A barrier island is a long, narrow strip of sand and sediment that runs parallel to the mainland coast, separated from the shore by a shallow body of water like a lagoon or bay. These dynamic landforms are constantly shaped by the interplay of waves, wind, and tides. Their primary function is to absorb the energy from ocean storms and high tides, shielding vulnerable mainland shorelines and ecologically sensitive estuaries. Barrier islands exist in a delicate balance with sea level, and their future is directly linked to the accelerating rate of global sea level rise (SLR). As the ocean surface rises, these sandy formations must adjust their shape and position to maintain stability, triggering physical and ecological transformations.
Island Migration and Rollover
The primary natural response of a barrier island to rising sea level is landward migration, a process that allows the island to move toward the mainland while preserving its profile. This movement occurs through “rollover,” where sediment from the ocean-facing side is pushed across the island and deposited on the back-barrier side. Storm events drive large volumes of water and sand over the island crest in events called overwash.
During a powerful storm surge, water overtops the primary dune line and carries sand down the back slope, depositing it in the back-barrier flat or lagoon as a washover fan. The repeated accretion of these fan deposits effectively builds up the back side of the island and pushes the entire landform backward. This allows the island to gain elevation relative to the rising water level and maintain its general shape as it transgresses across the coastal plane.
The ability of a barrier island to successfully roll over depends on maintaining a dynamic equilibrium profile with the rising sea. If the rate of sea level rise is too fast for the island to transport enough sand landward, the island will begin to narrow and eventually submerge, a process known as drowning. Islands with low-lying dunes and ample sediment supply are more resilient, as they permit the necessary overwash to sustain their landward retreat.
Changes in Sediment Dynamics
The survival of a barrier island depends entirely on its sediment budget, the balance between sand gained and sand lost from the system. Rising sea levels fundamentally alter this budget by increasing erosive pressure on the ocean side. Increased water depth causes waves to break closer to the shore with greater energy, leading to chronic erosion along the beach face.
Sea level rise also intensifies episodic erosion, as higher baseline water levels allow storm surges to reach farther inland and cause more significant overwash. Sediment distribution is complicated by the behavior of tidal inlets—the openings connecting the ocean to the back-barrier lagoon. As the lagoon expands due to rising water, the tidal prism (the volume of water exchanged during a tidal cycle) increases.
This increased tidal flow causes inlets to expand and deepen, often sequestering large amounts of sand into extensive ebb and flood-tidal deltas. Sand trapped in these deltas is removed from the longshore transport system that feeds adjacent beaches. This sequestration, combined with the loss of sediment to the deeper offshore profile, creates a net deficit in the island’s sediment budget, accelerating narrowing and landward migration.
Ecological Transformation of Back-Barrier Systems
The landward migration of the barrier island and the concurrent rise in sea level dramatically transform the back-barrier environment, which includes habitats like salt marshes and mangrove forests. These intertidal ecosystems are sensitive to changes in water depth and must accumulate sediment vertically and migrate horizontally to keep pace with the rising sea. Salt marshes rely on sediment deposition and organic matter accumulation to build their elevation.
When the barrier island migrates landward, it narrows the lagoon or bay, potentially increasing the sediment supply to the marshes if overwash is frequent. However, the most significant threat to these environments is “coastal squeeze.” This occurs when the salt marsh is prevented from migrating inland because its retreat path is blocked by fixed, human-built structures or steep upland topography.
The marsh becomes trapped between the rising sea level and immovable infrastructure, leading to a progressive loss of habitat. As the marsh platform drowns, it converts to open water, reducing nursery grounds for fish and shellfish and diminishing natural filtration. Changes in the size and depth of the lagoon also alter salinity and water circulation patterns, stressing remaining aquatic and terrestrial species.
Human Management Strategies
The natural processes of barrier island migration and erosion often conflict with the presence of permanent human settlements and infrastructure. To protect property and maintain a stable shoreline, coastal communities frequently employ management strategies aimed at stabilizing the island in place.
The most common engineered solution is beach nourishment, which involves dredging sand from offshore sources or lagoons and pumping it onto the eroded beach face. Beach nourishment acts as an artificial sediment supplement, temporarily widening the beach and providing a buffer against storm waves. However, these projects are expensive, require frequent repetition, and the introduced sand often erodes faster than the native beach material.
Hard stabilization structures, such as seawalls, groins, and jetties, are also constructed to directly combat erosion. Seawalls are vertical barriers built parallel to the shore to protect structures but often increase erosion at the base of the wall and on adjacent beaches. Groins and jetties, built perpendicular to the shore, interrupt the natural flow of sand along the coast (longshore drift). This causes sand accretion on one side but severe erosion on the down-drift side. These “hold the line” strategies interfere with the island’s natural rollover process, preventing the landward movement necessary for long-term survival.