A river flood occurs when the water level exceeds the top of the river banks and flows onto the floodplain. The floodplain is the land alongside the river that floods regularly, depositing silt. Flood hydrographs show the relationship between rainfall and river discharge over time, with lag time being the delay between peak rainfall and peak discharge.

1. Flooding What is meant by a river flood? A river floods when the level of the water exceeds bankfull and flows onto the floodplain.

2. Floodplain Rivers regularly flood. When they do so they deposit some of their load. This material is known as silt . The land on either side of a river that floods is known as its floodplain. Floodplain Higher land

3. Floodplain Notice how brown the water is. It is this colour because of the silt it is carrying..

4. Our school is on the floodplain of the River Thames but is protected by flood defences Edge of floodplain The Thames Dagenham Park Dagenham Heathway

5. Floodplain worksheet Complete the floodplain worksheet

6. Flood hydrographs These are used to show the relationship between rainfall and the discharge of a river Discharge is the volume of water flowing past a point in a river during a period of time. It is usually measured in cubic metres per second i.e. cumecs

7. TIME DISCHARGE (cumecs) FLOOD HYDROGRAPH

8. TIME days or hours DISCHARGE (cumecs) FLOOD HYDROGRAPH Bankfull level Lag time Peak discharge Peak rainfall

9. Key terms Peak rainfall Peak discharge Lag time Bankfull level Write out simple definitions of these terms.

10. A hydrograph shows us the difference between the peak rainfall and peak discharge. This difference is called lag time . The greater the lag time, the less chance there is of a flood; while a short lag time shows that water has reached the river channel quickly. A rise in discharge on the graph is called the rising limb ; the falling limb shows a decrease in discharge. The flood hydrograph below shows: rainfall peaking at six hours discharge peaking at 11 hours lag time - the time between peak rainfall and peak discharge - is five hours.

11. There is a lag time because ___________ __________________________________ __________________________________ Extension for the clever ones There are two peaks to my hydrograph because __________________________ __________________________________ __________________________________ __________________________________

12. Deforestation and flooding Rock Trees River Soil

13. When it rains water falls to the ground. In a forested drainage basin the trees ___________________________ _______________________________________________ _______________________________________________ _______________________________________________ _______________________________________________ _______________________________________________ _______________________________________________ Less water reaches the river and the water that does arrive takes a long time to reach it. This means that the river has A chance to discharge the rainwater and so flooding is less likely. intercept transpiration roots

14. Rock Concrete Drains Urbanisation and flooding River

15. TIME DISCHARGE (cumecs) FLOOD HYDROGRAPH Bankfull level Lag time Peak discharge Peak rainfall

16. Explain why there a time lag between peak rainfall and peak discharge? Use your diagram of the drainage basin to help you.