MORPHOLOGICAL IMPACTS DOWNSTREAM OF MEROWE DAM

Abstract

Morphological changes on the River Nile in response to Merowe Dam operation had been investigated using a numerical one dimensional model. The study covers the river channel from the dam site to Dongla town about 313 km downstream the dam. MIKE11–GST software had been used in calculating the river bed changes downstream of the dam. The hydrodynamic and sediment model had been calibrated. Using the mean outflow from the dam and applying zero bed load release from the dam; a long term simulation for calculating the river bed changes had been done. The simulation had been carried out over 100 years of the dam operation. A simulation of 50 years using the maximum out flow had been carried out to study the sensitivity of the bed to the high discharges. The result of the model had shown that the river reach form the dam to Dongla is divided into three zones depending on the morphological impact. Degradation zone extend from the dam site down to some 50 Km after 5 years and 100 km after 15 year of the dam operation and remains with no increase towards the downstream. The degradation zone is followed by an aggradation (sedimentation) zone which extends about 50 km right on the big bend of the river Nile around Al-Daba town. The sedimentation zone increases with time towards the downstream. Following the aggradation zone there is a stable bed level zone extending from chainage 150km to Dongla. The stable zone decreases with time. The maximum degradation in the first 10 km after 100 yeas is about 5m associated with water level drawdown of about 3m. The average degradation from chainge 19.5 km to chainage 100km is 1.3m. The entire river reach at the end of the model is in an aggradations zone with 0.84 m increase in the river bed elevation. The high sedimentation over a long area of the model domain indicates to a high degradation process in the first 100 km downstream Merowe Dam. The degradation had been found sensitive to the high discharges, accordingly reducing the high flood peaks will significantly reduce the degradation downstream the dam.