8.1 Waves, marine and sub-aerial processes

Waves result from friction between wind and the sea surface. Wave height is an indication of wave energy. It is controlled by wind strength, fetch (the distance of open water a wave travels over) and the depth of the sea. Waves of up to 12–15m are formed in open sea and can travel vast distances away from the generation area, reaching distant shores as swell waves, characterised by a lower height and a longer wavelength. In contrast, storm waves are characterised by a short wavelength, high amplitude and high wave frequency.

l As wave fronts approach the shore, their speed of approach will be reduced as the waves ‘feel bottom’. l Usually, wave fronts will approach the shore obliquely – this causes the wave fronts to bend and swing round in an attempt to break parallel to the shore. l The change in speed and distortion of the wave fronts is called wave refraction (Figure 8.1). l If refraction is completed, the fronts will break parallel to the shore.

Concentration of wave energy on to headland Reduced wave energy in embayment Refracted wave fronts

Concentration of wave energy on to headland

Revised

Wave height or amplitude is the distance between the trough and the crest. Wavelength is the distance between two successive crests or troughs. Wave frequency is the number of waves per minute.

Parallel wave fronts

Source: Advanced Geography: Concepts & Cases by P. Guinness & G. Nagle (Hodder Education, 1999), p.294

Figure 8.1 Wave refraction

Wave refraction distributes wave energy along a stretch of coast. Along a complex transverse coast with alternating headlands and bays, wave refraction will concentrate wave energy and therefore erosional activity on the headlands, while wave energy will be dispersed in the bays. Hence deposition will tend to occur in the bays.

l Spilling breakers are steep waves (large height relative to wavelength) associated with gentle beach gradients. They are characterised by a gradual peaking of the wave until the crest becomes unstable, resulting in a gentle spilling forward of the crest. l Plunging breakers are waves of intermediate steepness that tend to occur on steeper beaches than spilling breakers. They are distinguished by the shoreward face of the wave becoming vertical, curling over and plunging forward and downward as an intact mass of water. l Surging breakers have low steepness and are found on steep beaches.

In surging breakers the front face and crest of the wave remain relatively smooth and the wave slides directly up the beach without breaking. In surging breakers a large proportion of the wave energy is reflected at the beach.

Once a breaker has collapsed, the swash will surge up the beach with its speed gradually lessened by friction and the uphill gradient. Gravity will then draw the water back as the backwash. Constructive waves (Figure 8.2) tend to occur when wave frequency is low (6–8/minute), particularly when these waves advance over a gently shelving sea floor (e.g. formed of fine material, such as sand). Because of the low frequency, the backwash of each wave will be allowed to return to the sea before the next wave breaks – i.e. the swash of each wave is not impeded and retains maximum energy.

Orbital motion of wave becomes more eliptical with sea bed contact Strong swash Foreshore Surf zone Breaker zone Berm Foredune Low flat waves spill over Swash zone Strong swash transports sand up the beach to form a berm Inshore Nearshore Backshore

Longshore bar

Small longshore bar (breakpoint bar) Material from offshore bars moved onshore Weak backwash much percolation through sand, little transport of sand down beach

Source: Advanced Geography: Concepts & Cases by P. Guinness & G. Nagle (Hodder Education, 1999), p.294

Figure 8.2 Constructive waves

Destructive waves (Figure 8.3) are the result of locally generated winds, which create waves of high frequency (12–14/minute). As the backwash is stronger than the swash, material is eroded from the beach.

Large steep wave plunges over

Eroded material deposited offshore in longshore bars Original profile Beach cliff forms Foredune

Weak swash

Strong backwash Little percolation through sand

Source: Advanced Geography: Concepts & Cases by P. Guinness & G. Nagle (Hodder Education, 1999), p.294

Figure 8.3 Destructive waves

1 Define wavelength and wave frequency. 2 Explain the meaning of the following terms: swash, backwash, fetch. 3 Distinguish between plunging breakers and surging breakers. 4 Describe the main changes that occur as a result of wave refraction.

Tested

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Hydraulic action (wave pounding) is an important process as waves break onto cliffs (Figure 8.4). As the waves break against the cliff face, any air trapped in cracks, joints and bedding planes will be momentarily placed under very great pressure. As the wave retreats, this pressure will be released with explosive force. Abrasion (corrasion) is the process whereby a breaking wave can hurl pebbles and shingle against a coast, thereby abrading it. Attrition takes place when the eroded material itself is worn down. Solution (corrosion) is a form of chemical erosion which affects calcareous (lime-rich) rock. Waves speed up the process.

The terms corrasion and corrosion are easy to confuse – and mis-spell. It is better to use abrasion and solution as you are less likely to get them mixed up.

Sub-aerial – surface runoff – rain wash – weathering by wind and frost – mass movement – soil creep, landslides, slumps

Corrasion – salt crystallisation disintegrates weaker layers – blue-green algae help break down rock Biotic factors – burrowing and browsing organisms Abrasion/corrasion – wearing away of cliff by material (rocks, sand) hurled against it Currents – generated by waves and tides Human activity – can increase runoff and erosion – sea defences

Hydraulic pressure – compression of trapped air and sudden release

Wave pounding (shock waves up to 30 tonnes/m 2 )

Attrition – wearing down of broken material into smaller rounded particles Solution – dissolving of limestones and other minerals by carbonic acid in sea water Source: Baker, P et al., Pathways in Geography, Nelson, 1997

Figure 8.4 Types of erosion

Sub-aerial, or cliff-face, processes include: l salt weathering – the process by which sodium and magnesium compounds expand in joints and cracks, thereby weakening rock structures l freeze–thaw weathering – the process whereby water freezes, expands and degrades jointed rocks l biological weathering – carried out by molluscs, sponges and urchins, and very important on low-energy coasts l solution weathering –the chemical weathering of calcium by acidic water; it tends to occur in rockpools due to the presence of organisms secreting organic acids l slaking – materials disintegrating when exposed to water, which can be caused by hydration cycles

Mass movements are also important in coastal areas, especially slumping and rock falls (see pages 45–49).

Coastal zones are complex – they are affected by marine, terrestrial and atmospheric processes. All need to be taken into account when explaining coastal processes and features.

Sediment sources are varied and include: l onshore transport by waves l longshore transport by waves l rivers l glacial and periglacial deposits l wind-blown deposits l artificial beach replenishment

Sediment transport is generally categorised into two modes: l Bedload –grains transported by bedload are moved with continuous contact (traction or dragging) or by discontinuous contact (saltation) with the seafloor. Traction, in which grains slide or roll along, is a slow form of transport. l Suspended load –grains are carried by turbulent flow and generally are held up by the water. Suspension occurs when moderate currents are transporting silts or strong currents are transporting sands.

Deposition is governed by sediment size (mass) and shape. In some cases sediments will flocculate (stick together), become heavier and fall out in deposition.

The coastal sediment system, or littoral cell system, is a simplified model that examines coastal processes and patterns in a given area (Figure 8.5). It operates at a variety of scales from a single bay, e.g. Turtle Bay, North Queensland, Australia, to a regional scale, e.g. the south California coast. Each littoral cell is self-contained, with inputs and outputs balanced.

Dune erosion River sediment transport

Dredging and sand mixing

Marine erosion Longshore drift

Artificial beach nourishment Inlet filling

Onshore transport Internal biogenic input (shells, etc,)

Offshore transport

Littoral drift in

Submarine canyon Solution (dissolving of sediments)

Marine processes Sediment input and output

Figure 8.5 Sediment cells

If refraction is not complete, longshore drift occurs (Figure 8.6). This leads to a gradual movement of sediment along the shore, as the swash moves in the direction of the prevailing wind, whereas the backwash moves straight down the beach, following the steepest gradient.

Cliff erosion

Littoral drift out