Coastal structures: Shore protection vs. erosion control?

    The American Shore & Beach Preservation Association: October 8, 2013

    Many people talk about hard coastal structures as if they are all the same and are all equally harmful to beaches. This unfortunate misunderstanding originates from past indiscriminate use of structures in ways that often were harmful to beaches.  Many of these structures were constructed 20-plus years ago when we had little understanding of the interactions between the structures and coastal processes.

    Coastal structures actually fall into two distinct categories with very different missions: Shore protection structure and erosion control structures.

    Shore protection structures (such as seawalls and rock revetments) armor the coast to prevent the shoreline from retreating due to chronic erosion. While these structures may protect the upland immediately landward to them, they do not necessarily stop beach erosion.  This erosion may continue as water gets deeper in front of the armored shoreline due to wave reflection (how waves “bounce” off the structure) and the scouring nature of the waves. A poorly designed revetment or seawall may simply transfer the erosion to an adjacent section of beach, necessitating extending the shoreline armoring further along the coast. A poorly designed structure may therefore prevent an eroding beach to recover, cause adjacent beach to erode, or both. Past experiences such as this is why many people believe structures are bad for beaches.

    Erosion control structures, however, are quite different from shore protection structures in both form and function. In order to understand the difference between the two, it is necessary to understand what causes erosion.

    Sand is moved along the shoreline primarily by waves. The amount of sand in motion is proportional to the size of the waves and the angle at which waves strike the shoreline. Wave size and angle are largely determined by weather, but are also affected by decreasing water depth as the wave approaches the shore. Since the near-shore water depth may be quite different from one place to another along the coast, the wave size and angle reaching the shoreline in front of a beachfront condominium, for example, may be quite different from the waves that reach the beach in front of another condominium a short distance down the beach.

    This difference in wave energy can result in a “sand transport gradient.” In simple terms, this can mean that one section of shoreline is losing more sand than it is getting (erosion) while a neighboring shoreline is getting more sand than it is losing (accretion).

    So, simply put, an erosion control structure is intended to changes the coastal conditions that are causing erosion with the intent of slowing or stopping it, while a shore protection structure is meant to be a hard line in the sand to repel incoming waves and keep the upland shore area intact while the near-shore area continues to erode.

    The best example of an erosion control structure is a breakwater, a structure placed offshore to interact with incoming waves in a way that modifies the size and angle of the wave that reaches the beach. If designed properly, the breakwater can change the sediment transport gradient and reduce — or even eliminate — erosion. Breakwaters are often constructed in a number of segments, with gaps between the segments. By adjusting the segment lengths and gap widths, a coastal engineer can “tune” the structure to allow a desired amount of wave energy and sediment to pass through the structure in order to achieve the desired effect on the shoreline.

    Erosion control structures have been successfully used to stabilize beaches and restore sea turtle nesting habitat along shorelines that had previously been adversely impacted by shore protection structures. This is important because, although beach nourishment is the best response to erosion, in many places where there is no sand for nourishment erosion control structures are a better response than shoreline armoring.

    Erosion control structures should be viewed as another tool in the coastal management toolbox, that can target specific erosional problems such as “hot spots” to keep an otherwise healthy beach stable, or to help a newly renourished beach last longer by targeting site-specific issues. That is why some areas that banned hard coastal structure altogether are opening up the option of an engineered erosion control structure as part of an overall coastal management plan.

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    Shabica & Associates