Estimation of sedimentation rates under Mediterranean conditions deduced from the Mishmar Ayyalon Reservoir, Israel

The Mishmar Ayyalon Reservoir was constructed in 1955 and its watershed drains the upper basins of Nahal Ayyalon and Nahal Nachshon (160 km²). It has a water storage capacity of 7.3 × 10⁶ m³ and is the only reservoir located within the western-flowing drainage basins that drains the mountain backbone of central Israel. Because the reservoir has never overflowed, the sediments that accumulated on its floor during the last 44 years provide a unique record of sedimentation rates and total sedimentation. A pit 3.3 m deep was dug into the sediment and variable-thickness layers, associated with specific years or flood events, were identified. Based on the sediment that was exposed in the pit and considering the original volume-elevation data of the reservoir, we have estimated the total sediment volume. Specific sediment layers were correlated with specific storms (and their resulting floods), based on three distinctive historical marker beds. The total sediment volume, which has been deposited by at least 22 significant floods, was calculated to be about 456,000 m³. Approximately 30% of the sediment reached the reservoir during the two largest consecutive floods of the winter of 1963/4. Approximately 70% of the sediment was produced by a small number of very large floods. The mean annual specific suspended sediment yield for the catchment of the reservoir is 114 t km^-2 year^-1, which is a typical sediment yield for land under minimal cultivation.