The drainage basin as an open system

A drainage basin is the area drained by a river and its tributaries, bounded by the watershed. CCEA wants you to treat it as an open system with inputs, outputs, stores and transfers of energy and matter.

• Inputs: precipitation (and solar energy driving evaporation).
• Outputs: river discharge to the sea, evapotranspiration.
• Stores: interception, surface storage, soil moisture, groundwater, channel storage.
• Transfers (flows): infiltration, throughflow, percolation, groundwater flow, overland flow, stemflow.

Open, not closed: matter and energy cross the watershed boundary, so the basin gains inputs and loses outputs. The water balance links them: precipitation = discharge + evapotranspiration ± change in storage.

Which is a transfer, not a store?

Storm and annual hydrographs

A storm hydrograph plots river discharge against time after a rainfall event. Key features: the rising limb, peak discharge, the lag time (delay between peak rainfall and peak discharge) and the recessional limb, sitting above the steady baseflow.

CCEA asks you to explain the factors that shape hydrographs and annual regimes:

• Relief: steep basins = faster overland flow, shorter lag, higher peak.
• Basin size and shape: small, circular basins = shorter lag than long, thin ones.
• Soil and geology: impermeable rock/clay = more overland flow, flashier peak.
• Land use: urbanisation and deforestation reduce infiltration, raising peaks.
• Drainage density and precipitation: denser networks and intense rain shorten lag.

Calculate — drainage density

Calculate — hydrograph lag time

Erosion, transport and deposition

Three families of process shape channels:

• Erosion: abrasion/corrasion (bedload scours the bed), attrition (particles collide and round), hydraulic action (water pressure prises rock), solution/corrosion (chemical dissolving).
• Transport: traction (rolling), saltation (bouncing), suspension (carried in the flow), solution (dissolved load).
• Deposition: when velocity and energy fall, load is dropped — coarsest first.

The Hjulström curve links water velocity to whether a particle is eroded, transported or deposited. Note two ideas: fine clays need surprisingly high velocities to entrain (they are cohesive), and once moving, particles stay in transport at lower velocities than were needed to lift them.

Exam nuance: the critical erosion (entrainment) velocity is higher than the settling (deposition) velocity for the same particle — the gap between the two curves is why sediment keeps travelling.

Sort the river process

Tap a term, then tap whether it is a form of erosion, transport or deposition.

Reading the Hjulström curve

Landforms of erosion and deposition

CCEA names specific landforms you must explain:

• Waterfalls and rapids: resistant rock over weaker rock; undercutting and collapse cause retreat, leaving a gorge.
• Meanders, pools and riffles: helicoidal flow erodes the outer bank (river cliff) and deposits on the inner bank (slip-off slope); pools and riffles set up alternating deep/shallow reaches.
• Ox-bow lakes: neck of a meander is cut through, then sealed by deposition.
• Levées and floodplains: over-bank flow drops coarse sediment at the channel edge (levées) and fines across the floodplain.
• Deltas: sediment deposited where the river meets standing water — arcuate (fan-shaped) and bird’s foot forms.

Match the landform to its origin

Tap an item on the left, then its matching partner on the right.

Why and how rivers are channelised

Rivers are channelised to reduce flood risk and improve navigation. CCEA lists three hard methods:

• Realignment — straightening the course to speed flow through a settlement.
• Re-sectioning — deepening or widening the channel to raise capacity.
• Dredging — removing bed sediment to lower the water level.

These often just move the flood problem downstream and damage habitats — hence the shift to environmentally sensitive, sustainable strategies.

A sustainable strategy

Sustainable management and flood case studies

Sustainable strategies include washlands (temporary flood storage), afforestation (raising interception and infiltration to lengthen lag) and land-use zoning (keeping high-value development off the floodplain).

CCEA requires one flood case study — either an LEDC example (such as the Pakistan floods of 2010) or an MEDC example (such as the Somerset Levels of 2014). For each, learn the causes (physical and human), the effects on people, property and land, and the responses.

Keep figures honest: in the exam, use the causes, impacts and management of your chosen flood qualitatively and comparatively unless you can quote a figure you have verified from a reliable source — do not fabricate statistics.

Afforestation and the hydrograph

The big ideas to know

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