Managing River Floods Natural Physical Causes of Floods Human Causes of Floods Factors that increase the risk of floods becoming a disaster Flood management techniques

Natural Causes of floods Prolonged periods of heavy rainfall Over a long period of time meaning that soils become saturated. High intensity rainfall meaning that the water cannot infiltrate the soil or drain away quickly. Heavy rainfall can be very localised leading to local ‘flash floods’ as in the above picture.

Natural Causes of floods Extreme weather systems such as tropical cyclones or thunderstorms. Extreme weather tropical cyclones (aka hurricanes) are characterised by heavy rainfall. This exceeds soil’s infiltration rate resulting in overland flow and sudden flash floods

Natural Causes of floods Sudden snow or ice melt Sudden snow melt results in heavy runoff. This increases the risks of floods as the meltwater flows into river channels.

Natural Causes of floods Rain falling on saturated soil. When soil is saturated, overland water flow into rivers occurs.

Natural Causes of floods Large areas of low lying flat land When the land is flat the flood waters spread extensively causing widespread damage. Eg Ganges Delta - Bangladesh

Natural Causes of Floods High discharge where two or more tributaries meet at confluences Rising tides preventing the discharge of water into the sea may lead to ‘back-up’ the river.

Human causes of floods Large scale deforestation Change in land use from woodland to farmland Large scale ‘urbanisation’ into the drainage basin. Poorly constructed/maintained drainage system Building on floodplains Ineffective or poorly maintained flood protection scheme. Other forms of river management Untimely release of water from dams/dam failure, earthquakes, landslides or storms Sedimentation of river channels due to soil erosion

Factors that increase the risk of a flooding disaster Lack of awareness of the flood risk. Lack of preparation. Lack of disaster planning – Inadequate evacuation, emergency transport Lack of emergency supplies – medication, water purification tablets. Contamination of drinking water Loss of power supplies/fuel. Disruptions or damage to transport infrastructure. Damage to crops, loss of livestock Dead bodies and animals increase risk of disease. Flooding of key buildings eg hospitals.

Flood Management Techniques Planning regulations – control building on flood plains. Build flood defences Re-afforestation Diversionary river channels or spillways. Relocation of high risk settlements Drainage basin modelling Flood forecasting and development of warning systems. Temporary flood defences eg sand bags, gates. Flood shelters Buildings flood proofed eg built on stilts. River restoration projects Emergency planning Government and or overseas AIDS.