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Home , Breaking wave, Clapotis, Coast, Coral reef, Current (stream), Longshore drift

CHAPTER 2

2. Coastal processes

Coastal result from by weakening the surface and driving nearshore the interaction between coastal to facilitate further processes. and processes and sediment movement. movement. Biological, biophysical are also significant contributors, Hydrodynamic (, tides and biochemical processes are and are indeed dominant in coastal and currents) and aerodynamic important in , and estuarine environments, (wind) processes are important. and environments. respectively, but the action of contributes significantly waves is dominant in most settings. to sediment transport along rocky Waves Information Box 2.1 explains the , either directly through waves are the principal technical terms associated with of , or indirectly agents for shaping the regular (or sinusoidal) waves.

INFORMATION BOX 2.1 TECHNICAL TERMS ASSOCIATED WITH WAVES

Important characteristics of regular, Natural waves are, however, height (Hs), which is or sinusoidal, waves (Figure 2.1). highly irregular (not sinusoidal), defined as the average of the • (H) – the difference and a range of wave heights highest one-third of the waves. in elevation between the wave and periods are usually present The crest and the wave trough (Figure 2.2), making it difficult to off the coast of south-west • wave length (L) – the distance describe the wave conditions in England, for example, is, on between successive crests quantitative terms. One way of average, 1.5m, despite the area (or troughs) measuring variable height experiencing 10m-high waves • wave period (T) – the time from is to calculate the significant during extreme . one wave crest to the next wave crest measured at a fixed point. This is easy to measure in the Wave length field: simply count and time the passage of a large number of waves (at least ten) past a Crest Crest fixed point (e.g. a harbour wall) Wave height Amplitude and divide the time by the Still number of waves level • wave steepness is the ratio of wave height to H/L; ‘steep’ waves are short Trough Trough Trough and large, while ‘’ waves are long and small. Figure 2.1 The characteristics of a regular wave.

1.8 Hs = 0.41m 1.7

1.6

1.5

Water depth (m ) 1.4

1.3 0620 40 080 100 120 Time (s)

Figure 2.2 Irregular and asymmetric wave heights measured just outside the on a sandy over a period of two minutes. Source: Holden, 2008.

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The size of the waves produced The slowing down of the wave and In shallow water, waves break by a given depends on three the shortening of the wave length in a depth slightly larger than their factors: wind speed, wind duration causes an increase in wave height height and the region on the beach (how long the wind blows for) and and a deformation of the wave shape where waves break is known as fetch (the distance over which the that makes the waves asymmetrical the surf zone. The wave front is wind blows). Where strong and ‘peaked’ – this process is too steep to maintain itself and the blow for a long time over long known as shoaling (Figure 2.4). The water particles at the crest of the distances, large waves are produced. asymmetry of the waves means that wave move forward faster than the Such waves are characterised both the onshore push is stronger than wave itself, causing collapse and by their large height and by their the offshore pull, thus and the generation of bubbles and foam. long periods. The longer the wave are driven onshore, ultimately Wave breaking is an important period the faster the wave will be resulting in the formation of process, because when waves travelling. The carried by a . Without this asymmetry, break, their energy is released and wave is proportional to its height there would be no mechanism to generates nearshore currents and squared, thus a slightly bigger wave transport to . sediment transport. will have a lot more energy – a doubling of the wave height results in a fourfold increase in wave energy. Orbital Wave peaking as In the UK, the waves with the largest movement of H/L increases fetch are those that hit the south-west wave particles coast, having travelled 7000km across the . Large, long-period waves generated by extreme storms have incredible , and it is this Horizontal that transports large movement of Elliptical wave particles amounts of beach sediment, thus movement of changing the shape of the coast. wave particles Waves behave differently according to the depth of water. In Figure 2.3 The motion of water particles under waves. Source: Holden, 2008. deep water, the waveform moves forward, but the water particles themselves have an almost closed , with virtually no forward trajectory. These circular orbits (shown in Figure 2.3) decrease in diameter with depth, until all wave motion ceases. The water depth at which the wave motion is no longer felt is known as wave base and is generally 10–20m. Beneath this, the sediment on the remains undisturbed. As waves are the most significant geomorphological agent acting on the coast, causing , transportation and of sediment, the calculation of the wave base is critical in understanding the impact of waves along the coast. As waves move shoreward, the water depth becomes less than the wave base, thus the seabed begins to affect the wave motion. The circular movement of water particles becomes increasingly elliptical, and eventually horizontal, Figure 2.4 just offshore the surf zone of sandy beach. Note how and the wave length and wave peaked and asymmetric the shoaling waves are, with the wave crest being sharp velocity decrease (see Figure 2.3). and short, and the wave troughs being flat and long. Photo: © G. Masselink.

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Wave types and the part of the wave that is in topographic depressions, referred A continuum of breaker shapes shallower water travels more slowly to as divergence, which leads occurs in ; however, three than the part of the wave in deeper to relatively sheltered shoreline main breaker types are commonly water. This results in a rotation conditions. On the other hand, recognised (Figure 2.5). of the wave crest with respect convergence, where wave energy The type of breaker is connected to the wave bottom contours, or, is concentrated, will occur when with the distribution of wave energy in other words, a bending of the waves travel over relatively shallow and beach gradient. Spilling breakers wave rays (Figure 2.7). water (such as a sand on the are associated with wide, low-angled is an important process because floor), resulting in an increase beaches where energy is gradually it controls the angle at which the in the wave energy and wave height dispersed. Plunging and surging waves break. This is important (see Chapter 3). breakers often break on steep for generating longshore currents At the shoreline, surf zone beaches soon after reaching wave and . Drift-aligned waves break onto the ‘dry’ beach base and significant amounts of barriers are oriented obliquely to in the form of . Swash energy may be reflected back to sea. the crest of the prevailing waves motion has an onshore phase in When waves encounter the vertical (Figure 2.7c). The shoreline of which velocity slows (uprush), and face of a , however, wave drift-aligned coasts is primarily an offshore phase characterised approaches 100%. This may controlled by longshore sediment by acceleration under gravity give rise to motion transport processes, forming spits (backwash). For a number of () in front of the seawall and/ and bars (see Chapter 4). reasons (such as effects, or complicated criss-cross wave Refraction also determines transport of sediment within the patterns where the waves arrive at the distribution of energy along main water flow and an angle to the structure (Figure 2.6). particular stretches of coast. from the surf zone into the swash A complex seabed topography zone), more sediment is transported Wave refraction can cause waves to be refracted by the uprush than the backwash. Wave refraction is another important in complicated ways and produce A balance with the weaker backwash process that takes place during significant variations in wave height is achieved by the relatively steep shoaling. It occurs where the wave and energy along the coast. Here, gradient that generally occurs in front approaches the coast obliquely, wave energy is directed away from the swash zone.

Spilling breakers are associated with gentle beach gradients and high- Spilling breakers steepness waves (e.g. during storms). A gradual peaking of the wave occurs until the crest becomes unstable, resulting in a gentle forward spilling of the crest and the production of bubbles and foam.

Plunging breakers occur on steeper beaches than spilling breakers, with waves of intermediate steepness. The shoreward face of the wave becomes vertical, curls over, and plunges forward Plunging breakers and downward as an intact mass of water. Plunging breakers are the most desirable type of breaker for surfers, because they offer the fastest rides and, as they plunge over, they produce ‘tubes’.

Surging breakers are found on steep beaches with low-steepness waves Surging breakers (e.g. summer waves). The front face and crest of surging breakers remain relatively smooth and the wave slides directly up the beach without breaking or producing foam.

Figure 2.5 The three main types of breakers: spilling, plunging and surging. Source: G. Masselink.

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Figure 2.6 Waves reflecting off a seawall at an angle, giving rise to a criss-cross wave pattern. Photo: © G. Masselink.

In calm conditions, the up-beach movement of sediment heat, reflected at the shore or backwash returns to the sea before and can enhance the contribution dissipated due to wave breaking. the next incoming , of the seaward-flowing backwash. Much of the energy from breaking thus there is no interference and This encourages beach erosion and waves is used for generating a greater amount of sediment is results in a flattening of the beach. nearshore currents and sediment transported up the beach than is At the risk of overgeneralising, transport, ultimately resulting in brought down. In this way, steep constructive waves tend to have a the formation of distinct coastal beaches can be constructed as low wave steepness and destructive morphology. The intensity of these waves take sediment from low waves a high wave steepness. currents increases with increasing down the beach and deposit it at incoming wave energy level – the landward edge. In stormier Nearshore currents thus the strongest currents are conditions, breaking waves and In the surf zone, incoming wave encountered during storms. turbulent swash motion inhibit the energy is lost through noise and There are three types of wave-

a b c Reduced wave Refracted energy in wave fronts embayment Shoreline

Concentration of wave of wave energy on to energy on to headland Longshore in surf zone

Waves refracting in shallow water

Figure 2.7 Wave refraction: (a) The wave crest breaks on the headland while the rest of the wave continues into the ; (b) Waves fanning out into bay but clear line of breaking waves indicates shallow depth; (c) Generation of longshore drift by refracting waves. Photo: surftime.ru

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induced currents that dominate natural disaster in Europe in modern the net water movement in the Although ocean waves are the times. Under extreme conditions (such nearshore, as seen in Figure 2.8. principal agents for shaping the coast as during hurricanes or cyclones), Nearshore currents are and driving nearshore sediment storm surge levels can be in excess capable of transporting large transport, the contribution of storm of 5m, leading to extensive coastal quantities of sediment. This is surge is significant; raised water levels flooding and erosion. The maximum partly due to their significant flow may last several days. During severe storm surge associated with Hurricane velocities, but also because the storms, the water level near the shore Katrina in the USA in 2005 appears stirring motion of the breaking can be significantly higher than to have been in excess of 10m. waves intensifies the entrainment tidal predictions, and the difference The maximum water level at the of sediment. The amount of between the measured and the coast during storm surge depends sediment transported by longshore predicted water level is the storm on four main factors: currents is known as the littoral surge. Figure 2.9 indicates the storm 1. The severity of low – drift and is generally of the surge levels observed at the Hook of will rise approximately of 10,000–100,000m3/yr. Such large Holland, January–February 1953. This 1cm for every 1mb fall in air transport rates have a major effect disastrous storm also hit the east pressure. Storms are always on coastal morphology and coast of England, causing 300 deaths characterised by low pressure shoreline change. (2100 deaths Europe-wide), the worst and hence the water level always rises under a storm. 2. Onshore wind – if the wind is directed shoreward it can Plan view Longshore currents are shore-parallel water against the coast, causing flows within the surf zone. They are an increase in the water level. rests driven by waves entering the surf zone 3. Coastal topography – the effect Wave c with their crests aligned at oblique of the storm surge on the coast angles to the shoreline; the larger the wave angle, the stronger the currents. depends greatly on coastal Longshore current configuration. Relatively low-

Surf zone gradient, funnel- and bay-shaped coastal settings are particularly prone to extreme surges. Examples of these are the Bay Cross-section The bed return flow (sometimes of (), of misleadingly referred to as ) Mexico () and the North is an average flow near the bed flowing Sea (Europe). Straight coastlines Onshore Surf zone offshore. The current is part of a mass and promontories are generally transport circulation of water characterised by in waves onshore flow in the upper part of the less sensitive to storm surge. 4. The state of the – very high Bed return and seaward flow near flow the bottom. It is the bed return flow that tides exacerbate the takes sediment eroded from the beach impact of a storm surge. during storms to the edge of the surf zone where it may end up in nearshore Tides breaker bars. The tidal rise and fall of the ocean Plan view Rip currents are strong, narrow currents surface are barely noticeable in the that flow seaward through the surf zone deep ocean, but on shallow continental in channels and present a significant shelves, along coastlines and within

Rip head to swimmers. As water comes , tidal processes can be the onshore over bars, longshore feeder dominant morphological agent. The currents direct it away from the bars and two driving for ocean tides are Rip neck into the rip itself, which transports water Bar the gravitational attraction of the offshore. Maximum current velocities –Moon and the Earth–Sun in rip currents may reach up to 2m/s Longshore feeder current systems, with the latter having almost Surf zone under extreme storm conditions. Typical velocities are 0.5–1m/s with half the impact of the former. The flows generally stronger during low tide is rather complicated, than during high tide. therefore it is more useful to describe how tides are manifested Figure 2.8 Wave-induced currents in the nearshore – longshore, bed return flow, and rip. along our coasts, than be concerned Source: Holden, 2008. with how they are generated.

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