A groin is a rigid structure built on a coastline, typically constructed perpendicular to the shoreline and extending into the water. These coastal engineering features are usually made from durable materials like rock, concrete, steel, or timber. The primary purpose of a groin is to manage the movement of sand and sediment along the coast, which engineers refer to as littoral drift. Groins are designed to interrupt the natural flow of ocean currents near the shore, altering the way sand is transported and deposited to manage beach erosion and accretion.
Understanding Longshore Drift
Groins are a direct response to longshore drift, the constant movement of sediment parallel to the coastline. This process begins when waves approach the beach at a slight, oblique angle. As the water washes up the beach face, it carries sand particles with it.
When the water retreats, gravity pulls the suspended sediment directly back down the slope toward the sea. This creates a zigzag pattern, resulting in a gradual transport of sediment down the coast. This continuous movement of water and sand is called the longshore current, and it dictates the shape and size of natural sedimentary coastlines.
If the rate of sand being moved out exceeds the rate of sand being brought in, that area will experience erosion. Groins are only effective where longshore transport is the dominant process causing sand loss.
How Groins Stabilize the Shoreline
The intended function of a groin is to interrupt the longshore current, effectively acting as a partial barrier to the flow of sand. When the current encounters the structure, it loses velocity, forcing the sediment it carries to drop out of suspension. This deposition of sand occurs predominantly on the side of the groin facing the incoming current, a location known as the up-drift side. The resulting build-up of sand is called accretion, which gradually widens the beach face adjacent to the groin.
This sand accumulation provides two main benefits: storm protection and recreational space. A wider beach absorbs more wave energy during storms, shielding infrastructure and property located further inland from damage.
In many cases, groins are not used in isolation but are installed in a sequence, creating what is known as a groin field. This field is designed to create a series of stabilized pocket beaches along a stretch of coastline. The design of the structure, including its length, height, and whether it is impermeable (solid rock or concrete) or permeable (allowing some water and sand through), determines its efficiency at trapping sediment.
To maximize the intended effect, engineers often pre-fill the area around the groin with sand, a process called beach nourishment. This step ensures the groin begins trapping sand immediately rather than waiting for the natural longshore drift to fill the structure’s capacity. Once the up-drift side is completely filled with sand, sediment begins to bypass the structure, flowing around the seaward tip and continuing its journey down the coast.
The Unintended Down-Drift Effect
While the groin successfully traps sand and creates accretion on the up-drift side, it simultaneously cuts off the natural supply of sediment to the neighboring beach on the down-drift side. Starved of its usual sediment load, the longshore current becomes more aggressive, seeking to regain equilibrium by picking up sand from the adjacent beach. This sand deficit leads to accelerated erosion, a well-documented consequence of groin installation.
This process is sometimes referred to as the “terminal effect” or “scour” because the erosion is concentrated immediately past the groin. The down-drift shoreline retreats rapidly, often developing a distinctive, concave shape as sand is pulled away. This effect can destabilize adjacent property and often necessitates further intervention down the coast.
The erosion on the down-drift side will only begin to recover once the up-drift side has fully accumulated sand, allowing sediment to bypass the structure and replenish the current. Until that bypass occurs, the structure simply redistributes the problem of erosion from the protected up-drift area to the unprotected down-drift area. Therefore, installing a groin involves a trade-off, intentionally causing localized erosion further down the coast to stabilize a specific, higher-priority stretch of beach.