3.1.3.1, 3.1.3.2 - Coastal systems and energy
Understand how energy and sediment move through a coast, why waves and tides vary, and how feedbacks connect individual landforms into a changing landscape.
A coast as an open system
A coast exchanges energy and material with the sea, land and atmosphere, so its behaviour depends on where we draw the system boundary.
Coastal system
A connected set of coastal stores and transfers with inputs and outputs of energy or material across a defined boundary.
Sun-driven winds transfer energy to waves. Gravity drives tides, flowing water and falling cliff material. Sediment enters through cliff erosion, rivers, seabed transport or human nourishment. Beaches, dunes and nearshore bars store sediment; waves, currents, wind and gravity transfer it. Material can leave a frontage alongshore or move into deeper water. A beach losing sand during a storm may therefore have transferred it to a nearby submerged bar rather than lost it from the whole coastal system.
Define the time period as well as the place. A bay's beach might gain sediment over a summer but lose it over a decade. A sediment budget compares all relevant inputs and outputs; an energy budget concerns energy transfers and dissipation. They are related because energy enables transport, but a quantity of sediment is not a quantity of energy.
A landform is an individual feature such as a cliff or spit. A landscape is the spatial combination of landforms and their relationships: an eroding cliff can supply a beach, which supplies a spit enclosing a sheltered marsh. Explaining the landscape requires the transfers connecting those features, not only naming them.
Feedback and changing balance
A feedback occurs when a system's response changes the conditions producing further change.
Negative feedback counteracts a disturbance. Storm waves may transfer beach sand offshore into a bar. If that bar makes waves break farther out, less energy reaches the beach, reducing further erosion. Recovery is possible when later lower-energy conditions move sediment landward. This is conditional: sand carried beyond the active profile or blocked by structures may not return.
Positive feedback reinforces a disturbance. Erosion can breach a protective dune ridge, allowing more overwash and flooding, which further lowers or widens the breach. Positive does not mean beneficial, and negative does not mean harmful. The terms describe the direction of the feedback.
Dynamic equilibrium
A fluctuating balance in which opposing transfers broadly maintain a system's characteristic form or store over the specified time and area.
Equilibrium is not a motionless shoreline. Seasonal beach changes can fluctuate around a longer-term balance. A persistent reduction in sediment supply can shift that balance towards a smaller beach. A threshold is a point at which the response changes substantially, such as overtopping a dune crest; equal increments of sea level need not produce equal landscape changes near that threshold.
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