The Causes of Flooding Today we will: Consider the physical causes of flooding events. Explore how human impacts can intensify flooding events. Consider approaches to the management of flooding .

RECAP: Cause of UK floods 2007 On June 25th 2007 an unseasonably low pressure depression moved across England. The front settled across eastern England and more than 100 mm of rain fell in some places. High rainfall, coupled with high water levels in rivers from rainfall earlier in June, led to extensive flooding across the UK. The Midlands and Yorkshire were worst affected. Teacher’s notes: The Association of British Insurers estimated the cost of the damage due to flooding in June and July 2007 to be over £3 billion. According to the emergency services, the flooding caused the greatest number of search and rescue missions in the country since the Second World War, stretching resources to the limit. Since the flooding the Environment Agency have: built 54 new flood defences, increasing protection for over 38,000 properties established an Extreme Rainfall Alert pilot service with the Met Office shared maps with Local Resilience Forums, which identify areas susceptible to surface water flooding continued to raise public awareness of flood risk through information campaigns and guidance about what to do before, during and after a flood signed up 78,600 properties to a free flood warning service spent around £131 million maintaining 22,800 km of main river in England and Wales. Photo: © David Miller June 2007. This image is reproduced under the terms of the Creative Commons License. A copy of the license can be read at this address: http://creativecommons.org/licenses/by-sa/2.0/ Flooding continued until the end of July 2007. What do you think the cost of the flooding was? What was done in the aftermath?

Term Definition Lag time Loss of water from ground via a combination of evaporation and transpiration Discharge Plants, trees, or buildings blocking precipitation from reaching ground Velocity Volume of water flowing through a river channel Interception Amount of water a channel can hold Evapotranspiration Time required for ground water to reach channel Channel Capacity Filtering of water through the ground and the tiny pores in the soil Infiltration Loss of water through plants’ “breathing” water into atmosphere Overland Flow Speed of water flowing through a river channel Transpiration Flow of water on the surface of the ground

Interception

Causes of Flooding Physical Causes Human Causes Are there any interactions or feedbacks between causes? Red = links between physical causes Blue = links between human causes Annotate Green = links across physical/human causes Do you see any patterns to the causes of flooding? Hint: amount of water

TASK: Use pages 192 and 193 of the textbook to answer the following questions: Why is the flood risk rising in the UK? Describe the distribution of the 8 areas of the UK expected to be at risk of flooding by 2080 (Fig 17.1). Define the term Flood Return Period and briefly explain the impact of this on decision making in river valleys.

Factors affecting flood hydrographs Photos: drainage basin © Thierry Maffeis, shutterstock.com; human activity © stock.xchng; physical factors, NASA, the image was taken by the Aqua MODIS instrument on September 4, 2003. This photo is in the public domain.

Independent Study Complete the hydrograph lesson task (email by the same name). THIS IS COMPULSORY AND ESSENTIAL – failure to complete this by next lesson = CAUSE FOR CONCERN & phone home If you have not received the email, contact me ASAP. Geofactsheet 71

Heavy Precipitation, which exceeds infiltration capacity Heavy Precipitation, which exceeds infiltration capacity. Often as a result of large convectional thunderstorms in semi-arid areas. Storm Surges coming into river mouths and estuaries can cause a backlog in the river and lead to flooding. Snowmelt due to sudden temperature rise. Infiltration is often reduced by frozen soil magnifying the problem Prolonged rainfall, which saturates the soil. Overland flow leads to rapid water transfer. Soil type and geology determine how quickly soil is saturated.

Deforestation Urbanisation Deforestation reduces interception and evapotranspiration, leading to a shorter lag time, and more water reaching the river. Soil erosion increases and this aggrades the river bed, reducing channel capacity and making flooding worse. Urbanisation Urbanisation leads to less infiltration due to an increase in impermeable surface coverage through roads, roofing etc.; increase in run-off through drainage systems and sewers. This shortens the lag time and increases the peak discharge, often leading to flooding during heavy rain.

Channel Alterations Global Warming River channels are often straightened and channelised with concrete as part of urbanisation. This increases the velocity of the water in the river and quickly transports the water downstream from the urban area. Downstream areas are then put at an increased risk of flooding. Global Warming The warming of the atmosphere through global warming has led to the increased melting of the polar ice caps and glaciers. This has led to a rise in sea levels which increases the risks of flooding by slowing the discharge of flood waters into the sea. The increasing warmth has also caused the sea water to expand, further raising sea levels. This is also a factor in increasing risk.

Flood Management Rivers can be managed in an attempt to reduce or minimise flood risk. The two main approaches to this are either: Direct (hard engineering) such as building dams, raising levees artificially and flood overspill reservoirs. Indirect (soft engineering) such as Afforestation, land-use management, Planning and control of urbanisation, flood warning systems. Both have strengths and weaknesses often a sustainable combination works best.