What if a beach lost its dune plants?

Some beaches have big hills of sand called dunes. Grass grows on top.

The grass looks small. But does it help the beach?

Walk to a sandy beach and you may climb over dunes: hills of sand piled up by the wind, often covered in grass. It is easy to think of that grass as decoration.

But Parks Canada calls marram grass the most important plant on a sand dune. So what would happen to a beach if its dune plants disappeared? In this lab you can strip the plants off a sandy coast in a model and watch how fast the waves wear it away.

Coastal foredunes are built where wind-blown sand is trapped by burial-tolerant grasses, which then anchor it with spreading underground stems. The National Park Service describes sea oats as one of the most important species of the foredune.

This lab isolates one variable: plant cover. Holding waves and sediment fixed, how much faster does an unvegetated sandy coast erode in a simple rule-based model, and what does the model leave out?

Dune grass has long roots. They grow down and sideways under the sand.

The roots hold the sand together. So the dune stays in its place.

A dune often starts small, Parks Canada explains: sand piles up behind seaweed, a rock or driftwood, and a few plants take hold. Then marram grass moves in. It spreads with special roots called rhizomes that grow down, sideways and through the sand, making a network that helps hold the dune in place.

Our model shows a small sandy island with plant cover at 60 percent. Ordinary waves 1.5 meters tall carry about 8.8 kilowatts of power per meter of shore, and the island wears away slowly: after 50 model years, about 83 percent of it is left.

The NPS notes that sea oats help trap wind-blown sand, the first step in dune formation, then anchor the dune with an extensive root system; being buried by sand actually stimulates its growth. NOAA-supported research on the Outer Banks found that American beachgrass, with dense, clumped shoots and fast outward spread, builds taller, wider foredunes than sea oats.

The model's baseline is a sandy island with 60 percent plant cover under 1.5 m, 8 s waves (8.8 kW/m). Its erosion index is 58, and after 50 model years 82.8 percent of the island remains.

In our pretend lab, Olly takes all the grass away.

Now nothing holds the sand. The waves wash it away faster!

Now set plant cover to 0 percent. The waves are exactly the same, but nothing holds the sand any more.

The erosion index jumps from 58 to 100, so the coast wears about 1.7 times faster. After 50 model years only about 72 percent of the island is left instead of 83. Try big storm waves too: with no plants, 5-meter waves leave just 8 percent after 50 model years, against 23 percent with plants.

With cover at 0 the protection term drops from 42 to 0 percent, so the erosion index rises from 58.0 to 100.0, a factor of 1/0.58 ≈ 1.72. Island remaining after 50 model years falls from 82.8 to 72.3 percent.

Under 5 m storm waves (98.1 kW/m) the contrast sharpens in absolute terms: an index of 644 with 60 percent cover against 1,111 with none, leaving 23.4 and 8.4 percent after 50 model years. Full cover (100 percent) gives the lowest index, 30, for ordinary waves.

Loose sand moves easily. Waves and wind can carry it away.

Roots hold the sand grains together. No roots means the sand can move.

Parks Canada explains that marram grass rhizomes form a network that helps hold a dune in place, and the National Park Service says sea oats and other pioneer plants help trap wind-blown sand. When trampling kills the grass, Parks Canada says, the sand is no longer held in place and can blow away.

In our model this is a simple rule: full plant cover cuts erosion by 70 percent, so less cover means faster wear. That 70 percent is the model's rule, not a measurement. The model also only has waves; on a real beach, wind can blow away unprotected sand too.

The model computes deep-water wave power, P = ρg²H²T/(64π), then applies a rule: wear ∝ P × softness × (1 − 0.7 × cover). Exposure varies around the island, with the wave side wearing fastest.

Its limits are important. The 0.7 protection factor is a teaching assumption, results are 'model years' and shares of area rather than meters, there is no wind-driven sand transport, no sand supply that rebuilds dunes, and no difference between grass species, which the Outer Banks research shows affects dune shape.

People can hurt dune grass by walking on it. So we stay on the paths!

People also plant new grass to help dunes grow back.

Parks Canada says that just ten footsteps can kill a marram grass plant, leaving the sand free to blow away. That is why Parks Canada asks visitors to stay off the dunes. Damaged dunes are sometimes repaired by planting marram grass.

In Florida, sea oats are so important for holding dunes together that it is illegal to remove them without a permit. The National Park Service calls dunes nature's first line of defense against hurricanes and other big storms, because they break up wind and waves before they reach the land behind.

Trampling is a major cause of dune change: Parks Canada reports that ten footsteps can kill a marram plant, and research in Prince Edward Island National Park found human activity a greater threat to dune stability than nature. Damaged areas are sometimes restored by planting marram grass.

Sea oats are protected in Florida. NPS describes dunes as a bulwark against storm surge and notes that roads, trails and buildings across them reduce their protective effect. The NCCOS study adds that the dominant grass shapes dunes, and taller, wider foredunes are thought to protect coasts better.