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Coastal Systems: Waves, Tides, Sediments, Cells

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Coastal Systems: Waves, Tides, Sediments, Cells SEPTEMBER 2008 Online 575 Geofile Lucy Prentice COASTAL SYSTEMS: WAVES, TIDES, SEDIMENTS, CELLS The narrow strip where the sea and Figure 1: The coastal system land interact is shaped and influenced by both natural and human variables rosion/ Wind Tides E within a powerful system. The action strength weathering of waves, tides and currents provides an input of energy which is then used through the processes of erosion, Wave Wave Fetch Sediment weathering, transportation and type action supply/input deposition to produce the morphology of the coastal zone above and below the Transfers Fluvial waves. The coastal system is driven by Wind Transportation sediments wave energy within the nearshore direction Refraction (breaker zone) and foreshore (intertidal) zones. Figure 1 shows how Deposition the components of the system are related and interact. The processes within the system and the appearance of the coastline will be controlled by a Sink/store number of physical variables and possibly influenced by human activity. open sea there is no actual movement length becomes shorter and the wave Physical variables of water, just a movement of energy. height increases to compensate. The GeoFile Series 27 Isscircularue 1 motion of the wave becomes • Climate/weather patterns/seasons An imaginary particleFig would 575_01 move Mac/eps/i in a llustramoretor 1 1elliptical s/s as the wave base drags • Wave type and strength clockwise direction betweenNELSON wave THOR crest,NES PUBLIon SHINGthe sea bed and the wave velocity • Wind direction trough, then back to the Acrestrtist: Davof theid Russell Illudecreasesstration (Figure 2a). The wave • Fetch length and direction wave, but would not move forward in steepens further, until the ratio of • Tidal range/flow the ocean; these are called oscillation height to length is 1:7. Eventually the • Currents waves. The orbit of the particle varies body of the wave collapses forward, or • Geology of coastline from circular to eliptical; the base of the breaks, and rushes up the beach. • Concordant/discordant orbit is called the wave base (Figure 2). Movement of water up the beach is • Availability of sediment from called swash. Movement of water down marine, coastal and fluvial sources The height of the wave is an indication the beach is called backwash (Figure • Erosional and weathering of energy and depends on the fetch (the 2b). processes. distance over which the wind blows), the strength of the wind, duration of the Sea bed topography can also influence Human influences wind, and sea depth. Strong winds will how a wave breaks. A sudden reduction create steep waves which, when the in water depth over a steeper shingle • Coastal engineering and winds ease, will decrease in height and profile will produce a taller, steeper management increase in wavelength. These waves are wave which is more likely to plunge. A • Groynes called swell. Swell waves effect the gently shelving sea bed, with a long • Sea walls Atlantic coasts of Britain even in the run up, is more likely to encourage • Disruption of sediment supply quieter summer months. lower-profile waves. • Dredging • River dams Wave refraction occurs where the There are two types of wave: • Cliff protection undersea topography causes the wave constructive and destructive, which • Non-management fronts to slow, bend and aim to break shape beaches by the removal, addition • Blocking structures parallel to shore. This effect is most and movement of sediment. Figure 3 • Jetties often seen in a headland and bay shows their characteristics and how • Harbour walls. coastline. Wave energy tends to be they shape beaches. concentrated on the headlands hence Waves more erosion, with lower energy levels Constructive/spilling waves occurring within the bays and • Long wavelength Waves are caused by the surface of the deposition occurring. If the waves break • Low in height sea exerting frictional drag on the at an angle within the bays, then • Strong swash pushes sediment up lowest layer of the wind. Higher layers longshore drift occurs. the beach of the wind then move faster over the • Backwash soaks into beach on lower levels and fall forward, pushing Types of wave return. Sediment not pulled back down on the sea surface, creating a • Lower energy waves , commonly wave. As the wind blows on the back of As a wave approaches the shore, and swell waves the small ripple, the wave grows. In the the water depth decreases, the wave • 6–10/minute Geofile Online © Nelson Thornes 2008 September 2008 no.575 Coastal Systems: waves, tides, sediments, cells Figure 2: Constructive/destructive waves berm may form when material is flung to the top of the beach a Constructive waves (Figure 3). Foreshore Inshore Most British beaches will be subject to both types of wave during the year, with Nearshore Backshore Swash higher-energy destructive waves zone dominating during the stormier winter Breaker zone Surf zone Strong swash transports sand up the beach to months and constructive lower-energy Orbital motion of wave form a berm Foredune becomes more eliptical Low flat waves waves during the calmer summer with sea bed contact spill over Berm months (Figure 3). Strong wash Longshore These points may explain why sandy bar Weak backwash beaches are eroded so badly during the Small much percolation longshore bar through sand, little winter when high-energy destructive Material from offshore bars (breakpoint bar) transport of sand waves are combined with a gentle sandy moved onshore down beach profile. The percolation rate on the b Destructive waves backwash is low and therefore material Large steep wave can be dragged from the beach. As plunges over smaller particle sizes do not require Beach Foredune much energy to be transported, cliff forms beaches can be depleted quickly. Original profile During stormy conditions, sand and Weak swash larger material is thrown up the beach to create a storm beach of larger Strong backwash pebbles. During lower-energy Eroded material deposited offshore little percolation in longshore bars through sand conditions with constructive waves the sandy beach can be replenished by the strong swash of constructive waves. Source: Guinness and Nagle, 2000, p. 116 Figure 3 shows typical characteristics of beaches on the south coast of • Most effective over a gentleGeoFile shelving Series 27• IssConstructiveue 1 waves have a stronger Fig 575_02 Mac/eps/illustrator 11 s/s England and how they are dependent sea bed. NELSON THORNES PUBLIswashSHING and a weaker backwash, on seasons and sediment size. Artist: David Russell Illucarryingstration material up the beach but Destructive/plunging waves not having enough energy to carry • Short wave length it back down. Tides • Steep wave faces and high wave • Destructive or plunging waves have The ocean’s tides are controlled by the height a weak swash, with a small swash gravitational pull of the Moon, and to a • Wave crashes downwards into the distance, and a strong high energy lesser extent the Sun. The Moon pulls trough of the wave with little swash backwash which draws material the water in the ocean towards it, • Backwash is very strong and drags back down the beach. creating a bulge of water; a high tide. material back down the beach • Swash, whether from constructive The Moon not only pulls the water but • Backwash interferes with swash of or destructive waves, will tend to be also pulls the Earth towards it, this next wave stronger and backwash weaker on a creates a second bulge of water and the • Higher energy waves generate shingle beach due to high second high tide on the other side of the localised storm conditions percolation rates. Earth. • 11–15/minute • Sandy beaches will tend to have • Most effective over a steeply strong swash with a long run up due Twice a month the Earth, Moon and Sun shelving sea bed which causes a to the flat profile and a similar are aligned: this puts an extra rapid increase in friction and a strength backwash due to low gravitational pull on the tidal bulge, to steep wave front. percolation rates on compressed produce an extra high tide called a spring sand. Material will be combed back tide. When the Sun and Moon are at Influence of waves and down the beach, but returned with right angles to each other, neap tides the next wave. occur, when the tidal range is lowest. sediment on beach • Sediment will be moved up a morphology shingle beach. High percolation Figure 4 shows the influence of the rates on the backwash will be too Moon and Sun on the Earth’s tides. Beach morphology is dependent on weak to remove sediment. When a spring tide coincides with an several factors: wave type, energy, • Finer sediments do not require so onshore gale, a storm surge can occur, sediment type and sea bed much energy to be eroded and which can lead to exceptionally high morphology. It is a complex transported. Higher energy seas and flooding, as in the East coast relationship, but some key environments therefore are floods of 1953 and the ‘near miss’ of relationships can be found: characterised by coarser sediment November 2007. • Sand forms wide, gentle gradient sizes. beaches, whereas shingle beaches • Most changes in beach morphology The tidal range is the vertical distance are narrower and have a steeper occur within the sweep zone between high tide and low tide, and this angle of rest due to their larger between high and low tide. Above coincides with the sweep zone for the particle size (Figure 3). the high tide mark a storm beach or beach (Figure 3). The slope of the Geofile Online © Nelson Thornes 2008 September 2008 no.575 Coastal Systems: waves, tides, sediments, cells shoreline and the tidal range determine Figure 3: Beach morphology and sediment type the amount of shore exposed to wave Beach profiles and particle size action A low tidal range tends to Seasonal beach profile produce a narrower beach, which is Material Diameter Beach prone to higher erosion; such beaches (mm) angle are found on the shores of seas such as Cobbles 32 24° the Mediterranean, rather than oceans.
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