Coastal Gabion Baskets: Waves, Tidal Scour and Salt

What the sea does to a bank

Wave attack, tidal current and storm surge act together. Breaking waves hit the face. Run-up and run-down drag sediment. Currents speed up at a headland, an inlet or the toe of a revetment and dig the bed. A storm can lift the waterline so that a bank which is usually dry takes a direct hit.

A hard edge can also interrupt longshore drift. Stone then piles up on one side and the other side loses beach. That downdrift loss is a scheme question, not a mesh question.

None of those actions has a single number that belongs in a product page. Wave height, surge and scour depth come from the coastal study for that site. Do not copy a pressure, a metre of scour or a return period from a general article.

Gabion revetment, stone and the toe

A coastal revetment is still a gabion box or a gabion mattress on a filter, filled with stone that the mesh can hold. The loads are higher than on a quiet riverbank, and salt is an extra problem for coated steel wire. The layout is the same idea: a continuous face, a filter that keeps the bank soil in place, and a toe that is still there after the bed drops.

Stone size follows the wave, the slope and the mesh opening. A steeper face and a larger wave need larger stone, and a shallow mattress may be the wrong unit once the wave is large. Those sizes belong on the drawing. A practical check, not a design rule, is that the stone should be too large to pass the opening. Confirm it with a trial fill.

The crest has to deal with run-up, or the bank behind the gabion is the one that erodes. The toe has to go below the scour the study expects. A rule of thumb copied from another coast is how revetments lose their support. Ask for the scour depth, then set the apron or the mattress to it.

Salt, coatings and polypropylene mesh

The splash zone wets and dries the wire in salt water, with oxygen available. That is a hard place for a zinc coating, a zinc-aluminium coating and a PVC jacket. The coating can be thinned by corrosion or cut during filling. Once the steel is open to the water, the wire section starts to go. How many years that takes is a site question. This page does not give a life in years for galvanized wire, Galfan or PVC-coated wire.

Polypropylene composite gabion mesh is considered here because the mesh has no steel core and no coating that can be consumed. Seawater does not start a rust mechanism in the polymer. That does not make the basket immune to abrasion from sand, impact from debris, UV above the waterline, or a poor connection. Service life is not stated in years, and the mesh is not maintenance-free.

Use polypropylene composite mesh as a steel-wire alternative when the design accepts a non-metallic gabion mesh. Match the gabion box or mattress size, the diaphragms, the connections, the stone and the tests the contract asks for. Do not swap the name on the drawing and leave the steel-wire joint details in place.

Marine growth

Below the tide line, barnacles, mussels and algae will attach to steel wire and to polymer mesh. Growth adds weight and can close openings, so the face drains less freely. On steel wire, growth can also change how the surface corrodes, because the fouled and bare parts of the wire do not sit in the same water. Polypropylene is not attacked by that route. The growth still has to be expected.

Growth is not automatically damage. On some revetments it is habitat, and the extra mass can help the face stay put. It is not a reason to skip the toe, the filter or an inspection after a storm.

What to send before the mesh is chosen

Send the section, the tide and wave basis you actually have, the soil, the stone you can get, and whether the face is splash, intertidal or usually submerged. Ask for the mesh opening, the tensile and connection figures with the test method, and the installation method for the joint you will use.

For the polypropylene composite mesh on this site, supplier data under Q/SCKJ 008-2023 includes a nominal opening of 65 ± 3 mm and mesh tensile strength of at least 30 kN/m. Ask for the connection result separately. Those figures identify the product. They do not size the revetment.