Modelling the water dynamics of a tidal lagoon: The impact of human intervention in the Nador Lagoon (Morocco)

Abstract

The Lagoon of Nador (Morocco) has been subject to relevant man-made changes in recent years, which consisted of closing the inlet and opening a new one, wider and deeper, with long lateral jetties. In this paper, we study some of the consequences of this modification, using a 3D hydrodynamic model that allows a comparison of the two configurations (old and new inlet). This would be not possible analysing only the observations and the approach we followed is a general method that can be applied wherever man-made or natural changes affect a coastal system. We have created two numerical grids, one with the old inlet and one with the new inlet, both covering the entire lagoon and a wide part of the coastal area. Using these two configurations, fully baroclinic simulations were run for one year. The results show a significant increase of the tidal amplitude and of the water circulation inside the lagoon, with a tidal prism four times larger. The change of the tidal asymmetry from positive to negative values (−0.7 in the inlet) causes a shift toward the ebb dominance, with implications for the mid-term morphological evolution. With the new inlet the water renewal time of the lagoon decreased from 57 to 15 days and the water temperature got warmer in winter (+1 °C) and colder in summer (−2 °C). As a consequence of this, the evaporation lowered and the salinity reduced by 0.8 PSU. The seasonal variability of the water renewal time shows that the tidal forcing, the baroclinic circulation and the wind contribute to the mixing processes. The mixing capacity of the lagoon (0.7) did not change because it is an intrinsic property of the lagoon morphology and of the atmospheric forcing. Finally, we found that the change of the water renewal time, with the new inlet, is higher than the change predicted, with the old inlet, under the RCP8.5 climate change scenario.