Lagunas litorales intermitentes en Andalucíaun modelo para la gestión

Résumé

Coastal lagoons are transitional water bodies located on coastal areas isolated from the sea by a barrier but joined to it through an inlet. The conjunction of an important number of agents in these areas stimulates the appearance of gradients and a high productivity. This fact lets coastal lagoons interchange mass, momentum and energy between the ocean and the continent and also with the atmosphere. This task has a great importance for the maintenance of environmental processes with interest at the local and the global scales, being defined as essential and unique. Andalusian coastal lagoons have the main following characteristics: smallness, intermittence, shallowness, high area/depth ratio, one dimensional morphology and no appearance of flood or ebb-deltas. Actually, authorities are giving importance to wetlands due to their ecological and socio-economical functions. If a lagoon accomplishes these functions, it has a regular behavior; if not, the system moves into a failure mode. This mode entails some environmental or socio-economic risks, which depends on the probability of failure and its consequences. The analysis of the theoretical behavior of coastal lagoons and the development of some tools to simulate the response of the system to different agents is a necessary requirement for integrated coastal managing purposes. This reasoning constitutes the main justification of this thesis. In order to facilitate managers' decision process, a simple numerical model reproducing the most important responses of coastal lagoons is implemented (Chapter 3). Before that, a qualitative description of these systems (Chapter 1) and of the interactions between coastal lagoons and agents (Chapter 2) is developed, being focused all the work on Andalusian coastal lagoons. Finally, the model has been applied to a specific coastal lagoon located on the Atlantic coast of Andalusia (Chapter 4). Agents causing evolution of coastal lagoons are: astronomical tide, meteorological tide, storm surge, waves, rainfall and river discharge, solar radiation and evaporation. Instantaneous, state and basic variables are identified for each of the agents when possible. The variables describing the system are also exposed. The responses of coastal lagoons to the agents are described in more detail, taking special relevance inlet opening and closure and inlet equilibrium. To simulate coastal lagoons quantitative responses, I have implemented a numerical model reproducing hydrodynamic, sediment transport and energy balance in Matlab language. The geometry of most of Andalusian coastal lagoons makes a 1-D approach accurate enough, and therefore, Saint-Venant equations are solved using a numerical scheme: MacCormack-TDV (García-Navarro et al., 1992) with the improvements of the improved mass conservation method (Tseng, 2003). The Courant-Friedrichs-Lévy condition (CFL) is used as stability criterion. Dry and wet technique is implemented to simulate intermittency. This model is verified with some the analytical solutions of Ippen (1966) for simple cases. Sediment transport is computed by Van Rijn (1984a, 1984b) empirical formulation; Exner equation is used for sediment conservation. Energy is computed according to the eulerian balance proposed by Stull (2000). Eventually, the model is applied to a real study case: Zahara de los Atunes lagoon (Barbate, Cádiz). This coastal lagoon is closed almost all the year except during the summer, when it is artificially open. The bay is 1300 m long, 50 m width and 0.6 m depth. Inlet, when open, is 160 m long and 13 m width. This lagoon has some environmental problems related to water quality and flooding. The numerical model is calibrated with measured data collected by three current meters (two vectors and an ADCP) and a meteorological station during a field trip specifically desing to this purpose. Manning's roughness coefficient is used for calibration. After that, some tests are carried out to understand the lagoon hydrodynamics when working on operational or extreme conditions. The main results of the application of the model to the Zahara de los Atunes coastal lagoon are that the bay tide shows a pumping-mode response and an asymmetric damped oscillation when compared with the ocean tide. The lagoon is flood-dominated, which means that it is a sink for marine sediments and that the inlet tends towards closure. This hypothesis has been also checked with the Escoffier curve and the stability criterion of Byrne et al. (1980). Moreover, the water renewal in the upper part of the lagoon is scarce, causing water quality problems even when the lagoon is open. Regarding the sea level rise, if Losada et al. (2008) hypothesis of 2-3 meters of sea level rise by 2300 become true, Zahara de los Atunes village will result almost completely flooded by then. The recommendations for this lagoon are to increase the inlet cross-sectional area, when the inlet is artificially opened, to values near the Escoffier curve stability point. In this way, the lagoon would remain open for a longer period of time and the tidal prism would be higher, facilitating water renewal and improving water quality. It would be also interesting to open the lagoon not only during the summer, but also during the winter. Events and mainly storms would remove sediments from the inlet, contributing to the opening maintenance. In the long term, preventive policies should be prepared to minimize the economical losses and the Zahara de los Atunes population migration.