Coastal Geomorphology

The shape of the coast and its landforms is a direct result of:

Geology + Marine processes + Sub-aerial processes + Man + (Climate)

Marine processes

Constructive waves

These are depositional waves as they lead to sediment build up, and are most common where a large fetch exists.

They tend to have a low gradient, a larger swash than backwash, low energy and an elliptical orbit.

The wave period is long, with 6-8 waves breaking in a minute.

Destructive waves

These act as agents of erosion, because backwash is greater than swash.

They are most common where fetch is short, have a mainly circular orbit, a steep gradient, and 'plunge' onto the beach.

The wave period is short, with 12-14 waves breaking per minute.

Wave definitions

Wave fetch: The distance of open water over which a wave has passed. Maximum fetch is the distance from one coastline to the next landmass, it often coincides with prevailing wind direction (South West in the UK).

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.

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 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.

Summary of coastal erosion processes

Coastal erosion

Abrasion/corrasion: When waves approach the coastline they are carrying material such as sand,

shingle, pebbles and boulders. Abrasion occurs when this material is hurled against cliffs as waves hit them, wearing the cliff away.

Hydraulic pressure: Cliffs and rocks contain many lines of weakness in the form of joints and cracks. A

parcel of air can become trapped/compressed in these cracks when water is thrown against it. The increase in pressure leads to a weakening/cracking of the rock.

Corrosion/solution: Seawater contains carbonic acid, which is capable of dissolving limestone. The

evaporation of salts in seawater produces crystals and their formation can lead to the disintegration of rocks.

Sub-aerial: Coastal erosional processes that are not linked to the action of the sea. Erosion

occurs via rain, weathering by wind and frost. Its impact is often seen in soil creep, slumping and landslides.

Human activity: Much building and recreation occurs at the coast, and this increases pressure on

cliff tops, making them more liable to erosion and subsidence. The building of sea defences upsets the dynamic equilibrium of the coastline.

Influences on the rate of erosion The rate at which a stretch of coastline

is eroded is related to the following factors: The point at which the wave breaks - (if at the foot

of a cliff, the cliff is subject to maximum energy and most erosion).

Steepness of the wave. Depth of sea, fetch, aspect. Amount of beach material - (a wide beach protects

a cliff more than an arrow beach). Rock type and structure - (hard rock such as

granite is far more resistant to erosion than soft rocks, such as clay).

The sub-aerial processes

Sub-aerial processes

Sub-aerial processes = weathering + mass movement + humans*

Weathering = physical + chemical

Mass movement = falls + flows + slides + slumps + creep

*Pressure on cliffs (construction work)Sea Defences (although designed to reduce erosion - in some instances (e.g. the use of groynes) can increase erosion by starving down stream areas of sediment

Weathering – basic summary Weathering Processes: Weathering is the breaking up of rock in situ Freeze-thaw processes (as water freezes and thaws in

cracks in cliffs, the expansion and contraction forces the rock open, making them more susceptible to the action of waves).

Chemical weathering - water running down the face of cliffs, either from surface runoff from above or from rainfall can result in solution so that rock cracks are enlarged, leaving the remaining rock loosenedBiological weathering - burrowing organisms and roots of vegetation forces open cracks, making them more susceptible to wave erosion) [NOT sub-aerial]Human Activity

The weathering processes

Physical Weathering Definition: Form of weathering responsible for the mechanical breakdown of rocks but involving no chemical change.

Forces acting on rock exposed on the Earth's surface open up any weak points in the rock and cause pieces to be broken off. Piles of jagged rock fragments called scree are formed.

Freeze-thaw ( a fatigue process) ↑ 9% on freezing.7-15kg/cm2 (2115 kgcm-2 @ -22oC) Possibility of granular disintegration

Salt crystallisation↑ Volume not accretionSedimentary more than igneous

Insolation weatheringIn presence of H2O only – granular disintegration

Surface unloading (exfoliation)Pressure release

HydrationWater absorption – clays - kaolinitee.g. SiO2 + 2H2O i Si(OH)4

The weathering processes

Chemical WeatheringDefinition: Form of weathering brought about by chemical attack on rocks, usually in the presence of water. Chemical weathering involves the breakdown of the original minerals within a rock to produce new minerals (such as clay minerals, bauxite, and calcite). Solution - the general term for chemical dissolving of rock minerals

Hydrolysis•H+ ion - e.g. the hydrolysis of feldspars e.g. orthoclase feldspar produces clay minerals such as kaolinite,+ H+ and silicic acid.•Particularly significant on igneous rocks eg. Basalts of Northern Ireland

Oxidation•O2 in gaseous form or dissolved in water will reacts with metal ions;

•4FeO + 3H2O + O2 → 2Fe2O3.3H2O

Carbonation•CO2 + H2O → H2CO3 leads to

•CaCO3 + H2O → CO2 + Ca (HCO3 ) 2

•Particularly significant on sedimentary rocks eg. Limestone and chalk of the Jurassic coast

Sub-aerial processes

Mass Movement

Cliffs – erosional landform

Factors determining cliff formation Geological. Sub-aerial. Marine. Meteorological. Human activity.

The rule of thumb!Cliffs are steep if removal of material at its base is greater

than supply.Cliffs are shallow if the supply of material is greater than

removal.

Mass movement

Mass Movement

This is a combination of a rockfall & earthflow

Types of cliff

Soft Geology Cliff

Fairlight Cove is mostly softer unconsolidated rocks.

These rocks are less resistant to erosion & landslips are common

Typical of NE England

Cliff & Wave Cut Platform

Wave attack constantly erodes the cliff base, creating a Wave Cut Notch.

As the cliff is undercut, it collapses, leaving rockfalls on the beach below.

As the cliff retreats it leaves behind a gently sloping platform – Wave Cut Platform

Rotational Slumping

The full extent of (rotational) slumping can be seen on the west side of the cove.

Coastal transportCoastal TransportWeathering and erosion at the coast produces large amounts of material which is moved along the coast line and out to sea by the action of waves.Up and down the beach: Material is moved up the beach in the swash and back down the beach in the backwash. Constructive waves move large amounts of material up the beach as they have a strong swash, whereas destructive waves move large amounts of material down the beach due to their strong backwash.The movement of material by waves: The movement of material within the waves themselves depends on (i) the size of the material and (ii) the energy available for transporting the material. There are four possible types of transport within the water:

Solution - material carried dissolved in the waterSuspension - fine material carried suspended within the water itselfSaltation - material which is too heavy to be continuously held in suspension is bounced along the sea bed.Traction - the heaviest material is simply rolled along the sea bed

Coastal transport

1. Waves break on the beach at an angle controlled by the prevailing wind direction. Material is moved up the beach in the swash.2. Material then moves straight back down the beach in the backwash under the influence of gravity.3. As the processes continues material moves along the coast in a zig-zag movement.