Coastal Processes Lecture Notes

1. [ENVIRONMENTAL GEOLOGY] KJD 2016
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ENVIRONMENTAL GEOLOGY
COASTAL EROSION
Coastal erosionisthe wearingawayandbreaking up of rock along the coast. Destructive waves
erode the coastline in different ways.
1. Hydraulic action- the sheer weight and impact of the water against the coastline. It is greatest
under storm conditions when hundreds of tonnes of water may hit the rock face. Also, air
trappedincracks and cavesis suddenlycompressedbythe breaking waves which increases the
pressure on the rock.
*note: one cubic meter of water weighs one tonnes
2. Abrasion- alsoknownas corrosion.The breakingwavesthrow sandandpebblesagainst thr rock
face. These break off pieces of rock and cause the under cutting. In large storms boulders will
also be flung against the cliff face causing even greater damage.
3. Attrition- particles carried by the waves are reduced in size as they collide with the rock face
and one another. Boulders and pebbles are broken down into sand- sized particles which are
easier for the waves to carry away.
4. Solution- this is the chemical action on rocks by sea water and is most effective on limestone
rocks which are carried away in solution.
Erosional features
Waves usually contain sand and are moving rapidly, so they can be very abrasive.
Coastlines are zones of very intense erosion due to waves: much of the energy from marine
storms travels away from the storm and affects coasts.

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• Wave cut cliffs Wavescut at the shore,and the cliff collapsesabovethe resultingnotch,leaving
a cliff.
• Wave cut platformsWave erosionlargelystopsbelow sealevel (depth of wave base, actually),
so the retreat of a cliff leaves behind a flat area slightly below sea level.
• Seacaves,arches,and stacks: Sea caves resultwhere aneasilyerodedpiece of rockisremoved.
A sea arch happenswhena sea cave or two goes all the way through. If the arch collapses, the
seaward part is still there and is a stack. Stacks also happen when some chunk of rock doesn't
get eroded as rapidly as its neighbors. (Sea stacks are erosional remnants; with a cave cutting
through, it's a sea arch.)
COASTAL DEPOSITION
Coastal deposition happens when sea losses energyand drops the sand, rock particles and
pebbles it has been carrying. At this time, swash is stronger than the backwash and is associated with
constructive waves
It is likely to occur when:
 Waves enter an area of shallow water
 Waves enter a sheltered area
 There is little wind
 There is a good supply of material
Depositional Land features
• Spits and baymouth bars: sand is moved along by
longshore drift. Where itencountersdeeper water (as at a
bay), it drops the sand, and a spit forms, which can
eventually go all the way across the bay, forming a
baymouth bar.
• Sand bars and barrier islands:A sandbar is a submarine
dune. A barrier island is a sand bar that makes it above
a sea level, usually due to storms and swells and high
tides.

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COASTAL INUNDATION
Coastal inundation is the flooding of
normallydry,low-lyingcoastal land,primarilycaused
by severe weathereventsalongthe coasts,estuaries
and adjoining rivers
COASTAL FLOODING
It occurs when normally dry, low-
lying land is flooded by sea water. The
extent of coastal flooding is a function of
the elevationinlandfloodwaterspenetrate
which is controlled by the topography of
the coastal land exposed to flooding. The
seawater can inundate the landvia several
different paths.
 Direct inundation— where the sea height exceeds the elevation of the land, often where
waves have not built up a natural barrier such as a dune system
 Overtopping of a barrier — the barrier may be natural or human engineered and
overtopping occurs due to swell conditions during storm or high tides often on open
stretchesof the coast. The heightof the wavesexceeds the height of the barrier and water
flowsoverthe topof the barrierto flood the land behind it. Overtopping can result in high
velocityflowsthatcanerode significant amounts of the land surface which can undermine
defense structures.
 Breaching of a barrier — again the barrier may be natural or human engineered, and
breachingoccurson open coasts exposed to large waves. Breaching is where the barrier is
broken down by waves allowing the sea water to extend inland.
Coastal flooding is largely a natural event, however human influence on the coastal environment can
exacerbate coastal flooding. Extraction of water from groundwater reservoirs in the coastal zone can

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enhance subsidence of the landincreasing the risk of flooding. Engineered protection structures along
the coast such as seawalls alterthe natural processesof the beach,oftenleadingto erosion onadjacent
stretches of the coast which also increases the risk of flooding.
Causes of coastal Flooding
 Storm surges
 Sea level rise
 Tsunami
COASTAL SUBSIDENCE
Coastal subsidence is the
motion of a coastal surface as
it shifts downward relative to
a datum such as sea-level.
GeologicProcessesDriving
Coastal Subsidence
 Physical processes
drivingsubsidence
such as faultingor
sedimentcompaction
can occur individually
or in combination,creatingacomplex systemof drivers.
 Processesdeepinthe Earth'scrust (for example,faultingorflexure of the lithosphere)
contribute relativelylittle toland-surface subsidence rates(typicallylessthan1millimeterper
yearin aggregate).
 Deltaicsedimentloadingleadstoshort-termlithosphericsubsidence rates.
 The common, long-termprocessthatdriveslithosphericsubsidence isthe isostaticadjustment.
 Sedimentcompaction,fluidextraction,growthfaulting,andsaltwaterintrusionpotentially
contribute significantlytocoastal subsidence butthe relativeimportance of theseprocessesis
still poorlyunderstoodbecause of theirspatialandtemporal variability.