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Flooding and River Management


What is a river flood?

Why do rivers flood?

What caused the Boscastle flood?

What were the effects of the Boscastle flood?

How can the flood risk be managed?

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A flood occurs whenever a river overflows its banks (exceeds its ‘bankfull discharge’).

What is a flood?

However, a flood becomes a problem when the water rises to a level where it threatens property and/or lives.







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Rivers usually flood due to a range of physical factors.These physical factors can be divided into climatic factorsand drainage basin characteristics. (Human intervention can also make flooding worse).


For example – A major flood devastated the village of Lynmouth in Devon in August 1952. 229mm of rain fell in 24 hours. NOTE: The impact of this was increased because narrow bridges across the River Lyn acted asdams and stopped the water flowing away quickly.


Physical causes of river flooding







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The Boscastle flood, 16th August 2004

Boscastle is situated on the North Cornwall coast.

About 90% of Boscastle’s economy is now reliant on tourism.

On Monday 16th August 2004 major flooding occurred in North Cornwall. The small village of Boscastle was hit hardest – it was devastated.


What were the causes of the Boscastle Flood?

Climatic conditions were the main reason for the floods in Boscastle. 200mm fell over Ottersham Moor, to the east of Boscastle, within a 4 hour period. This was combined with - 1. Onshore coastal winds; 2. A rising tide; and 3. The ground was also already saturated from previous rainfall events.

Explain why these conditions would cause major flooding.


What human and physical factors increased the risk of Boscastle flooding?


Physical or human factors (sorting task)


Examination Question

Explain the causes of a flood event that you have studied.







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Effects of the Boscastle flood


What were the short - and long-term effects?







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How can river flood risk be managed?


Dams on the Rhône

This is a dam generating HEP near Beauchastel.

Dams can be multi-purpose:

They hold back the spring meltwater and so stop the floods.

They release this water in the dry summer to irrigate the intensive fruit and vegetable growing areas. The water is also used for cooling in the two nuclear power plants near Genissiat – Beugy and Creys-Malville.

In addition, the dams produce hydroelectric power (HEP).

What are the disadvantages of building dams?


Floodplain zoning


After the Lynmouth floods in 1952

revetments

widerchannel

concrete banks

Study the photograph.

Explain how the changes will help to prevent flooding.

What action was taken after the Lynmouth floods in 1952?


How should the flood risk be managed?

Which scheme received the best score? Explain why.


Examination question

Study the diagram of a flood management scheme.

Explain why the meander loop has been dredged.

List two other ways of managing river flooding.


Test your knowledge of river flooding?


Key ideas

A river floods when it exceeds its bankfull discharge.

Flooding can be a result of both physical and human factors.

Physical factors include climatic conditions and drainage basin characteristics.

The effects of flooding are short-term and long-term.

Flood management schemes can aim to prevent flooding or reduce its impact.

Flood management schemes can have both positive and negative effects.

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