1.c Coastal sediment is supplied from a variety of sources

What is sediment budget?

• The balance of the sediment volume entering and exiting a section of the coast

What are the terrestrial sources of coastal sediment?

• Fluvial, aeolian deposition

• Weathering, mass movement

• Marine erosion, longshore drift

What are fluvial processes?

• Processes associated with rivers and streams

What is fluvial deposition?

• River enters the sea - reduction in velocity - flowing water enters the static body of sea water

• Energy is reduced - sediment load is deposited

Why might (fluvial) deposition occur slower with tides and currents?

• They move in the opposite direction to the river flow

• Major resistance to its forward movement

What is the pattern of sediment (fluvial) deposition?

• Sequential

• Largest particles are deposited first

• Finest particles are carried out to sea

What is flocculation in fluvial deposition?

• Fine materials clump together - electrical charges in saline conditions (freshwater and saltwater meet)

• They sink to the seabed

What are the different processes of weathering?

• Physical

• Chemical

• Biological

What is weathering?

• The breakdown of rock in situ

What is physical weathering?

• The breakdown of rock without any chemical alteration

What is freeze-thaw weathering? (physical weathering)

• Water enters cracks, expands by ~10% when it freezes

• Confined spaces - pressure is exerted, rock splits

What is pressure release? (physical weathering)

• Overlying rocks face weathering and erosion

• Underlying rock expands - fractures parallel to the surface (pseudo-bedding planes)

• Exposure of subsurface rocks - dilation

What is thermal expansion? (physical weathering)

• Rocks expand when heated - contracts when cools

• Frequent cycles of temperature change - outer layer cracks

• Insolation weathering

What is salt crystalisation? (physical weathering)

• Salt solutions (etc., sodium sulphate) enters the pores in porous rocks

• Salts precipitate (can expand up to 300%), crystals form, rock stresses and disintegrates

What is chemical weathering?

• The breakdown of rock involving chemical reactions between moisture and some minerals

What does Van’t-Hoff’s law state about chemical reactions?

• A 10°c increase in temperature leads to a x2.5 increase in the rate of chemical reaction (up to 600°c)

What is oxidation? (chemical weathering)

• Minerals in rocks react with oxygen (air/water)

• Iron is susceptible - soluble under acidic conditions (iron-rich cements)

What is carbonation? (chemical weathering)

• Rainwater + dissolved carbon dioxide = carbonic acid

• Carbonic acid + calcium carbonate = calcium bicarbonate

• Reversible

What is solution? (chemical weathering)

• Some salts - soluble

• Other materials (iron) - soluble in acidic (pH 3) water

What is hydrolysis? (chemical weathering)

• Rock minerals, water

• Silicates + water = secondary minerals (clay)

What is hydration? (chemical weathering)

• Water molecules + rock minerals = new minerals of a larger volume

• Surface flaking in many rocks - minerals expand by 0.5% as they absorb water

What is biological weathering?

• The breakdown of rocks involving the physical actions of organisms (etc., growth of tree roots, cheltation by organic acids)

What do tree roots do in biological weathering?

• They grow into cracks and exert outward pressure on rock and soil

What do organic acids do in biological weathering?

• Soil water becomes more acidic - reacts with minerals in cheltation

What is mass movement?

• Resultant force of gravity acting on slope material exceeds the friction trying to keep the material on the slope

What are the different processes of mass movement?

• Rockfall

• Slides

• Slumps

What is rockfall? (mass movement)

• Cliffs (~40°) - rocks detach from the slope by physical weathering and falls under gravity

• Waves usually remove this material, or it may accumulate

What are the different types of slides? (mass movement)

• Linear (straight line slip plane)

• Rotational (curved slip plane)

What are slumps? (mass movement)

• Rotational slides

• Weak rocks (clay), heavier when wet, increases the downslope force

What are the different processes of marine erosion?

• Abrasion, attrition

• Hydraulic action

• Pounding

• Solution

What is abrasion? (marine erosion)

• Corrasion

• Rock particles scour the rocks against the coastline

What is attrition? (marine erosion)

• Rock particles collide against each other

• Worn away, smoother and rounded

What is hydraulic action? (marine erosion)

• Waves break against a cliff-face

• Air is trapped in cracks, crevices are compressed

• Waves recede - pressure is released, crack is widened

What is pounding? (marine erosion)

• Mass of a breaking wave exerts pressure on the rock which weakens

• <30 tonnes per m² can be exerted

What is solution? (marine erosion)

• Corrosion

• Minerals dissolve in coastal rock

• The pH of seawater is 7-8 - limited significance; unless the water is polluted

What are aeolian processes?

• Processes involved with the wind

What is aeolian deposition?

• Material carried by wind will be deposited - wind speed falls (surface friction)

• Coastal areas - occurs inland, friction greater

What is longshore drift?

• Waves approach the coast at an angle - dominant wind

• Swash carries particles up the beach

• Backwash moves them back down

What angles are the swash and the backwash at?

• Swash - 45° (diagonal)

• Backwash - 90° (perpendicular)

What is transportation?

• The movement of material shorewards

What is solution? (transportation)

• Materials that have been dissolved into the mass of water

What is suspension? (transportation)

• Small particles of sand, silt and clay carried by currents

What is saltation? (transportation)

• Irregular movements of material carried by turbulent flow

• Other particles may be dislodged by the impact

What is traction? (transportation)

• Large particles in the load are pushed along the sea floor

What are the offshore sources of coastal sediment?

• Marine deposition

What is marine deposition?

• Material is deposited - loss of energy, decrease in velocity/volume of water

When does (marine) deposition take place?

• Rate of sediment accumulation > the rate of removal

• Waves slow down after breaking

• Top of the swash - water is no longer moving

• Backwash - water percolates into the beach material

• Low-energy movements/areas sheltered by the wind

What is the settling velocity in marine deposition?

• The velocity at which sediment particles are deposited

What are the human sources of beach nourishment?

• Sediment brought in by lorries - bulldozers

• Sand, water - pumped onshore by pipeline

• Low bunds hold the mixture in place