Physical Influences on Coastal Landscapes (DP IB Geography)

Waves & Tides

Waves

• Waves form our coastlines through erosion, deposition and transportation

• Wind causes waves to move over the surface of the sea:

  • Wind creates friction, causing the water to ripple

  • These ripples grow into waves

• Waves can have different energy levels depending on:

  • Wind strength

  • The length of time the wind blows

  • The fetch (the distance the wave has travelled)

• If the fetch is larger, the wave will be more powerful

• If the wind is powerful and blows over a longer period, the wave will be larger

• The swash is when a wave moves up the beach

• The backwash is when the wave pulls back out to the ocean

• Waves slow down as they reach a coastline and the water becomes shallower: 

  • The bottom of the wave experiences friction with the seabed below

  • The top part of the wave (crest) gains momentum, pushing over the bottom of the wave

  • This is when the wave will break

Diagram of a wave

• There are two types of waves:

  • Constructive waves – constructive waves construct or build up beaches through deposition processes

  • Destructive waves – destructive waves destroy the coastline through erosion processes

Table comparing constructive and destructive waves

• Waves can reflect, refract, diffract and interfere:

  • Waves can change direction when they slow down through refraction, often seen at headlands and bays

Diagram showing wave refraction

• Waves can reflect, typically seen if a wave collides with a cliff

• Two waves from different directions may meet, causing wave interference

• Waves can also bend, or diffract, when they meet an obstacle or a gap

Tides

• Tides are when the surface of the sea moves up and down

• Gravitational pull (from the moon) controls tidal movements

• The rotation of the earth causes different tidal patterns across the globe

• When the water reaches its highest level, this is high tide:

  • As different areas of the earth face the moon, gravity will pull the ocean in the moon’s direction 

  • Directly on the other side of the earth, high tide will also occur 

  • High tide causes ocean waters to bulge outwards

  • High tide causes the water to spread up the coastline

• As the water sinks back to its lowest level, this is low tide:

  • Low tides are located between the two high tides

• The difference between high and low tide is the tidal range

• Spring tides occur if the moon, earth and sun all line up, causing tides to be higher

• Neap tides occur when the moon, earth and sun are at right angles to each other, causing tides to be lower

Tidal differences as the earth rotates

Sediment Supply

• Sediment is the material on a coastline, e.g. sand, shells, silt 

• The coast is a system of inputs, transfers and outputs of sediment:

  • Sediment cells (littoral cells) are areas of coastline where this system takes place

  • Sediment moves around in each cell

  • Each cell is typically a closed system that recycles sediment

Diagram of a sediment cell

• Sediment supplies and sources (inputs) come from:

  • Rivers

  • Erosional processes and mass movement

  • Onshore currents

  • Subaerial processes (weathering)

  • Aeolian processes (wind)

  • Biological processes (particles from dead marine life)

  • Human activity (hard and soft engineering)

• Sediment transfers occur along the coastline by:

  • Longshore drift

  • Swash and backwash

  • Wind

  • Ocean and tidal currents

• Output of sediment occurs through:

  • Deposition. It produces sinks (depositional landforms)

  • Ocean currents

Sediment cells of England and Wales

• The sediment budget is the balance of sediment that enters and exits the system

  • If more sediment enters than exits the system, this will produce a positive sediment budget

  • A negative sediment budget occurs when more sediment exits than enters the system

  • The system is in equilibrium when inputs and outputs are in balance. Sediment moves equally through the system

  • Changes to inputs and outputs affect the sediment budget

  • Positive and negative feedback loops can alter the equilibrium by heightening and balancing changes within the system:

    • Positive feedback can cause instability, moving the system out of equilibrium

    • Negative feedback brings the system back into equilibrium

Coastal Lithology

• Lithology is the study of the physical characteristics of rocks

• Discordant coastlines contain both hard and soft rock types:

  • Discordant coastlines have more bays and headlands 

• Concordant coastlines contain one type of rock

Map showing concordant and discordant coastlines on the south coast of England

• The rate of recession describes the speed at which coastlines retreat 

• Sedimentary rocks, like sandstone, erode more easily:

  • Softer rocks contain more faults, like cracks, which increase the rate of erosion 

  • Rocks like limestone are more vulnerable to chemical weathering 

  • They are clastic rocks 

  • Rocks like gravel or sand are not cemented well together, making erosion easier

  • Sedimentary rocks will likely form bays or beaches as waves easily erode the cliff

• Igneous rocks, like granite, are harder to erode:

  • They are formed of interlocking crystals which are more resistant to erosion

  • There are fewer weaknesses and faults

  • Headlands will more likely to form due to erosion resistance 

• Metamorphic rocks are not eroded as easily:

  • They contain crystals, which reduce the rate of erosion

• The type of rock on a cliff face can also impact the shape of the cliff

Table showing rock type and its effect on cliff shape

Coastal Vegetation

• Vegetation is vital for stabilising coastlines 

• Vegetation reduces the coastline’s vulnerability to erosion and increases deposition:

  • Roots of plants can bind the sediment together

  • Stems and leaves protect the ground

  • Plants shelter the sediment from the wind

  • Plants can help to slow down wind and water, increasing deposition 

  • Once vegetation dies, organic matter is recycled back into the soil 

  • Vegetation protects coastal landforms like sand dunes, salt marshes and mangroves

• Coastlines are harsh environments for vegetation e.g. high salt levels

• Hardy, adaptable plants colonise areas like this

• If a landform has existed for a long time, plants will more likely colonise

• Plant succession is the colonisation and development of new plants: 

  • Pioneer plants colonise in bare environments, e.g. sediment

  • They help to stabilise the sediment and improve the fertility of the soil

  • Other species can then begin to colonise 

  • As more species colonise, the hardier they become

  • The climax community settles at the end of the succession