IB Geography B.2.1 Physical Influences on Coastal Landscapes Notes

Wave Energy Depends on Wind and Fetch

1. Wind transfers energy to the sea surface through friction, while most water particles move in small orbits rather than travelling with the wave.
2. A longer fetch, stronger wind and longer wind duration usually produce waves with greater height and energy.
3. As a wave enters shallow water, friction with the seabed slows its base, shortens its wavelength and increases its steepness.
4. The crest eventually moves faster than the base, so the wave breaks and releases energy across the shore.

Swash and backwash redistribute sediment

1. Swash is the movement of water and sediment up the beach after a wave breaks.
2. Backwash is the downslope return flow under gravity.
3. Constructive waves usually have a relatively strong swash and weak backwash, so deposition tends to exceed removal.
4. Destructive waves usually have a relatively weak swash and strong backwash, so beach sediment is more likely to move offshore.

Wave Processes Erode, Transport and Deposit

Erosion attacks exposed rock

1. Hydraulic action occurs when waves force water and compressed air into joints, increasing pressure until fragments break away.
2. Abrasion occurs when sand, pebbles and larger clasts carried by waves strike or scrape the coast.
3. Attrition occurs when transported clasts collide, becoming smaller, smoother and rounder.
4. Solution occurs when seawater dissolves soluble minerals, especially in rocks such as limestone or chalk.

Transport links one stretch of coast to another

Oblique waves create littoral drift

1. Prevailing winds often make waves approach the shore at an angle.
2. Swash carries sediment diagonally up the beach in the direction of wave approach.
3. Backwash returns downslope approximately at right angles to the shoreline under gravity.
4. Repeated swash and backwash move sediment along the coast in a zigzag path.
5. Traction rolls large clasts, saltation bounces smaller grains, suspension carries fine particles, and solution transports dissolved material.

Tides Shift the Zone of Coastal Action

Tidal range controls exposure

1. The tidal range is the vertical difference between high tide and low tide.
2. A large tidal range spreads wave action across a wider vertical section of cliff or beach.
3. A small tidal range concentrates repeated wave attack within a narrower zone.
4. High tides allow waves to reach the upper beach and cliff foot, while low tides expose platforms, bars and intertidal sediment.

Sediment Supply Creates a Coastal Budget

Sources and sinks determine the balance

1. Rivers, cliff erosion, offshore deposits and biogenic material such as shells can add sediment to a coastal system.
2. Littoral drift, tidal currents and wind transfer sediment between beaches, dunes, bars, spits and offshore zones.
3. Deep water, estuaries and permanently stable landforms can act as sinks where sediment is stored for long periods.
4. A positive sediment budget occurs when inputs exceed outputs, so beaches tend to accrete.
5. A negative sediment budget occurs when outputs exceed inputs, so beaches tend to narrow or retreat.

Lithology and Structure Control Resistance

Rock properties affect erosion rates

1. Well-cemented and crystalline rocks generally resist erosion better than weakly consolidated sediment.
2. Permeable rock allows water to infiltrate, whereas impermeable layers can concentrate runoff or create saturated slip surfaces.
3. Closely spaced joints, faults and bedding planes provide weaknesses that hydraulic action and weathering can exploit.
4. The direction and angle of dip influence whether a cliff face is supported or whether blocks can slide seaward.

Geological arrangement influences planform

1. On a concordant coast, rock bands run parallel to the shoreline, so one resistant outer band may protect weaker rock behind it.
2. On a discordant coast, alternating resistant and weaker rocks meet the shore, encouraging headlands and bays.
3. Wave refraction concentrates energy on projecting headlands and disperses energy inside bays.

Vegetation Modifies Water, Wind and Sediment

Plants can increase surface stability

1. Roots bind sand, soil and fine sediment, increasing resistance to wind and water erosion.
2. Stems and leaves increase surface roughness, reducing wind speed and encouraging suspended sand to settle.
3. Salt-marsh vegetation slows tidal flows, traps fine sediment and can promote vertical accretion.
4. Mangrove roots dissipate wave energy and trap sediment in tropical intertidal environments.
5. Loss of vegetation through trampling, grazing or clearance exposes sediment and can reactivate erosion.

Subaerial Processes Weaken Cliffs from Above

Weathering prepares material for removal

1. Mechanical weathering breaks rock without changing its chemistry, as in salt crystallization or repeated wetting and drying.
2. Chemical weathering alters minerals through reactions such as carbonation and solution.
3. Biological weathering includes root growth, burrowing and organic acids that widen weaknesses.

Mass movement transfers weakened material downslope

1. Rockfall occurs when detached blocks fall from a steep cliff along joints or bedding planes.
2. Landslides move material along a relatively planar failure surface.
3. Rotational slumping occurs when saturated weak material moves along a curved slip plane.
4. Heavy rainfall can raise pore-water pressure, reduce friction and trigger slope failure.
5. Wave undercutting steepens the cliff and removes support from its base.

Coastal Form Reflects Interacting Controls

1. High wave energy can produce limited erosion where resistant, unjointed rock and a protective beach reduce attack.
2. Lower wave energy can still cause change where unconsolidated sediment, saturated slopes or a negative sediment budget create vulnerability.
3. Tides determine when and where waves act, while vegetation and sediment storage can reduce the energy reaching exposed material.
4. The dominant control can therefore change across short distances and between calm, storm and seasonal conditions.

Wave Refraction Redistributes Energy

1. Wave fronts slow first where they enter shallow water, causing them to bend towards the coastline.
2. Refraction concentrates wave energy on headlands and spreads it across sheltered bays.
3. Greater erosion can make headlands more exposed, while deposition inside bays builds beaches that absorb wave energy.
4. Coastal shape therefore modifies wave energy, and wave energy then modifies coastal shape.

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