Transcript Coasts3

Section 3 - Coastal processes 2
How can Geology affect the rate of erosion?
Differential erosion occurs when rocks wear away at different rates.
Limestone has joints which can be exposed by the sea
Granite and chalk tend to withstand erosion compared to softer clay
based rocks.
If rocks lie parallel to the coast or at right angles they will produce very
different coastlines.
In Purbeck the southern coast has rocks lying parallel and this is called
concordant. The stronger Limestone has only been broken in a few
places e.g. Lulworth Cove where it reaches the weaker clay.
The rocks on the Eastern side run a t right angles known as
Discordant. The result is large bays followed by headlands that stick
out into the sea.
The Dip of the rock is
extremely important is
the shape and sturdiness
of a cliff.
Longshore Drift (Littoral Drift)
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Waves approach the shore at an
angle most of the time due to sea
bed, refraction etc
Material will be pushed up the beach
at the same angle by the Swash.
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Backwash is influenced by gravity and
so falls back at a right angle to the
beach line
Over time there is a lateral shift of
material
If the material that is
being moved away is
greater than the material
arriving beach erosion
will occur.
In south England the
prevailing wind ensures
LD that is West to East
Groynes are used to
control beach material.
They are large wooden
blocks heading out to sea.
Accumulation of material
will occur on the windward
side.
It is particularly useful if the
beach is a large tourist
destination
Beaches
Constructed from sand and shingle.
Sand - has lower profile as its small
size means it will compact when wet
and allow little percolation.
Most of the swash therefore returns
as backwash and little energy is lost
in friction due to the smooth surface.
Material is often carried down the
beach
This will cause the
development of ridges and
runnels at the lower water
mark running parallel to the
shore.
Shingle – The larger the material
generally causes a steeper gradient.
Because more water will percolate
through the shingle the backwash is
weaker and thus less material is moved
down the beach.
During the spring tides the highest waves will throw material to the back
of the beach. This creates a storm beach with a steep profile. The
ridges that follow show the successively lower high tide marks. These
are called berms. The result of constructive waves.
Cusps are semicircular shaped
depressions formed when waves
break directly onto the beach
They have both a strong swash and
backwash. Once developed the
swash is channeled into the centre
and so a stronger backwash forms
in this part increasing the erosion.
Ripples form on the
sand by the action of
wave movement in the
lower part of the
beach.
Spits
A spit is a long ridge of land that has
one end joined to the mainland and
projects out into the sea across an
estuary.
They composition of a spit depends on
the source of the material that has
been eroded away.
Longshore drift moves material along the coast. When the coastline
changes direction (more North – South) the material is carried into the
more sheltered water and begins to build up eastwards. Storms bring
more material that rises above the high water mark.
Over time the feature develops and the finer material continues to build
eastwards.
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The end of the spit curves due
to wave refraction carrying the
material round into more
sheltered water. Wind direction
may also play a role here.
The river carries material out
from the estuary into the sea
and prevents the spit
connecting to the land
There may be several curves
due to periods of dominant
weather / winds changing the
direction
Sediment stars to infill the water
behind the spit and salt marshes
may develop.
Dunes can develop if strong winds
lift sand and propel it inland
If the spit joins an island to the
mainland then a tombolo is formed. If
the spit runs across a bay because the
flow of water is not strong from an
estuary then it is called a bar.
Changes is sea level since the ice age
is thought to have contributed to
development of these features.
Sand Dunes
Sand is blown inland from the
process of saltation. If there is a
large tidal range sand dries out
and is exposed to wind.
Obstacles such as seaweed and
beach debris trap sand at the back
of the beach e.g. high berm. Sand
has a high PH (carbonate from
seashells) and is not hospitable.
Pioneers hardy plants are the only
ones to colonise the area e.g
Lyme Grass, Sea Rocket, Salwort
The first dunes to develop are Embryo dunes. They are colonised by
grasses such as couch, lyme and marram. They are able to grow upwards
through the wind blown sand and give the ground some stability.
These plants have long roots (marram), leaves that contain water (sea
couch). This helps with water retention and adds organic matter to the
dunes making it more hospitable for later plants.
The next type of dune is known as a Fore
dunes. These are initially yellow but as
more organic matter is formed they turn
to a grey colour. The smaller ones are
around 5mts but can grow to 20 mts on
the main ridge. They have a slightly
alkaline PH. Much less bare sand will be
exposed here.
The Grey dunes further inland
become more fixed. With increasing
organic matter a humus layer
develops. This improves the nutrient
supply and more water is retained.
Plants that could not survive in
previous conditions now flourish. E.g.
include lichens, moss, red fescue
Behind the grey dunes the sand
supply is cut off giving smaller
dune features. Often referred to as
the dune heath (wasting dunes).
Heather is usually a big plant
species.
Dune Slacks develop in between
dunes where conditions are
damp. Mosses, willows and
reeds tend to favour these
conditions. The water table is
closer to the surface here.
Blowouts may be visible where
the wind has been funnelled and
removed sand to leave the dune
bare.
Beyond the dune heath a
woodland may develop and this is
the beginning of the climatic
climax vegetation for the British
isles. This is known as a
psammosere. If humans have
interfered and this stage is not
reached i.e. farm animals or
human planted species stop it the
development is known as
plagioclimax.
Salt Marshes
These are formed in sheltered river estuaries or behind spits. Mudflats
are formed from rivers or gentle tides depositing silt and mud. Vegetation
develops in a succession known as a Halosere.
Plants able to tolerate slat and the occasional submergence by the sea
start to colonise. E.g. glasswort, seablite, spartina. These help secure the
mud and bind it together – Spartina in particular.
Vegetation covers up to 15cm high as more organic matter and sediment
builds up. 25mm growth upwards a year is common for a developing
mudflat. Sea lavender, sea thrift and red fescue develop with the Spartina
and early colonisers.
The tidal currents are slowed by the vegetation, helping more material
build up.
As mud levels rise the complex creek systems develop and this channels
the tides as the marsh grows higher. If the land can build up above high
tide colonisation can lead to growth of tress such as oak.