The Effects of Annual Extreme Weather Conditions on the Exchange Flows between Lake Bardawil and the Mediterranean Sea

Abstract

Lake Bardawil is a hypersaline, shallow coastal lagoon located along the coast of the northern Sinai Peninsula in Egypt. Coastal systems are understood to be hypersaline when the salinity exceeds values of 30 parts per thousand throughout the year.
Being located in the arid climate region of North Africa the lagoon is subject to year-round extreme weather conditions which are dominated by high air temperatures, extreme evaporation rates, and limited precipitation. All the above in combination with the lagoon’s shallow bathymetry and restricted freshwater input causes Lake Bardawil to inhibit hypersaline conditions throughout the whole year. For Lake Bardawil this means average salinity values in the order of 42 – 51 parts per thousand.
The here conducted study is the first in the line of five successive studies focusing on three-dimensional hydrodynamics. Making use of the three-dimensional hydrodynamics, the movement of the water and salt through the lagoon is considered. Here the response of the system to the extreme meteorological conditions around Lake Bardawil is investigated. The meteorological forcing causes subtidal flows in coastal waters which are responsible for the propagation of waterborne materials such as salt and pollutants.
The results show that during periods of extreme meteorological forcing the response of the lagoon and its inlets is in line with expectations one might have from literature. During periods and events of high evaporation, the lagoon shows an overall increase in top and bottom layer salinity. Furthermore, the largest compensation flow due to evaporative losses of approximately 34 m3/s from the Mediterranean Sea into the lagoon is observed. While the lagoon waters act rather inert as shown for the average conditions during the month of April, it is the wind that initiates the motion. Successively the phenomenon of gravitational circulation in the deeper areas around the inlets can be observed with more saline waters propagating towards the inlets, along the bottom layer. At times, these processes cause subtidal velocities up to 0.15 m/s, which were computed in the inlets.
Crucial for the overall response of Lake Bardawil to meteorological forcing is the complex lagoon geometry and shallowness. Here the tidal divide, on the verge of the western bottleneck to the eastern basin is found to be of great importance. At this location very calm flow conditions are present promoting stagnant waters. The eastern basin with its vast areal extent experiences wind-driven circulations, while the western bottleneck is bound by its narrowness and tide dominance. As a consequence, the western inlet Boughaz 1 can be described as continuously well-mixed, while its eastern counterpart, Boughaz 2, experiences periodic stratification.
The application of system adaptations as proposed by The Weather Makers (TWM) proves to increase the exchange flow with the Mediterranean Sea. Here the dredging of two new inlets as well as dedicated tidal gullies through the lagoon established a more dynamic system where the inner lagoon connectivity is significantly enhanced. This is evident by the widespread presence of Mediterranean waters and the decreased salinity in a broad stretch along the barrier island as well as on the full lagoon scale. However, the lagoon waters in remote locations along the fringes of the large eastern basin remain of high salinity and show still little interaction with the tidal flow.