Evaluation of the equilibrium of the River Nile morphological changes throughout “Assuit-Delta Barrages” reach

Abstract

When river channel morphology is changed under dynamic hydrologic conditions, the river re-adjusts itself in terms of dimension, profile and pattern to recover its former equilibrium. In general, a river always tends to experience a state of equilibrium through a process of erosion and deposition. Erosion at one location is almost balanced by deposition at another. The present study aims to evaluate the dynamic morphological equilibrium status of the River Nile over a specific period of time, in terms of riverbed and plan form. To achieve this purpose, Reach (4) of the river between Assuit and Delta Barrages throughout the period between 1982 and 2004 was selected as a case study. Two sets of 296 cross sections (x-secs) of 1982 and 2004 representing the reach were obtained. Also, 3 discharge values of 1982 (low, medium, and high) and their corresponding water surface profiles (WSPs) were collected. The study used the volumetric approach in comparing the aggradation/degradation volumes. X-sec water areas and top widths were computed and compared for the same x-secs of 1982 and 2004 under the 3 WSPs. Then, the volumes of sediment deposition/erosion along the river were computed by the summation of the computed water area difference (WAD) at each x-sec (+ve WAD means deposition & −ve WAD means erosion) considering the third dimension (i.e. length) equal to unity. Also, the plan form changes at each x-sec (top width difference) between the two comparison years were computed. The final comparison results showed satisfactory equilibrium rates, especially during the release of low discharge, where the equilibrium percentage (EP) was 95% and 92.2% with respect to both riverbed and plan form changes, respectively with an average EP = 93.6%. As for the medium discharge, the EP achieved 100% and 74%, respectively (Average EP = 87%). For the high discharge, the EP was 80% and 67%, respectively (Average EP = 73.5%). Finally, it was concluded that the more the discharge released, the less the average EP. Also, the study concluded that the equilibrium status of a river reach depends on the released discharges as every case reveals a different status of equilibrium depending on the inclusion or exclusion of the adjacent floodplain areas. In short, the floodplain areas that come under water due to discharge fluctuations turned out to have an essential impact on the evaluation of river equilibrium status. Keywords: River Dynamic Equilibrium, Morphological Equilibrium, Morphological Balance, Aggradation, Degradation, Erosion, Deposition, Riverbed, Plan form Morphological Changes