Gf575_coastal Systems Waves, Tides, Sediment Cells

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The narrow strip where the sea andland interact is shaped and influenced

by both natural and human variableswithin a powerful system. The actionof waves, tides and currents providesan input of energy which is then usedthrough the processes of erosion,weathering, transportation anddeposition to produce the morphologyof the coastal zone above and below thewaves. The coastal system is driven bywave energy within the nearshore(breaker zone) and foreshore(intertidal) zones. Figure 1 shows howthe components of the system arerelated and interact. The processeswithin the system and the appearanceof the coastline will be controlled by anumber of physical variables andpossibly influenced by human activity.

Physical variables

Climate/weather patterns/seasons Wave type and strength Wind direction Fetch length and direction Tidal range/flow Currents

Geology of coastline Concordant/discordant Availability of sediment from

marine, coastal and fluvial sources Erosional and weathering

processes.

Human influences

Coastal engineering andmanagement Groynes Sea walls

Disruption of sediment supply

Dredging River dams Cliff protection

Non-management Blocking structures Jetties Harbour walls.

Waves

Waves are caused by the surface of thesea exerting frictional drag on thelowest layer of the wind. Higher layers

of the wind then move faster over thelower levels and fall forward, pushingdown on the sea surface, creating awave. As the wind blows on the back ofthe small ripple, the wave grows. In the

open sea there is no actual movementof water, just a movement of energy.

An imaginary particle would move in aclockwise direction between wave crest,trough, then back to the crest of thewave, but would not move forward inthe ocean; these are called oscillationwaves. The orbit of the particle varies

from circular to eliptical; the base of theorbit is called the wave base (Figure 2).

The height of the wave is an indicationof energy and depends on the fetch (thedistance over which the wind blows),the strength of the wind, duration of thewind, and sea depth. Strong winds willcreate steep waves which, when thewinds ease, will decrease in height andincrease in wavelength. These waves arecalled swell. Swell waves effect theAtlantic coasts of Britain even in thequieter summer months.

Wave refraction occurs where theundersea topography causes the wavefronts to slow, bend and aim to breakparallel to shore. This effect is mostoften seen in a headland and baycoastline. Wave energy tends to beconcentrated on the headlands hencemore erosion, with lower energy levelsoccurring within the bays anddeposition occurring. If the waves breakat an angle within the bays, thenlongshore drift occurs.

Types of wave

As a wave approaches the shore, andthe water depth decreases, the wave

length becomes shorter and the waveheight increases to compensate. Thecircular motion of the wave becomesmore elliptical as the wave base dragson the sea bed and the wave velocitydecreases (Figure 2a). The wavesteepens further, until the ratio ofheight to length is 1:7. Eventually thebody of the wave collapses forward, or

breaks, and rushes up the beach.Movement of water up the beach iscalled swash. Movement of water downthe beach is called backwash (Figure2b).

Sea bed topography can also influencehow a wave breaks. A sudden reductionin water depth over a steeper shingleprofile will produce a taller, steeperwave which is more likely to plunge. Agently shelving sea bed, with a longrun up, is more likely to encouragelower-profile waves.

There are two types of wave:constructive and destructive, whichshape beaches by the removal, additionand movement of sediment. Figure 3shows their characteristics and howthey shape beaches.

Constructive/spilling waves Long wavelength Low in height Strong swash pushes sediment up

the beach

Backwash soaks into beach onreturn. Sediment not pulled back Lower energy waves , commonly

swell waves 610/minute

SEPTEMBER 2008

575

Lucy Prentice

Geofile Online Nelson Thornes 2008

COASTAL SYSTEMS: WAVES, TIDES,SEDIMENTS, CELLS

GeofileOnline

Figure 1: The coastal system

TransfersTransportation

Windstrength

Winddirection

Wavetype

Waveaction

Refraction

Tides

Deposition

Sink/store

Erosion/weathering

Sedimentsupply/input

Fluvialsediments

Fetch


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Most effective over a gentle shelvingsea bed.

Destructive/plunging waves Short wave length

Steep wave faces and high waveheight

Wave crashes downwards into thetrough of the wave with little swash

Backwash is very strong and dragsmaterial back down the beach

Backwash interferes with swash ofnext wave

Higher energy waves generatelocalised storm conditions

1115/minute Most effective over a steeply

shelving sea bed which causes a

rapid increase in friction and asteep wave front.

Influence of waves andsediment on beachmorphology

Beach morphology is dependent onseveral factors: wave type, energy,sediment type and sea bedmorphology. It is a complexrelationship, but some keyrelationships can be found:

Sand forms wide, gentle gradientbeaches, whereas shingle beachesare narrower and have a steeperangle of rest due to their largerparticle size (Figure 3).

Constructive waves have a strongerswash and a weaker backwash,carrying material up the beach butnot having enough energy to carryit back down.

Destructive or plunging waves havea weak swash, with a small swashdistance, and a strong high energybackwash which draws materialback down the beach.

Swash, whether from constructiveor destructive waves, will tend to bestronger and backwash weaker on ashingle beach due to highpercolation rates.

Sandy beaches will tend to havestrong swash with a long run up dueto the flat profile and a similar

strength backwash due to lowpercolation rates on compressedsand. Material will be combed backdown the beach, but returned withthe next wave.

Sediment will be moved up ashingle beach. High percolationrates on the backwash will be tooweak to remove sediment.

Finer sediments do not require somuch energy to be eroded andtransported. Higher energyenvironments therefore arecharacterised by coarser sediment

sizes. Most changes in beach morphology

occur within the sweep zonebetween high and low tide. Abovethe high tide mark a storm beach or

berm may form when material isflung to the top of the beach(Figure 3).

Most British beaches will be subject toboth types of wave during the year, withhigher-energy destructive wavesdominating during the stormier wintermonths and constructive lower-energywaves during the calmer summermonths (Figure 3).

These points may explain why sandybeaches are eroded so badly during thewinter when high-energy destructivewaves are combined with a gentle sandyprofile. The percolation rate on thebackwash is low and therefore materialcan be dragged from the beach. Assmaller particle sizes do not requiremuch energy to be transported,beaches can be depleted quickly.

During stormy conditions, sand andlarger material is thrown up the beachto create a storm beach of largerpebbles. During lower-energyconditions with constructive waves thesandy beach can be replenished by thestrong swash of constructive waves.Figure 3 shows typical characteristicsof beaches on the south coast ofEngland and how they are dependenton seasons and sediment size.

Tides

The oceans tides are controlled by thegravitational pull of the Moon, and to alesser extent the Sun. The Moon pullsthe water in the ocean towards it,creating a bulge of water; a high tide.The Moon not only pulls the water butalso pulls the Earth towards it, thiscreates a second bulge of water and thesecond high tide on the other side of theEarth.

Twice a month the Earth, Moon and Sunare aligned: this puts an extra

gravitational pull on the tidal bulge, toproduce an extra high tide called a springtide. When the Sun and Moon are atright angles to each other, neap tidesoccur, when the tidal range is lowest.

Figure 4 shows the influence of theMoon and Sun on the Earths tides.When a spring tide coincides with anonshore gale, a storm surge can occur,which can lead to exceptionally highseas and flooding, as in the East coastfloods of 1953 and the near miss ofNovember 2007.

The tidal range is the vertical distancebetween high tide and low tide, and thiscoincides with the sweep zone for thebeach (Figure 3). The slope of the

September 2008 no.575 Coastal Systems: waves, tides, sediments, cells


Figure 2: Constructive/destructive waves

Swashzone

Foreshore

Foredune

Foredune

Backshore

Longshorebar

a Constructive waves

Smalllongshore bar

(breakpoint bar)

Nearshore

Breaker zone Surf zone

Low flat wavesspill over

Large steep waveplunges over

Original

profile

Beachcliff

forms

Material from offshore barsmoved onshore

Eroded material deposited offshorein longshore bars

Berm

Strong swash transports

sand up the beach toform a berm

Strong wash

Strong backwash

little percolationthrough sand

Weak swash

Weak backwashmuch percolationthrough sand, littletransport of sand

down beach

Orbital motion of wavebecomes more eliptical

with sea bed contact

Inshore

b Destructive waves

Source: Guinness and Nagle, 2000, p. 116


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September 2008 no.575 Coastal Systems: waves, tides, sediments, cells


shoreline and the tidal range determinethe amount of shore exposed to waveaction A low tidal range tends toproduce a narrower beach, which isprone to higher erosion; such beachesare found on the shores of seas such asthe Mediterranean, rather than oceans.Higher tidal ranges are found on oceancoasts, such as the Atlantic coasts ofBritain and Canada.

Sediments and sediment cells

One of the main activities of the coastalsystem is the sourcing, transfer anddeposition of sediment along a stretchof coastline called a sediment or littoralcell.

DEFRA (the Department forEnvironment, Farming and RuralAffairs) defines a sediment cell as:

A length of coastline and its associatednearshore area within which the movement of

coarse sediment (sand and shingle) is largely

self-contained. Interruptions to the movement

of sand and shingle within one cell should not

affect beaches in a neighbouring sediment

cell.

The English and Welsh coastlines aredivided into 11 cells, which are thendivided into subcells or managementunits. Sediment cell theory is a keycomponent of shoreline managementplans, which determine future

strategies (see Geofile no. 537). Figure5 shows the main inputs, transfers andstores within a sediment cell.

The key characteristics of sedimentcells are as follows.

Cells are discreet and functionseparately from each other. Thesediment cells are geographicallybounded by significant disruptionsto the coastline, such as headlands,estuaries or a convergence ofcurrents or longshore drift

direction. Within the cell, sediment is

sourced, transferred and stored.Coarse sediments are not exchangedbetween cells, but finer sediment insuspension can be.

Over time, sub-sinks will erode andthe sediment will re-enter the cellssystem.

The sediment in the sink is awayfrom wave action and longshoredrift, it becomes essentially anoutput, as it is no longer beingworked by the processes within thecell.

The amount of sediment availableto the sediment cell is called thesediment budget. The sediment cellwill produce depositional features

which are in equilibrium with theamount of sediment available. If thebudget is decreased then the waveswill continue to move sediment,

causing erosion in some areas. If thebudget increases, then moredeposition is likely.

High tide (spring)

High neap

Main sea level

Low neap

Low tide (spring)

CobblesPebblesCoarse sandMedium sand

Fine sandVery fine sand

Upper sweep profile

(summer)

Lower sweep profile (winter)

sweep zone

Beach profiles and particle sizeSeasonal beach profile

324

20.2

0.020.002

241775

31

Material Diameter

(mm)

Beach

angle

Figure 3: Beach morphology and sediment type

Source: Guinness and Nagle, 2000

Moon

Lowtide

a The gravitational pull of the moon

b Spring tides

Lowtide

Gravitational

attraction

Moon

Maximumtidal range

Sun

cNeap tides

Moon

(Not to scale)

Minimumtidal range

Sun

Hightide

Hightide

Earth

Earth

Earth

Figure 4: Causes of tides

Source: Waugh 1995, p. 130

Figure 5: Sources of sediment, transfers and sediment sinks and stores within sediment cells

INPUTSSource of sediment

TRANSFERSTransportation

STORESSinks

Cliff erosion Fluvial sediment Eroding

depositionalfeatures e.g. Beaches Dunes Spits

Beach recharge Offshore bars and

sediment Erosion of wave

cut platforms

Longshore drift: Themovement of materialcaused by the approachof swash at an angle tothe shore and thesubsequentperpendicular backwashdown the steepestbeach gradient whichmoves the materiallaterally downdrift. Aidedby wave refraction.

Currents Saltation: Transportation

of sand along the shoreby the wind.

Sinks/permanentstorage: Estuary Submarine canyon Offshore bar/bank Dredging

Sub-sinks andtemporary stores: Sedimentary

featuresBeaches, dunes, spits,

bars


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Gf575_coastal Systems Waves, Tides, Sediment Cells