Coastal Processes and Landscapes

Weathering is defined as the breakdown of rocks in situ, meaning the disintegration occurs in the exact location where the rock is situated without immediate displacement by an external agent.
Erosion is defined as the breakdown and subsequent removal of rock particles by an external medium, such as seawater, rivers, wind, or ice.
Coastal zones experience continuous mechanical, chemical, and marine degradation through interacting weathering and erosional agents.

Mechanical weathering involves the physical disintegration of rock without altering its chemical composition.
Freeze-thaw weathering is a primary mechanical weathering process operating in coastal environments:
- Occurs in environments where the ambient temperature repeatedly fluctuates above and below the freezing threshold of water, specifically $0\,^\circ\text{C}$ .
- Liquid water enters existing cracks, joints, and fissures within exposed coastal rock formations (e.g., granite).
- Upon freezing at or below $0\,^\circ\text{C}$ , the water expands by approximately $9\%$ to $10\%$ , exerting substantial internal pressure against the confining walls of the fissure.
- Upon thawing as temperatures rise above $0\,^\circ\text{C}$ , the ice melts, contracts, and releases the pressure on the rock walls.
- The repeated cyclical application and release of expansionary pressure progressively widens the cracks until structural integrity fails, causing rock fragments to detach and shatter.

Chemical weathering involves the breakdown and decomposition of rock through alterations to its internal chemical composition.
Carbonation weathering is an important form of chemical weathering that operates predominantly in warm and wet climatic conditions:
- Atmospheric carbon dioxide dissolves into falling rainwater, producing a weak solution of carbonic acid.
- The carbonic acid comes into direct contact with rocks containing calcium carbonate, such as carboniferous limestone.
- A chemical reaction converts insoluble calcium carbonate into soluble calcium bicarbonate, causing the rock structure to dissolve and wash away in solution.

Mass movement is the downslope shifting and displacement of rock, soil, and unconsolidated regolith under the direct influence of gravity.
Mechanical initiation:
- Occurs when the downslope gravitational force acting upon the cliff material exceeds the resisting shear strength and supporting forces holding the slope in place.
- Promotes rapid coastal cliff retreat and shoreline repositioning.
- Saturation by water significantly accelerates mass movement: water acts as a lubricant between geological strata and adds immense dead weight to the slope material.
Specific types of mass movement:
- Slides: Material shifts rapidly downslope in a straight line along a discrete, planar slide plane.
- Slumps: Material rotates downward and outward along a concave, curved slip plane, frequently producing a prominent scarp (a steep, exposed cut or rupture surface in the side of the slope).
- Rockfalls: Material detaches and breaks up entirely, frequently along horizontal or angled bedding planes, falling vertically or bouncing down a steep cliff face.
Coastal management interventions, such as those implemented at Mappleton, are frequently required to address the severe cliff instability and rapid coastal retreat caused by mass movement and wave attack.

Marine erosion occurs when waves break directly against the shoreline, utilizing three primary mechanisms:
- Hydraulic power: High-energy waves strike cliffs, forcing water into cracks and rapidly compressing pockets of trapped air. When the wave retreats, the compressed air expands explosively. This continuous pressure fluctuation widens the fissures, dislodging blocks of rock over time.
- Abrasion: Waves fling suspended sediment, pebbles, and rock fragments against cliff faces and shore platforms. The abrasive friction scrapes and gouges the rock, chipping away fine particles and smoothing hard surfaces.
- Attrition: Rock particles and pebbles carried by wave turbulence constantly collide with one another. These impacts shatter angular projections, causing the sediments to break down into progressively smaller, smoother, and more rounded pebbles and sand grains.

Longshore drift is the principal process responsible for transporting sediment along the coastline:
- Approaching waves align with the direction of the prevailing (most common) wind.
- Waves approach and break upon the shoreline at an oblique angle (any angle that deviates from a perpendicular $90^\circ$ orientation).
- The swash drives sediment up the beach face at the same oblique angle as the oncoming wave.
- The backwash subsequently pulls water and sediment straight back down the beach slope under the vertical pull of gravity, moving perpendicular ( $90^\circ$ ) to the shoreline.
- Continuous repetition of this asymmetrical cycle moves sediment in a distinct zigzag pattern down the coast.
Four additional hydrodynamic processes transport sediment depending on water velocity and particle mass:
- Traction: The rolling or sliding of large, heavy clasts (such as boulders) along the seabed by the continuous shear force of powerful moving water.
- Saltation: The bouncing or hopping motion of intermediate pebble-sized particles along the sea bed, temporarily lifted by water pulses before dropping back down.
- Suspension: The transport of fine, lightweight sediment particles (such as silt and clay) held aloft within the turbulent water column without touching the seabed.
- Solution: The transport of fully dissolved chemical compounds and soluble minerals (such as limestone components) within the water chemistry itself.

Wave formation is governed by the friction of wind blowing across the sea surface:
- The fetch represents the uninterrupted distance of open water over which the wind blows.
- An increase in fetch generates significantly more powerful waves with higher energy.
- Storm surges represent temporary, extreme sea level rises caused by low atmospheric pressure combined with strong winds driving large masses of seawater onshore.
Destructive waves:
- Associated with high-energy environments and coastal erosion.
- Feature a high wave frequency, high wave height, and steep wave fronts.
- The backwash is significantly stronger than the swash, removing more sediment down the beach profile than is carried up, leading to net beach scour and erosion.
Constructive waves:
- Associated with low-energy environments and coastal accretion.
- Feature a low wave frequency, low wave height, and long wavelength.
- The swash is significantly stronger than the backwash, driving sediment up the beach and leaving it behind, resulting in net deposition.

Deposition is the process whereby sediment carried by water is dropped onto the seabed or shoreline.
Deposition takes place when sediment-transporting water loses kinetic energy, slows down, and can no longer overcome the gravitational pull acting on the load.
Coastal accretion occurs whenever the rate of sediment deposition exceeds the rate of marine erosion.
The volume of deposited material increases under specific coastal conditions:
- High rates of erosion occurring updrift or in adjacent coastal sectors, supplying abundant sediment into the local coastal system.
- Large amounts of sediment transported directly into the area via longshore drift or river inputs.
- Dominance of constructive waves, where the swash systematically deposits more sediment than the weak backwash can drag offshore.