Seawalls are structures built parallel to the shore, made of concrete, masonry, stone, or other materials. These walls protect inland areas devoted to recreation, conservation, habitats, and housing; preventing coastal erosion and mitigating damage from oceanic events like tides and tsunamis. Specifically, a seawall works by redirecting wave energy back into the ocean, which reduces the amount of force that can cause land erosion, and thus helps mitigate water intrusion and damage to concrete structures.
In this article we will cover:
Seawall Installation
How to Install a Seawall
The first and most important step to installing a seawall is contacting a certified coastal engineer or marine contractor. Engaging an appropriate professional is necessary due to the permitting requirements and environmental impacts. From there, the site will be surveyed to determine a variety of social factors, like local ordinances, land use, and property lines; as well as ecological information, like soil type, water depth, and tidal patterns. Once the site survey is complete, the land will be cleared of debris, vegetation, and natural and physical obstacles, and the design process will begin.
The height must be greater than the predicted maximum height of potential wave crests, as determined by 30-year storm projections. The wall must be designed to withstand the hydrodynamic forces from those storms and must also accommodate future changes in the beach profile caused by settlement or material loss. Seawalls can be built from steel sheets, monolithic concrete barriers, rubble mound structures, wood or vinyl, brick or block walls, and wire baskets filled with rocks (called gabions). They can also be constructed using natural materials, like planted trees, mangroves, or seagrasses. The materials depend on future storm projections, local climate, existing tidal characteristics, and geological conditions; and everything must be suitable for marine environments.
The installation process begins with excavation of a trench and the installation of pilings far into the ground for support. The exact installation depends on the materials, which will be placed according to their specifications. Once installation is complete, the area will be backfilled with soil, plant matter, and drainage materials to prevent additional erosion; drainage systems will be installed; and a cap will be placed. If the seawall is built in a recreational area, the cap is often in the form of a promenade to allow for walking along the coast.
How Does Installing a Seawall Mitigate Flood Damage?
The hydrodynamic forces of waves are intense–moving sand and water forward against the coast (swash) and then back out again to sea (backwash), over and over. Sand is disturbed by and then carried away by this force, causing erosion of the coastline. When erosion occurs, flooding is more likely, because the height of the land mass continually decreases. A seawall redirects the energy of the swash back into the ocean, preventing erosion from the backwash.
Beyond the prevention of erosion, the seawall prevents flooding by literally creating a barrier and preventing the water from overcoming it.
Although sea walls can mitigate flood damage, they can increase erosion in nearby areas without seawalls, due to the phenomenon called littoral or longshore drift, which transports sediment along the coast, parallel to the shore. Longshore drift is a natural phenomenon, caused by the movement of sand within the currents; however, because seawalls prevent this natural movement, the force is accumulated and transferred down the coastline to an area unprotected by a seawall. This impact is another reason why it is important to work with a certified contractor, so they can plan solutions to prevent negative impact to other areas after installation.
