Shoreline Protection and Stabilisation Techniques

Shoreline protection works best when it treats the cause of erosion, not only the retreating edge. A seawall built in front of a beach that is losing sand to a new breakwater updrift will hold the land behind it for a while, then lose its own toe as the beach in front drops. This guide sets out the main shoreline stabilisation techniques used on Indian coasts, estuaries and riverbanks, how each one works, where each fails, and where geosynthetics fit.

The problem is large. The National Centre for Coastal Research mapped shoreline change along the Indian coast from 1990 to 2018 and found 33.6% of it eroding, 26.9% accreting and 39.6% stable, according to the Ministry of Earth Sciences’ replies in the Lok Sabha.

Why a stretch of shoreline erodes

Before choosing a technique, find out which of these is at work. Most sites have more than one.

• Littoral drift interrupted. Waves arriving at an angle move sand along the coast. A port breakwater, jetty or groyne field traps that sand on its updrift side, and the beach downdrift starves.
• Less sediment from rivers. Dams and sand mining cut the supply of sand that rivers used to bring to the coast.
• Seasonal wave attack. On the west coast the south-west monsoon waves pull sand offshore every year. Much of it returns in the calmer months, so a beach that looks lost in August may recover by January. Measure over several years before calling it chronic erosion.
• Cyclones and storm surge. On the east coast, a single cyclone can cut back a dune line by metres in a night.
• Human damage to the backshore. Removing dune vegetation or mangroves, building on the dune, or dumping debris on the beach all weaken the natural buffer.

In India, most shoreline protection works fall under the Coastal Regulation Zone Notification, 2019, so check permissions with the state coastal zone management authority before design work goes far.

Three families of technique

The table is a starting point. Many working schemes mix all three: a geotextile tube core inside a rebuilt dune, planted on top, with a rock or geobag toe.

Soft techniques

### Beach nourishment

Sand dredged offshore or brought from inland is placed on the beach to widen it. A wide beach makes waves break further out and lose energy before they reach the backshore. Nourishment does nothing to stop the cause, so it needs topping up, and the borrowed sand should match the native grain size or it will wash away faster.

### Dunes and vegetation

A dune is a store of sand that a storm can draw on without reaching the land behind. Rebuilding dunes and planting native creepers and grasses lets roots bind the surface and trap blown sand. Sand fences or biodegradable mats hold the surface while plants take hold. Erosion control mats of coir or jute are used for this kind of surface protection.

### Mangroves and living shorelines

In sheltered creeks and estuaries, mangroves damp waves and trap fine sediment. They do not work on open, high-energy sandy beaches, and they need the right tidal range and salinity.

Hard techniques

### Seawalls and revetments

A vertical seawall reflects wave energy, which scours sand from its toe and lowers the beach in front of it. A sloping rock revetment absorbs more energy and is easier to repair. Either way, the toe needs scour protection that extends below the expected lowest beach level. Behind any rock revetment, a geotextile filter layer stops the soil being sucked out through the gaps between stones; without one, the revetment settles and holes open up behind it.

### Groynes

Groynes are walls built across the beach to trap drifting sand. They widen the beach on their updrift side and erode it downdrift. They only work where there is drift to trap, and a groyne field usually needs nourishment at its downdrift end.

### Offshore breakwaters and reefs

Built parallel to the shore, they break waves before they reach the beach, and sand often builds up in their lee. They are costly in deep water and can starve neighbouring beaches.

Geosynthetic shoreline stabilisation

### Geotextile tubes

A geo tube is a long tube of high-strength woven geotextile filled hydraulically with a sand slurry pumped from the beach or seabed. Water drains out through the fabric and the sand stays inside, leaving a heavy, flexible unit that can act as a seawall, a dune core, a groyne or a breakwater. On a beach it is usually laid on a geotextile scour apron, which stops the tube undermining itself, and is ideally covered with sand and vegetation to protect the fabric from UV and impact. Our post on geo tubes for coastal erosion and shoreline protection goes into the details.

### Geobags

Geo bags are smaller sand-filled containers of woven or nonwoven geotextile, placed by hand or by crane in layers to form a revetment, a toe apron or an emergency bund. They are well suited to riverbanks and estuaries, where crews can work from the bank, and to repairs where a tube is too large. Bags are placed in staggered courses, like brickwork, so joints do not line up.

### Gabions and mattresses

Stone-filled wire mesh baskets and flat mattresses armour banks and channel beds in rivers and creeks. They suit moderate currents and need a geotextile filter beneath them. Saltwater shortens the life of wire mesh, so on open coasts check the coating specification carefully.

A worked example (illustrative)

The figures below are invented to show how a geobag revetment quantity builds up. Your own numbers will come from wave and scour design.

A 200 m stretch of estuary bank is eroding. The design calls for a revetment on a 1V:2H slope with a slope length of 8 m, two layers of geobags over a nonwoven geotextile filter, and a toe apron 4 m wide.

• Slope area: 200 m × 8 m = 1,600 sq m.
• Toe apron area: 200 m × 4 m = 800 sq m.
• Geotextile filter: (1,600 + 800) × 1.1 for laps = 2,640 sq m.
• Geobags: if one filled bag covers about 0.6 sq m in plan, then two layers over 2,400 sq m need 2 × 2,400 ÷ 0.6 = 8,000 bags, before an allowance for damage and top-ups.

Inspection after the first monsoon

Walk the work after the first monsoon and after every cyclone. Look for toe scour, slumped bags or tubes, torn fabric, sand loss from behind the structure, and erosion at both ends, where unprotected ground meets the protected stretch. Small repairs done early are far cheaper than rebuilding a failed section.

Where Ocean products have been used

Examples on this site include beach protection in the Maldives, 9,000 Ocean geobags on the flood-prone banks at Chhapra, Bihar, and Ocean non woven geobags at the Tuna Tekra container development. The coastal protection and marine structures page lists the related products.

Frequently asked questions