UPPER SIXTH
COASTS - 2

Erosion Landforms

       Sea cliffs can be regarded as slopes with normal processes in operation, and wave action providing the mechanism for basal removal.
       The energy of the waves is expended in reducing the debris to a size which is suitable for transport. Mass wasting becomes the dominant process on the cliff.
       The relationship between mass wasting and basal removal determines the nature of the cliff.
       If there is an abundance of energy, the debris is removed, and the foot of the cliff exposed to active abrasion.
       In macrotidal environments, this results in an undercutting of the cliff about the high tide line.
       This is known as a wave cut notch, and is recognisable by the smooth nature of the rock.
       Weakness in the rock are picked out and eroded at a faster rate; forming sea caves.
       In weak or unconsolidated coasts, these features do not form, but basal failures result in slumps along dipped bedding planes.
       Plunging cliffs descend directly in to deep water. There is no change in depth, and waves do not break. Instead, they reflect off the cliff, and interfere with the advancing wave to set up a standing wave called a clapotis. The water appears to oscillate about stationary nodes, one wavelength apart.
      Slope over wall cliffs are a result of the effect of two stage cliff development. The slope was formed under colder conditions, and the cliff formed in the present climactic conditions.
       Both lithology and structure play an important role in cliff development.
i        When rocks dip seawards, the cliff develops as a long slab.
ii       Inland dipping or horizontally bedded strata form steeper cliffs.
       In plan, the greatest influence is tectonic.
i        Where the folds are parallel with the trend of the coast, it is a Dalmatian coast, after an area of the former Yugoslavia near Split on the Adriatic.
ii       If the folds are perpendicular, then the series of bays and headlands associated with south west Ireland, the Atlantic coastline, is found.
       Headlands and bays are formed by differential erosion of rocks which have different resistance values.
       The less resistant rock erodes to form a bay, and the resistant rock is left as a headland. See Lulworth Cove.
       Many cliffs have areas of gently sloping platform from their base, particularly associated with the wave cut notches.
       These are known as wave cut platforms and usually have common features;
i        As the wave cut notch is increased, the cliff collapses, and leaves a relatively shallow (>4) even platform.
ii       It cuts across rocks, regardless of lithology.
iii     It is usually deeply dissected by abrasion.
iv     As it widens, the incoming waves break further out, and dissipate their energy.
v      This reduces erosion of the headland, and limits further extension of the platform, approximately limiting the platform to a km.
       Wave cut platforms are also known as inter-tidal platforms. Normal wave cut platforms as found in the British Isles are recognised as inter-tidal platforms. Two other types are recognised;
i       High tide platforms; found about the mean high tide level in microtidal swell wave environments. They are horizontal, and caused by processes other than abrasion. They usually have a rim, which means that part of the platform is not subject to regular wave action. This may be weathered by salt crystallisation processes, and assisted by biological processes.
ii       Low tide platforms are found in tropical seas with calcareous rock. Biological weathering and wave action combine to produce a platform backed by a visor at the level of highest bio-activity.
       Negative sea level changes, from isostatic and eustatic means, leave former wave cut platforms above current sea level as raised beaches.

BEACHES

The reason for beach accumulation may fall in to two groups; it may be a store of sediment trapped in a bay, or it may be a mobile stream of sediment which represents a transport pathway for sediment moving along the coast.
Sources of Beach Material;
(i)    Material eroded from headlands by contemporary wave attack is only important for soft or unconsolidated rocks.
(ii) Sediment may have been brought ashore from the offshore zone. This will have occurred as the sea levels rise, since waves have no effect below 10m depth.
(iii) Large rivers deposit large volumes of alluvial sediment in to the coastal zone.
(iv) Beach material cycled along through long shore drift.
The Nature of Beach Materials;
Majority are sand sized (1/16 mm to 2 mm) or of pebbles (4-64 mm). It is unusual to find a mixture of sand and pebbles on the same beach.
The high sorting of beaches is due to the differentiating effects of swash and backwash. Coarser material is moved up the beach by strong swash, but the backwash is weakened by percolation. The coarsest materials tend to be found at the limit of wave action, while the finer ones are moved in to the lower energy zones offshore.
Beach Profile;
Wave period determines dominance of backwash. If the wave breaks as the backwash is underway, the swash is limited. If the backwash has completed its run and then the wave breaks, then the swash is very effective. This results in construction.
W. V. Lewis [1931] determined that constructive waves have a period of 8-10 sec, and destructive have periods of 4-5 sec.
Onshore winds tend to produce steeper waves, which are usually destructive.
Profiles in Equilibrium is a simple concept. It advocates the idea of the beach as a dynamic self regulatory system, which adapts to changes provided that external conditions remain constant. If the beach profile is steep, then the waves tend towards the destructive type, which flatten the beach. The waves are then constructive, and rebuild the beach. The mid-point is the profile of equilibrium, which is where the profile is neither being raised or lowered.
Beach Profile Definitions;
i.       Beach face - the zone affected at high tide by wave action
ii.     Berm - gently inclined area at the top of the beach. Slopes seaward, and does not usually receive wave attack.
iii.    Summer Profile - the profile of a beach in the summer
iv.   Winter profile - the profile of a beach in the winter
v.     Sweep zone - the zone of change between summer and winter profiles.
Longshore Drift [Littoral Drift]
Oblique wave angle creates a lateral movement of near shore current. Longshore drift is predominantly in one direction. It often coincides with the direction of prevailing winds.
Swash carries material up the beach at an oblique angle, but the backwash follows gravity, and it acts at right angles to the coastline. The material is moved along the beach. It is usually reduced in size and angularity.

BEACHES IN PLAN
Swash aligned beaches;
       Smoothly curving concave beaches.
       Beach face is orientated parallel to the fronts of the dominant waves.
       Beaches which face the waves are termed swash aligned.
Drift aligned beaches;
       Some beaches show oblique alignments to the dominant wave fronts.
       Usually occurs where the beach gradient is steep and the wavelength is short.
       This is because the crest breaks at different times along the beach.
Changes in beach alignment;
       Longshore transport of sediment can determine the orientation of beaches.
       The most effective direction of wave advance for long shore sediment movement is between 40-50.
       If the angle of wave advance is less that this, transport can longer take place, and deposition occurs.
       The accumulation continues until the angle narrows to the optimum of 40-50.
       The usual result of this is that a spit is built out from the coast in a continuation of the line of the best orientation for drift in relation to the waves.
       If the cost turns such that the angle of wave approach increases, deposition occurs again and the angle continues to become greater.
       If swash alignment is acheived, accretion build the beach seawards at a constant angle.
SMALL SCALE LANDFORMS ON BEACHES
Beach cusps;Formed when a swell approaches a beach with almost perfect refraction, beach cusps are a series of small arcuate embayments between 5 and 50m across. The sides of the cusp channel swash in to the centre of the arc. The backwash in the centre is stronger, and deepens the cusp.
Rip channels;Swash piles water up in the surf zone. Water tries to escape laterally, but is opposed by the water flowing in the oposite direction. The two together have sufficient energy to break through the surf as a well defined zone of high velocity water. They can scour channels, and be as fast as 8m/s.
Ripples;Symmetrical ripples are produced when a wave oscillates back and forth in shallow water. They are elongated in the direction of the crest. They are small scale, perhaps 10-50 cm.Asymmetrical ripples are produced where there is a resultant directional flow. The current moves particles up the gentle stoss slope, and then deposits them in the eddy zone in the lee side of the ripple.
CONSTRUCTIONAL LANDFORMSDepositional features are named after their shape, as opposed to their origin.
Spits;e.g. Spurn Head, Holderness, Yorkshire.Dominant waves push material along the coast in longshore drift. Coast turns inwards, and the material continues to be deposited in line with the coastal trend. Finer materials are deposited in the sheltered side of the spit. Marshes can build up, and this forces the river to the other side of the estuary.
Proximal end of the spit - the bit that meets the land.Distal end of the spit - the tip, usually curved.
As spits build further in to deeper water, they require more sediment to build above the HWM. The waves have greater energy to attack the distal end, turning it inwards. Sometimes, the curves are due to the changing of the predominant wind direction to another, causing waves, and thus deposition to occur an a different angle. These recurves are sheltered by the spit and become prominent features, e.g. Hurst Castle Spit, The Solent.
Double Spits; Both Poole Harbour, and Christchurch harbour have spits which have grown from both sides. Both show attributes which indicated that they are swash and drift aligned.
Forelands; Low lying triangular areas of coarse deposits. The updrift side traps sediment and slowly builds forward, whilst the downdrift side is eroded. The balance between erosion and deposition determines the size of the foreland.e.g. Dungeness, Hastings-Dover, SE England. Benacre Ness, Suffolk. Nr. Southwold.
Barrier beachesBeach which stretches across from one embayment to the other, and encloses a lagoon behind it, e.g. Loe Bar, near Porthleven, Cornwall. Feature is swash aligned, and has probably been driven landwards from the offshore zone.
BarsAccumulations of sediment parallel to the coast and submerged during the tidal cycle. Assumed to be the precursor of barriers and being driven landwards.