Hydro-sedimentary functioning of the Salines lagoon in Martinique

The need

The Conservatoire du Littoral (Coastal Conservatory) and BRGM wished to acquire new knowledge about the hydro-sedimentary functioning of the Salines lagoon, which is an important part of Martinique's historical heritage and represents an environmental challenge as a resting place, breeding ground and feeding ground for migratory birds and aquatic populations.

The results

Through the observations and analyses we carried out, we improved our knowledge of the hydro-sedimentary functioning of the Salines lagoon and gained a better understanding of its historical development over recent years. 

With regard to hydrodynamics, the work carried out confirms that:

• the annual inflow of fresh water into the Salines lagoon is much greater than the total volume of the lagoon, and constitutes essential physical and environmental forcing. However, these contributions show considerable seasonal variation: for example, the theoretical monthly recharge is twice as high during the wet season as during the dry season;
• The tides observed in the lagoon are determined by the tides of the ocean along the coast and give rise to non-linear flow dynamics. The channel linking the lagoon to the sea acts as a dynamic filter, determining the currents and the fluctuations in water level. Within the lagoon, the tidal range (around 0.07 m) is much smaller than that observed at sea (around 0.60 m), with a significant time lag (around 4 hours and 30 minutes);
• Seasonal fluctuations in water levels have also been observed and are directly linked to variations in sea levels. Sea levels between December and July are lower than those measured between July and November, forced by the effect of atmospheric pressure across the Caribbean and by regional circulation. There are seasonal water level variations in the lagoon, with an amplitude of around 0.10 m.

With regard to morphological changes, a significant reduction in the surface area of the water in the lagoon was observed between 1992 and 2006 (estimated at around 84,000 m², i.e. 10% of the current surface area of the lagoon). The creation of a hill reservoir to the north-east of the lagoon in the 1980s represents a significant usable resource and an obstacle to natural run-off. The silting described a few years ago is therefore not directly linked to poor communication between the sea and the lagoon, but to a reduction in freshwater input resulting from the development of the catchment area. In recent years (between 2006 and 2017), it would appear that the morphology of the lagoon has regained a certain balance, to the detriment of some parts of the lagoon.