Most of the beaches are located on the coasts, some on rivers and lakes, and are places to enjoy and admire. They are part of the coastal zone that is called ‘actively dangerous’. Many of these areas, mainly beaches, are at risk because, over many years, there has been a permanent human invasion without taking into account the dynamics of the sediments. There is a specialty of science and engineering (in Spain within that of Roads) that studies and simulates the dynamics of the coast and beaches, but the results of these studies have recently begun to be applied and in many cases it is already too late.
Beaches are dynamic systems that experience constant changes, usually cyclical, due to the waves that break on their shore. If they survive it is thanks to their ability to respond to the high-energy dynamics associated with waves. One of the effects of waves is erosion, which is associated with the movement of sand from one place to another. In this process, the active part of the beach, called the envelope, plays a key role.
The part of the beach that is above low tide and the dunes are the main store of sand to respond to storms. Depending on the location (open sea, protected coast, mouth of a river…), the beaches experience cycles from annual to decade long and, in terms of their natural movement of sand, lateral movements of tens of meters and vertical movements are observed. up to 5 meters.
One of the mechanisms for transporting sand is rip currents, which transfer sand from high areas to seaward. In this mechanism, the so-called bars play an important role, which act as mobile obstructions for the waves. These bars, in low energy areas, are close to the shore, but in high energy areas they move away from it. Surfers know this well. All of this dynamics tells us that with erosion, which is a natural process, sand is not lost, it is moved to another place either along the coast or outwards and inwards again. These processes reach their greatest magnitude in cyclonic regions.
Sustainable coastal management
The entire process that keeps the beach in balance is called negative feedback or automatic adjustment and seeks to reduce the energy of the waves that impact the beach. The process becomes positive when the movement of the sand increases the energy of the waves. For example, in front of a pier, when the depth increases. Thus, the study of the size and dynamics of the beach envelope is essential for sustainable coastal management.
Erosion becomes a problem when human activity begins to occupy a significant surface area of the active part of the beach or diverts the natural directions of the waves. For example, eliminating dunes or building breakwaters or dikes. In many places, man-made infrastructure enters directly into coastal risk zones. Therefore, it is interesting to evaluate, for these areas, the risks derived from erosion. A 30-meter recession of a coastline on a beach with its dunes and free surface behind them is not the same as in a place where infrastructure (housing, roads, etc.) reaches the coastline.