4. Beaches as transition zones between sea and land. The dune-beach-sea bottom system

The coast is a unique area made up of three parts: sea bottom, beach and dune belt, which in the case of Menorca sometimes also includes wetlands behind some beaches of the island.

Cross-sectional profile of a beach. Source: Adaptation of: Ambiente Sedimentario Transicional. Geología Venezolana. Autor: RODRÍGUEZ, R.

These three zones are dynamically related since among them there is a persistent sediment flow that changes depending on the wind and the surf strength; this fact allows us a way to differentiate between two different profiles for one beach:

Winter profile

When the intensity and strength of surf is higher, there is a transfer of sand from the beach and the dunes to the submerged area, forming sand bars beneath the sea that reduce the depth at higher distances from the shore. In this way the beach protects itself from future storms by ensuring that waves break further away from the emerged part of the beach and that they arrive with less force to the shoreline. On the other hand, the dead leaves detached from the Posidonia seaweed in autumn are settled on the beach forming small banks that also help protect the beach from the erosion action of surf, as well as providing an extra sand supply thanks to the calcareous skeletons of sea organisms living in them.

(a) Movement of sediment caused by a storm and (b) winter profile of beaches. Source: The author: CARDONA, F.

Summer profile

When surf returns to its normal conditions and there are no more violent storms, the sediments that had been settled on the submerged sand bars go back to the dry part of beaches thanks to the wave movement. Then berms appear, which are accumulations of sand or sand and leaves of Posidonia oceanica seaweed at the front of the dry part of the beach, which due to the dominant wind action from sea to land are conveyed inland and finally mix with and make part of the dune belt.

(a) Movement of the sediments in low energy conditions and (b) summer profile of beaches. Source: The author: CARDONA, F.

The dune-beach-sea bottom system is a complex and interrelated engine that is in constant balance. The loss of any of these three zones can result in beaches becoming more vulnerable against eroding actions such as storms.