River Flooding for IGCSE A. Ramdial

What do you need to know? Demonstrate an understanding that rivers present hazards and offer opportunities for people. Causes of hazards including flooding and river erosion Opportunities of living on a floodplain, a delta or near a river Explain what can be done to manage the impacts of river flooding Case Study required: the opportunities presented by a river, the hazards associated with it and their management

Key Words Flood: A discharge great enough to cause a body of water to overflow its channel and submerge (flood) the surrounding area. A submerging of usually dry land by a large amount of water that comes from an overflowing river or lake, an exceptionally high tide, melting snow or sudden excessive rainfall

Key Words Flood Control: The regulation of excessive run off of water in order to prevent inundation of the land e.g. by the building of river barrages; the deepening of existing and the cutting of new channels to speed river flow; the making of temporary storage basins; the conservation or planting of vegetation to slow down run off. The building of sea walls etc. to prevent inundation by exceptionally high tides.

Key Words Flood Hazard: The dangerous chance of the inundation of dry land by water from: an overflowing river or lake (e.g. caused by a break in a dam) melting snow sudden excessive rainfall an exceptionally high tide.

Study Questions What are the different causes of flooding? Why are some places more susceptible than others? What is climate change and how is it related to flooding? Are there positive results from some flooding?

Physical Causes of Flooding Rain Ice Melt Snow Melt Climatological Estuarine interactions between stream flow and tidal conditions Coastal storm surges Causes of flood Part- climatological Earthquake Landslide Dam Failure Other

of flood/intensifying Human Causes of Flooding More rapid discharge in urban areas due to impermeable surfaces and an increased number of drainage channels Urbanisation and urban growth (Increases the amount of impermeable surfaces) Human-related causes of flood/intensifying Floodplain development (increases the risk of damage) Bridges, dams and other obstructions Changes in vegetation cover e.g. agriculture River engineering works e.g. levées Human-induced climate change

Causes of Floods in Urban Areas

Factors that increase the risk of flooding Do we remember these factors from flood hydrographs? What did they cause?

Types of Flooding River (Fluvial) Flooding: occurs when the river has burst its banks. It is due to sustained or intense rainfall that has run off the land surrounding the river (this area is called the river valley and its boundary is called the watershed). River flooding is made worse when river bank defenses fail; it is also made worse by concrete (as rain can’t soak into the soil and more heads to the river).

Types of Flooding Coastal Flooding: this can sometimes happen when there is a high tide and a storm is blowing at the same time. Winds drive the high tide further inland than usual. Even if there is no high tide, the strong winds found in tropical storms and hurricanes can push sea water onto the land as a storm surge.

Types of Flooding Coastal Flooding con’t: Tsunamis are very large sea waves caused by earthquakes at sea; the readjustment of the crustal plates jolts the seabed by several metres and displaces hundreds of cubic kilometres of sea water which form waves moving out from the earthquake's epicentre. In deep water, the waves move quickly but as they approach the coastline the sea bed in shallow water slows the waves, causing them to increase in height. undersea volcanoes and; underwater landslides

Types of Flooding Pluvial (Surface Water) Flooding (Ponding): This occurs when an extremely heavy shower saturates urban drainage systems and the excess water cannot be absorbed. The excess water will collect in hollows and depressions where homes are located causing local floods to occur. This can happen far from a rivers and other natural watercourses. Lots of persons are at risk from pluvial flooding and don’t realise it. Blocked drains and sewers make things worse as rain cannot drain away.

Study Questions What are the different causes of flooding? Why are some places more susceptible than others? What is climate change and how is it related to flooding? Are there positive results from some flooding?

Why do some places flood more than others? Some countries in the world are more prone to flooding than others. This includes islands formed from coral reefs which are relatively low-lying e.g. The Maldives, Kiribati, Seychelles. Small sea-level rises of 1 – 3 metres will make some island uninhabitable. http://www.businessinsider.com/islands-threatened-by-climate-change-2012-10?op=1

Why do some places flood more than others? There are other countries where large sections of the population live in low-lying areas close to major rivers such as floodplains and deltas. These countries also experience climates where intense, sustained rainfall is concentrated within a relatively short period of time viz. monsoon climates. During these times, rivers often flood as rainfall exceeds infiltration rates, the ground becomes saturated and human factors intensify flooding. Examples include India, Bangladesh and China

Why do some places flood more than others? Bangladesh, for example, lies on the Ganges-Brahmaputra Delta. 80% of the country is a floodplain. 70% of the country is less than 1m above sea-level. The Himalayas lie to the north. The country thus receives monsoon rains, mountain runoff from snow and ice melt and is experiencing rising sea-levels in the Bay of Bengal.

Study Questions What are the different causes of flooding? Why are some places more susceptible than others? What is climate change and how is it related to flooding? Are there positive results from some flooding?

What is climate change? Climate change, also called global warming, refers to the rise in average surface temperatures on Earth. An overwhelming scientific consensus maintains that climate change is due primarily to the human use of fossil fuels, which releases carbon dioxide and other greenhouse gases into the air. The gases trap heat within the atmosphere, which can have a range of effects on ecosystems including: rising sea levels, severe weather events, and droughts that render landscapes more susceptible to wildfires.

How is climate change related to flooding? Flooding occurs in a number of ways, and each may be affected by climate change. Surface water flooding occurs where heavy rainfall can't absorb into the ground or drain away. River flooding, closely linked to surface flooding, occurs when streams burst their banks. Coastal flooding results from high tides, storm surges and sea level rise.

How is climate change related to flooding? It has been predicted that climate change will cause: a rise in global sea-levels As temperatures rise, the sea will absorb heat from the atmosphere, causing it to expand and therefore creating sea level rises. Recent studies show that the ice sheets in Greenland and Antarctica are melting faster than the snow is replacing the mass. Land glaciers will continue to melt over the coming century which will increase the level of the seas. Sea level rises increase the risk of flooding and coastal erosion Some islands will be affected by sea level rises significantly and their habitats will be threatened. One example is of the small island - Tuvalu in the Pacific, which is already experiencing severe flooding which is damaging their homes and affecting drinking water. The islanders have already started to leave and the rest will have to do so in coming years if the trend continues.

How is climate change related to flooding? It has been predicted that climate change will cause: an increase in the occurrence of extreme weather events because: A warmer atmosphere holds more moisture According to the Intergovernmental Panel on Climate Change (IPCC), the atmosphere is about 0.75 degrees warmer than it was at the start of the century, which means it can hold 5% – 6% more moisture. This doesn't automatically mean more heavy rainfall for countries because complex weather patterns govern the amount, timing and distribution of rainfall. But it does mean that with more water in the atmosphere, the volume of rainfall may increase when it does pour.

How is climate change related to flooding? Climate change has contributed to a rise in extreme weather events - including higher-intensity hurricanes in the North Atlantic and heavier rainfalls across the country. Scientists project that climate change will increase the frequency of heavy rainstorms, putting many communities at risk for devastation from floods. As rains become heavier, streams, rivers, and lakes can overflow. Cities like New York City and Chicago, where older sewer systems carry sewage and rain water in the same pipes, are at greater risk for sewage spills. During heavy rains, these pipes cannot handle the volume of storm water and wastewater, and untreated sewage is often discharged into local waters where people swim and play.

How is climate change related to flooding? Flooding isn't just about rainfall; other human factors contribute as well. Flooding and extreme precipitation go hand in hand, but they aren't the same thing. While climate change may directly alter precipitation in some countries, flooding is a consequence of heavy rainfall which also has a human component. The rising cost of damages associated with flooding is a perfect example of this. The rise could be down to a number of factors, not simply the amount of rain that falls. People are getting better at reporting damages, but human activities have a major impact too.

How is climate change related to flooding? Flooding isn't just about rainfall; other human factors contribute as well. Changes in land use, such as building houses on flood plains and paving over natural surfaces, are making people more vulnerable to flooding. Deforestation can also have a large impact as upland forests can retain a lot of water. Removal of vegetation causes increased runoff and a greater risk to homes and people. Wetlands can also soak up a lot of moisture, but so many are now drained to make room for development that their disappearance also increases the risk of flooding.

How is climate change related to flooding? According to a report by the European Environment Agency, climate change could contribute more in the future, but for now, land use changes are the reason behind rising damage costs in the UK.

Study Questions What are the different causes of flooding? Why are some places more susceptible than others? What is climate change and how is it related to flooding? Are there positive results from some flooding?

Positive Effects of Flooding Renewal of Wetlands Floods contribute to the health of ecologically important wetland areas. Healthy wetlands promote healthy water supplies and even affect air quality. Floods inundate wetlands with fresh waste. They also carry and deposit nutrient-rich sediments that support both plant and animal life in wetlands. In addition, flooding adds nutrients to lakes and streams that help support healthy fisheries.

Positive Effects of Flooding Returning Nutrients to Soil Floods distribute and deposit river sediments over large areas of land. These river sediments replenish nutrients in topsoil and make agricultural lands more fertile. The populations of many ancient civilizations concentrated along the floodplains of rivers such as the Nile, the Tigris and the Yellow because periodic flooding resulted in fertile, productive farmland. The construction of the Aswan High Dam in Egypt prevented the Nile from flooding major population centers downriver, but it also depleted once fertile agricultural lands along the banks of the river.

Positive Effects of Flooding Preventing Erosion and Maintaining Land Mass Elevation Soil deposited by flood waters prevents erosion and helps maintain the elevation of land masses above sea level. The rapidly receding land of the Mississippi River Delta is a direct result of man-made flood controls and levees that prevent topsoil-replenishing sediments from being deposited in the delta.

Positive Effects of Flooding Recharge and Replenish Ground Water Many population centers depend upon ground water and underground aquifers for fresh water. Flood waters absorb into the ground and percolate down through the rock to recharge these underground aquifers, which supply natural springs, wells, rivers and lakes with fresh water.