P5 Flashcards
A number of physical factors lead to very high flows of water in a drainage basin. If the discharge is of sufficient quantity to cause a body of water to overflow its channel and submerge the surrounding land, flooding is deemed to have occurred.
There are a number of environments which are more at risk:
• Low-lying parts of flood plains and river estuaries.
These are not only subject to river flooding, but also to groundwater flooding after the ground becomes saturated from prolonged heavy rainfall.
• Where low-lying areas are partially urbanised with impermeable surfaces, there is a greater danger of temporary surface water flooding as intense rainfall has insufficient time to infiltrate the soil, so flows overland.
• Small basins, especially in semi-arid and/or arid areas, are subject to flash flooding. These are floods with an exceptionally short lag time - often minutes or hours - which are therefore extremely dangerous. They are usually associated with very intense convectional storms, so again infiltration, especially on semi-impermeable surfaces, and steep, unvegetated slopes, is very limited, allowing surface overland flow to develop very rapidly.
Groundwater flooding:
Flooding that occurs after the ground has become saturated from prolonged heavy rainfall.
Surface water flooding:
Flooding that occurs when intense rainfall has insufficient time to infiltrate the soil, so flows overland.
Flash flooding:
A flood with an exceptionally short lag time - often minutes or hours.
Causes of flooding
Physical factors
The primary causes of floods are either meteorological (short-term weather events) or longer-term climatic causes such as rainfall patterns.
In areas such as the UK, the usual cause of flooding is the prolonged and heavy rain associated with the passage of low-pressure systems or depressions. The traditional time of year for this sequence, known as a progressive cycle, is autumn or early winter but, as a result of unusual positions in the jet stream, this sequence can occur at other times of the year (for example the summer floods of 2007 and 2008). The degree of flooding also depends on the precise depression sequence - sometimes a succession of very intense storms, as occurred in the UK from October to December 2015, has a cumulative effect on the drainage system. This resulted from a very sinuous jet stream in a fairly constant track, which means that all high-pressure systems (anticyclones) were ‘blocked’. Data for Northern England from the Centre for Hydrology and Ecology reported many rivers with flows up to 50 times higher than normal, some experiencing their highest ever recorded flows.
In other areas, in particular southern and eastern Asia, intense seasonal monsoonal rainfall can result in widespread, damaging flooding. Around 70 per cent of the average annual rainfall occurs during 100 days - usually from July to September. The low-lying plains of the larger rivers in India, Pakistan, Bangladesh and Vietnam, as well as China, are most at risk. Around 80 per cent of Bangladeshi people are exposed to flood risk.
As Figure 2.11 shows, there are a variety of flood types in Bangladesh, with some areas affected by more than one type of flood. The highest flood risks are along the river courses and at the edge of the delta. This is hardly surprising as half of the country is less than 12.5 m above sea level. Pakistan suffered disastrous floods in July 2010. The primary cause was the 9000 mm of water received in one week, about ten times the yearly average. Local flash floods and landslides also contributed to significant flooding on all major rivers downstream.
Excess rainfall across large river basins is also associated with tropical cyclones (mainly late summer in the sub-tropics). In Southern Africa regular rainfall is sometimes supplemented by tropical cyclones, for example in Mozambique in 2000, when there was significant flooding.
Snow and ice are responsible for many food events, usually in higher latitudes or mountainous areas.
Melting snow in late spring regularly causes extensive flooding in the continental interiors of Asia and America. The great north-flowing Siberian rivers, such as the Ob and Yenisei, cause vast annual flooding in the plains of Siberia. The quick transition from winter to spring upstream causes rapid snow melting, while their lower reaches remain frozen, with very limited infiltration. Flood water is often held up by temporary ice dams. Sometimes rain falls on melting snow when a rapid thaw occurs and this combination can cause heavy flooding. In the UK, spring floods in York are frequently intensified by rapid snow melt in the higher parts of the River Ouse catchment.
In the Himalayas, glacial outburst floods occur as ice dams melt, leading to catastrophic draining of glacial lakes. Sometimes the flooding is exacerbated by landslides or earthquake-induced dam failure. In Iceland, glacial outburst floods are particularly frequent because of volcanic activity, which generates melt water beneath the ice sheets and acts as a trigger for ice instability and the sudden release of melt water, known as a jökulhlaup.
Floods frequently occur in estuarine areas as very high river flows interact with high tidal conditions or coastal storm surges, so the causes here are only partly climatological.
As well as climatic causes of flooding (primary factors) there are also secondary physical factors which affect flooding levels, which tend to be basin specific (Figure 2.12). This makes certain river basins more flood prone than others. Geology, soil, topography and vegetation all play an important role as they combine with precipitation characteristics to determine key features of a flood, such as speed of onset, peak flow and flood duration.
Humans and flooding
There are a number of human actions that can exacerbate flood risk. These are summarised in Figure 2.13.
A combination of economic growth and population movements throughout the twentieth century have caused many flood plains to be built on and many natural landscapes to be modified for agricultural, industrial and urban purposes.
Of these changes, many researchers have suggested that urbanisation is the key factor, for the following
reasons:
• Creation of impermeable surfaces - roofs, pavements, roads and car parking space. It has been estimated that in London, the land taken up by car parks is around 25 square kilometres as around 60 per cent of residents have paved over their front gardens for car parking.
• Speeding up the drainage of water in built-up areas via artificial conduits, e.g. drains and sewers.
• Impeding channel flow by building alongside the river, e.g. building bridge supports and carrying out structural engineering by building levees.
• Straightening channels (realignment) to increase the flow, which results in flooding downstream
- this could be regarded as mismanagement in some ways. Resectioning by dredging involves widening and deepening the channel to increase efficiency by increasing capacity and moving water away at a faster rate - but at considerable environmental cost.
• Changing land use associated with agricultural development. Deforestation, overgrazing, ploughing or drowning wetlands usually occurs upstream from urbanised flood plains, which has a knock-on effect downstream with increased run-off and increased levels of sediment (which are washed into rivers and block river channels).
The fact that urbanisation is concentrated on lower-lying land within drainage basins (especially on flood plains) means that natural and human factors coincide, which enhances both the frequency and the magnitude of flood risk. In 2015 the causes of recurrent flooding in areas in which flood defences were built only a decade ago (Carlisle, Cockermouth), and how to make them safe from future flooding, became a major topic for discussion in the UK and EU. It has also been noted that some of these flooding events (Cumbria) seem to have been of a higher magnitude than ever experienced previously.
In the British media both public and social blame for the floods of 2015 has been allocated to:
• extreme weather induced by climate warming
• budget cuts in the amount of money being spent on flood defences
• the green priorities of the EU Water Framework Directive, which puts environmental concerns before regular maintenance (i.e. dredging), although it does advocate making space for water to flood lowland
areas
• poor land management, for example blocking ditches or improving pasture then overgrazing it.
Skills focus: Calculating flood frequency
The size of the largest flood event for each year for a particular location is placed in rank order, with Rank 1 being the largest for all available records for any given location.
The following calculation is applied to calculate the time interval between floods of similar size:
T = n+1/ m
Where
• T = recurrence interval
• n = number of years of observation
• m = rank order
The calculated recurrence level indicates the number of years within which a flood of this size might be expected.
However, it is a probability based on existing historic evidence and does not mean that similar floods will now occur more or less frequently, as with climate warming increased frequency is the more likely option. The recurrent interval is expressed as a 1 in 25, 50, 100, 500, 1000… event. The floods of highest magnitude will have much longer return periods: while they have the highest impact, they may be less likely to occur.
Flood return periods are average recurrence intervals. It is possible to get two ‘100 year’ floods very close together.
This happened on many English rivers in 2007 and 2012.
However, over centuries such floods should occur on average once every 100 years.
Urbanisation:
The increase in the number of people living in towns and cities compared to the number of people living in the countryside.
Key concept: Understanding flood risk and return periods
Like all hazards, flooding has a frequency and a magnitude, both of which are important in assessing the risks involved. The flood return period, also known as the flood recurrence interval, is an estimate of the likelihood of a flood of a certain size recurring. A flood likely to happen once in ten years has a ten per cent chance of happening in any one year. However, this is not a forecast and
such a flood may happen more than once in the same interval or may not occur at all.
A river may flood on average every two or three years, with significant flooding only every 50 to 100 years. One way to illustrate this is shown in Figure 2.14. Floods may only reach the furthest edge of the floodplain once in 500 years.
The impacts of flooding
Floods are a common environmental hazard due to the widespread distribution of river flood plains and other low-lying areas. In the period between 1990 and 2010, the Emergency Events Database (EM-DAT) recorded over 3000 flood disasters worldwide. These were collectively responsible for 200,000 deaths and 3 billion people (around half of the world’s population) being adversely affected - note this total includes many who were affected several times. Around 900 million people live in flood-prone areas and, of these, up to 75 million are exposed to flooding every year.
About 90 per cent of all flood deaths and 50 per cent of the economic damage occurs in Asia, notably in China, India, Bangladesh, Pakistan and Vietnam.
However, damaging floods are not confined to emerging nations. Floods are the most frequent environmental disaster in Europe, and while fatalities are usually under 30 for each event, damage costs are very high. For instance, a cost of £1.3 billion has been quoted for the most recent of the series of flood events in the UK in December 2015. In Australia, average annual flood losses are A$372 million.
Socio-economic impacts
The degree of threat posed by a flood event depends on the depth and velocity of the water, the duration of the flood, and the quality of water (sediment load, presence of raw sewage, toxic chemicals, pollution from oil, etc.).
Research on the impact of river depth and velocity suggests that water 0.5 m deep can wash cars away, and that foundations of buildings start to collapse at velocities of 2 m per sec. Stresses on structures such as bridges are also very closely related to depth and velocity.
Flood depth also has a very clear link to mortality. In many developing countries, people have not learned to swim, and can be also be killed by poisonous snakes in the flood waters. Children and old people are particularly vulnerable. Post-flood morbidity is also extremely likely in low-income countries, mainly from water-borne diseases, which are secondary flood hazards. In developed countries, psychological stress is very common among flood victims.
Floods can affect people’s livelihoods in many ways.
Direct structural damage to property in countries at all stages of development is the major cause of tangible food losses. There are all sorts of concerns post-fooding about getting flood insurance against future events, and also coping with reduced property values when a flood-prone property is resold.
Crops, livestock and agricultural infrastructure suffer major damage in intensively farmed rural areas. Where farming is subsistence, there is a direct loss of food supplies and famine can occur. In more developed countries (MDCs) floods can lead to escalating food prices as shortages of key products occur, as in the Big Dry in the Murray-Darling Basin of Australia in 2006. In Cockermouth, Cumbria, the destruction of a key bridge connecting different parts of the town made communication and transport very difficult. In addition, flooded electricity substations meant that many people endured power shortages for up to three days.
Infrastructural losses are often extremely high in mega cities such as Mumbai, as growth has outstripped flood defence systems.
A further loss of livelihood can occur when services and businesses are flooded - often putting them out of action for up to six months. In Carlisle, the McVitie’s biscuit factory was flooded, leading to the temporary loss of over 1000 jobs.
Many areas that flood earn substantial income from tourism. Negative images of flood-affected areas lead to many cancellations in the short term - except for keen geography teachers and students out taking pictures of floods! Cumbria experienced a drop in tourism for up to a year after the floods.
Environmental impacts of flooding
In contrast to the horrendous tales of death and destruction from the socio-economic impacts of floods, there are some positive environmental impacts. In many natural ecosystems floods play an important role in maintaining key ecosystem functions and biodiversity, by linking the river with its land surroundings.
The floods can recharge groundwater systems, fill wetlands, increase connectivity between aquatic habitats, and move sediment and nutrients around the landscape and into marine environments. For many species, flood events trigger breeding, migration and dispersal. Many natural ecosystems are resilient to the effects of moderate flooding, which can lead to increased productivity and maintenance of recreational environments.
However, in environments degraded by human activities, the impacts of flooding become more negative. Intense flooding, caused by excessive overland How, can lead to oversupplies of sediment and nutrients, with possible eutrophication and the destruction of aquatic plants, as well as introducing pollution from nitrates, chemicals and heavy metals, which can degrade aquatic habitats.
Several research reports comment on the impacts on wildlife living in the soil, such as earthworms, moles, voles, hedgehogs and badgers, which can be poisoned by polluted waters.
In developing countries, many subsistence farmers have developed agricultural practices that rely on the annual inundation, which brings sediment and nutrients to the fields, so working with nature. When the Aswan Dam was built in the Nile Basin, one of the aims of this multipurpose scheme was to control flooding in the Nile. This had a negative impact on subsistence farmers, and also on sardine fishermen, as the sardines migrated away from the Nile Delta because of the loss of nutrient supply.
