Trap efficiency of reservoirs on the Nile River
The reservoir trap efficiency is defined as the ratio of deposited sediment to the total sediment inflow for a given period within the reservoirs economic life time. The curves presented by Brune are still widely used to estimate the reservoir trap efficiencies. These curves are based on data collected from 40 normal ponded reservoirs in the USA. In the Nile River, the transported sediment is mainly cohesive material from which about 85% to 95% are suspended sediment. Data from the Roseires Reservoir on the Blue Nile show that trap efficiency decreased from 45.5% after 10 years to 26% after 30 years of operation. However, by applying Brune’s curves the estimated trap efficiency is about 79%. Recently, Siyam (2000) showed that Brune’s curves are a special case of a more general trap efficiency function which can be described by an exponential decay function. The so-called sedimentation factor β which is integrated in the equation of Siyam reflects the reduction in reservoir storage capacity. The upper and lower Brune’s trap efficiency curves can be well described with β=0.0055 and β=0.015 respectively for normal ponded reservoirs. Siyam (2000) provided an explanation for Brune’s extreme data in the semi-dry reservoirs (β=0.75) and de-silting basins (β=0.00012). The observed trap efficiency in the Roseires reservoir can be well estimated using a value of β=0.056. The simulation of the long-term morphological changes in the Nile River due to construction of Merowe and Shereik dams in Sudan using 1D numerical morphological model revealed that the trap efficiencies of these reservoirs did not follow Brune’s curves for normal ponded reservoirs. The calculated sedimentation factor β has a range between 0.015 and 0.056. In addition, the relation between trap efficiency and years of operation in these reservoirs is presented.