Beaches are quite remarkable since they are formed by waves bringing sand from the sea bed onto the coast against the force of gravity. In fact waves both bring sand to the shore and take it away again and it only accumulates to form beaches when the conditions are right. Some of the factors which influence this are the slope of the underlying bedrock, typical wave strength, concave coast line and offshore structures such as rocky outcrops and coral reefs.
As soon as Mauritius formed, the soft rocks that form soils inland were washed away around the coast exposing the underlying basaltic rock which was slowly eroded by wave action into black basalt sand, as found in Sable Noires. Initially the beaches of Mauritius would have resembled those of the Canary Islands; indeed, dig deep enough at many beaches around the island and you will find a substrate of black sand. As the protective fringing reefs slowly formed out to sea by massive and encrusting corals, the lagoon became tranquil enough for relatively fragile, fast growing coral to form on exposed rock. These were grazed by lagoon fishes, such as parrot fish, which crushed them into powder as they digested them. This is the main source of fine white coral sand on many beaches. Coarser sand formed as waves from storms and cyclones smashed these fragile corals, while flecks of colour in the sand are mostly from broken shells.
It is important to remember that beaches are a dynamic process rather than a fixed feature and the sand below the high tide mark is constantly being moved around. Waves which hit the beach obliquely and long-shore currents displace sand along the coast. Sand is also lost from the beach as the wind blows it into sand dunes and even further in land. It is also lost from the lagoon by being pulled by currents through passes in the fringing reef. These are natural exits for water to leave the lagoon, especially between high to low tides and also contribute to the long-shore currents which flow parallel to the coast.
Long term changes in sea level caused by ice ages and interglacial periods have modified the position of high and low tides and the shape of the coastline. However, the processes of sand formation have generally outweighed those of sand loss, allowing the beaches to slowly accumulate. In some areas, so much sand has been deposited over time that the coastline has grown outwards into the sea for hundreds of metres. A good example is Pointe aux Canonniers whose whole extent is basically one massive sand bank covered with a relatively thin layer of top soil.
Historical human impact in Mauritius has resulted in an enormous sand deficit and coral loss within the lagoon as they were mined for aggregate for construction and to produce lime for sugar production. Although this is now outlawed, sand is still being lost from beaches at unnaturally high rates because of accidental transportation on people’s feet, shoes and the tyres of vehicles. It is also being lost from the lagoon through artificial passes that have been blasted through the fringing reef and natural ones that have been widened and deepened for larger pleasure craft. These have also increased the strength of long-shore currents, transporting sand along the coast. Human impact has also reduced the rate at which sand is produced both by the dramatic reduction in coral growth within the lagoon and the almost complete loss of coral grazing fish.
Rising sea levels would normally cause beaches to migrate inland exposing the sand that had previously accumulated there. Without any understanding of long term beach dynamics, we have constructed buildings on much of this buffer area, preventing migration. Whenever the sea reaches a solid wall, incoming waves deposit less sand and reflected waves carry it away at an increased rate causing beaches to rapidly disappear. This can be seen at various private coastal developments where walls have been constructed to retain soil that has been placed to create level gardens. Groyns (cages of rock) and solid jetties have been used to try to interrupt long-shore currents. In the short term they cause local sand accumulation, at the expense of beaches downstream, but storms and cyclones will inevitably remove this trapped sand.
Strategically placed offshore structures can be used to enhance sand deposition in suitable locations, hence preserving current beaches and perhaps creating new ones in the short term. Care must be taken to prevent the formation of new long-shore currents and ensure the one beach isn’t preserved at the cost of losing another further along the coast. Under current conditions, however, such solutions would only be temporary.
Sea level rise due to climate change threatens the survival of our natural beaches in the medium term. To reverse the anthropogenic component, it is imperative that we persuade other countries to help restore atmospheric carbon dioxide levels to their pre-industrial levels. We will only succeed in this if we set an example in making the transition from fossil fuels to renewable energy and engaging in aggressive reforestation. In the mean time, we must take fairly drastic adaptation measures by allowing our beaches to migrate inland into the buffer zones of accumulated sand. This requires strict coastal management, the demolition of coastal property and restoration of the profile of the original sand slopes.
Even without rising sea levels, in the long term, our beaches will slowly shrink due to a shortfall in the sand production-loss balance. To reduce loss inland we must make sure we leave sand on the beach by removing it from our feet as we leave and avoid parking vehicles on the sand. Artificial and artificially enlarged passes through the fringing reef must be closed to retain sand within the lagoon and restore long-shore currents to their natural states. To restore sand production, it is vital to encourage the growth of coral within the lagoon which means drastically reducing the factors which limit it (see Coral Reefs) and replenishing the lagoon with coral grazing fish.