1. Solutions to flooding Key idea: There is discussion about the costs and benefits of hard and soft engineering and debate about which is the better option
2. Lesson objectives <ul><li>To be able to describe hard engineering strategies – dams, reservoirs and channel straightening </li></ul><ul><li>To be able to describe soft engineering strategies – flood warnings, preparation, flood plain zoning, do nothing </li></ul><ul><li>To be able to explain the costs and benefits </li></ul><ul><li>To evaluate which is the better option and understand situations where one solution outweighs the other. </li></ul><ul><li>Textbook reference: </li></ul><ul><li>Pg 117 </li></ul>
3. What causes flooding? Impermeable rock deforestation Arable farming Building on floodplains Heavy rainfall Lots of tributaries Marshy areas Very wet soil Hard dry soil steep slopes
4. Recap … What factors increase The risk of flooding?
5. What factors increase The risk of flooding? * Impermeable rock * Hard dry soil * Very wet soil *steep slopes * Cutting down trees Building in the drainage basin * Many tributaries
6. Reducing The Risk? <ul><li>What is the difference between prevention and protection? </li></ul><ul><li>Methods of prevention? </li></ul><ul><li>Methods of protection? </li></ul><ul><li>Hard / soft engineering </li></ul>
7. Starter activity What do we mean by river basin management? What do you think is meant by ‘hard engineering’ and ‘soft engineering’? In pairs discuss the questions What strategies for flood control are there?
8. Flood protection *Dams hold back flood waters *reservoirs store floodwater *straighten channels Increases speed of flow *Dredging Makes channel deeper so can hold more water *levees and embankments Prevents river from overflowing *land use zoning Restricts development to uses unaffected by flooding *afforestation Increases interception etc, reduces run off
9. Flood management strategies <ul><li>The aim of flood management is to protect homes, businesses and the environment from flooding </li></ul><ul><li>Hard engineering defences involve the use of technology to control rivers - building structures that reduce flooding: </li></ul><ul><ul><li>Dams and reservoirs - designed to store water and therefore control the discharge of the river. </li></ul></ul><ul><ul><li>Levees - increases the height of the river banks and therefore the river can contain more water </li></ul></ul><ul><ul><li>Dredging the channel – keeps the channel free from sediment so more water can flow through it. </li></ul></ul><ul><ul><li>Straightening rivers - increases the speed of the river to remove water from affected areas. </li></ul></ul><ul><ul><li>overflow channels which allow rivers to flood areas of unused land or areas which do not cause much damage. </li></ul></ul><ul><ul><li>Building flood walls </li></ul></ul><ul><ul><li>Hard engineering approaches tend to give immediate results and control the river but are expensive </li></ul></ul>
10. General disadvantages of engineering defences <ul><li>Expensive to build and maintain, need technical skill </li></ul><ul><li>Floods happen less often – but when they do occur they tend to be more hazardous … imagine if a dam fails </li></ul><ul><li>Natural processes are disrupted – crops don’t get fertile silt </li></ul><ul><li>Ugly structures </li></ul>
12. Dams They’re very expensive Land is flooded when a reservoir is created. Farmland may be destroyed People may be forced to move elsewhere Wildlife may be affected – fish migrating upstream to breeding grounds Dams trap sediment normally carried in rivers. This can cause the dam to fail. It can also cause increased river erosion downstream as there is less sediment being deposited. Turbines are often built into the dams which generate electricity Steady water release allows irrigation of land below the dam throughout the year. People can use the reservoir for recreational activities e.g. sailing Dams are huge walls built across rivers. A reservoir is formed behind the dam. Floodwater is caught by the dam which prevents flooding downstream. The water is released as a steady flow throughout the year Disadvantages Extra benefits How it works
13. To control the water level <ul><li>Dam and pumping station </li></ul>
14. Channel straightening Flooding may happen downstream instead as flood water is carried there faster. More erosion occurs downstream because the river flows faster Altering river channels disturbs wildlife habitats It takes less time to navigate the river because it has been made shorter Meanders are removed by building artificial cut- throughs. This makes the water flow faster which reduces flooding because water drains downstream more quickly and doesn’t build up to a point where the channel can’t contain it any more Disadvantages Extra benefits How it works
15. Channel Straightening
16. Alter the river’s channel
17. Levees They’re quite expensive There’s a risk of severe flooding if the levees are breached …. Think back to New Orleans They allow the floodplain to be built upon Levees are embankments built along rivers. The river can hold more water without overflowing and so it floods less often disadvantages Extra benefits How it works
18. Diversion spillways An increase in discharge when the diverted water joins another river (or rejoins the same one) could cause flooding below that point. If spillways are overwhelmed water will flood areas not used to flooding which could cause even bigger problems These are channels that take water elsewhere if the water level in the river is too high. Water is normally diverted around an important area or to another river. They prevent flooding because river discharge is reduced. The spillways often have gates that can be opened so the release of water can be controlled disadvantages Extra benefits How it works
19. How can the risk of flooding be reduced? <ul><li>Alter the RIVERS CHANNEL </li></ul><ul><li>Straighten it – to speed up flow of water </li></ul><ul><li>Widen and deepen it </li></ul><ul><ul><li>Levees and embankment </li></ul></ul><ul><ul><li>Dredging </li></ul></ul><ul><li>Build BARRIERS </li></ul><ul><li>Build up the banks with earth or concrete to make embankments to keep water IN </li></ul><ul><li>Build flood walls to keep water OUT </li></ul>
20. Build barriers
21. Flood barriers in Bewdley
22. Straightening meanders <ul><li>This represents a small scale approach to managing rivers. </li></ul><ul><li>Water in a meander takes longer to clear an area than water in a straight section of a river. </li></ul><ul><li>A possible solution to flooding in areas where there are many meanders is to straighten them artificially. </li></ul><ul><li>The river is made to follow a new shorter straight section. </li></ul>
23. Channel Diversion
24. Levee on the Mississippi Note the school bus for scale and the seepage along the side of the levee.