3.1.3.2 - Sediment cells and coastal processes
Trace sediment from its sources through a coastal cell, explain the processes changing and transporting it, and use a budget to distinguish movement from net gain or loss.
Sources, cells and sediment budgets
A coastline's sediment supply depends on sources, routes and sinks, not simply on how strongly waves break.
Cliff erosion supplies material whose size depends on geology. Rivers bring material from catchments; offshore deposits can be moved landward; shell fragments add biogenic sediment; wind can move beach sand into dunes. Nourishment deliberately adds sediment, while dredging or extraction can remove it. Some material remains in the active beach system, while fine particles can be transported much farther offshore or into estuaries.
Sediment cell
A coastal area within which sediment transfers are strongly interconnected and whose boundaries substantially limit exchange with neighbouring areas.
Headlands, deep water and changes in transport direction can help define boundaries. Cells are useful management models, not sealed boxes: sediment may cross a boundary during storms or through offshore exchange. A subcell is a smaller management or transport unit. The East Riding monitoring programme calls its area subcell 2a; this should not be confused with the independently numbered Shoreline Management Plan 3.
Here is change in a specified sediment store, total inputs and total outputs over the same interval, in consistent mass or volume units.
For an original illustrative annual beach budget, cliff input is 8,000 m³, alongshore input 12,000 m³ and nourishment 5,000 m³. Alongshore output is 18,000 m³ and offshore output 9,000 m³. Inputs total 25,000 m³ and outputs 27,000 m³, so storage falls by 2,000 m³. This is a net budget: gross transfers are much larger. Avoid adding a within-system transfer to boundary inputs or mixing tonnes with cubic metres without an appropriate density conversion.
Weathering, runoff and mass movement
Processes operating above the waterline can prepare and deliver sediment that waves later remove.
Weathering breaks down rock in place. Physical weathering includes freeze–thaw where water in cracks freezes repeatedly, and salt crystallisation where saline water evaporates and growing crystals stress pores. Chemical weathering includes dissolution of susceptible minerals, enhanced when water is mildly acidic. Biological weathering includes root growth in fractures and organisms producing acids. Climate, rock permeability and jointing control which mechanisms matter; frost weathering should not be presumed dominant on every coast.
Mass movement transfers material downslope under gravity. Rockfalls detach from steep jointed cliffs. Slides move along a relatively planar surface; rotational slumps move along a curved slip surface and can leave backward-tilted blocks. Flows involve less coherent, often water-rich material. Saturation adds weight and raises pore-water pressure, reducing effective friction. Marine undercutting removes support from a slope's foot, while runoff concentrates water, washes exposed particles downslope and can erode channels.
At Aldbrough, East Riding, BGS describes late-Devensian tills, including Skipsea Till overlain by Withernsea Till. Differences in their properties help produce stepped cliffs and different styles of failure. This is a named example of interacting geology, water and marine erosion, not proof that rainfall alone causes every retreat event. After a slump, fallen debris may briefly protect the foot; removal by waves renews exposure. The timing of rainfall, failure and wave removal can therefore differ.
How waves erode rock
Marine erosion involves several mechanisms whose effectiveness depends on weaknesses, wave impact and available sediment.
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