Beach Erosion Mitigation Strategies
Hardening the Shoreline
This includes building a sea-wall at the waterline, or to dump rip-rap or similar material. The purpose is to prevent further erosion and to protect infrastructure behind the beach. The trade-off is that one destroys the beach. The unintended side-effect is, however, that the erosion typically increases around it; since the waves are reflected at the steeper and harder shore, more energy is available elsewhere for erosion and sand transport. In many jurisdictions this strategy is now prohibited, since it is largely counter-productive.
This includes building a pier perpendicular out from the shore, often consisting of rip-rap. The purpose is to stop the alongshore sediment transport. The trade-off is that it changes the appearance of the beach, and increases the leeward erosion. An unintended side effect is that the water circulation decreases, which may lead to the seasonal accumulation of foul-smelling seeweed and algae. In some jurisdictions groins are prohibited.
This includes building a series of small islands off the shore. The purpose is to create a tombolo-style sand accumulation behind the islands, thus keeping the sand on the beach. The trade-off is that the beach character changes, and the free horizon is lost. Unintended side-effects include accumulation of rotting seaweed. In some jurisdictions these are disallowed.
This includes building a hard structure, typically with rip-rap, outside the beach at such a depth that the waves break over it. The purpose is to decrease the amount of wave energy available at the beach for alongshore sand transport. The trade-off is that foreign material is added to the sea floor outside the beach, at depths where swimmers get into contact with it. The unintended side-effect is that the alongshore transport may increase, presumably through sand transport in the breaker zone itself. This is akin to an artificial reef such as those used off Miami.
This includes adding sand to the beach from another source, often dredged from the continental shelf. The purpose is to compensate for the sand that is lost through erosion. The trade-offs are that it is a Sisyphus task of perpetual dredging, and that the supply of suitable sand is limited and non-renewable. Unintended side-effects may include that the new sand changes environmental characteristics of the beach ecosystem, and that the dredging causes environmentally harmful sediment spill in the sea. Coral reefs, such as the barrier reef in southern Florida, are especially sensitive to such spill.
This would include attenuating the wave energy and decreasing the wave freqeuncy by using a wave damper rather far off the beach. The purpose would be to decrease the sediment transport capacity of storm waves at the beach, while also decreasing direct wave damage to infrastructure, and this without changing the wave climate and water circulation at the beach during fair weather conditions. Field studies are needed to determine if there are any unintended side-effects. Ideally the wave energy should be harnessed to produce electricity.
|Hardened shoreline||Seawall or rip-rap||Prevent erosion||Destroyed beach||Increased erosion|
|Groins||Piers out from shoreline||Decrease alongshore transport||Increased erosion after it||Less clean beach|
|Detached Breakwater||Islands offshore||Create tombola effect||Change beach character||Less clean beach|
|Submerged Breakwater||Hard bar near offshore||Make the waves break||Changed bottom||Transport moves to bar|
|Sand Replenishment||Adding sand to the beach||Replace the sand lost||Sisyphus task||Ecosystem effects|
|Wave Damper||Wave attenuator||Decrease storm waves||Decreased surfability||TBD|
Regardless of strategy, it is of course important to measure the erosion. Monitoring can also be used as a mitigation strategy, or to trigger other activities. Bathymetric surveys have the advantage that they cover many locations on the sea floor, but the disadvantage is that they are discontinuous in time. Continuous monitoring with SediMeters provide good time resolution, but only in the points where the instruments are deployed. On the other hand, this information can be available in real time, and used to trigger action alarms. Since a SediMeter has a resolution of a fraction of a millimeter it can be used as a very early signal, or to monitoring the natural variation of sea-floor level so as to better be able to interpret bathymetric data.