Landforms of low-energy coasts

Overview

Sand beach and a shingle beach in terms of profile, storm beach and formation of a spit.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   How does a beach form, and why are beaches typically found in bays?

   Model answer

   A beach is an accumulation of sand and shingle deposited between the low water mark and the highest point reached by storm waves, formed where wave energy is low enough for the sediment supplied to settle rather than be carried away. Beaches are typical of bays because wave refraction round the flanking headlands spreads the wave energy over the longer curved shoreline of the bay, so waves arriving at the bay head are low, constructive and dissipated. Sediment eroded from the headlands and delivered by rivers and by drift is therefore deposited in this low-energy zone, and constructive waves with a swash stronger than their backwash push it onshore and build it up.

2. 2

   Distinguish between a sand beach and a shingle beach in terms of profile, and explain the difference.

   Model answer

   A sand beach is wide and gently sloping, often at only two to five degrees, while a shingle beach is narrow and steep, sometimes over ten or fifteen degrees. The reason is percolation. Sand particles are small and pack tightly, so pore spaces are tiny and water cannot soak in quickly; the swash therefore returns almost entirely as backwash, which spreads material out into a low-angle profile. Shingle has large particles with large spaces between them, so much of the swash percolates straight into the beach and the backwash is much weaker than the swash; material is carried up and left there, piling into a steep profile. Sorting follows from the same logic, with the coarsest material thrown highest up the beach.

3. 3

   What is a berm, and what does the presence of several berms tell you?

   Model answer

   A berm is a ridge or terrace of sediment running along the beach parallel to the shoreline, marking the highest point reached by the swash of a particular set of waves. Several berms at different heights up the beach record different tidal and wave conditions: the highest is usually the storm beach, thrown up by the largest storm waves and made of the coarsest material, and lower berms mark successive high tides or calmer periods. A well-developed sequence of berms indicates a beach that has been built up by constructive waves over a period without a major storm to comb it down.

4. 4

   What is a storm beach, and how is it formed?

   Model answer

   A ridge of the coarsest material on the beach, often large shingle and cobbles, lying at the very top of the beach above the reach of normal tides. It is thrown up there by the exceptional energy of storm waves at high tide, which have the force to hurl large clasts far up the profile, and it is left stranded because ordinary waves cannot reach it to move it back down. It is therefore a record of the largest events rather than of everyday conditions, and it acts as a natural sea defence protecting the land behind.

5. 5

   What conditions are needed for sand dunes to develop behind a beach?

   Model answer

   A large and continuing supply of sand. A wide, gently sloping beach with a large tidal range, so that an extensive area of sand is exposed and has time to dry out at low tide. A dominant onshore wind strong enough to move dry sand landwards by saltation and surface creep. An obstacle at the back of the beach, such as a line of driftwood, shingle or the first pioneer plants, to trap the moving sand and cause it to be deposited. And enough space landward for the dunes to build and migrate, without a cliff or a sea wall immediately behind.

6. 6

   Describe the succession of dunes from the beach inland and explain why the conditions change.

   Model answer

   Nearest the sea are embryo dunes, low, unstable mounds of fresh sand around a first obstacle, colonised by salt-tolerant pioneers such as sea rocket and sand couch. Behind them are foredunes or yellow dunes, higher and dominated by marram grass, whose long roots bind the sand and whose stems trap more of it, so these are the tallest and most actively growing dunes. Further inland are grey dunes, so called because accumulating humus and colonisation by mosses and lichens darken the surface; they are largely fixed, with a more or less continuous plant cover. Beyond them, mature or wooded dunes carry shrubs and trees. Between ridges lie dune slacks, damp hollows scoured down to near the water table where marsh plants grow. The conditions change with distance inland because salt spray, sand burial and wind exposure all decrease while organic matter, moisture retention, soil development and shelter all increase, so a greater range of species can survive and the substrate becomes stable.

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