Hydrodynamic and ecological functioning of the Tunis North Lake by a numerical modelling approach
This work deals with the modelling of the hydrodynamic and ecological functioning of the Tunis North Lake. The Tunis North Lake is situated north-east of Tunisia, its surface is about 24 km2 and the average depth is 1.5 m. The lake is connected to the Gulf of Tunis by the Khereddine channel through ten sluices functioning under tide’s effect. In fact, five of these sluices are opened on flood tide and the others on ebb tide. These sluices were built with a separation dam, in the centre of the lake, during the period between 1985 and 1988, in order to improve the water circulation and the quality of the lake. We then studied the actual hydrodynamic functioning of the lake by using a 2D vertical-integrated model. The numerical software used is the «Surfacewater Modeling System» (SMS). The initial code, SMS, does not simulate the change of the boundary conditions through the time of simulation, as it really occurs on the Khereddine sluices, which open in an automatic way according to the tide: high tide (opened northern sluices and closed southern sluices) and low tide (closed northern sluices and opened southern sluices). Therefore, first of all, we modified the source code of software «SMS» to take into account this specification of the Tunis North Lake. After these modifications, we carried out the calibration of the model by basing outselves on recent hydraulic measurements performed in December 2005. The model’s calibration enabled us to simulate the average hydrodynamic functioning of the lake, by imposing average boundary conditions: An average tide of marling 20 cm at the Khereddine sluices and a constant flow on the power electric station of La Goulette. The results of these simulations showed «a correct» functioning of the Tunis North Lake through an exchanged flow with the sea of 66 m3/s, an exchanged daily volume of 1.65 Mm3/day, a velocity field that scanned all the surface of the lake and an average residence time of water in the lake of approximately 21 days which can reach 16 days with maximal tide and western wind of intensity 6 m/s. The following stage consisted in the development of a zonal ecological model taking into account specificities of the lake and the data which are available. This model was coupled with the hydrodynamic model that is presented in the preceding stage, in particular with regard to the exchanges between the zones (or sea) defined in the ecological model. The ecological model computes the evolution of the biomasses of the macroalgae, phytoplankton and zooplankton as well as nutrients in detritus, water column and sediments, the salinity, the dissolved oxygen and the chlorophyll-a concentrations. In this model, the growth of phytoplankton and macroalgae depends on the water temperature, light intensity and nutrient. Competition between phytoplankton and macroalgae for nutriments is simulated. The computation was carried out in two steps. The first step consists in the preliminary calibration of the model on the basis of measurements of water quality performed during the year 2002 and the parameter update used by HALCROW (Sir William Halcrow & Partners Ltd) (in 1990) which enabled us to bring closer the results of the model to the actual characteristics of the lake. The cyclical nature of the population density of macroalgae was reproduced, with a bloom throughout the spring especially in the southern part of the lake. In the second step, we carried out a five-year predictive simulation of the ecological functioning of the lake from the year 2002. The results of this simulation showed us that the lake presents good ecological conditions. The results of the present ecological model are the best results that we can obtain using the available data, but further measurements of phytoplankton, zooplankton, macroalgae and nutrients in the sediments can improve considerably the results of this model.