Flooding and River Management These icons indicate that

Flooding and River Management These icons indicate that teacher’s notes or useful web addresses are available in the Notes Page. This icon indicates the slide contains activities created in Flash. These activities are not editable. For more detailed instructions, see the Getting Started presentation. 1 of 27 © Boardworks Ltd 2005

Did you know? Why should we study flooding? 2 of 27 © Boardworks Ltd 2005

Learning objectives 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? 3 of 27 © Boardworks Ltd 2005

What is a flood? A flood occurs whenever a river overflows its banks (exceeds its ‘bankfull discharge’). However, a flood becomes a problem when the water rises to a level where it threatens property and/or life. 4 of 27 © Boardworks Ltd 2005

Learning objectives 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? 5 of 27 © Boardworks Ltd 2005

Why do rivers flood? Rivers usually flood due to a range of physical factors. These physical factors can be divided into climatic factors and drainage basin characteristics. Human intervention can also make flooding worse. A major flood devastated the village of Lynmouth in Devon in August 1952. 229 mm of rain fell in 24 hours. The impact of this was increased because narrow bridges across the River Lyn acted as dams and stopped the water flowing away quickly. 6 of 27 © Boardworks Ltd 2005

Physical causes of river flooding 7 of 27 © Boardworks Ltd 2005

Learning objectives 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? 8 of 27 © Boardworks Ltd 2005

The Boscastle flood, 16 th August 2004 Boscastle is situated on the North Cornwall coast. For a hundred years the village was a thriving port but the coming of the railways soon saw its decline as a trading port. About 90% of Boscastle’s economy is now reliant on tourism. On Monday 16 th August 2004 major flooding occurred in North Cornwall. The small village of Boscastle was devastated. 9 of 27 © Boardworks Ltd 2005

What were the causes of the Boscastle Flood? Climatic conditions were the main reason for the floods in Boscastle. 200 mm fell over Ottersham Moor, to the east of Boscastle, within a 4 hour period. This was combined with coastal winds and a rising tide. The ground was also already saturated from previous rainfall events. Explain why these conditions would cause major flooding. 10 of 27 © Boardworks Ltd 2005

What factors increased the risk of flood? 11 of 27 © Boardworks Ltd 2005

What factors increased the flood risk? 12 of 27 © Boardworks Ltd 2005

Examination Question Explain the causes of a flood event that you have studied. 13 of 27 © Boardworks Ltd 2005

Learning objectives 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? 14 of 27 © Boardworks Ltd 2005

Effects of the Boscastle flood 15 of 27 © Boardworks Ltd 2005

Effects of the Boscastle flood 16 of 27 © Boardworks Ltd 2005

Effects of the Boscastle flood 17 of 27 © Boardworks Ltd 2005

Short- and long-term effects 18 of 27 © Boardworks Ltd 2005

Learning objectives 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? 19 of 27 © Boardworks Ltd 2005

How can flood risk be managed? 20 of 27 © Boardworks Ltd 2005

Dams on the Rhône Dams can be multi-purpose: They hold back the spring meltwater and so stop the floods. This is a dam generating HEP near Beauchastel. What are the disadvantages of building dams? 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). 21 of 27 © Boardworks Ltd 2005

Floodplain zoning 22 of 27 © Boardworks Ltd 2005

After the Lynmouth floods in 1952 What action was taken after the Lynmouth floods in 1952? concrete banks revetments wider channel Study the photograph. Explain how the changes will help to prevent flooding. 23 of 27 © Boardworks Ltd 2005

How should the flood risk be managed? Which scheme received the best score? Explain why. 24 of 27 © Boardworks Ltd 2005

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. 25 of 27 © Boardworks Ltd 2005

Test your knowledge of river flooding? 26 of 27 © Boardworks Ltd 2005

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. 27 of 27 © Boardworks Ltd 2005