The Nile River bifurcates at El-Qanater city (about 20 km north to Cairo) into two branches which are Rosetta branch (the western) and Damietta branch (the eastern), the two branches enclosing the Nile Delta and forming the Nile Estuary. The two branches discharge the Nile water into the Mediterranean Sea. The discharge of the two branches is controlled through several water structures.
The interaction between the Nile and the Mediterranean Sea considered in this research includes water and salinity transport. Considered as a large scale case with a complex geometry, multi-dimensional models for the Nile Estuary were set up in this research, and the TELEMAC-MASCARET modeling system was used for this purpose. The main aim of the research was to simulate the current conditions of flow and salinity transport in the Nile Estuary to improve the process understanding and to investigate possible changes due to the anticipated sea level rise. The Nile Estuary is a tideless estuary, this type of estuaries are more complex in terms of salinity transport than tidy estuaries.
The current status was first modeled based on the mean conditions of flow and sea water level, in order to investigate the propagation of saltwater inside the Nile. Then scenarios for the sea level rise were assessed to evaluate the influence of sea level rise on the propagation of gravity currents.
A two-dimensional model was first set up using TELEMAC2D, although the salinity transport is density-driven flow and therefore it is mainly a three-dimensional phenomenon. The capability of TELEMAC2D to simulate flow driven by horizontal density differences was first checked using two case studies (rectangular and trapezoidal cross sections) and finally it was verified and hence it was used to simulate the Nile Estuary.
As the density-driven flows (gravity currents) are the major phenomena that govern the transport of the saline water between the Nile and the sea, a 3D model to simulate these phenomena in two cases of lock-exchange experiments was first set up using TELEMAC3D in order to verify it. The results of the numerical model were compared with experimental results. The model showed high accuracy and it was concluded that the TELEMAC3D is capable of simulating such phenomena. It was also concluded that a non-hydrostatic simulation and the use of complex turbulence model achieve higher accuracy. Thereof, a 3D model for the Nile Estuary was set up to simulate the gravity currents and to assess the stratification of salinity in the Nile. The model simulated complex phenomena in a complex natural geometry.
It was concluded that the salt wedge is a stratified fluid in which the salinity decreases from the bottom towards the surface where a layer of less brackish water exists. The salt wedge was fluctuating, although steady boundary conditions were imposed, and it was not stagnant as it was expected, which could be caused by the weak balance between barotropic and baroclinic gradients in tideless estuaries as the case of the Nile Estuary.
For both the 2D and the 3D models of the Nile Estuary, three scenarios for the sea level rise were also analyzed to study its impact. Based on the results of the models, it was found that there was an intrusion for the saltwater inside the Nile, and any increase in the sea level will cause further intrusion. The intrusion length increased by 1.2 km, 5.1 km, and 6.6 km in case of sea level rise of 0.24 m, 0.69 m and 1.0 m, respectively. To keep the current status of balance, in terms of saltwater intrusion inside the Nile in case of sea level rise, the discharge of Edfina barrage has to be increased. However that would affect the water budget of the country as this water will be discharged into the sea being considered as losses.
Comparing the 2D and the 3D model, the hydrodynamic results were similar in terms of water levels and discharge, however the flow field was different as the secondary currents could be seen only in the 3D model. For the salinity transport, the intrusion length was much higher in the 3D model. The stratification of the salt wedge can only be seen in the 3D model. So, the 2D model can be used for calculating water levels and average water velocity. For the salinity transport, the 3D model is more suitable, the 2D model can only be used as a rough estimation.