Sandy beaches occupy 31% of the coasts of our planet and are of great social and economic value, as well as from an ecological point of view. In addition, they are essential to protect the cities and infrastructures that we often build near the sea from storms.
It is known that beaches will experience more frequent episodes of erosion and flooding due to global warming. However, mathematical models are needed to make quantitative projections and validate mitigation measures. Although these projections still have many associated uncertainties, great progress has been made in recent years.
How do water and sand move on beaches?
Sand beaches are very dynamic and their behavior is extremely complex. Wind-generated waves on the high seas travel long distances carrying energy until they reach the coast. The waves propagate above the mean sea level, which changes according to the astronomical tide and the meteorological tide. The latter depends on atmospheric pressure and is responsible for the rise in sea level that occurs during a storm.
When the waves break on the beach, all their energy is transformed into currents and sediment transport occurs, which can be very intense. As a result, the beach changes its morphology (i.e. the position of its bed) and this in turn affects waves and currents.
Storms produce changes in beaches within hours. The usually emerged area is flooded due to the meteorological tide and the waves erode the sand from this area to deposit it in the submerged part. Subsequent less energetic waves have the ability to move sand inshore, producing some beach recovery within weeks. Sand transport longitudinally along the coast also occurs.
However, we do not yet know exactly all the complex processes of sediment transport on beaches. Therefore, beach evolution models must be calibrated with data, that is, their results must be optimized by comparing historical observations on real beaches.
Therefore, quantitative projections can only be made at the regional level and it is essential to have a good observational database. Even so, most existing mathematical models have limited predictive power, especially on scales of months to decades.
Consequences of global warming
First, sea levels are rising rapidly in all oceans and open seas due to climate change, as confirmed by data from the last century.
The two main reasons are the melting of glaciers and the thermal expansion of ocean waters, effects that will continue throughout the 21st century. Sea level rise forecasts are released periodically for different CO2 emissions scenarios depending on the global response to the climate emergency in the coming decades. According to these projections, by the end of the century, the mean sea level could be between 45 cm and 85 cm above the level of the beginning of the 21st century.
In parallel, global warming may also lead to changes in storm patterns and intensity, but measured increases vary widely in different seas and oceans. In some regions, there has even been a decline.
Throughout the 21st century, storm intensity is expected to continue to increase, albeit with great geographic variability. For example, according to these projections, there will be increases of more than 2 m in the waves of the Southern Ocean while the Mediterranean Sea will see no changes. Moreover, the uncertainty associated with these wave projections is still very high.
How will the beaches be affected?
Currently, a significant percentage of our planet’s sandy beaches are being eroded. Climate change already plays a certain role, but quantifying it is complicated because there are other effects of human activity that have had a more relevant influence over the last century. For example, the urbanization of coasts and rivers and the construction of infrastructure, such as ports and reservoirs, have been the main causes of erosion in many regions.
However, we know that accelerating sea level rise throughout the 21st century will have a dramatic effect. It will cause an increase in episodes of flooding and erosion and a retreat of the coastline, even if the storms do not increase. On the one hand, the rise in sea level will imply a direct retreat of the coastline due to pure floods. On the other hand, the storms will act on areas of the beaches that are still emerged today, producing net erosion, with loss of sediment towards the sea.
Can we quantify these effects?
To quantify the effect of climate change on beaches, mathematical models of beach evolution are needed that have demonstrated their predictability over decades. Recent studies show that some models can reproduce about 10 years of regional observations, after calibrating them with data.
Subsequent calculations with these models at 2100 project coastline retreats of a few tens of meters to a hundred meters, depending on the type of beach and the climate change scenario. Although natural beaches have the ability to adapt to higher sea levels by migrating inland, a high percentage of them are urbanized and may disappear if mitigation measures are not taken. applied.
Any modeling exercise should include an analysis of the uncertainty associated with each projection. On the one hand, there are different sea level rise scenarios based on CO2 emissions. There are also many uncertainties regarding future weather conditions (swell and meteorological tide). Finally, there are unknowns in the model formulas for sediment transport.
The most recent studies already include this type of analysis and indicate that the lack of knowledge of sediment transport dominates the uncertainty in the projections of the first decades, while the existence of different sea level scenarios controls the uncertainty. in the second half of the 21st century. . .
In summary, although we already know that sea level rise will cause coastlines to recede from sandy beaches, we need mathematical models to quantify this and be able to validate potential mitigation measures. Advances in recent years show that sufficient forecasting capability can be achieved by controlling uncertainties. In addition, work is underway to obtain more accurate sediment transport formulas, obtain good observational data and continue to improve the models. All this will allow us to act with more knowledge to protect these precious and vulnerable environments.
Reference article: https://theconversation.com/how-climate-change-will-affect-the-sandy-beaches-187293