Coastal Systems: Waves, Tides, Sediments, Cells

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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. Pebbles 4 17° Higher tidal ranges are found on ocean Coarse sand 2 7° coasts, such as the Atlantic coasts of Upper sweep profile Medium sand 0.2 5° (summer) Fine sand 0.02 3° Britain and Canada. Very fine sand 0.002 1°

Sediments and sediment cells High tide (spring) High neap One of the main activities of the coastal system is the sourcing, transfer and Main sea level deposition of sediment along a stretch sweep zone Low neap of coastline called a sediment or littoral Low tide (spring) cell. Lower sweep profile (winter) DEFRA (the Department for Environment, Farming and Rural Source: Guinness and Nagle, 2000 Affairs) defines a sediment cell as: ‘A length of coastline and its associated Figure 4: Causes of tides GeoFile Series 27 Issue 1 Fig 575_03 Mac/eps/illustrator 11 s/s nearshore area within which the movement of NELSON THORNES PUBLISHING coarse sediment (sand and shingle) is largely a The gravitational Artist: David Russebll SpIlluringstra tidestion self-contained. Interruptions to the movement pull of the moon Maximum of sand and shingle within one cell should not tidal range affect beaches in a neighbouring sediment

cell.’ Earth Sun Low tide Moon The English and Welsh coastlines are divided into 11 cells, which are then Gravitational divided into subcells or management High Earth High tide tide attraction c Neap tides units. Sediment cell theory is a key Moon Moon component of shoreline management Minimum plans, which determine future Low tidal range strategies (see Geofile no. 537). Figure tide

5 shows the main inputs, transfers and Earth Sun stores within a sediment cell.

(Not to scale) The key characteristics of sediment Source: Waugh 1995, p. 130 cells are as follows. • Cells are discreet and function Figure 5: Sources of sediment, transfers and sediment sinks and stores within sediment cells separately from each other. The sediment cells are geographically GeoFile Series 27 Issue 1 bounded by significant disruptions INPUTS Fig 575_04TRANSFERS Mac/eps/illustrator 11 s/s STORES Source of sediment NELSONTransportation THORNES PUBLISHING Sinks to the coastline, such as headlands, Artist: David Russell Illustration estuaries or a convergence of currents or longshore drift • Cliff erosion • Longshore drift: The Sinks/permanent direction. • Fluvial sediment movement of material storage: • Eroding caused by the approach • Estuary • Within the cell, sediment is depositional of swash at an angle to • Submarine canyon sourced, transferred and stored. features e.g. the shore and the • Offshore bar/bank Coarse sediments are not exchanged • Beaches subsequent • Dredging between cells, but finer sediment in • Dunes perpendicular backwash suspension can be. • Spits down the steepest Sub-sinks and • Beach recharge beach gradient which temporary stores: • Over time, sub-sinks will erode and • Offshore bars and moves the material • Sedimentary the sediment will re-enter the cell’s sediment laterally downdrift. Aided features system. • Erosion of wave by wave refraction. Beaches, dunes, spits, • The sediment in the sink is away cut platforms • Currents bars • Saltation: Transportation from wave action and longshore of sand along the shore drift, it becomes essentially an by the wind. output, as it is no longer being worked by the processes within the cell. • The amount of sediment available which are in equilibrium with the causing erosion in some areas. If the to the sediment cell is called the amount of sediment available. If the budget increases, then more sediment budget. The sediment cell budget is decreased then the waves deposition is likely. will produce depositional features will continue to move sediment,

Figure 6: South Downs Sediment Cell 4d:Selsey Bill to Beechy Head Human activity and sediment e

e cells R. Adur Shoreham Brighton m Human activity can interfere with the k R. Arun Littlehampton Marina c processes within a sediment cell by Pagenham R. Ouse New- u haven C Beachy

disrupting the supply of sediment and Harbour . Head therefore the sediment budget of the R cell. Groynes, jetties and harbour walls Back eddy will block the movement of sediment, Key which can lead to beach erosion further Dominant wind and Chalk cliffs Selsey wave direction Dominant drift direction downdrift. Groynes are used to trap Bill Pier/breakwater sediment in areas where a beach is Cell division considered essential, either for the • Dominant drift and transport direction • The rivers Ouse and Cuckmere west to east ( 48%) (5000m3) provide sediment to the east protection of cliffs, defences, leisure From E/SE 28% amenity or economic prosperity. More GeoFile Series 27of Iss Brightonue 1 Marina. From south 16% . Fig 575_06 Mac/eps/i•llu Ofstra thetor 22km 11 s/s of cliff in the cell 8km is built-up coastal areas tend to have more • The tidal range increases NELSONand floods THORNES PUBLIprotected;SHING this affects the quantities of groynes than more rural coastlines, and from west to east. The fallingArti ebbst: Davtideid Russelflintsl Illustra enteringtion the cell from cliff these areas often have problems of beach flows to the west. erosion. Possible input of 5000 m3. • Onshore shingle movement or shingle erosion. • Rottingdean, Saltdean and Seaford creep in water of 15m or less and kelp have become seriously depleted and rafted sediment (10,000m3). have been artificially replenished. Sediment input supply can also be • The tidal inlet of Pagham Harbour and • The construction of the breakwater at disrupted by river dams, which cut the rivers Adur (2800m3) and Arun Newhaven undoubtedly helped starve down on the amount of fluvial sediment provide fluvial sediments into the cell Seaford beach. to the west of Brighton Marina. • Wave cut platform erosion may entering the coastal system. Protecting • Dredging Shoreham Harbour contribute 400m3 of sediment. soft cliffs can prevent cliff falls and (32000m3), Brighton Marina (5000m3), • Where the rivers enter the sea they reduce the amount of sediment entering Newhaven Harbour. block the eastwards movement of the system. • Bypassing of breakwaters pushes long shore drift and spits/beaches sediment offshore. build-up to the west of the channel • Up to 5000m3 gravel and sand lost outlet. The South Downs sediment round Beachy Head to Eastbourne • Abraded and recently eroded finer beaches. particles of sand and chalk are cell • Major store of gravel at Birling Gap is transported by suspension past being depleted. beaches,barriers and headlands. To The South Downs Shoreline • A back eddy in the lee of Beachy Head next subcell or offshore stores. Management Plan occupies sub-cell 4d deposits sediment to the south of along the Sussex coast of England Pagham Harbour. between the cliff headlands of Selsey Bill (Source: SDCG and SCOPAC, and Beachy Head. The shoreline figures are estimates) management plan further splits the cell in half to the east and west of Brighton Marina, forming two further subcells. familiar coastlines will change as tidal www.geog.sussex.ac.uk/BERM The subcell beaches are heavily ranges, weather patterns , sediment Bishop and Prosser (2003), Landform defenced with rock reefs, wooden and supplies and wave energy all change. Systems, Collins rock groynes along all urban sections. Guinness and Nagle (2000), AS Geography Concepts, Hodder and Beaches are composed of pebbles swept References onshore at the end of the last ice age, as Stoughton sea levels rose to give an extensive fossil SCOPAC: Standing Conference on Waugh (1995), Geography: an Integrated beach with sand exposed at low tide. To problems associated with the coastline: approach, Nelson Thornes the east of Brighton Marina the chalk www.stream.port.ac.uk DEFRA: www.DEFRA.gov.uk cliffs continue to Beachy Head and South Downs Coastal Goup: Google Earth: Close-up aerial views of include the famous Seven Sisters Cliffs. www.sdcg.org.uk the British coastline Figure 6 outlines the key features of the BERM project: cell. F ocus Questions Conclusion The coastal system is a complex and 1 Describe how the action of the sea interacts with the coastline through dynamic system which will adapt the coastal system. according to wave energy levels and sediment supply. A change in one part 2 How do wave type and sediment size affect beach morphology? of the system will cause the whole system to work harder to compensate 3 (a) Define the term sediment cell. for the change and achieve equilibrium. (b) What are the three main components of a sediment cell, and how do they This can be inconvenient for the interact? millions of people who live along the (c) How can people affect the equilibrium of a sediment cell? world’s coastlines as they may find they no longer have a beach, a safe harbour, 4 (a) Identify the sources, transfers and sinks within the South Downs or even a home. With global warming sediment cell 4d from the information provided. and the predicted rise in sea levels, (b) Suggest how and why human activity has affected this cell.