題目 1 · Explanation
8 分Option A: Coastal Landscapes. Explain how material flows (transfers) operate within a coastal sediment cell.
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解題
A coastal sediment cell (or littoral cell) is a stretch of coastline where sediment movement is largely self-contained, meaning it acts as a closed system for material. Within this system, material flows (transfers) represent the active movement of sediment linking inputs (sources) to outputs (sinks).
First, sediment is introduced via sources (inputs) such as cliff erosion, river discharge, wind-blown sand, and offshore marine deposition. Once in the system, sediment is transferred along the coastline. The primary transport mechanism is longshore drift (lateral transfer), which occurs when waves strike the shore at an oblique angle, transporting sediment along the beach in a zig-zag pattern. Additional transfers include offshore-onshore currents, tidal currents, and aeolian (wind) transport, which move sand inland to form dunes or offshore to form sandbars.
These flows continue until sediment reaches a low-energy environment, which acts as a sink (store or output). Sinks include beaches, spits, tombolos, dunes, and estuaries. The overall balance between these inputs, transfers, and outputs is known as the sediment budget. Under normal conditions, these transfers operate to maintain a state of dynamic equilibrium. If a disruption occurs (e.g., a storm increases cliff erosion and sediment input), negative feedback mechanisms (such as increased deposition and wave dissipation) work to restore the cell's balance.
First, sediment is introduced via sources (inputs) such as cliff erosion, river discharge, wind-blown sand, and offshore marine deposition. Once in the system, sediment is transferred along the coastline. The primary transport mechanism is longshore drift (lateral transfer), which occurs when waves strike the shore at an oblique angle, transporting sediment along the beach in a zig-zag pattern. Additional transfers include offshore-onshore currents, tidal currents, and aeolian (wind) transport, which move sand inland to form dunes or offshore to form sandbars.
These flows continue until sediment reaches a low-energy environment, which acts as a sink (store or output). Sinks include beaches, spits, tombolos, dunes, and estuaries. The overall balance between these inputs, transfers, and outputs is known as the sediment budget. Under normal conditions, these transfers operate to maintain a state of dynamic equilibrium. If a disruption occurs (e.g., a storm increases cliff erosion and sediment input), negative feedback mechanisms (such as increased deposition and wave dissipation) work to restore the cell's balance.
評分準則
Level 3 (6-8 marks): Demonstrates detailed, accurate, and coherent knowledge of coastal sediment cells as systems. Clear explanation of material flows (transfers) with precise terminology (e.g., longshore drift, offshore-onshore movement, aeolian transport) and links them effectively to sediment sources and sinks. Demonstrates a strong understanding of system concepts like the sediment budget and dynamic equilibrium.
Level 2 (3-5 marks): Shows reasonable knowledge of sediment cells. Explains some material flows (such as longshore drift) but the connections between sources, transfers, and sinks are less integrated. Terminology is used but may lack precision.
Level 1 (1-2 marks): Basic description of coastal transport or deposition. Limited or no reference to sediment cells as systems or the concept of material flows. Points are outlined without clear explanation.
Level 2 (3-5 marks): Shows reasonable knowledge of sediment cells. Explains some material flows (such as longshore drift) but the connections between sources, transfers, and sinks are less integrated. Terminology is used but may lack precision.
Level 1 (1-2 marks): Basic description of coastal transport or deposition. Limited or no reference to sediment cells as systems or the concept of material flows. Points are outlined without clear explanation.