Flooding. 5. What were the causes and effects of the floods on the River Ganges in 1998 and how have it since been managed?

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1 Flooding Key questions: 1. How are floods caused? 2. What are the effects of flooding? 3. How can the risk of flooding be managed? 4. What were the causes and effects of the floods of the River Severn in 2000 and how has it since been managed? 5. What were the causes and effects of the floods on the River Ganges in 1998 and how have it since been managed? Key vocabulary: Delta: A river formation at the mouth (where river meets the sea) of a river where deposition causes new land to be formed. Discharge: The amount of water flowing through a river, measured in cubic metres per second (cumecs). Groundwater flow: The flow of water through rocks. Hydrograph: A graph showing the discharge of a river over time. Causes of flooding The main cause of flooding is high levels of rain fall over a short period of time. Rock type can affect the chances of flooding. Areas with impermeable rocks will be more at risk from flooding and the ground will not soak up the rain. Urbanisation leads to more surfaces being made impermeable due to use of concrete. Water runs off quickly into gutters and reaches rivers much quicker. Vegetation cover will decrease the chances of flooding as the plants will take up the water through their roots. Tsunamis can cause devastating coastal flooding, tsunamis are caused by earthquakes which happen at sea and create giant waves. Strom surges also lead to coastal flooding. This is where a strong storm lifts up the sea water and causes sea levels to rise temporarily. 28

2 Deforestation means trees are cut down to create more farm land, this increases the risk of flooding as there is less vegetation to take up the water. Relief of the land; steep valley sides means rain water runs into the river in a short space of time. The river does not have time to deal with such a large amount of water in a short space of time. Activity 1. Sort out the causes of flooding above into natural and human causes. Natural Human Extra 2. Explain how climate change can lead to increasing frequencies of serious flooding. Flood hydrographs Flood Hydrograph show how a river's discharge responds following a period of heavy rainfall. On a hydrograph, the flood is shown as a peak above the base (normal) flow of the river. 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 supplemented through inputs from throughflow and groundwater flow. 29

3 Effects of flooding Short term effects include: Loss of life Crop damage Roads closed Water borne diseases spread Businesses forced to shut Electricity cuts Long term effects include: People s homes remain ruin for many months Lack of future investment in area House and business insurance rises or is refused Activity Sort the effects above into economic and social. Create a key to show your categorisation. 30

4 Flood prevention Type of management Advantages Disadvantages Damn water is held back against a damn wall and only released when the river has the capacity for it. Levees banks built along the side of the river to enlarge its capacity. Tree planting Trees are planted to soak up excess water from rain. Dredging material removed from river channel to higher its water capacity. Straightening / widening river channels - river is made wider and channels straighten. Flood plain clearance keeping flood plains free from housing and business so damage from floods in minimal. Sand bags bags of sand handed out to households at risk of flooding, will hold back water from seeping under doors. Can also be used to create electricity. Very reliable at reducing flooding. Fairly cheap method. Fairly cheap method. Provides more trees which gives habitats and takes up CO 2. Soft engineering favoured by environmentalists. Simple way to increase the amount of water a river can carry. Allows water to move quickly out of urban areas where effects from flooding would be high. Soft engineering method. Allows land at flood plain to be used for parks and farm land. Can be given out when needed. Fairly cheap method. Can be targeted to people at risk. Very expensive, prevents the deposit of minerals from river into farm land, flooding behind damn can lead to settlements being lost, danger of damn breaking. Prevents water re-entering the river when rain stops. Takes time for trees to be established. Can only soak up a finite amount of water. Needs to be done regularly to keep channel deep. Can lead to greater flooding down steam as water arrives there quicker. Can lead to greater flooding down steam as water arrives there quicker. Reduced land available for housing. Does not stop the actual flood so can still be dangerous to people in the area. Not that effective in stopping water from entering homes. Only a short term solution. Activity 1. Sort out above management strategies into hard and soft engineering options (review meanings from coastal management if needed). Create a key to show your categories. Extra 2. How are the water levels of the River Trent controlled (it is not one of the above!)? 31

5 Flooding on the River Severn in 2000 Case study The River Severn begins in the Cambrian Mountains in Wales and flows out through England and the Bristol Channel. It is the longest river in Great Britain. Some of the most serious effects of the 2000 floods were felt in the tourist town of Bewdley. Causes: Highest levels of rainfall in over 270 years Location in the West of the UK close to Wales leads to high levels of rainfall all year round Land use urban land use on the banks of the Effects: river, further afield mostly farm land, little forest Insurance costs rise Dredging ceased in the 1950s due to high costs 140 homes flooded damage Climate change blamed for increase in stormy to buildings weather conditions Loss of income for the area Narrow river channel in Bewdley from tourism and agriculture Flood prevention: Loss of personal belongings Due to the effects of the flooding shown above the for residents government saw the need to put in flood defence Transport links cut off mechanisms. The chosen method was a 2.7 metre high demountable flood wall. It acted in a similar way to levees, increasing the amount of discharge the river could carry before if would flood the outside areas. The wall was made demountable so it was not an Bewdley during eyesore to tourists when there was no flood risk and 2007 It can be erected in 24hours floods. It dramatically reduced the potential effects of the high river levels in Without the wall the Go to the following web pages for town would have flooded severely again. more The net benefits of the wall were calculated to be Flooding on the River Ganges 1998 Case 0.6 study million following an initial cost benefit ces/flooding/bewdley/whathappened analysis. 32

6 Background Between July and September 1998, Bangladesh suffered one of its worse ever floods. Despite being flooding being common in this country, the floods of 1998 were particularly severe resulting in over 1000 deaths and 30 million people being made homeless. Physical causes Effects Over two thirds of the land was underwater including Dhaka covered by 2m. 30 million made homeless with many more damaged homes. Official death toll of 1, 070. Deaths caused by; drowning, contamination of water leading to water board diseases e.g. cholera, lack of access to medical care for non-flood related conditions due to break down of transportation. Food supplies severely affected as farm land an crops are flooded. Economy suffers as businesses and factories are flooded, 20% drop in exports. Transportation cut off in many areas leading to relief efforts not being able to reach areas. Bangladesh is a low lying country with 70% of its land at less than 1m above sea level and 80% being on the floodplain of the Ganges and Brahmaputra rivers. Bangladesh is set on the drainage basins of the rivers Ganges and Brahmaputra and so is at risk of both these rivers flooding. Bangladesh has a monsoon climate meaning heavy annual rains which often result in the rivers exceeding their carrying capacity. In spring melting snow from the Himalayas further increases the amount of water in the two rivers and the chance of flooding. Human causes Population increase in Bangladesh, Nepal and India has led to more deforestation. Deforestation in areas of Nepal and India further upstream means that less water is intercepted and more ends up in the rivers. Deforestation has also led to more erosion upstream leading to silt being carried downstream. This is then deposited in the channel later on reducing the amount of water it can carry. Bangladesh is an LEDC and has high national debt. It therefore has little money to spend on flood defences and protection methods. Short term response International food aid. Volunteers and aid agencies try to rebuild flood defences in case of further flooding. Make shift medical centres set up. Possible long term solutions Creation of more levees (embankments which raise sides of river increasing its channel capacity. Constructing flood protection shelters in higher land to shelter people and animals in times of emergency. Emergency flood warning systems and Exam plans to practice be put in place 2003 and practiced annually. Providing medical stores in villages. Building flood proof storage shelters for food and emergency 33 supplies.

7 1. Study the map and news item below. Maximum and minimum discharge of the Parana River (at Corrientes) a) How did the maximum discharge of the Parana River change between 1951 and 2001? Refer to figures in your answer. (2) 34

8 b) Choose two of the causes given for increased flooding in the news article. For each cause, explain why it could increase flooding on the Parana River. (4) c) Study the map below which shows the location of Resistencia. i) What is being done to protect the town from flooding? (1) ii) Suggest two other ways of protecting the town from flooding. Explain how each would work. 35

9 (4) 2. Case study: A place where a serious flood has affected people and the environment. Name a place where you have studied a serious flood. Describe how the flood affected people and the environment. Explain what people could do or have done to prevent flooding from affecting them in the future. Named place: (8) 36