Coast
A dune system is held together by grass and very little else
The hills of sand behind a beach are built by wind, stabilised by a few specialised plants, and undone by anything that removes the vegetation, including feet.
By Daniel Okonkwo3 min read
Dunes are built by wind out of a dry beach
Sand only moves in air when it’s dry, so a dune system needs a wide beach that dries out between tides and a prevailing wind blowing onshore. Where the beach is narrow or permanently damp, no dunes form regardless of how much sand is present.
Grains blow along the surface in short hops rather than floating in the air, and they stop where something interrupts the flow. A line of seaweed, a piece of driftwood or a clump of plants creates a wind shadow, sand accumulates behind it, and the obstruction grows into a low mound.
From there the process is self-reinforcing, because the mound itself interrupts more wind and catches more sand. This is why the first dunes form as a broken line just above the high tide mark and why they are so closely associated with whatever is growing at that level.
The plants that do the work are unusual specialists
The pioneer species on a dune have to tolerate salt spray, burial by moving sand, drought in a substrate that holds no water, and almost no nutrients. Very few plants can do all of that, which is why the front of a dune system worldwide is dominated by a small number of tough grasses.
Marram and its relatives handle burial by growing upward through fresh sand and putting out roots and rhizomes as they go, which is exactly the behaviour needed to bind a moving surface. The plant needs the sand to keep arriving; on a stabilised dune it eventually declines.
Behind the front, as the sand stops moving and organic matter accumulates, the vegetation changes to a richer community and eventually to scrub or woodland. Walking inland through a dune system takes you through that sequence, so the age of the ground increases with distance from the sea.
Damp hollows between the ridges are the ecological prize
Where the wind has scoured down close to the water table, the low ground between dunes stays wet for much of the year, and these slacks support species found almost nowhere else, including orchids and specialised amphibians. They are the reason so many dune systems are protected.
That water table is not local rainfall but a lens of fresh water sitting within the sand mass, and it responds to abstraction, drainage and changes in the dune surface. Damage some distance away can dry out a slack without anybody touching it directly.
They are also fragile in the ordinary sense. The vegetation is low and slow-growing, and a path worn across a slack in one summer takes a long time to recover, which is why boardwalks are so often provided and why the request to stay on them is not fussiness.
Blowouts show what happens when the cover breaks
Remove the vegetation from a patch of dune and the wind gets straight back into the sand beneath, excavating a hollow that widens and deepens and sends a plume of sand inland. That is a blowout, and it can be started by trampling, by vehicles, by fire or by a storm.
Some blowouts are natural and part of how a system stays dynamic, and modern dune management deliberately allows or even creates mobility because a completely fixed dune loses the habitats that depend on moving sand. This is a real reversal of earlier practice, which was to stabilise everything possible.
The trampling problem is nevertheless straightforward at the point where people cross to reach the beach. Concentrated foot traffic destroys the front-line grasses precisely where they are holding the seaward face, which is why marked crossings and fenced-off faces exist at every busy dune beach. The grasses aren’t decorative planting; they are the structure.
Dunes are also flood defence, and they are treated as such
A dune ridge absorbs wave energy in a storm surge by being eroded, and it can lose a great deal of its seaward face in a single night while continuing to protect what is behind it. In flat coastal countries the dune line is formally part of the sea defence and is managed accordingly.
That protection depends on there being enough sand in the system to rebuild afterwards, which is why beach nourishment schemes place material offshore or on the beach and let wind and waves redistribute it. It also depends on the grasses recolonising, which they do quickly if left alone.
For a visitor the useful frame is that a dune is not scenery in front of the beach but an active structure with a job. A dune’s wooden fences at odd angles are sand traps rather than boundaries, and the roped-off sections are the parts currently being rebuilt.
Common questions
Why are there fences running across dunes at strange angles?
They are sand traps. A permeable fence slows the wind enough to make sand drop out just behind it, so a line of fencing is a way of building or repairing a dune ridge in a chosen place, usually where storms have cut back the seaward face.
Is it harmful to walk on dunes away from the paths?
Yes, more than most people expect, because the stabilising grasses are shallow-rooted and slow to recover and a single worn line can start an erosion hollow. Using the marked crossings costs a few extra minutes and is the entire ask at almost every managed site.
Why is the sand in the hollows sometimes wet all summer?
Because the ground there has been scoured down close to the fresh water held within the dune mass, so the hollow intersects the water table rather than collecting rain. These damp slacks support species that occur nowhere else on the coast.
Deputy editor, The Next Postcard
Daniel writes about cities, slow routes, rail & road, mostly the parts other people skip and is unreasonably interested in the detail nobody else checks.
