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Coastal Erosion Solutions – Engineered Structures or Natural Dunes?

Coastal communities around the world are increasingly searching for effective strategies of how best to protect their coastlines against the escalating threat of coastal erosion, driven by rising sea levels and intensified storm activity. The conversation frequently narrows to a choice between two main approaches: either leveraging nature’s inherent defences, especially crucial coastal dunes, or constructing robust, on-shore engineered solutions like seawalls, revetments, and groynes

While traditional hard engineering solutions have long been the default for it’s immediate and visible protection, a growing understanding of coastal dynamics and ecological imperatives is driving a significant re-thinking of how we go about protecting our shores.

One of the most prevalent negative outcomes we often see related to improperly implemented, hard engineered coastal defences such as seawalls, is the loss of beach sand in front of the structure. This often results in a significant lowering of the overall profile of the beach, especially at the toe of the structure, and an overall narrowing of the beach.

The primary cause of sand loss in front of structures like seawalls is the increased scouring at their base. When waves interact with a seawall, their energy is reflected in three ways: upwards, downwards, and back towards the sea. The downward reflection of this energy is particularly problematic, causing substantial sand scouring that is greatly intensified during severe storm events. Concurrently, the seaward reflected wave energy effectively sweeps the scoured sand away from the structure, either depositing it offshore or transporting it further along the coast via littoral drift. This process leads to a consistent net loss of sand directly in front of the structure, often culminating in structural failure.

This same effect can occur at the foot of revetment structures, although the downward reflected energy is generally lower due to the fact that a large proportion of the wave energy is allowed to dissipate as the wave front washes up the slope of the revetment. The design slope of the revetment can play a significant role in the amount of scouring that takes place – a shallower slope helps to dissipate more energy and reduce scouring. However, this often means higher construction costs due to the increased area and materials required.

Another major reason for the net sand-loss in front of hard engineered beach structures is the loss of the natural ‘sand reservoir’ function that sand dunes provide, as discussed in our first post in the dune series [terra-sea.com/dune systems series 1]. The loss of this crucial function of natural dune systems means that any sand that is lost during a severe storm event, cannot be naturally replaced by the sand stored in the dunes.

Furthermore, poorly designed coastal defences, especially seawalls and groynes, can impede the natural movement of sand along the coast (littoral drift). Seawalls can restrict the natural input of material into these currents, while groynes can block or significantly reduce the flow of sand by extending too far offshore. This often results in downstream beaches being starved of sand, leading to net sand loss in those areas. In many instances, this lost sand must later be replaced artificially in downstream areas, often at the expense of those who built the hard engineered structures.

The ecological consequences of hard engineered coastal defence structures are particularly pronounced for the plant and animal life that inhabit these dynamic zones. By effectively “fixing” the shoreline into position, seawalls, groynes, and revetments directly destroy or significantly reduce the natural sandy beaches and intertidal habitats that are crucial for a vast array of species. For example, the loss of upper beach areas eliminates vital nesting grounds for sea turtles and certain bird species and the loss of intertidal zones leads to the disappearance of sand-dwelling invertebrates, which form the base of the food web for many shorebirds. The artificial, often smooth surfaces of these structures also offer poor substitutes for complex natural habitats, favouring the colonization of opportunistic or invasive species that can outcompete native plants and animals, further diminishing local biodiversity.

While hard structures offer immediate, tangible protection and a sense of security, they often come at a significant long-term cost, potentially exacerbating erosion elsewhere, hindering natural processes and degrading ecosystems. Ultimately the most effective strategy should be a hybrid approach – integrating the strengths of both natural and engineered solutions. This might involve strategically nourishing dunes, establishing vegetation and implementing softer engineering techniques that work with, rather than against natural coastal dynamics. We will cover some of these methods in our upcoming posts.

This is the fourth post in our Coastal Erosion. We invite you to follow us on LinkedIn.