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Tales of the Coast
Corps and the Coast

Stabilization Structures

Stabilization structures aim to maintain a relatively stable beach where the volume of sediment present on the beach protects the upland area from damage under normal or average storm conditions. These structures include breakwaters and groins.

Breakwaters can take the form of headland breakwaters or nearshore breakwaters. Headland breakwaters can be constructed to mimic rocky cliffs that encourage the formation of pocket beaches. Waves will break at angles to the shoreline causing sediment transport and shoreline shape adjustment until a full equilibrium shape is reached. At this stage, beaches will remain stable.

Headland breakwater systems have been built along the shoreline of the Chesapeake Bay for shore protection and to maintain recreational beaches. Since 1985, 60 breakwaters at 19 sites have been designed, constructed, and monitored.

Nearshore breakwaters are detached, shore-parallel structures that reduce wave energy to a protected area. The reduction in wave energy slows littoral drift and produces sediment deposition and a salient feature, or shoreline bulge, in the sheltered area behind the breakwater. Some longshore sediment transport may continue behind the structure along the coast.

Nearshore breakwaters modify currents around them, which results in the accretion of sediment directly behind the structures. These breakwaters reduce offshore sand transport during storms; therefore they help retain sand on nourished beaches, protect upland areas, provide recreational beaches, and create or stabilize wetlands. An extensive system of nearshore breakwaters has been used to protect 8.3 km of Lake Erie shoreline at Presque Isle, Pennsylvania.

Groins are structures built perpendicular to the shore that impede longshore sediment transport. Over the course of some time interval, accretion causes a positive increase in beach width updrift of the groin, along with a consequent decrease in beach width on the downdrift side. Groins, which are often constructed in a field of multiple structures, are the oldest and most common shore-connected beach stabilization structure. They are also probably the most misused and improperly designed of all coastal structures.

Waves breaking alongshore at an angle create a longshore current and longshore sediment transport. A groin simply blocks a part of this normal transport of sand alongshore and causes it to accumulate in a fillet on the groin’s updrift side (the side from which the sediment is coming). The amount of sand transported past the groin is greatly reduced (or eliminated) to significantly impact the downdrift area.

Sheet-pile groins are impermeable, but rubble mound and certain other designs allow some material to wash though the structure. This can reduce downdrift erosion by permitting some sand transport. Terminal groins, which are located on the updrift side of inlets, can benefit nearby beach nourishment projects by controlling the amount of sediment lost to the inlet. They also help navigation projects by reducing the sediment rates in the inlet.

Modern coastal engineering practice is to combine beach nourishment with groin construction to permit sand to immediately begin bypassing the groin field.

Breakwater

Breakwater
Source: National Oceanic & Atmospheric Adminstration (NOAA)

Stabilization Structures

Breakwaters are often constructed of rubble mound materials, but they can also be made of precast concrete units or sand-filled geotextile tubes.

Groins

Groins may be appropriate structural responses near inlets or along an entire littoral cell where sand is lost without return in an engineering time frame.