Inputs of Sediment into the System
- Rivers carry sediment from inland.
- Sea level rise can flood river valleys, forming estuaries.
- Eroded material from cliffs by waves, weathering and landslides.
- Waves, tides and currents can transport sediment into the coastal zone from offshore deposits.
Sediment Budget – difference between the amount of sediment that enters the system and the amount that leaves.
Sediment Cells –
- Coast is divided into sediment cells.
- Lengths of coastline that are entirely self-contained for the movement of sediment.
- Processes going on in one cell don’t affect the movement of sediment in another cell.
Six Ways Waves Erode the Coastline
- Abrasion- rock and sediment smash and grind against rocks and cliffs, breaking bits off and smoothing surfaces.
- Hydraulic action- air in cracks in cliffs is compressed when waves crash in. The pressure exerted by the compressed air break off pieces of rock.
- Cavitation- as waves recede, the compressed air expands violently, again exerting pressure on the rock, causing pieces to break off.
- Wave quarrying- energy of a wave as it breaks against a cliff is enough to detach bits of rock.
- Solution- soluble rocks get gradually dissolved by the seawater.
- Attrition- bits of rock smash against each other and break into smaller bits.
Transportation is the Process of Eroded Material Being Moved
- The energyprovided by waves, tides and currents transports eroded material. There are four transportational processes:
- Traction- very big rocks/boulders are rolled along the sea bed by the force of the water.
- Saltation- pebbles and gravel are bounced along the sea bed by the force of the water.
- Suspension- very fine material like silt and clay, is whipped up by turbulence and carried along in the water.
- Solution- substances that can dissolve, such as limestone, are carried along in the water.
- Longshore drift- transportation of sediment along the coastline:
- Swash carries sediment up the beach, parallel to the prevailing wind.
- Backwash carries sediment back down the beach, at right angles to the shoreline.
- When there’s an angle between the prevailing wind and the shoreline, a few rounds of swash and backwash move the sediment along the shoreline.
Deposition is the Process of Dropping Eroded Material
- Deposition- when material being transported is dropped on the coast.
- Marine deposition- when sediment carried by seawater is deposited.
- Aeolian deposition- when sediment carried by wind is deposited.
- Happens when the sediment loadexceeds the ability of the water or wind to carry This can be because sediment load increases, or because wind or water flow slows down.
- Friction increases- if waves enter shallow water or wind reaches land, friction between the water/wind and ground surface increases, which slows down the water or wind.
- Flow becomes turbulent- if water or wind encounters an obstacle, flow becomes rougher and overall speed
Sub-Aerial Weathering Occurs Along the Coastline
- Sub-aerial weatheringthe gradual break down of rock by agents such as ice, salt, plant roots and acids. Weathering weakens cliffs and makes them more vulnerable to
Salt Weathering
- Caused by saline water.
- Enters pores/cracks at high tide.
- Tide goes out, rocks dry and water evaporates, forming salt crystals. As they form, they expand, exerting pressure on the rock causing pieces to break off.
Freeze-thaw Weathering
- Occurs in areas where temperatures fluctuate above and below freezing.
- Water enters the joints and crevices in rocks.
- If the temperature drops below 0C, the water in the cracks freezes and expands.
- Repetition weakens the rocks causing pieces to fall off.
Chemical Weathering
- Breakdown of rock by changing its chemical composition.
Wetting and Drying
- When clay gets wet, it expands, and the pressure caused by this breaks fragments of the rock.
- Mass movement is when material moves down a slope:
- Mass movement – the shifting of material downhill due to gravity.
- Slides- material shifts in a straight line.
- Slumps- material shifts with a rotation.
- Rockfalls- material breaks up and falls.
- Mudflows- material flows downslope.
- Unconsolidatedrocks are prone to collapse as there’s little friction between particles to hold them together.
- Heavy raincan saturate unconsolidated rock, further reducing friction and making it more likely to
- Runoffcan erode fine particles and transport them downslope.