Why do Rivers Flood and How Can They be Managed? Objectives: Explain the causes of flooding and describe the effects of the flooding on the environment and people

How can we predict a flood? STORM HYDROGRAPHS Draw you own graph Use a ruler, Be accurate, Add Labels Copy diagram at base of page 296

What are Hydrographs? The amount of water in a river at any given point and time is known as the discharge which is measured in cumecs (cubic metres per second). Hydrographs show river discharge over a given period of time and show the response of a drainage basin and its river to a period of rainfall. A storm hydrograph shows how a river's discharge responds following a period of heavy rainfall.

What are Hydrographs? The flood is shown as a peak above the base (normal) flow of the river. Analysis of hydrographs can help hydrologists to predict the likelihood of flooding in a drainage basin. Lag time is how long it takes the river level to respond to the extra water from a storm. The time between peak rainfall and peak discharge.

What are Hydrographs? Rivers with a short lag time respond rapidly to rainfall events and are therefore more prone to flooding than rivers with a longer lag time River discharge does not respond immediately to rainfall inputs as only a little of the rainfall will fall directly into the channel. The river will start to respond initially through inputs from surface runoff (the fastest flow of water) and its discharge will later be added to through inputs from throughflow and groundwater flow.

Your Turn... Read page 280 of Waugh – New Wider World Answer questions 3a – 3c page 297

Flood Case Study 1: Boscastle 2004 Watch this 5 minute clip of the floods http://www.youtube.com/watch?v=F8Fy6dYRp5w In August 2004 a Cornish Village called Boscastle suffered a ‘flash flood’ The River responded rapidly to a large rain storm Your task is to identify why. – Use the A3 sheet provided Read the account and complete the text boxes to the side

Flood case study 2: Flooding in the news Queensland Australia, Jan 2011 Why did it rain: http://www.youtube.com/watch?v=F8Fy6dYRp5w Initial impacts: http://www.youtube.com/watch?v=c-BEMTwh4hI Flood worsens: http://www.youtube.com/watch?v=NO8fPW2lY2c&NR=1&feature=fvwp

What are the impacts of flooding? Social Impacts Economic Impacts Environmental Impacts

Outcomes: I can explain how flooding is caused I can explain the shape of a hydrograph I can categorise the impacts of flooding Next Lesson – How do humans increase flooding and How do we manage floods?