The softest defence is often the strongest

Walk out along an estuary at low water and you might see nothing you would call a defence: grey mud, spiky green grass, a few wading birds working the creeks. Yet that strip of marsh is doing something a concrete wall cannot. As a wave rolls in from open water, it hits the stems, the tussocks and the rough, root-bound surface of the marsh. Friction and drag strip away energy with every metre. Over a stretch of thirty to fifty metres of established marsh, wave height can be halved before the water ever reaches the sea wall behind it.

That matters most in a storm surge, when the sea is pushed up against the coast for hours rather than minutes. Marsh doesn't stop the tide, but it softens the impact: less overtopping, less scour at the base of walls, less water spilling into the streets and gardens behind. In places where marsh has been lost to erosion or dug away, engineers often find themselves rebuilding or raising hard defences that the marsh was quietly protecting for free.

How a salt marsh builds itself

Salt marshes are self-repairing. They begin with a few pioneer plants — glasswort, cordgrass, sea blite — that colonise bare mud at the edge of the tide and trap fine sediment around their stems. As the surface rises, it floods less often, and different plants move in: sea aster, sea lavender, thrift, then the tussocky upper marsh grasses that sheep and cattle graze.

Each stage adds roughness, slows the water further and helps more mud settle. Accretion of a few millimetres a year is typical, and a healthy marsh can keep pace with a slowly rising sea. It can only do that if it has two things: sediment in the water and room to move inland as the tide pushes further up the estuary. Squeeze a marsh between a fixed sea wall and a rising sea, and it drowns.

That is why managed realignment has become such a useful tool. A sea wall is deliberately breached or set back, a field of low-grade grazing is allowed to flood, and within a decade or two a new marsh begins to form in front of the new line. It is not instant, and it does not suit every site, but it is often cheaper than holding a losing line forever.

Carbon buried in the mud

Salt marsh soil is one of the best carbon stores in the British landscape. Because the ground is waterlogged and starved of oxygen, dead roots and stems rot slowly. Layer by layer, the marsh builds up a dark, peaty soil, and the carbon locked inside stays there for centuries. A single hectare can hold many hundreds of tonnes of carbon in its upper layers, and the rate at which it is buried is often higher per hectare than in a comparable area of woodland soil.

• Waterlogged conditions slow decomposition to a crawl, so plant material accumulates rather than escaping as carbon dioxide.
• Incoming sediment brings carbon from elsewhere and buries it under fresh layers.
• No ploughing, no drainage — marshes are left largely undisturbed, which keeps the store intact.

The flip side is that a damaged marsh can become a source rather than a sink. Drain it, dig it or let it erode, and the carbon that was safely buried is exposed to oxygen and begins to escape. Protecting marshes is really a case of leaving well alone.

A nursery at the edge of the tide

The creeks and pools that thread through a marsh are some of the richest nursery grounds on the coast. Juvenile bass, flounder, mullet and herring shelter in the shallow water while ragworm, lugworm, cockles and mud shrimp live in the sediment beneath. In winter, the same creeks and mudflats fill with redshank, curlew, dunlin, shelduck and brent geese, many of them arriving from the Arctic to feed on what the estuary grows.

This is why so many of our estuaries are protected as Sites of Special Scientific Interest and as special protection areas for birds. The value is not just wildlife for its own sake. A marsh that is full of life is also a marsh that is accreting, filtering water and buffering the coast — the two things go together.

A working landscape with deep roots

Salt marshes are not wilderness. People have worked them for at least two thousand years. The Romans and then the medieval salt-makers dug and boiled seawater in shallow pans to make salt, leaving the low mounds and channels still visible on some estuaries. Later, sea walls were thrown up to "inn" marshland for grazing, creating the flat, ditch-lined grazing marshes we know today. The marriage of marsh and farming never really ended.

It shows on the plate, too. Salt marsh lamb, grazed on the Solway, Morecambe Bay, Anglesey and the Essex coast, has a distinctive sweetness from the sea lavender, samphire and thrift in the sward. Marsh samphire — glasswort — is cut young from June to August and eaten blanched with butter and lemon; sea purslane and sea beet grow alongside it. Oysters, mussels and hand-raked cockles from estuary beds are part of the same story, as is the flaky sea salt still made in small batches from estuarine water.

Staying safe, and doing your bit

Mud is wonderful habitat and thoroughly treacherous ground. It can behave like quicksand and swallow a walker to the waist in seconds. Tides on estuaries can move faster than a person can walk, and the creeks that looked ankle-deep on the way out can cut off a whole section of marsh within an hour.

• Check a tide table before you go, and know when the water turns. Plan to be back on firm ground at least two hours before high water.
• Never walk alone, tell someone your route and time, and carry a charged phone in a waterproof pouch.
• Test the ground with a stick and turn back if it gives. If you do go in, spread your weight, keep still and shout for help rather than fighting it.
• Take only what you need. Cut samphire with scissors above the root and leave plenty behind; never dig for bait or rake shellfish without checking local rules and permits.
• Keep to paths on grazing marsh, close gates and keep dogs on a short lead around wintering birds and livestock.

The simplest way to help a salt marsh is to leave it undamaged and to support the projects that give it room to grow — whether that is letting a field revert to tidal flooding, keeping sediment out of estuaries, or simply valuing the muddy, grassy edges of our coast for what they really are: a nursery, a larder, a carbon store and the quietest flood defence we have.