River Flood Risk Lab

From source to sea, and flood risk

The River Exe rises on Exmoor and flows south through Devon to the sea at Exmouth, passing the city of Exeter. Along its course it shapes the land, but it can also flood. This module looks at flood risk on a river like the Exe, and at the two big ways of managing it: hard and soft engineering.

Key terms

Four ideas run through flood management:

Where does the flood plain form?

Along a river long profile, where is the wide, flat flood plain, the area most at risk of flooding, found?

• In the lower course, near the mouth
• In the upper course, near the source
• Only at the exact source
• Rivers do not have flood plains

Match each management method

Hard vs soft engineering

There are two contrasting ways to manage flood risk.

Spot the soft methods

Select the TWO soft-engineering approaches to flood management.

• Flood-plain zoning, keeping building off the flood plain
• Afforestation, planting trees across the catchment
• Building a large concrete dam
• Building high flood walls along the banks

A named river: the Exe

In the exam, use a named example. The River Exe drains Exmoor, whose steep, wet and impermeable moorland sends water quickly downstream. After heavy rain, the Exe has flooded Exeter, damaging thousands of homes in the past. Flood defences, such as walls and channel works, were later built to protect the city. Naming the river and the flooding is what earns the marks.

Why choose soft engineering?

Why might a council choose soft engineering rather than hard engineering to manage flood risk?

• It usually costs less and is more sustainable, working with natural processes
• It completely guarantees that no flooding will ever happen
• It uses more concrete than any other method
• It stops all rivers from flowing at once

Complete the picture

The chance that a river will overflow and cause damage is called flood _____. Building structures such as dams and flood walls is _____ engineering, while working with nature by planting trees is _____ engineering. The wide, flat area beside a river that floods is the flood _____. On the River Exe, heavy rain on Exmoor can flood the city of _____.

How a flood develops

Put the stages of how heavy rain leads to a river flood in order.

• Heavy, prolonged rain falls across the catchment
• The ground becomes saturated and cannot soak up more water
• Water runs quickly overland into the river as runoff
• The river discharge rises and the water level climbs
• The river overtops its banks and floods the land

Tap the soft approaches

A town is planning its flood management. Tap the TWO soft-engineering approaches.

• Plant woodland across the upper catchment to slow the runoff.
• Build a tall concrete flood wall along the river bank.
• Use planning rules to stop new houses being built on the flood plain.
• Straighten and deepen the channel with heavy machinery.
• Construct a large dam and reservoir upstream.

Manage the flood risk

Recommend a flood-management approach for each place, and think about why.

• A historic city centre, already crowded with buildings, floods regularly. Which is most realistic?
• A council wants a cheap, sustainable, long-term scheme for a rural catchment. Which fits best?
• A developer wants to build new homes on flat land right beside the river. What is the wisest planning decision?

Evaluate the options

For a named river such as the River Exe, evaluate hard and soft engineering as ways to manage flood risk.

• Name the river and describe one way it is at risk of flooding
• Give one hard-engineering method, with an advantage and a disadvantage
• Give one soft-engineering method and an advantage of it
• Reach a justified conclusion about which approach is better here