A2 CAMBRIDGE GEOGRAPHY: COASTAL ENVIRONMENTS - WAVE, MARINE AND SUB-AERIAL PROCESSES. An overall presentation of the first sub-chapter of Coastal Environments chapter.

1. A2GEOGRAPHY
COASTALENVIRONMENTS
8.1 WAVEMARINE
ANDSUBAERIALPROCESSES

2. INTRODUCTION
Coastal environments are influenced and affected by some important factors,
physical and human processes.
Landscapes vary in terms of:
• Lithology
• Geological structure
• Processes
• Sea-level changes
• Human impacts
• Ecosystem types

3. LITHOLOGY
Lithology is the study of the general physical characteristics of rocks.
Hard rocks: granite or basalt (rugged landscape aspect).
Soft rocks: sands or gravels (low, flat landscape aspect).
AWHITU Regional Park New Zealand

4. GEOLOGICAL STRUCTURE
Concordant coastlines
A concordant, longitudinal, or Pacific
type coastline occurs where beds, or
layers, of differing rock types are
folded into ridges that run parallel to
the coast. The outer hard rock (for
example, granite) provides a
protective barrier to erosion of the
softer rocks (for example, clays)
further inland.
Discordant coastlines
Discordant (Atlantic-type) coastlines
occur where the geological strata are
at right angles to the shoreline.

5. PROCESSES
Erosional landscapes contain many rapidly retreating cliffs, whereas areas of
rapid deposition contain many sand dunes and coastal flats.

6. SEA LEVEL CHANGES
They interact with erosional and depositional processes to produce advancing
coasts (those growing due to a deposition and/or relative fall in sea level) or
retreating coasts (those being eroded and/or drowned by a relative rise in sea
level). Many examples in New Zealand coastlines.

7. HUMAN IMPACTS
Some
coasts are
extensively
modified
whereas
others are
more
natural.

8. ECOSYSTEMS
They can be mangroves, corals, sand dunes, saltmarshes and rocky shores,
adding variety to coastlines.

9. COASTAL ZONES
The coastal zone includes all
areas from the deep ocean (up
to 320km offshore) to 60km
inland.
At the coast there is the upper
beach or backshore (backed by
cliffs or sand dunes), the
foreshore (periodically exposed
by the tides) and the offshore
area (covered by water)
The coastal zone is a dynamic
area with inputs and processes
from land, sea and atmosphere.

10. WAVE, MARINE AND SUB-AERIAL PROCESSES
Waves are a medium through which energy is transferred.
They are created by the wind blowing across the surface of the sea.
Frictional drag increases as the wind speed increases, making the wave bigger.
Wave energy depends upon three things:
• The strength of the wind.
• The length of time the wind has blown for.
• The fetch of the wind (the distance it blows over).

11. Raglan Surfers’ Beach

12. THE WAVE ORBIT
The wave orbit is the shape of the wave: varying between circular and
elliptical.
The orbit diameter decreases with depth to a depth roughly equal to
wavelength, at which point there is no further movement related to wind
energy – this point is called the wave base.

13. WAVE CHARACTERISTICS
The wave orbit is the
shape of the wave:
varying between
circular and elliptical.
The orbit diameter
decreases with depth
to a depth roughly
equal to wavelength,
at which point there
is no further
movement related to
wind energy – this
point is called the
wave base.

14. WAVE DEFINITIONS
Wave fetch: The distance of open water over which a wave has passed.
Wave crest: Highest point of a wave.
Wave trough: Lowest point of a wave.
Wave height: Distance between trough and crest.
Wave length: Distance between one crest/trough and the next.
Swash: Water movement up a beach.
Backwash: Water movement down a beach.

16. WHY WAVES BREAK?
When out in open water there is little horizontal movement of ocean water,
the bulk of the motion is up and down or vertical. This changes slightly when
waves approach the coastline.
As the water approaches the coastline it encounters increasing contact with
the shelving sea bed, which exerts a frictional force on the base of the wave.
This changes the normal circular orbit of the wave into an elliptical orbit. As
the waves gets closer and closer to the coast the impact of friction grows, with
the top of the wave moving faster than the base of the wave. Eventually a
critical point is reached where the top of the wave (the CREST) curves over
and creates a breaking wave.

17. SPILLING BREAKERS
Spilling breakers are associated with gentle beach gradients and steep waves
(wave height relative to wave length).
They are characterized by a gradual peaking of the wave until the crest
becomes unstable; resulting in a gentle spilling forward of the crest.

19. PLUNGING BREAKERS
Plunging breakers: tend to occur on steeper beaches, with waves of
intermediate steepness.
They are distinguished by the shore-ward face of the wave becoming vertical,
curling over, and plunging forward and downward as an intact mass of water.

20. SURGING BREAKERS
Surging breakers: are found on steep beaches with low steepness waves. In
surging breakers the front face and crest of the wave remain relatively smooth
and the wave slides directly up the beach without breaking.
Once the breaker has collapsed, the wave energy is transmitted onshore as a
‘wave of translation’.
The swash will surge up the beach, with its speed gradually lessened by
friction and the uphill gradient.
Gravity will draw the water back as the backwash gradient.

22. WAVES OF TRANSLATION
Constructive waves
- with a short amplitude and a long wavelength.
They have a low frequency of around 6-8 waves per minute, particularly when
these waves advance over a gently shelving sea floor (formed of fine material:
sand). These waves have been generated far offshore creating a gradual
increase in friction and thus a gradual steepening of the wave front.
This creates a spilling breaker, where water movement is elliptical. As this
breaker collapses, the swash surges up the gentle gradient with maximum
energy.
Constructive waves produce a strong swash, but a weak backwash, which
produces a gentle beach as material is deposited but not removed from the
beach.
The supply of new material and the constant action of pushing the material up
the beach eventually produces berms.

23. PLUS OXFORD DIAGRAMS

24. WAVES OF TRANSLATION
Destructive waves
Destructive waves are the result of locally generated winds
- They have a high amplitude and a short wavelength.
They also have a high frequency of 10-14 waves per minute, resulting in a
steeply shelving coastline, where rapid friction and steep circular plunging
breakers are formed.
The waves have a strong backwash but a weak swash, so they remove a lot of
material from the beach producing a steeper beach profile.
The force of destructive waves can fire material to the back of the beach
where a ridge known as a storm beach forms

25. PLUS OXFORD DIAGRAMS

26. PROCESSES AND LANDFORMS IN COASTAL AREAS

27. SPRING TIDES
Tides are regular movements in the sea’s surface – the rise and fall of sea
levels, caused by the gravitational pull of the moon and sun on the oceans.
Out of the two, the moon accounts for the larger share of the pull.
When the earth, moon and sun are aligned the gravitational pull is at its
greatest.
This creates a Spring tide.
A Spring tide results in a high, high tide and low, low tide.
This creates a high tidal range (difference between the highest and lowest
tide).

29. NEAP TIDES AND TIDAL CYCLE
Low spring tides occur just after a new moon whereas high spring tides occur
after a full moon - when the Sun and moon are aligned.
When the sun and moon are at a right angle to the earth we experience Neap
tides. The gravitational pull of the sun partially cancels the moon’s.
This results in a low, high tide and a high, low tide.
This creates a low tidal range and results in weaker tidal currents than normal.

31. WHAT INFLUENCES TIDES?
Tides are influenced by:
• The size and shape of ocean basins.
• The characteristics of the Shoreline
• Coriolis forces
• Meteorological conditions.
In General:
Tides are greatest in bays and along funnel-shaped coastlines.
In the Northern Hemisphere water is deflected to the right of its path.
During low pressure systems water levels are raised 10 cm for every decrease
of 10mb.

32. CORIOLIS FORCE (EFFECT)
Is an effect whereby a mass
moving in a rotating system
experiences a force (the Coriolis
force) acting perpendicular to
the direction of motion and to
the axis of rotation.
On the earth, the effect tends to
deflect moving objects to the
right in the northern
hemisphere and to the left in
the southern and is important in
the formation of cyclonic
weather systems.

33. TIDES AND THE TIDAL CYCLE
The difference between high tide and low tide is called the tidal range.
Tidal range varies with distance from the amphidromic point (place where
there is no tidal range) & according to the shape of the coast; the strength of
tidal currents varies enormously.
If the coast is funneled, a tidal bore can be created due to tide advances being
concentrated in a narrow space.
Coastal areas can be classified into…
• Micro-tidal: (very low tidal range – less than 2m)
• Meso-tidal: (2-4m)
• Macro: (over 4m)

34. TIDAL RANGE’S INFLUENCE ON COASTAL PROCESSES
It controls the vertical range of erosion and deposition.
Weathering + biological activity is affected by the time between tides.
Velocity is influenced by the tidal range and has an important scouring effect.

35. RIP CURRENTS
Rip currents are strong offshore flows, and often occur when breaking waves
push water up the beach face.
This piled-up water must escape back out to the sea as water seeks its own
level.
Typically the return flow (backwash) is relatively uniform along the beach, so
rip currents aren't present.
However, If there is an area where the water can flow back out the ocean
more easily, such as a break in the sand bar, then a rip current can form.
When water from the highest sections of breakers travels upshore upon
returning as backwash it moves through the points where lower sections have
broken, creating a strong backwash current.
Once rip currents are formed they modify the beach by creating cusps which
perpetrate the currents.

37. DISCORDANT COASTS
On a discordant coastline, alternating layers of hard and soft rock are
perpendicular to the coast.
Because the soft rock is exposed, it is eroded faster than the hard rock.
This differential erosion creates headlands and bays along discordant
coastlines.

38. CONCORDANT COASTS
Concordant coasts have alternating layers of hard and soft rock that run
parallel to the coast.
The hard rock acts as a protective barrier to the softer rock behind it
preventing erosion.
If the hard rock is breached though, the softer rock is exposed and a cove can
form. A good example is Cathedral Cove in Coromandel, New Zealand.

39. STORM SURGES
Storm surges are changes in the sea level, caused by intense low pressure
systems and high wind speeds.
For every drop in pressure of 10mb, sea water is raised by 10cm.
Therefore during tropical cyclones, pressure may drop by 100mb resulting in a
sea level rise of 1m!
Storm surges can bring catastrophic consequences and are intensified on
funnel shaped coastlines.

41. WAVE REFRACTION
It is very rare for waves to approach a regular uniform coastline, as most have
a variety of bays, beaches and headlands.
Because of these features, the depth of water around a coast varies and as a
wave approaches a coast its progress is modified due to friction from the
seabed, halting the motion of waves.
As waves approach a coast, due to the uneven coastline, they are refracted so
that their energy is concentrated around headlands but reduced around bays.
Waves then tend to approach coastline parallel to it, and their energy
decreases as water depth decreases.
However, due to the complexities of coastline shapes, refraction is not always
fully achieved resulting in long shore drift (which is a major force for
transporting material along the coast).

43. COASTAL EROSION PROCESSES
How did the original headland shape become eroded to the present coastal
landscape? A number of stages are involved:
• All rocks have lines of weakness. The sea and its waves use hydraulic
action, abrasion, attrition and solution to erode along any lines of
weakness. Undercutting takes place all around the headland.
• These lines of weakness get enlarged and develop into small sea caves.
• The caves are deepened and widened on both sides of the headland until
eventually the sea cuts through the headland, forming an arch.
• The rock at the top of the arch becomes unsupported as the arch is
enlarged, eventually collapsing to form a stack.

44. COASTAL EROSION PROCESSES cont.
• The stack gets eroded until only a stump remains.
• Over time the stump will disappear.
• As the headland retreats under this erosion, the gently sloping land at the
foot of the retreating cliff is called a wave-cut platform.

46. HYDRAULIC ACTION
As waves break against the face of cliffs, any air trapped in cracks, joints
and bedding planes is momentarily placed under great pressure.
As the wave retreats this pressure is released with explosive force.
This stresses the coherence of the rock, weakening it and aiding erosion.
This is particularly obvious in well bedded rocks such as limestone,
sandstone, granite and chalk, as well as poorly consolidated rocks such as
clays and glacial deposits.
It is most notable during times of storm wave activity

48. ABRASION / CORRASION
This is the process where a breaking wave hurls pebbles and shingle against
the coast breaking bits off and smoothing surfaces.

49. ATTRITION
Takes place as other forms of erosion continue.
Rocks and pebbles constantly collide with one another as they are moved
by waves action, resulting in reduced size of beach material and increased
roundness as the impact of 2 hitting smooth's away rough edges.

50. SOLUTION
Is a form of chemical erosion whereby rocks containing carbonates such as
limestone and chalk are dissolved by weak acids in the water – supplied by
organisms such as barnacles and limpets.
Calcium carbonate + weak acids = Calcium bicarbonate (soluble).

52. SUB-AERIAL PROCESSES
Sub-aerial processes refer to the processes of weathering and mass
movement.
Weathering is the breaking down of rock in situ.
It can be divided into mechanical and chemical weathering.
Mechanical weathering refers to physical processes like freeze-thaw action
and biological weathering.

53. SALT CRYSTALLISATION
The process where sodium and magnesium compounds expand in joints
and cracks by 300% when temperatures fluctuate between 26-28 degrees
thereby weakening rock structures.

54. FREEZE THAW
Water becomes trapped in cracks in rocks, freezes when temperatures drop
below 0 degrees and therefore expand by 10%. As they expand they put
extra pressure onto the rock, until the ice melts when temperatures rise.
After many repeated cycles the rock fragments and weakens.

55. BIOLOGICAL WEATHERING
Where living organisms – such as mollusks, sponges and sea urchins on low
energy coasts physically break up structures.

56. SOLUTION WEATHERING
The chemical weathering of calcium carbonate by acidic water, which tends
to occur in rock pools due to the presence of organisms secreting organic
acids.

57. MASS MOVEMENT
Mass movement can
be defined as the
large scale movement
of weathered material
in response to gravity.
Essentially, it’s when a
cliff or other structure
that is not
horizontally
orientated has been
weathered to the
point at which it
starts to collapse.

58. ROCK FALLS Freeze thaw weathering
on a cliff breaks the
rocks up into smaller
pieces which can then
free fall.
This occurs commonly
on cliffs with lots of
joints as the joints make
it easier to break up the
rock.
If the cliff is undercut by
the sea, it can loose
some of its stability,
increasing the likelihood
that a rock fall will occur.

59. SOIL CREEP
Soil creep is an
incredibly slow
process.
It occurs on very
gentle slopes
and produces an
undulated
(wavy) surface.
Damp soil moves
very slowly
down hill due to
the increase in
its mass (since
it’s wet).

60. LANDSLIDES
After being soaked by water, cliffs made from soft rock will begin to slip due
to the rock being lubricated.

61. ROTATIONAL SLUMPING
Slumping happens for similar reasons to landslides.
Heavy rainfall makes the rock heavier due to it absorbing the water and the
water also acts as a lubricant.
The difference with slumping is that it happens on a concave surface, which
causes the cliff to form a crescent shape.

62. MUDFLOW
Mudflow is a very dangerous form of mass movement which occurs on
steep slopes with saturated soil and little vegetation.
The lack of vegetation means that there is nothing to bind the soil together,
promoting mass wasting.
The saturated soil becomes heavier and is lubricated, leading to the rapid
movement of a lot of mud downhill.

63. WAVE TRANSPORTATION
When waves transports sediments and other materials.

64. TRACTION
Grains slide or roll along the sea floor – a low form of transport.
In weaker currents sands may be transported, whereas in stronger currents
pebbles and boulders may be transported.

65. SALTATION
Is when grains hop along the seabed in a skipping motion.
Moderate currents may transport sand, whereas strong currents may
transport pebbles and gravel.

66. SUSPENSION
Suspension is when grains are carried by turbulent flow and are held up in
the water.
Suspension occurs when moderate currents are transporting silts or strong
currents transporting sands.
Grains transported as wash loads are permanently in suspension and
typically consist of clays and dissolved materials.

67. SOLUTION
Solution is when particles, invisible to the naked eye are carried in the
waters current.

68. LONGSHORE DRIFT
Longshore drift is a process responsible for moving significant amounts of
sediment along the coast.
This usually occurs in one direction as dictated by the prevailing wind.
For example the prevailing wind along the Holderness Coast is north-
easterly.
As the result waves break on to the beach obliquely at an angle of around
45 degrees.
The swash moves beach material along the beach and the backwash, under
gravity, pulls the material back down the beach at right angles to the
coastline.
Over time this creates a net shift of material along the coast.

70. COASTAL SEDIMENT CELLS
`The coastal sediment system’ or `littoral cell system’ is a simplified model
that examines coastal processes and patterns in a given area.
Each sediment cell is a self-contained cell in which inputs and outputs are
balanced.
There are 11 such cells around the UK and many more in coastal New
Zealand.

72. DYNAMIC EQUILIBRIUM
The concept of dynamic equilibrium is important to Littoral cells.
The concept states any system is as a result of inputs and processes
operating within it.
The sediment increase has a knock on effect on processes such as
Longshore drift, and a resulting change in landforms will occur.

74. HUMAN IMPACTS
Dams have reduced the supply of sediment to the beaches by 33%.
Building, houses, boats, pools, protection schemes and roads are
destabilising the cliffs.