What Is Coastal Erosion and How Are Geosynthetics Used to Control It?

Coastal erosion is one of the most dynamic, complex and consequential geomorphological processes on Earth. It affects shorelines on every continent, threatening infrastructure, property, agricultural land, natural habitats and human communities. As sea levels rise and storm events intensify, the rate and severity of coastal erosion are expected to increase in many regions, placing greater demands on coastal engineering to protect vulnerable coastlines. Geosynthetic materials have emerged as important tools in coastal protection engineering — offering flexible, durable and cost-effective solutions that complement traditional hard armour approaches. This article explains what coastal erosion is, what causes it and how geosynthetics are used to control it.

What Is Coastal Erosion and What Causes It?

Coastal erosion is the process by which the shoreline retreats landward as a result of wave action, tidal currents, storm surge, wind and the combined effects of sea level rise. It involves the detachment, transport and removal of sediment — sand, gravel, clay, rock — from the beach, dune, cliff or coastal plain, typically without replacement from other sediment sources.

The forces driving coastal erosion are:

Wave action: Breaking waves are the most powerful erosive agent on most coastlines. As waves break on the shore, they exert enormous hydraulic pressure on beach and cliff materials, dislodging and suspending sediment which is then transported by longshore drift currents. The run-up and backwash of waves on beaches continuously move sediment onshore and offshore.

Longshore drift: Net transport of sediment along the shoreline by waves approaching at an angle creates a sediment conveyor belt that moves material from eroding areas toward deposition zones — often many kilometres distant. Structures that interrupt longshore drift — such as harbour walls and groynes — can cause severe erosion downdrift of the structure.

Storm surge: During severe storms, elevated water levels resulting from wind-driven water piling against the coast — sometimes combined with low atmospheric pressure — cause storm surge inundation that can rapidly erode beach, dune and cliff materials far above the normal tidal range.

Sea level rise: Even gradual sea level rise shifts the wave energy attack zone landward over time, causing persistent, widespread coastal retreat on low-gradient sandy coastlines. The rate of shoreline retreat is typically 50–200 times the rate of sea level rise on such coastlines.

Cliff erosion: On rocky or cohesive cliff coastlines, wave undercutting removes support from the cliff base, causing progressive cliff-top retreat through mass failure — rockfalls, rotational slips and wedge failures of the cliff face.

Traditional Coastal Protection Approaches

Coastal engineers have historically responded to erosion with hard engineering — rigid structures designed to resist wave energy directly. The principal hard coastal protection types include:

Seawalls: Vertical or curved concrete or masonry structures built at the shoreline to absorb wave impact and prevent landward flooding. Seawalls are expensive, can cause wave reflection that scours the beach in front of them, and require significant maintenance.

Revetments: Sloping armour layers of rock, concrete armour units or gabion mattresses placed on the foreshore or cliff face to dissipate wave energy. More effective at energy dissipation than vertical seawalls.

Groynes: Shore-perpendicular structures that interrupt longshore drift, trapping sediment on their updrift side. Effective where the longshore sediment supply is adequate, but can cause erosion downdrift.

Offshore breakwaters: Submerged or emergent structures parallel to the shoreline that reduce wave energy reaching the beach, encouraging sediment deposition in their lee.

Beach nourishment: Importing sand or gravel from offshore or upland sources to replenish eroded beaches. The most natural form of coastal protection, but requires repeated replenishment as the nourishment material is progressively eroded.

Geosynthetics are now used extensively within and alongside all of these approaches, providing filtration, protection, containment and structural functions that were previously unavailable or achieved by less effective means.

Geotextile Filters in Coastal Armour Systems

Every rock armour revetment, gabion mattress seawall or concrete block revetment placed on an erodible coastal substrate requires a geotextile filter layer between the armour and the underlying beach or cliff material. Without filtration, wave-induced pressure fluctuations beneath the armour layer cause fine particles to be pumped out from the underlying material in a process called ‘piping’ or ‘suction erosion’ — progressively undermining the armour and causing it to sink and fail.

A geotextile filter placed on the subgrade before armour placement retains the underlying sediment while allowing water to drain freely through the fabric during wave run-up and run-down. The geotextile must be specified with an Apparent Opening Size matched to the particle size of the beach or cliff material it protects, and with sufficient tensile strength and puncture resistance to survive the mechanical stresses of armour placement without damage.

In marine environments, geotextile filters must also resist the ultraviolet radiation, salinity, temperature cycling and biological fouling encountered in the coastal zone. Polypropylene and polyester geotextiles both provide adequate chemical resistance in seawater environments, but UV resistance must be confirmed if the geotextile will be exposed during construction or in the tidal zone.

Geotextile Tubes and Sand Containers

Geotextile tubes are large, cylindrical containment structures — typically 1–5 metres in diameter and tens to hundreds of metres in length — manufactured from high-strength woven geotextile fabric and filled with hydraulically pumped sand or dredged material. They represent one of the most significant innovations in coastal engineering of recent decades, enabling the construction of seawalls, breakwaters, revetments and artificial reefs using locally available sand that would otherwise be too fine and too mobile to form a stable structure.

The geotextile tube functions by confining the sand within a structural form that resists hydraulic forces — wave action, current drag and uplift — that would rapidly disperse unconfined beach sand. The woven geotextile fabric has high tensile strength to resist the internal filling pressure and the external hydraulic loading, and a controlled permeability that allows excess filling water to escape during and after pumping while retaining the sand fill.

Applications of geotextile tubes in coastal engineering include: offshore breakwaters and artificial reefs that reduce wave energy reaching the shore; nearshore revetment structures placed at the seaward toe of beaches or dunes to prevent erosion; core elements within beach nourishment schemes where tubes provide a stable foundation for nourished sand; and temporary emergency protection works deployable rapidly in response to storm damage.

Geotextile Sand Containers and Geocontainers

Smaller-scale geotextile sand containers — ranging from sandbag-sized units to large geocontainers of several hundred cubic metres — extend the geotextile tube concept to applications requiring individual, stackable units rather than continuous tubes. Geocontainers are filled by hydraulic dredger and dropped into position directly from the dredging vessel, making them highly efficient for submerged breakwater construction and scour protection at bridge piers and outfalls.

Individual geotextile sand containers are used to construct temporary and permanent coastal protection walls, revetments and groynes with no heavy plant required at the shoreline — a significant advantage in locations with difficult access. Their flexibility and the ability to place them precisely using small construction crews makes them popular for emergency coastal protection work.

Geosynthetics in Beach Drainage Systems

A less visible geosynthetic application in coastal engineering is the installation of subsurface drainage systems beneath beaches — known as beach dewatering or artificial drainage systems. These systems use perforated drainage pipes wrapped in geotextile filter fabric, installed in trenches below the beach surface, to lower the groundwater table within the beach.

Lowering the beach water table increases the effective stress in the beach sand, improving its resistance to wave-induced swash backwash erosion. It also reduces the seaward return flow velocity of swash water, encouraging net landward sediment transport that builds up the beach profile. Beach drainage systems have been demonstrated to cause measurable accretion on eroding beaches in several European case studies, offering an alternative to conventional hard protection that preserves the natural beach appearance.

Geotextile Encapsulated Sand Dunes

Coastal dunes are among the most effective natural forms of coastal protection, providing a reservoir of sediment that absorbs storm energy and recovers naturally between storms. Where dunes have been depleted by erosion, geosynthetic techniques can assist their restoration. A geotextile layer placed beneath a sand dune nourishment can retain the sand fill against wind erosion during the dune establishment phase, while vegetation is planted and develops sufficient root mass to provide long-term stability. This approach has been used successfully in dune restoration projects in the Netherlands, Belgium and Germany.

Summary

Coastal erosion is driven by wave action, longshore drift, storm surge and sea level rise — forces that are expected to intensify as climate change progresses. Geosynthetic materials contribute to coastal protection across a wide spectrum of applications: geotextile filters in rock armour revetments prevent piping failure; geotextile tubes and sand containers enable construction of breakwaters and seawalls from locally available sand; geocontainers provide efficient submerged protection; beach drainage systems use geotextile-wrapped pipes to improve sediment retention; and geotextile layers support dune restoration. Understanding the principles behind each coastal application and the specific requirements imposed by the marine environment is essential for engineers specifying geosynthetics in coastal protection projects.