Optional themes: Freshwater and/or Oceans and coastal margins
Freshwater and oceans and coastal margins
This optional theme examines water and coastal systems, their processes, landforms, hazards and management. Candidates usually study one option, but the same systems language applies: stores, flows, feedbacks and sustainability.
Core rules
- Hydrological cycle: stores include oceans, ice, groundwater and soil moisture; flows include evaporation, transpiration, condensation, precipitation and runoff. A drainage basin is an open system.
- Hydrograph: lag time is the delay between peak rainfall and peak discharge. Urbanisation, deforestation, compacted soil and steep slopes shorten lag time and raise peak discharge.
- Flood management ranges from hard engineering (dams, levees, channel straightening) to soft engineering (afforestation, floodplain zoning, sustainable drainage systems, SuDS).
- Coastal processes: waves are constructive or destructive; erosion operates by hydraulic action, abrasion, attrition and solution. Longshore drift moves sediment along the coast.
- Coastal landforms include cliffs, wave-cut platforms, spits, bars and salt marshes. Sediment cells are closed compartments of sediment movement.
- Coastal management includes hard defences (sea walls, groynes, riprap) and soft approaches (beach nourishment, dune stabilisation, managed retreat). Coastal squeeze occurs when sea-level rise and fixed defences squeeze habitats.
Worked example
A city upstream of a floodplain urbanises rapidly. Reasoning: impermeable concrete reduces infiltration, so more water reaches the river quickly. The hydrograph shows a shorter lag time and a higher peak discharge. Downstream, levees may protect built-up land but can raise water levels elsewhere and encourage further floodplain development. A systems answer links land-use change to the basin's stores and flows, then evaluates SuDS and floodplain zoning as softer alternatives.
Traps
- Reading a hydrograph as rainfall amount rather than discharge.
- Assuming hard engineering always reduces flood risk overall.
- Ignoring sediment movement when explaining coastal landforms.
- Treating water scarcity as only a physical shortage.