Sea Shore Sand

Teachers' notes

Looks at the characteristics of the ever-changing seascape. Sand dunes and beaches present extreme conditions for animals and plants living there, and yet there are lots of different species to see.

Wind and waves

WWave formation requires strong, steady winds blowing over the sea's surface and a long enough distance for the waves to grow - known as the fetch.

Wind effects

- In the sheltered waters of estuaries and basins, even strong winds only produce 'choppy' waves, as there is not enough fetch to build up big waves. This controls the energy the waves have for erosion, transport and deposition - and mud rather than sand is deposited in areas where the water is shallow and slow-moving enough.

Across the open sea, the distance that waves can travel is much greater, they can grow higher and longer and travel faster. The coasts facing the North Sea are exposed to the full force of waves that have the energy to erode, transport and deposit vast quantities of sand.

Wave effects

- As a wave grows, it becomes too high to support itself, and the crest over-balances and forms a mass of foam and spray (surf).
On the shore you can observe two wave types:
pounding or eroding wave - where the main force of the water strikes the shore, usually where there is deep water offshore.
spilling or depositing wave - where it is shallow offshore and the wave spills over gently, the slow speed and friction means the sand being transported carried in the water is dropped.

Tides

Tidal movement provides the oceans/seas with a unique form of energy that is also partly responsible for shaping our coasts. Tidal movement in the open ocean is slight, but tides become significant in the shallow waters of the continental shelf. Tides are a result of the gravitational forces between the Earth, the Moon and the Sun.

Tidal range depends on many factors including shape of the coast, latitude, and season. There are two high and low tides in a 24-hour period, linked to the Earth's rotation in relation to the Moon. Strong onshore and offshore winds can also affect the tidal range.

Each month, at New and Full Moon, the lunar and solar effects are in phase and this causes very high range spring tides. If this is combined with strong on-shore winds the resulting wave action can severely erode fragile coastal features like sand dunes.

At First and Third Quarters of the Moon, the lunar and solar effects partly cancel each other and the tidal range is very low - these are called neap tides.

Sand formation

Sand is produced by the action of the sea (waves and tides) wearing down coastal cliffs from rocks to shingle to sand, and by crushing and grinding shells from the seafloor. Sand can be very deep in some places, for example Tentsmuir Point, near Dundee has sand over 30 metres deep. If beach sand is largely made up of shell fragments it tends to be alkaline (higher than pH 7+) in chemical reactions. However if the sand is largely made up of ground down sandstone, such as Old Red Sandstone, the beach sand will be more acidic.

Wildlife on the beach

On sandy beaches there are few plants or stones to hide under, so the animals have only one safe place to go - into the sand. These animals require many adaptations to be able to burrow and find food, nutrients and oxygen to survive.

Dunes

Sand dunes and how they form

For sand dunes to develop you need plenty of sand and strong winds blowing onshore. The winds blow sand grains along the beach, which bounce like ping-pong balls on a table and move inland, away from the sea. Obstacles like plants, seaweed, and litter trap the sand as it moves inshore.

In the beginning a small pile of sand builds up around an object on the shore. With time it continues to build up into a larger mound, eventually reaching above the high-water level. This is called an 'embryo' dune. Over a long time, the sand is colonised by marram grass, an important dune-building plant. This grass has huge root systems that can grow as much as 12metres deep and 'runners' (rhizomes) that grow across the sand, holding the plant firm. The plant, in turn, holds the sand in place and keeps up with the new deposits of sand. It can grow up to a metre a year through the accumulating sand.

Specialisation - Most plants would be killed by the conditions that marram grass thrive in, but this plant is so specialised it grows best when it is repeatedly buried by loose sand. It has leaves that are adapted to help it survive the salty and dry conditions. The leaf is rolled up lengthways to conserve moisture when the sand is dry, together with a thick skin on the leaf surface to reduce water-loss through evaporation and transpiration.

As the sand becomes more stable, other plants are able to grow among the marram grass. Two of the most common are sea rocket and sea lyme grass. Eventually the dunes grow so high on the landward edge of a beach that wind speeds are reduced over the dunes in front, and they too can grow in a shoreward direction. This area is called the mobile dune.

Human-physical interactions

Mobile sand dunes are very fragile habitats. They may be damaged or broken through naturally, by extra strong winds or waves. 'Blow-outs' show evidence of this happening. They occur after a hole is blasted through the dune front by the wind. Blow-outs may be made bigger by the action of humans - such as walkers, sunbathers, trail bikers and quad bikers.

Stage 1 - Mobile (yellow) dunes

On the landward (sheltered) side of the mobile dunes, more plants are able to establish themselves. These are mainly perennials, which survive year after year and increase the stability of the dune until a complete cover of vegetation is achieved. These completely stable areas are called 'fixed' dunes.

Stage 2 - Dune grassland and dune heath

Fixed dunes are also called grey dunes because of the build-up of humus and the cover of lichens (e.g. dog lichen), which give giving a greyish hue to the area. As the distance from the shore increases, sand no longer accumulates and marram grass, unable to compete with the new colonising plants, becomes less common and other grasses, herbs and mosses increase. Common plants are buttercups, dandelions, bird's-foot trefoil, ragged robin, creeping fescue and sand sedge. This habitat is called dune grassland.

Old, drier, grey dunes are then colonised by woody shrubs such as heaths, gorse, broom, wild rose and bramble and non-native species like rosebay willowherb. This habitat is called dune heath. This tends to be wet or dry, depending on whether it is on the top of an old dune, or in the damp hollows between.

Dune slacks

Between the dunes there are natural hollows, which accumulate freshwater. These areas are called dune slacks. The water table is higher in these areas, and in winter shallow lochans form. This means colonising plants need to be adapted to waterlogged conditions. Mosses, rushes, and tree and shrub species like willows and alders are all found here.

Stage 3 - Woodland climax

Once the conditions are right for shrubby species then trees can also establish and woodland will eventually develop. Alder and willows can grow in the wetter areas and can survive water-logging in the winter. Birch can grow in damp areas and on well-drained sand, but conifers like the native Scots pine don't like to have their feet wet and only establish in dry, sandy areas.

Dune heath and dune grassland are rare habitats in Scotland and have often been lost because non-native conifers have been planted on them. Management of areas where this has occurred may involve felling the conifers in order to restore the dune heath and grassland.