investigation of rainfall, landslides, debris flows and channel suggests the effectiveness of such an event on the development of basin-scale morphology.The storm caused damage and casualties although it covered relatively narrow strips. Neither direct rainfall nor runoff measurements exist for the most severely affected area of Ramot Menashe, but the geomorphologic evidence combined with high-resolution meteorological radar data provides the basic understanding of the processes and hazardous conditions which prevailed at the time. In the storm core, based on estimation from meteorological radar data, 263mm of rain fell within 3h with a maximum intensity of 220mmh−1 for 10min, triggering both sporadic landslides at the soil/bedrock contact on the upper slopes and widespread landslides at the fractured/massive bedrock contact on the lower slopes. The 1st order channels on the alternation of chalk and marl also underwent erosion, and the produced sediment deposited on alluvial fans at the confluence with the main channel. The specific peak discharges for catchment size of 0.3–10km2 were 11 to 73m3s−1km−2, higher than any recorded floods in the Mediterranean climatic region of Israel. The effectiveness of the flood for geomorphic work, represented by shear stress and stream power per unit boundary area reached 87–398Nm−2 and 212–2134Wm−2, respectively. This kind of analysis can be applied to hazard prediction in other areas under similar geomorphological conditions.