1.1.3

• role of climate dorset:

  • Exposed to south westerly prevailing winds which bring storms to the UK from the Atlantic Ocean. High energy, destructive waves which increase erosion on the coastline. 
  • The Dorset coast receives relatively low amounts of rainfall, but it can experience very wet winters with heavy rainfall during storms. Prolonged heavy rainfall causes clay to become heavier, softer and more slippery, making mass movement more likely. During the winter rainfall there are often slides and slumps on the cliffs.
• role of climate dorset:

  Dorset has warm and dry summers (around 21℃ in July) and mild wet winters (average a minimum in January of 3℃). Salt weathering is the dominant form of physical weathering especially in the summer. Mild winters mean freeze thaw weathering is rare as temperatures rarely oscillate around 0℃.
• geology dorset:

  Geology (lithology): The coastline is made of bands of chalk, limestone, clays and sands. As a result, the coastline of Dorset experiences differential erosion. The concordant and discordant coastlines have a wide range of coastal landforms. 
• geology dorset:

  More resistant chalk and limestone cliffs are eroded more slowly creating exposed headlands. However, chalk and limestone are vulnerable to weathering and solution where rainwater and seawater chemically react with the rock, causing it to dissolve. 

  Less resistant rocks such as Greensand and Wealden clay and sands are easily eroded by hydraulic action and abrasion and have seen faster rates of erosion and the formation of bays.
• role of time dorset:

  -Recent glacials and interglacials have helped shape the coastline over the long term. 

  -During the ice age the Dorset coastline was frozen to great depth and sea level 120 metres below the present day. 

  -Approx. 11,700 years ago the frozen land thawed, and vast quantities of water were released from ice melt. 
• role of time dorset:

  -Fast-flowing rivers made their way to the sea, cutting through all the rocks, including the resistant Jurassic limestones. 

  This helped the sea break through the resistant rocks and form coves and bays. As sea level gradually rose, eventually wave action began to operate on the rocks, producing today’s landscape.
• Mass movement, such as rockfalls or landslides, depending on the rock type are common because of the prevailing steep, and therefore unstable slopes. All types of mass movement are episodic, occurring more commonly in winter as a result of more powerful waves undercutting the cliff base.
•  Rock erosion / removal processes depend on the action of the waves- energetic wave conditions in a concentrated micro-tidal environment are particularly effective, both in causing abrasion and hydraulic action.
• Rock breakdown processes are physical, chemical and biological (types of weathering) than weaken and loosen rock material, making it available for removal by marine processes
• high energy coastal environment:

  erosive and rocky coast

  physical chemical and biological weathering

  mass movement

  transportation
• low energy coasts:

  deposition dominates creating sandy coastlines and associated features such as sand dunes, spits, and bars

  estuarine coastlines low energy mudflats develop
• Deltas are areas where sediment is accumulating (river-derived) out from the land into the sea.

  They can be small or large-

  A small stream entering the sea can develop a delta of a few tens of square metres. 

  At the other extreme there are the deltas of continental-scale rivers such as the Nile, Mekong or Mississippi, which cover hundreds of square kilometres
• why does sediment build up delta

  -River-dominant deltas that have large catchments, where the river is discharging into a relatively low energy sea area. The Mississippi and the Danube are examples. Deltas that front on to open sea areas and receive high levels of wave energy have smooth coastlines.

  -Tidal energy is high and wave energy only moderate, sediment is deposited perpendicular to the coastline. Here tide-dominant deltas form such as the Ganges-Brahmaputra and Rhine.
• Nearly all present-day deltas are relatively young landforms. 

  Sea level has been at its current position for about the last 6000 years, so it is during this time period that deltas have developed, some involving very large volumes of sediment. 

  In the north-west of the Indian subcontinent, two major rivers share in the production of one of the largest deltas in the world. 

  The Ganges and Brahmaputra transport about 1.5 billion tonnes of sediment per year down to the Bay of Bengal.
• Estuaries are indentations in the coastline, often funnel-shaped, that are infilling looking inland with sediment.
• Most estuaries can be divided into three compartments.

  In terms of energy inputs;

  • Outer zone (near the sea) part receives lots of wave and tidal current energy
  • Inner zone (inland) part has considerable energy inputs from river currents 
  • Finer sediments are moved through these two areas into the less energetic central zone. It is here that most deposition occurs. 

  Coarse material, such as sands and gravel, tends to be deposited in the inner and outer zones
• types of estuary:

  • Stratified – very little mixing 

  Found in microtidal environments where neither the tidal nor the river current is strong enough to cause the turbulence necessary to bring about physical mixing. Fresh water lies above salt water. 

  • Partially mixed  and well-mixed. 

  As river and tidal currents increase, the degree of mixing increases. The resulting patterns of fresh, brackish (a mixture of salt and fresh water) and salt water are important influences on ecosystem development.
• wave refraction:

  • wave direction approaching shoreline is modified due to shape of seabed
  • depth of the water varies. friction from the seabed in shallow water slows the progress of waves
  • waves changes direction so that they approach the coastlines aligned parallel to it
  • distorts the spread of energy concentrated at headland disipated in bays
  • creates beaches
• Formation of a wave cut platform

  When high and steep waves break at the foot of a cliff they

  concentrate their erosive capabilities into only a small area

  of the rock face. This concentration eventually leads to the

  cliff being undercut, forming a feature known as a wave-cut

  notch. Continued activity at this point increases the stress

  on the cliff and in time it collapses. As the cliffs of solid rock

  recede over time from their original shore position they may

  leave a basal
• The wave cut platform is the base of the cliff that lies below

  the intertidal zone. The cliff base is gently sloping (less than

  5º). When viewed from a distance, the platform looks

  remarkably even as it cuts across the rocks, regardless of their hardness. On closer inspection, the platform is often

  deeply cut into by abrasion from the huge amount of

  material that is daily across it, and by the effects of chemical

  action. The platform continues to grow and, as it does, the

  waves break further out to sea and have to travel across

  more platform before reaching the cliff line.
• wave cut platform

  This leads to a greater dissipation of wave energy, reducingthe rate of erosion on the headland and slowing down thegrowth rate of the platform. There tends, therefore, to be alimit to how big the feature can grow and some expertshave suggested that growth beyond 0.5km is unusual.