Flood risk and hydrographs

Physical factors

A river floods when it carries more water than its channel can hold. Natural, physical factors that make this more likely:

Heavy or long-lasting rain: more water than the ground can soak up.
Saturated ground: after days of rain, the soil is full, so rain runs straight off the surface.
rocks such as clay and granite don't let water soak in.
Steep slopes: water runs off quickly into the river.
Snowmelt: a sudden thaw releases a lot of water at once.
Frozen ground acts like a hard surface, so water can't soak in.

Key terms

Human factors

People can make floods more likely:

roads, roofs and car parks are , so rain runs off fast into drains, which pour it straight into rivers.
with fewer trees, less rain is caught by leaves and taken up by roots, so more reaches the river, faster.
Farming: ploughing up and down slopes, and heavy machinery packing the soil down, increase runoff.
puts homes and businesses where rivers naturally flood.

Common mistake

Building on a flood plain doesn't cause floods, but it does put more people at risk when they happen.

Key terms

Storm hydrographs

A shows how a river's (the volume of water passing a point each second) changes after a storm.

A storm hydrograph (data written for this app).

discharge rises as rainwater reaches the river.
the highest discharge, here 34 m³/s.
the time from peak rainfall to peak discharge, here 12 hours.
discharge falls as less water reaches the river.
Base flow: the normal level, fed by water seeping through the ground.

Key terms

Flashy and gentle rivers

The shape of a hydrograph shows how quickly water reaches the river:

A flashy hydrograph	A gentle hydrograph
Short lag time, high peak	Long lag time, low peak
Steep rising limb	Gentle rising limb
Urban areas, steep slopes	Woodland, gentle slopes
Impermeable or saturated ground	Permeable rock, dry soil

A river with a flashy hydrograph is more likely to flood, because a lot of water arrives quickly.

Exam tip

Read figures off a hydrograph with units: "a lag time of 12 hours", "a peak of 34 m³/s".

Key terms

Exam practice

Plan your own answer first, then open the model answers to see what each level looks like.

6 marks · about 6 minutes

Explain how physical and human factors can increase the risk of river flooding.

Explain means: Give the reasons why something happens, developing each point with "because" or "so".

Level 1 (Basic)· 2/6 marks (band 1–2)

Heavy rain can cause floods. Building on flood plains also increases the risk.

Why this level: One physical and one human factor, stated simply. To reach Level 2: explain how each factor gets more water into the river more quickly.

Level 2 (Clear)· 4/6 marks (band 3–4)

Heavy rain can be more than the ground can absorb, so water runs off into the river quickly. Steep slopes also speed up runoff. People increase the risk: urbanisation covers the ground with impermeable roads and roofs, so rain can't soak in, and drains carry it straight to the river.

Why this level: Clear physical and human factors, each explained. To reach Level 3: develop more factors (saturated ground, impermeable rock, deforestation) and separate causes of floods from what puts people at risk.

Level 3 (Detailed)· 6/6 marks (band 5–6)

Physical factors can increase flood risk. Heavy or long-lasting rain can be more than the ground can absorb, especially when it is already saturated, so water runs over the surface straight into the river. Impermeable rock, such as clay, stops water soaking in, and steep slopes speed up runoff, so river levels rise quickly. Human factors add to this. Urbanisation covers land with impermeable roads and roofs, and drains pour rainwater quickly into rivers. Deforestation means less rain is intercepted by trees and taken up by roots, so more reaches the river faster. Building on flood plains doesn't cause floods, but it puts more people and property at risk when they happen.

Why this level: Detailed explanation of several physical and human factors, each linked to faster or greater runoff, with the useful distinction about building on flood plains. This is Level 3.

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