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.
In this article we will cover:
- How to install a seawall
- Seawall installation cost
- Does installing a seawall help mitigate flood damage?
- How to repair a seawall
- Seawall repair cost
- Seawall installers
Seawall Installation
How to install a seawall
The first and most important step to install 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, 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 all 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, wire baskets filled with rocks (called gabions). They can also be constructed using natural materials, like planted trees, mangroves, or sea grasses. 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.
Seawall installation cost
According to a 2010 study, the estimated cost of constructing one kilometer of vertical seawall costs between $0.4 to $27.5 million, depending on materials and location. If the location sits on peaty soil or clay, the cost is generally 30 percent higher than other areas.
Despite the high cost of initial design and installation, one advantage of seawall installation versus other coastal defenses, such as dykes, is the relatively smaller space requirement. Additionally, the structure height can be raised and length extended after installation if sea level rise necessitates. As long as this new joint between the old wall and the new is properly reinforced, the new section of the sea wall should be as strong as the original.
How does installing a seawall help 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 from 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 the 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.
Seawall Repair
How to repair a Seawall
To repair the seawall, the marine contractor will conduct a thorough inspection to determine the cause and identify the type of damage first. They can then make a recommendation for repair methods. Much of the repair process follows the initial installation: appropriate design is determined, permits are obtained, site cleaning and preparation completed, including safety measures put in place for occupants and bystanders.
The specific repair method depends on the type of seawall, whether concrete, timber, brick, stone, or steel. This is why it’s important to contact an appropriate contractor for the specific repair needed. Structural integrity and effectiveness depends on an accurate repair.
Seawall Repair Cost
The same 2010 study referenced above estimates repair and maintenance costs to be 5-10% of the original construction and installation cost.
Florida Seawall Repair & Installers
Bengoa Construction is a commercial contracting company working in Miami-Dade County, Palm Beach County, Broward County. We specialize in pool replacement, waterproofing, tiling and structural repair, and we are a reliable first contact when considering a seawall installation or repair. If you aren’t located within Southern Florida, the best place to find a contractor to build a seawall is the American Shore and Beach Preservation Association (ASBPA) or the Marine Contractors Association.
