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2B - Coastal Landscapes and Change (Sea-level change (Long-term changes…
2B - Coastal Landscapes and Change
The coast and littoral zone
All coastlines have show the
same littoral sub-zones
, but
not all coastlines have similar landscapes.
Littoral zone:
wider coastal zone, which includes the adjacent land areas, the shore and the shallow part of the sea just offshore.
It comprises of four sub-zones:
coast
;
back-shore
;
foreshore
and
nearshore
.
Classifying coasts
They may be classified in various ways, as for example by:
geographical characteristics; the impacts of sea-level changes; the dominant coastal process.
Common but simple coastal classification distinguishes between...
Rocky or cliffed
: clear distinction between
land and sea,
mainly due to
cliff heights
.
High-energy coastline
created from
exposure
to
erosive
forces
of sea, rain and wind.
Coastal plains
: where the
land slopes gently towards the sea
and there is an imperceptible transition from one to the other. These are often maintained in a state of
dynamic equilibrium
between the
sediment deposition by river stems
and
offshore sediment
and marine erosion (low-energy coastlines).
Rocky coasts and coastal plains
Rocky coasts
result from a
geology
that's
resistant
to
erosive forces
and
weather
in
high-energy
environments.
Coastal plains
are found in
low relief areas
and
depend
on the supply of
terrestrial sediment.
Geological structure and the development of coastal landscapes
Geological structure: arrangement of rocks in three dimensions. Involves 3 key elements...
Deformation
: degree to which
rock strata
have been
tilted
or
folded
by
tectonic activity.
Faulting
: presence of
fractures
along which rocks have
moved
.
Strata
: different
layers of rock
and how they
relate
to each other.
Concordant and discordant coasts
Concordant
: when
rock strata run parallel
to the coastline.
Typical
coastline =
generally smooth
or slightly indented.
Discordant
: when
different rock intersect the coast
at an
angle
, so that
lithology
varies along the coastline.
Typical
coastline =
bays and headlands.
Cliff profiles
Influenced by
two different aspects
of geology: the
resistance of the rock to erosion
and the
angle of rock strata in relation to the coastline
.
Other geological features that influence cliff profiles and rates of erosion...
Faults
: rocks
fractured
=
weakened
either side of the
fault line.
Joints
: in
most rocks
and are
potential lines of weakness.
Fissures
:
small cracks
in rocks represent
weakness
that
erosion can exploit.
Rates of coastal recession
Geological factors
Coastal erosion rate and recession
is a critical aspect of the coastal zone. Rate is
influenced by many factors
but the most significant is
lithology
(rock type).
Follow this link for a table on the rates of erosion for different rock types:
Permeability
: rocks such as
sandstone
and
limestone
allow
water to pass through them
.
Groundwater
flow through
permeable
rock can
weaken rocks
by removing the
cement
that binds the rock sediment. Outcome:
slumping
.
Coastal vegetation
Many coastlines are
protected
from
unconsolidated sediment
erosion by the
stabilising influence
of plants.
Many plants that grow in coastal environments are
halophytes
(can tolerate
salt water
) and
xerophytes
(can tolerate
very dry conditions
).
Through
plant succession
, sand dunes can
convert
a supply of sediment into
land
. It starts with
specialised halophytic plants
capable of
growing
in salty, bare sand.
Once
established
, they
trap more sand
and this leads to the
formation of embryo dunes
. The embryo dunes
alter the environmental conditions
to an environment in which
xerophytic plants can flourish.
Marine erosion
Waves
Caused by
friction
between
wind and water.
They directly influence the three marine processes of
erosion
,
transport
and
deposition
.
Wave size and strength depends on the following four factors...
Length of time the wind blows for.
Water depth
.
Strength of the wind.
Wave fetch
(uninterrupted distance across water over which the wind blows).
Two types of waves...
Destructive
waves:
relatively high, short wavelength.
Strong backwash
that
erodes
and
carries
away beach material.
Constructive
waves:
low height
,
long wavelength.
Strong swash
(flow of water up beach) and
weak backwash
(return flow of water down beach).
Beach morphology
Definition:
the
shape of a beach
, including its
width
and
slope
and
features
such as berms, ridges and runnels. It also includes the
type of sediment
(shingle, sand, mud) forming the beach.
Four diagnostic beach features of prevailing wind conditions...
Berms -
small
ridges
from
constructive
waves during
calm
weather.
Cusps -
gentle destructive
waves
eroding berms.
Storm beach
-
constructive
waves during
stormy
weather.
Offshore bars -
formed by
persistent destructive
waves.
Erosion processes
Waves cause erosion
which is not necessarily a
continuous
process. It occurs
during storms
and when:
waves approach at right angles; high tide; heavy rainfall weakens cliff rocks; debris at cliff foot has been removed and no longer protects this critical point.
There are four main types of marine erosion...
Abrasion
:
sediment being carried in the waves, has a wearing-down effect.
Attrition
:
wearing down of sediment as is moved around by waves.
Hydraulic action
:
wave quarrying when trapped air in joints and cracks is compressed by the force of waves crashing.
Corrosion (solution)
:
carbonate rocks (i.e. limestone) are dissolved by rainwater, sea spray and seawater.
Erosional landforms
Most
critical erosional feature
would be a
wave-cut notch
formed by
hydraulic action
and
abrasion
. As the notch becomes
deeper
, the rocks
overhanging
it become
unstable
and eventually
collapse
.
Follow this link for a full diagram of all erosional landforms in resistant sedimentary rocks:
Marine transport and deposition
Sediment transportation
Sediment transported in four different ways...
Saltation:
sediment bounces along the floor.
Suspension
:
fine sediment (silt, clay) is carried within the body of water
.
Traction
:
Heavier sediment rolls along sea floor, pushed by waves and currents.
Solution
:
dissolved sediment (calcium carbonate) is carried in the water as a solution.
Transportation
of sediment mainly takes place
along the coast
rather than into and away from the coast. This is known as
longshore drift.
Swash pushes sediment
obliquely up the beach and
backwash returns it directly to the sea
. This results in sediment slowly
moving along the coast
in the
same direction
as the waves approaching.
Depositional features
Main features...
Recurved hooked spit
:
spit built out into a bay
or across an estuary,
end curves landward
into
shallower
water.
Bar
:
sand
or
shingle
beach
extending across a coastal indentation
with a
lagoon behind.
Spit
:
sand
or
shingle
beach ridge
extending beyond a turn
in coastline.
Tombolo
:
sand
or
shingle
bar that
attaches a former offshore island
to the
coast
.
Bayhead
beach:
accumulation of sand
at the head of a
sheltered stretch of water
between two hadlands.
Cuspate foreland
:
triangular area
of
shingle
extending out from a shore line, possibly made by longshore drifts.
Sediment cells
Explanation
:
long stretches of coastline
that operate as
self-contained physical systems.
In each one
there are
sources
where sediment is
generated
, which are
stretches where sediment is moving along
the coast by longshore drift and currents, and
sinks
, locations where the
dominant process is deposition.
Sub-aerial processes
Weathering
Chemical
: involving a
chemical reaction
and
decomposition
.
Biological
: actions of
bacteria
,
plants
and
animals
, which
speed up
mechanical or physical weathering.
Mechanical
: rock breakdown by some form of
physical force
.
Mass movement - types common along cliffed coastlines
Rockfalls
: where
rock
on a cliff is being
undercut
is
weakened by weathering
. Falls can be
sudden
.
Rotational slides
:
slow downslope
movements of a mass of rock over a
curved plane.
Common
where permeable overlies unstable impermeable rock.
Landslides
:
sudden downslope surges
occurring when
weathered rock and soil
become
saturated
and
lubricated
by
water
.
Sea-level change
Change in sea level
can be brought about by a change either in
land level
(
isostatic
) or in the
volume of the sea
(
eustatic
).
Long-term changes
Marine regression
results from a
eustatic
(during glacial periods when water becomes locked up in ice and snow) fall in sea level and an
isostatic
(when ice sheets melt and the land rises) fall in sea level. Both movements
expose the seabed
and produce an
emergent coast.
See diagram:
Emergent and submergent coastlines
Marine transgression
results from a
eustatic
(end of glacial period) rise in sea level and an
isotatic
(when land sinks under the weight of accumulated snow and ice) rise in sea level. In both,
large areas
of land are
submerged
beneath the sea level, producing a
submerging coast.
See diagram:
Contemporary sea-level change
Most scientists attribute rising levels to the impact of global warming. There are two components:
thermal expansion of the oceans as they are warmed by change in climate; melting of ice sheets and glaciers increasing the water volume of the oceans
.
Coastal recession
Rapid recession
Threatens people, their property and their livelihoods.
It is caused by
physical factors
such as...
Strong
longshore drift.
Soft
or
unconsolidated
geology.
Long wave fetch
and
large destructive
waves.
Cliffs with
structural weaknesses
and
vulnerable to weathering
and mass movement.
It can also be accelerated by
human actions
, such as...
River
dams
reducing
the supply of
sediment
to the coast.
Coastal management
(construction of groynes).
Dredging
the
offshore seabed
for sand and gravel.
Coastal flooding
Factors increasing risk
Rising sea level
associated with global warming.
Human actions
such as the
removal of coastal vegetation
, the
building of coastal tourist resorts
and the general
pressure of population.
Climate change
Risks and their consequences for communities
Economic
costs:
loss of property
in the form of homes and businesses, the
loss of transport lines
, as well as the
loss of farmland
and other means of livelihood.
Social
costs: impacts on
people
, i.e. the costs of
relocation
and
community disruption
, as well as the impact on
health
and
well-being.
Environmental costs: the loss of coastal habitats and ecosystems.
Most at risk islands
of rising sea levels are the
Maldives
,
Tuvalu
and
Barbados
. Land will simply have to be
abandoned
and
left to disappear
beneath the sea, creating a
growing number of environmental refugees.
Different approaches to managing coastal risks
Hard engineering
Sea walls
: made of
reinforced concrete.
Rip-rap
(rock armour):
huge rock boulders
piled up at the base of a sea wall.
Rock breakwaters
: usually built of
huge boulders
, but
offshore
.
Revetments
: stone, timber or interlocking concrete
structures
on
dune faces
and
mud banks.
Groynes
: vertical stone or timber
fences
built at
90 degrees to the coast
and spaced along a
beach
.
Advantages
: obvious to those at risk that
something is being done to protect them
; it can be a
one-off action
that
protects for decades.
Disadvantages
:
construction and on-going maintenance costs are high; carefully designed engineering systems can fail; coastal ecosystems can be badly affected; defences can have adverse effects further along the coast.
Soft engineering
Cliff stabilisation
:
planting vegetation
through a tough, flexible
membrane
that holds
soil
and often
rock
in place; can also be done by
regrading and reducing
cliff
slopes
.
Dune stabilisation
: dunes are
effective
but are
easily degraded
; they can be
established
by
planting marram grass
and by constructing relatively
cheap dune fencing.
Beach nourishment
:
topping up
beaches with sediment transported from
elsewhere
.
Sustainable coastal management
Follow the link for a diagram describing different types of sustainable coastal management:
Integrated coastal zone management (IZCM)
It is a concept that has three main features. It recognises:
1) That the
entire coastal zone
needs to be
managed
, not just where erosion or flooding is taking place.
2) The
importance of the coastal zone
to people's
livelihoods
and
well-being.
3) The need to make the
management
of the coast
sustainable
.
The
IZCM is a joined-up approach
to coastal management which must: plan
long-term
; involve
all stakeholders
and ensure they have a
say in policy decisions
; follow an '
adaptive
' approach to
unforseen changes
; try to work
with natural processes
rather than
against them.
The hard part is the decision-making process to decide actions to take. There are
four different management options
to follow in the UK:
no active intervention
;
hold the line
(build coastal defences);
managed realignment
(allowing change but in a controlled way);
advance the line
(build new coastal defences).
The choice of management is dependent on a number of factors...
The
environmental sensitivity.
The
cultural
and
ecological
value
of the
land
.
Technical feasibility
of different
solutions
.
Pressure
from local communities, developers and environmental groups.
Economic value
of the
assets
that might be protected.
Problems with the ICZM
Dissatisfaction
will be rooted in the fact that
local councils and governments
have
limited funds
, so
not all coastal
areas can be protected.
Coastal managers are
forced to prioritise
which areas should be
protected
and which will have to be
ignored
.