Waves
Size depends on: wind speed, length of time wind blows in constant direction and
length of fetch
California: very big waves as come from Aleutian Islands, 4000m fetch so time for
wave energy to build up.
Mediterranean: very small waves as surrounded by islands so reduced fetch
available for wave generation.
wave form moves, water particles rotate in circular/elliptical movement. energy of
wave that moves towards shore. when wave breaks, energy translates into
movement of water towards shore.
Terminology
wave crest: top of the wave
wave trough: bottom of the wave
wave height: difference in height between crest and trough
wave length: distance between two crests
wave period: time taken for wave to travel through one wave length
wave velocity: speed of movement of wave crest. av wave length / av wave period
wave frequency: no. of waves that break on beach in given period of time
wave steepness: wave height / wave length. cannot exceed 1:7 as wave will break
wave energy: energy is proportional to wave length, multiplied wave height
squared. wave energy directly related to height; energy released when wave
breaks
plunge line: point at which wave breaks
swash: foaming water that rushes up beach when wave breaks, obtains energy
from energy released by breaking wave
backwash: water which returns down beach after wave has broken
Types of wave
wide, gently sloping beach can absorb wave energy so protects coastline.
if waves break against cliff/sea wall, wave energy directed at vertical surface,
effective erosion.
high energy waves: strong winds blowing onto coastline can produce storm waves.
when they break on steep sloping beach, plunging breakers are produced. example
of destructive waves.
destructive waves: remove sediment from beach. steep high waves. short wave
length. high wave frequency. weak swash and strong back wash. sediment
removed builds up as longshore bars near low tide mark.
low energy waves: if wind dies down over ocean, waves move in same direction
until reach coastline. wave height decreases and wave length increases. swell
waves when reach shore. smooth crest when approach coast, wave slides up beach
even if steep. called surging breakers. example of constructive waves.
constructive waves: low gentle waves. add sediment to beach. long wave length.
low wave frequency. spill up beach when they break. strong swash and weak
backwash. forms berms at top of the beach
Wave refraction
when waves approach an irregular coastline, they’re
refracted
waves slowdown in-front of headland as its shallow
water
wave energy concentrated upon headland, enhancing
erosion
refraction sets up longshore currents that move
sediment from headland into bays (forms bay beaches)
longshore currents help to maintain bay beaches by
providing them with sediment
Erosion
hydraulic action: big waves have a lot of energy. water is dense material. storm
waves hitting sea wall/cliff foot can generate large shocks. pressure leads to
significant erosion
cavitation: (wave quarrying) water trapped in cracks in the rock is compressed by
pounding of waves. when pressure released, bubbles form in water which escape
from crack with force. widens cracks quickly. air pockets compressed in cave at
high tide can weaken and ‘quarry’ the cave roof, leading to blowhole formation.
corrasion: (abrasion) waves throw sand/shingles/pebbles at base of cliff. effective
form of erosion. produces wave-cut notches and caves. wave-cut platforms
smoothed by this process.
attrition: rock falls and slumps provide material which builds up at base of cliff.
wave energy moves material around and particles become smaller as they rub
together. material moved by longshore drift is susceptible for this process. pebbles
become smaller and smoother as they’re moved further from sediment source.
solution: (corrosion) chemicals in sea water can dissolve rocks such as chalk and
limestone.
Weathering
freeze thaw weathering: water goes into cracks in cliff and freezes overnight. water
expands 9% when freezes. this widens and weakens the crack. repeated, the rock
crumbles and splits.
exfoliation: extreme changes in temperatures can cause expansion and contraction
of surface layer of rock on cliff face. the stresses weaken the rock and surface
layers peel off. increased availability of water can speed up process.
oxidation: rock contain iron compounds, oxidation of compounds into iron oxide
(rust), weakens and decomposes rocks. leads to staining and crumbling.
biological weathering: halophytic (salt-loving) plants can grow in cliff cracks which
widens them and leads to biological weathering.
chemical weathering: seabirds’ guano (poo) is very corrosive and can chemically
weather some types of rock.
salt crystals: salt water can soak into pores of rock in the spray zone near base of
cliff. when water evaporates, salt crystals form in the rock. crystals are perfect
cubes in irregular pores so leads to stresses. rock then decomposes and crumbles
Mass movement
marine erosion and weathering leads to mass movement. hard rock cliffs; rock falls
(after heavy rain). unconsolidated material cliffs (clay, glacial deposits); rotational
slumping. end result; cliff retreat
Transportation of coastal sediment
sediment sources: rivers flowing from land to sea, alluvium. erosion of the coast.
material moved onto coastline from sea bed
Movement of sediment up and down beach: swash carries material up and
backwash brings material down beach. waves sort the beach material. constructive
waves move material up the beach, forming berms. destructive waves move
material down, forming longshore bars. wind can blow sand up beach and deposit
it at top of the beach, forming sand dunes.
Movement of sediment along the coastline: longshore drift; oblique onshore winds
drive the wave crest at an angle to the shore. the swash of the wave carries the
pebble at an oblique angle up the beach. backwash brings pebble straight down
under influence of gravity. process is repeated so pebble moves along beach.
Marine transportation processes…
suspension: fine sediment is carried as a
suspension in the water, making it look muddy/
mercy.
solution: dissolved material is carried along in
solution, so you can’t see it.
traction: larger pebbles and cobbles are rolled
along sea bed.
saltation: small pebbles are moved when one
pebble hits another, causing it to bounce. bouncing can set up a chain reaction.
Types of sediment
sediment comes in range of sizes.
larger particles form a storm beach at top of beach whereas smaller particles form
and wide gently sloping lower beach.
Deposition of coastal sediment
takes place whenever movement of sediment slows or stops.
happens updraft of a groin or in sheltered locations (bay or estuary).
also occurs where coastline changes direction.
spits develop at these places as the beach is built out across the inlet, estuary or
bay.
wave refraction can lead to deposition between island and coast; cuspate tombolo.
Sediment cells
coastlines can be divided into a series of sediment cells, with smaller sub-cells
within.
sediment movement is self-contained in each cell.
within each cell: sediment inputs, sediment movement and sediment outputs.
sediment moved by LSD, tidal/longshore currents.
cell boundaries occur at headlands and estuaries.
sub cells form coastal systems and SMPs are based on these.
SMPs are essential step towards integrated coastal management by which
strategies can be made to protect one stretch of coastline and not damage others.
Movement of sediment within the sediment cell — processes…
coastline receives sediment from many sources
beach moves sediment along coast by LSD and currents, processes driven by wave
energy, wave direction is crucial
direction of sediment movement may change within seasons
fetch is important in deciding the direction of dominant wind
Loss of sediment — outputs from the sediment cell
sediment can’t cross boundaries so has to be lost to sea
tidal currents are very important as they surge up an estuary as tide comes in and
surge out again when tide falls, carry large sediment
amounts