Positive Feedback: Occurs when an initial change in the system triggers a response that amplifies or exaggerates that change, leading to further instability.
Negative Feedback: Occurs when the system responds to a change by initiating processes that counteract the change, effectively restoring the system to its original state of equilibrium.
Example of Negative Feedback: If a beach is eroded during a storm, the eroded sediment may form an offshore bar; this bar then causes waves to break further out at sea, reducing their energy and allowing the beach to recover.
Example of Positive Feedback: Increased cliff erosion releases more debris into the surf zone; this debris then acts as an abrasive tool (corrasion), which accelerates the rate of further cliff erosion.
| Feature | Open System | Closed System (Sediment Cell) |
|---|---|---|
| Definition | Exchanges energy and matter with surroundings | Exchanges energy but has minimal matter exchange |
| Scale | Global or regional coastal stretches | Discrete units bounded by headlands or deep water |
| Management | Difficult to manage due to external influences | Ideal for Shoreline Management Plans (SMPs) |
| Sediment Flow | Continuous flow across boundaries | Mostly self-contained recycling of sediment |
Identify the System Type: Always begin by stating whether the coastal area being discussed is being viewed as an open system or a closed sediment cell.
Link Energy to Landforms: When describing a landform, explain it through the systems lens: identify the energy input (e.g., high-energy waves), the process (e.g., hydraulic action), and the resulting store (e.g., a wave-cut platform).
Feedback Analysis: If a question asks about coastal change, look for opportunities to discuss whether the change is self-regulating (negative feedback) or self-reinforcing (positive feedback).
Check for Interconnectivity: Remember that coastal systems do not exist in isolation; mention links to the atmospheric system (wind/weather) or the fluvial system (rivers providing sediment).
Confusing Stores and Outputs: Students often mistake landforms like spits for outputs; however, because the sediment remains within the coastal zone, they are technically 'stores' or 'sinks'.
Assuming Equilibrium is Static: Dynamic equilibrium does not mean the coast never changes; it means the system fluctuates around a mean state while maintaining its overall character.
Overstating 'Closed' Systems: No sediment cell is perfectly closed; there is always some leakage of fine sediment to the deep ocean or across headlands during extreme storm events.
