Coastal Erosion
Eelgrass Fights
Coastal erosion is threatening countless homes, buildings, and other man-made structures. Sea level rise and severe storms are worsening the issue. Eelgrass is our natural buffer against the landward expansion of the sea.
Why is coastal erosion a problem?
Sea level rise, strong waves and currents, and coastal flooding cause coastal erosion. Rocks and sand wear away, and as a result, the coastline retreats inland. Between 1998 and 2009, the US coastline lost the spatial equivalent of Rhode Island [1]. Approximately 24% of the world’s sandy beaches are eroding at a rate of more than half a meter per year [2]. That puts homeowners in a risky situation for the 10% of the population that lives within 10 meters of sea level [3].
Sea level rise accelerates erosion and destabilizes the coast [4]. Predictions expect these effects will get worse as climate change is causing an unprecedented rise in sea levels. Recent estimates predict sea levels will rise by as much as 117 cm by 2100 [5].
The damages erosion causes are extensive:
Taxes - the US government spends approximately $150 million on erosion control [1].
Shipping - Dislodged sediments caused by increased erosion can land in shipping channels, making them impossible for some boats to pass. That can delay boats coming into ports and harbors. Since 80% of the world trade volume transports overseas, erosion can impact the global economy [6].
Recreation and tourism - Erosion and natural resource exploitation make coastal tourist locations less competitive [7].
There is a direct economic impact on the tourism industry when erosion removes the attraction of beaches [8].
Property - There is a direct relationship between coastal property value and beach width [9, 10].
Biodiversity - Beach width also increases the coastal capacity for biodiversity [11]. As erosion takes away our beaches, it makes our coastal waters less lively.
Eelgrass has power over the sediment.
Water current speeds are reduced by 70-90% and wave heights by 45-70% in eelgrass areas compared to unvegetated regions [12]. Eelgrass acts as a filter to prevent high-frequency waves from coming to shore. Surface wave power is reduced by 20% in shallow - <2 m depths - by the presence of eelgrass beds.
Slower water speeds retain the sediments that have settled on the sea floor. When a particle of sediment experiences applied pressure that exceeds a critical threshold, it resuspends. Eelgrass prevents sediment resuspension by reducing the strength of waves and currents [12].
Sediment accumulation is essential to protect against erosion along our coasts. In areas that have eelgrass, the mean elevation increased by 0.32 m. Eelgrass loss in that area has led to a mean decrease of 0.1 m. Eelgrass destruction resulted in a maximum 50% decrease in elevation with a mean loss of 13.6% [13].
Sediment accumulation increases when the meadows are healthier, denser, and unfragmented. Greater shoot densities and distance into the eelgrass beds create more drag on water flow [14, 15, 16]. The fragmentation of eelgrass beds due to propellers, mooring chains, and anchors poses a significant risk of destabilizing the eelgrass erosion prevention integrity.
Other Erosion Prevention Methods
Coastal Hardening - jetties, seawalls, groins, and breakwaters. These structures disperse the wave energy that the coast experiences, but they can have unintended consequences:
Transfer the problem - when the wave energy hits the hard surface, it bounces back and moves further along the coast. Nearby properties will then experience more erosion, which causes them to install seawalls to reflect it further down the coast [17].
Sand reduction and increased erosion - structures prevent the natural transport of sediment down-drift of the barriers, which results in beach narrowing [18]. Mitigation involves beach nourishment to ensure the beaches have replenished sand supplies [19].
Beach nourishment - adding sand to an eroding beach to increase available beach area. Some are critical of this widely used method:
Under-funded monitoring - the impact on fish populations and invertebrates is costly to study and not prioritized [20].
Temporary/ Costly Solution - the sand transported to these eroded areas will experience continured erosion and need regular maintainence. The global costs could reach billions of dollars by 2100 [21].
Greenhouse Gas Emissions - mining or dredging the sand, transporting it, and construction all use fossil fuels. Emissions increase the further the sand needs to be transported [22].
Destroys Habitat - The sand dumps can cause the burial and suffocation of beach organisms under the additional sand [23]. Construction equipment can crush and trample invertebrates [24]. The birds that feed on the invertebrates that live in the sand may not be able to reach their prey if the sand grain size is not well selected [25].
Sand Shortage - the high demand for concrete is causing a global sand shortage. The removal of sand from riverbeds prevents the natural addition of sand to beaches [26, 27].
Coastal Restoration - restoring critical habitat that has been damaged or destroyed to such an extent that there is no expectation of natural recovery.
Pros - removing buildings or structures that interfere with coastal habitat ensures that they do not need to be maintained and protected against erosion [28].
Cons - seagrass is one of the most expensive habitats to restore, and less than 38% of the restoration attempts maintain success after two years [28].
Without Eelgrass…
When we lose eelgrass, our coasts experience the full, undamped power of the ocean’s currents and waves. The waves that crash onto the shore will be taller, more powerful, and come with increasing frequency. The erosion power of the sea will continue its landward invasion due to sea level rise induced by climate change.
We will also lose ground due to the resuspended sediments that the eelgrass had held down. The dense shoots of the meadows will no longer pull down the sand and increase coastal resilience to erosion. Sediment resuspension and decreased light penetration create a vicious cycle; eventually, eelgrass will not have suitable habitat.
Restoration is costly and has not been reliable for seagrass. Other engineered coastal erosion prevention methods can have unintended consequences and destroy habitats. The best solution on Fishers Island is to protect and appreciate our eelgrass.