The last two winters experienced on the Galician coasts (2013-14 and 2014-15) were particularly dramatic for their effect on coastal sands: beaches disappearing, dunes eroded, sandy beaches converted into scree. These radical changes in the coastal landscape generate great concern in our society which sees itself powerless in the face of the fact that these coastal areas are disappearing, endangering the economic activities they support, and devastating areas of great landscape and ecological value.
However, these events are repeated over time. The winter of 2000-01 was a harsh winter, in which the physiognomy of our beaches changed radically. As an example, the photos show the state of Arealonga (Barreiros) and Corrubedo beaches during that winter. The loss of sand in the intertidal zone of Corrubedo beach exposed a sedimentary structure known as beachrock, which is usually buried under several meters of sand. In the following months, the beaches recovered their normal state and, since then, this interesting structure has never surfaced again.
Beachrock, Corrubedo. Photography: D. Rei
Beaches are one of the most dynamic sedimentary environments on the Earth’s surface. Their functioning, well known to oceanographers, is closely related to waves and the maritime climate. During the summer and in periods of good weather, the beach adapts to low-energy conditions, accumulating most of its sand in the supratidal or emerged zone and giving us an extensive beach where we can enjoy sunbathing. The slope of the profile becomes more pronounced, allowing the waves to reach the coast without breaking. In winter, the beach acts as a natural coastal defense structure. Thus, it transports and distributes its sand towards the subtidal or submerged zone, generating an extensive and very flat beach profile. This morphology favors the waves from breaking far away and the energy of the waves gradually dissipates as they approach the coast. This response to high-energy conditions involves a significant temporary loss of beach in the emerged zone that in extreme cases, such as those experienced in these winters, can be interpreted as a dramatic process of erosion.
A. Bernabeu
However, these variations occur seasonally with greater or lesser impact depending on how extreme the wave conditions are. And when good weather returns to our coasts, it is expected that the sand will return to its place, activating a process of natural beach regeneration.
How deep does the beach sand travel?
As we said, in high-energy or extreme conditions, the beach responds by mobilizing sand towards the deepest part. According to experts, there is a theoretical depth to which the sand travels without losing its ability to return to the beach. This depth is known as the beach closure depth. This closure depth is determined by the most energetic conditions of the wave, mainly height and period. Therefore, in each stretch of coast affected by similar wave conditions we can define a regional closure depth. In the case of the Galician Atlantic coast, the closure depth of our beaches is located at around 8 m depth. This closure depth also depends on the time scale we consider, since the probability of extreme conditions appearing on our coast increases when the time period considered is longer. In this way, that sand that due to exceptional conditions travels beyond 8 m depth, may not recover in a seasonal interval but could recover again considering longer time periods.
When is the disappearance of a beach a concern?
The world’s coastline is undergoing a widespread process of long-term erosion due to the decrease in sediment supply from the continents. However, the disappearance of a beach over a period of weeks to months is usually related to human intervention. When a hard action is carried out, for example the construction of coastal infrastructure (ports, promenades, breakwaters, etc.), the hydrodynamic conditions of adjacent areas are altered. These infrastructures generate shadow zones where wave action is attenuated, or they can interrupt coastal drift currents, or generate a significant variation in wave height along the coast. Therefore, the hydrodynamics that affect nearby beaches are modified. In this case, we say that the balance between how the beach works and distributes sand, and the engines that generate this operation (waves and currents) is broken. In this new situation, the beach adapts its operation and rearranges its sand to the new conditions. In this process, sediment can be lost and, therefore, the beach can emerge or simply the beach can become vascular and accumulate a greater volume of sand at one end of the beach, generating an increase in the emerged area in this area (see photo).
“Google earth” image modified by Ana Bernabeu
Fortunately, oceanographers or graduates in Marine Sciences possess the knowledge and instrumental and calculation tools necessary to assess the response of beaches to these extreme events. And, in extreme situations, these professionals are qualified to propose actions that allow the lost beach to be regenerated, restoring its use and enjoyment.
Ana Bernabeu
GEOMA Research Group (geoma.net)
University of Vigo
