What is happening to our beaches? Some are predicting that within a matter of decades they will all be eroded away. After one storm, the beach at Pointe aux Canonniers had lost an enormous amount of sand, making the predictions seem true. However, within weeks the sand was returning and the beach getting back to normal.
By thinking of our beaches as part of the mainland, we try to prevent them eroding by using many of the same methods. However, if beaches are significantly different, is it possible that we might be doing more harm than good?
Mauritius was formed by volcanic activity in three main stages between 10 million and 20,000 years
ago1 The three stages produced different types of basalt rock, none of them, however, contribute to the significantly to the sand of our beaches. They largely consist of sand and debris from dead coral and abandoned shells. Therefore, our beaches were not originally part of the island but are instead a gift of the ocean.
Simplistically, healthy beaches are in a state of dynamic equilibrium where the amount of sand being deposited by each wave as it moves up a beach is balanced by the amount it removes as it recedes again. Beaches exist where, at various points in time, this balance was in favour of deposition. In Mauritius, sand is naturally lost from this dynamic system by being blown in land or by being washed out of the lagoon. Fortunately, due to continuous coral (and shell) growth and destruction, the sand in the lagoon is being slowly replenished.
Travelling around the coast of Mauritius, it is clear that beaches do not form everywhere. Yet the bed of the lagoon contains an astonishing volume of coral and shell debris. So what causes the sea to deposit sand in some places but not others?
We can learn much by considering why beaches form in the places they do. While the topology of every beach is different and each must be considered uniquely, some general conclusions can be drawn. Deposits of sand tends to be found where:
- The coastline is concave
- The reef is near to the shore
- There are rocks near to the shore
However, sand tends not to be deposited:
- Where there is no fringing reef
- Where the water near the shore is very deep
- Where the slope of the shore is very steep
- Where the coast is convex
- At river mouths
- Where long shore currents flow within the lagoon
- Where trees are adjacent to the water
- Within inlets
Looking at the bed of the lagoon where there are trees near to the shore one finds soil mixed with sand to form a muddy sludge. Clearly these are the locations where the sea is actually eroding the land. These muds are far more stable than sand, especially when overgrown with marine vegetation, and cannot be moved and deposited on land by the waves.
So the first necessary condition for a beach to form is that the sand in the lagoon is clean.
Where the sand is not clean, it is a sign of land erosion. This is confirmed by the trees growing on very fertile land adjacent to the sea. Further erosion can be prevented by planting mangroves along the edge of these coasts using species appropriate to the conditions at each location. They will offer significant protection for the land from storms and tidal waves.
Considering more carefully the locations where beaches form, it becomes clear that sand deposition does not occur where the movement of water is too great, for example, where there is no fringing reef and where there are long shore currents within the lagoon. Equally, there is no sand deposition where in parts of inlets where the water is too still.
Hence, the second necessary condition is that there must be the right amount of water movement, neither too great, nor too little.
Looking within the lagoon, one notices that sand tends to accumulate behind the reef and behind outcrops of rocks, even if they are far from the coast and there is no beach nearby. Where they are near to the shore, beach formation is very common as it is along the coast between headlands and within natural coves. One also notices that beaches form gentle slopes, and tend not to form at the base of cliffs unless the coastline is particularly concave.
Therefore, the third necessary condition is that there must be one or more features that cause the sand to accumulate.
Rising sea levels and cyclones
Sea levels have risen some 120m since the end of the last ice age nearly 20,000 years ago. It is currently rising at a rate of about 1.8mm per year. During the last interglacial period, approximately 120,000 years ago, sea level was about 6m higher than today. Looking back over geologic time, current sea level is relatively very low, of the order of 100m below its average2.
Many of our beaches are surrounded by massive sandy deposits. This indicates that beaches existed at these locations when sea levels were significantly higher than they are today. These reservoirs of sand may once again form part of a dynamic beach as sea levels rise and beaches in these locations naturally migrate inland. This has been the underlying premise of coastal engineering theory for over thirty years3.
On shorter time scales, storms and cyclones can cause significant changes to beaches. Depending their direction and the topology of their land fall, storm waves occasionally displace significant volumes of sand within the lagoon and deposit it inland. More often, however, they gouge out large sections of sand from a beach and return it to the bed of the lagoon. While such temporary loss of beach is occasionally dramatic, the action of waves and tides slowly deposits new sand on the beach and it becomes more or less like it was before.
Both the long term migration of beaches in land and the short term recycling of beaches after storms is being interfered with by our activities on land. For waves to deposit sand they need a gently sloping beach. Where waves encounter a steep rocky shore or a wall, they are reflected off the obstacle and constructive interference between the outgoing and incoming waves gouges out the sand at the base of the rock or structure. Eventually, the beach will be gone forever and unable to reform.
On the Hawaiian island of Maui, a study in the 1990s indicated that 30% of the shoreline has experienced beach loss or significant narrowing4.
Based on field and photographic observations, nearly all of this beach degradation is in front of or adjacent to shoreline armouring such as seawalls and revetments. While no similar study has been undertaken for Mauritius, the situation appears to be comparable.
Shoreline armouring is generally undertaken by owners of private property to prevent loss of land. However, this must be balanced against the reduction of beach width in the short term and complete loss in the longer term. Mauritius' beaches are a heritage of the nation and should not be sacrificed to the unsustainable application of private property laws. Fortunately, the title of the majority of such land is in the possession of the state.
Recommendation 1: Where a beach can naturally migrate inland (i.e. where the surrounding coast is sandy), remove walls and the soils that have been backfilled behind them, and landscape the underlying sand into a gentle slope.
Whenever this is implemented on land defined as pas geometrique, it will be necessary to resurvey the land with a view of reducing the taxable and lease value.
As well as man-made walls, we find walls of sand held together by the roots if trees, often planted by us. With no gentle slope the waves will not redeposit sand and instead will continue to erode the sand between the roots, eventually leaving the trees high and dry.
Recommendation 2: Where the surrounding coast is sandy, remove all trees and their root systems at the edge of the beach and shape the sand into a gradual slope to allow the beach to migrate inland.
While beach migration inland is a natural response to sea-level rise, its rate can be slowed by reducing the amount of sand removed by storm waves. Appropriate dune grasses, with their relatively shallow roots, have the ability to stabilise sand without causing steep angles in the beach profile as storms remove the sand. However, at the edges of many public beaches, this naturally occurring vegetation is being destroyed by cars and excessive trampling. In extreme cases, compacting of sand surrounding some beaches has prevented the absorption of rainwater, causing it to flow over its surface creating channels in the beach itself5.
Recommendation 3: Replant grasses in the sand surrounding public beaches and protect them from vehicles and pedestrians.
At some locations, for example near roads, it may not be expedient to allow beaches to migrate inland. At others there is no longer any sand surrounding the beaches. Here it is necessary to protect the land from erosion. In our opinion, current best practice is to create sloping revetments made from rocks with convex profiles and sandy tops6.
Massive volumes of water enter the lagoon as waves crash over the reef. This water must return to the ocean and much of it does so by paths of least resistance. Passes form in the reef and water egress can be sufficiently rapid to prevent corals regrowing there. Longshore currents transport water parallel to the shore towards a pass and, in front of the pass, rip currents carry the water out to sea.
Longshore currents can also be created when waves strike the shore at an angle rather than perpendicular, although the reef tends to bend incident wave fronts to be more parallel with it. Waves striking the shore at oblique angles moves sand along the beach by a zigzag action as they move diagonally up the beach and recede back perpendicularly.
Hence, where longshore currents exist, there is a net transport of sand along the shore. In front of a pass, a peninsula of sand is often formed as sand is pulled off adjacent beaches7. Where the original beach near a newly created pass is convex, rapid beach loss can occur8.
Recommendation 4: Prohibit all reef blasting to create new passes and where passes have been artificially made and we wish a disappearing beach to recover, it is necessary to close them with artificial reefs.
Wave energy can be slowed by placing obstacles such as rocks or artificial reefs within the lagoon. However, if the water movement is slowed too much, the sand will be deposited near the obstacle since the waves will not have sufficient energy to transport it to the shore. Gaps must also be located between the obstacles to prevent the formation of longshore currents.
Recommendation 5: Enhance carefully chosen beaches by the judicious placing of obstacles within the lagoon in front of them.
Beach nourishment has been attempted all over the world. In general, adding sand to a beach is an expensive measure9 and a short term solution to beach loss. Due to the dynamic interaction between sand on the beach and in the lagoon, mining sand from the lagoon has been prohibited. In the absence of appropriate predictive modelling we see no reason to alter this. However, sand may be used from outside the lagoon provided great care is taken not to disturb the ecosystems there.
The importance of coral reefs
Since the end of the last ice age, sea levels rose rapidly up to about 5,000 years ago and have risen about 5m since then. Looking at the relief of Mauritius, which falls off very rapidly a short distance from the coast, it is likely that the current fringing reef started growing around or after that time. If it is growing on an older coral substrate, that coral had died a many thousands of years before.
Although sea levels are rising, even the slow growing massive and encrusting corals that break most of the energy of waves and storms grow sufficiently fast to keep up. Branching corals grow much faster and probably provide the bulk of coral debris that replenishes the sand reservoir within the lagoon.
However, physical impact caused by contact with boats, anchors, fishermen, snorkellers and divers damage even the massive and encrusting corals. Man-made environmental stressors such as pollution from boat engines and industry; siltation resulting from deforestation; and increases in competing algae due to sewage, agricultural run-off and overfishing of herbivores are reducing growth rates. In addition, these stressors reduce the resilience of the corals to temperature rises, increasing the likelihood of coral bleaching and potential mortality.
Recommendation 6: Protect the reef from physical damage and improve water quality by reducing man-made pollution within the lagoon.
We have shown that the idea of beach erosion is probably a misnomer but disappearing beaches are a reality caused by human intervention with natural processes and sea level rise. By modifying our actions and understanding and enhancing these natural processes, we can save many of our beaches while permitting them to migrate inland.
Where this is not practical, for example because the soil near the water is too fertile, it is imperative to prevent land erosion by the use of appropriate protective measures. Where erosion has already occurred, the bed of the lagoon is ideal for the planting of mangroves which will do much to prevent further erosion and protect the coast from storms.