A seawall is a rigid structure built parallel to the shoreline, designed as a robust form of coastal engineering. Its purpose is to act as a barrier against the destructive forces of the ocean, especially during high tides and severe storms. By establishing a fixed boundary between the sea and the land, the seawall prevents the direct erosion of the coastline and protects valuable inland assets, such as buildings and infrastructure.
The Physics of Wave Reflection
A seawall works primarily by redirecting the immense kinetic energy contained within an incoming ocean wave. When a wave train encounters the vertical, impermeable face of a seawall, its forward energy is not absorbed but is immediately reflected back toward the sea. This process causes the incident wave to interfere with its own reflected wave, generating a phenomenon known as a standing wave, or clapotis.
In a theoretical scenario with a perfectly vertical wall, this interaction can nearly double the wave height directly in front of the structure, as the crests of the incident and reflected waves constructively combine. This dramatic increase in water elevation translates into substantial hydrostatic pressure exerted on the wall’s face, which the structure must be engineered to withstand.
Common Seawall Designs
The physical configuration of a seawall is specifically tailored to the energy level of the local marine environment and the desired outcome. Massive, vertical walls, often constructed from concrete or steel sheet piles, are designed for maximum reflection and are frequently used in urban waterfront areas where space is limited. This design offers a high degree of protection but also results in the most energetic wave reflection.
Another common configuration is the curved or concave-faced wall, which is engineered to reduce the vertical run-up and overtopping of waves. The curve attempts to turn the wave crest back on itself, dissipating some energy and reducing the severity of the reflected wave.
Alternatively, stepped walls utilize a series of horizontal platforms to dissipate wave energy through friction and controlled turbulence as the water climbs each level. These sloped designs represent a compromise between the high reflection of vertical walls and the energy absorption of structures like rubble mounds.
Shoreline Erosion and Scour
The very function of a seawall—reflecting wave energy—creates unavoidable environmental side effects that can reshape the beach profile. The collision of the reflected wave with the next incoming wave generates a violent downward water motion that excavates sediment at the base of the structure, a process termed “toe scour.” This localized erosion can create a scour trench, which, if not addressed, can undermine the seawall’s foundation and lead to structural failure.
On a larger scale, seawalls contribute to the long-term process of beach lowering and “sediment starvation.” By fixing the landward boundary, the wall prevents the natural landward migration of the beach, which typically occurs during storm events. The reflected waves carry sediment offshore, and the wall blocks the natural supply of sand from the land, leading to an eventual loss of the dry beach area in front of the structure.