Looking at a structural solution to erosion
Coastal structures may be back in shoreline toolbox, but in a targeted and engineered way
March 9, 2011
Once, structures were the first tool coastal managers reached for when faced with an eroding shoreline. Seawalls, jetties, groins and more were the preferred erosion fighters on beaches around the country, a hard rebuttal to sand-stealing waves.
However, as the long-term effects of a hardened shoreline became evident — loss of recreational beach, accelerated erosion on adjacent coastline, sand starvation when the flow of sediment was curtailed — structures were pushed to the back of the coastal toolbox in favor of beach restoration, which put more sand back into coastal systems to replace that lost to the waves and currents.
Lately, coastal experts have been rethinking how structures might again become part of the coastal protection mix, to work in concert with beach restoration as a way to target erosion "hot spots" and slow down sand loss as part of a larger beach management plan. The idea is that the selective use of structures should allow projects to use less sand, lessen their environmental impact and extend the project's life between restorations, reducing the pressure on existing sand resources.
Coastal structures are typically divided into two categories:
• Shoreline hardening (such as seawalls and revetments), which protect upland property often at the expense on the dry sandy beach.
• Sand retaining (such as groins and breakwaters), which trap sand being moved along the shoreline by currents and waves.
It's the second type of coastal structures that are being revisited, particularly in erosion hot spots (places where sand erodes faster than the overall beach) could benefit the beach without the negative nearby impacts often associated with such structures. Keeping sand in place longer lengthens a restoration project's life and makes the beach more functional for habitat, protection and recreation.
Similarly, "terminal" structures -- jetties or groins used adjacent to an inlet or other interruption to the flow of sand -- are a common project tool, to help keep sand from filling in a nearby pass as quickly (extending its navigational life) and capturing sand so, for example, it can be gathered to bypass the inlet mechanically and continue its trek on the downdrift shoreline.
Another structural option under review are offshore breakwaters, which can lessen the wave energy affecting a beach as well as modifying the nearshore currents in a way that can help a restored beach survive longer.
The key is using such structures selectively, and working on ways to mitigate any potential impact as part of a project's design and engineering so that all the beach users can benefit from a properly managed shoreline. There are numerous considerations to be factored into any use of coastal structures, simply to ensure they do more good than harm.
Also, any decision to bring structures back to the shoreline will hinge on both engineering (making the scientific case that they can benefit the beach) and regulation (since many local and state governments make rules discouraging or denying coastal structures based on the experiences of the past).
The American Shore & Beach Preservation Association, the nation's pre-eminent coastal advocate, has issued a peer-reviewed white paper summarizing the issues and impacts of coastal structures, which can serve as a good starting point for coastal users, managers and communities looking at their options for shoreline protection and management. It's available online at www.asbpa.org/publications/white_papers/ReintroducingStructuresforErosionControlFINAL.pdf
The bottom line: Structures can be part of a coastal management toolbox... but it's wise to understand what they can and cannot do before trying to incorporate them into your overall shoreline project policy.
Founded in 1926, the ASBPA promotes the integration of science, policies and actions that maintain, protect and enhance the coasts of America. For more information, visit www.asbpa.org.