What are the hazards associated with flooding? Flashcards
1
Q
What are the physical causes of floods?
A
- Usually by altering the balance of stores, inputs and outputs in the system
- Climate – snow melt, heavy rain (thunderstorms), low evaporation, storm surges
- Previous weather conditions e.g. a long period of wet weather
- Relief – very flat, low lying area
- Drainage – density, regime, hydrograph, drainage pattern and density, drainage type
- Vegetation – grass versus trees
- Rock type – permeability, porosity, water table
- Soil conditions – wet versus dry, bakes impermeable by a drought
- Natural disaster – earthquake, landslide
- Subsidence – area is sinking (isostatic, removal of groundwater, etc.)
- Rising sea level – global warming melting ice sheets
2
Q
What are the human causes of floods?
A
- Usually by putting human activities in areas at risk and interfering with the natural cycle/hydrology
- Construction of impermeable surfaces, e.g. town, roads
- Removal of vegetation cover – deforestation, removal of coastal marshes/mangroves, farming
- Soil erosion, leading to silting of channels and storage lakes
- Drainage – ditches, waste disposal, drains, excessive irrigation
- Changing rivers – dams, diversions, embankments, removing or adding deposi
3
Q
What are primary impacts of flooding?
A
- Deaths – humans, pets and livestock
- Evacuation
- Destruction of buildings/possessions
- Disease from polluted water
- Stress
4
Q
What are secondary impacts of flooding?
A
- Transport links broken (bridges, roads, rail)
- Cost of clean up and replacement
- Loss of jobs
- Crops ruined (animals may starve)
- Rehousing costs
- May take over a year to dry out buildings
5
Q
What are tertiary impacts of flooding?
A
- Flood prone areas decline as property values decrease
- Difficult for owners to get insurance or sell property
6
Q
Responses to flooding - what is preventative planning?
A
- Planning – avoid flood plains and green sectors, e.g. parks
- Planned retreat – leave certain areas to flood
- Reduce surface flow, e.g. afforestation, contour ploughing
7
Q
Responses to flooding - what is structural planning?
A
- Channel modification, e.g. overflows, storage areas, dams, widen channels
- Embankments – raise them, reinforce them, put up flood gates
- Flood relief channels
- Flood barrages, e.g. on the River Thames
8
Q
Responses to flooding - what is management?
A
- Early warnings – better weather and river level forecasts, communications
- Flood insurance
- Public relief funds
- Accept the risk and live accordingly
- Ignore the risk (‘head in the sand’ approach)
9
Q
Responses to flooding - what is emergency rescue?
A
- Rescue people and animals
- Save property and possessions
- Use sandbags, erect flood barriers, pump out property
10
Q
Background to flooding on the River Severn
A
- The river Severn is the third largest British river
- It rises in the mountains of central Wales above 800m – On leaving the mountains, it initially flows east before turning south to its estuary in the Bristol Channel
- Two major reservoirs in the upper catchment provide flood water storage
- However, the lower catchment, especially between Worcester and Gloucester, fed by major tributaries such as the Avon and the Teme, is particularly vulnerable to flooding
11
Q
What physical factors affect flooding on the River Severn?
A
- Rock type, relief, climate and vegetation
- Large parts of the upper catchment in central Wales are above 300m, where slopes are steep and the main rock types have low permeability
- As a result, runoff is rapid and small tributaries, which feed into the main river, are highly responsive to rainfall and therefore liable to flash floods
- The main relief feature in the lower catchment is an extensive floodplain, only a few metres above sea level
o Much of the land adjacent to the River Severn floods regularly in winter
o Flooding is also more likely to occur in the lower catchment because of the convergence of a number of major tributaries in this area - The Warwickshire Avon joins the river Severn at Tewkesbury, and the River Teme’s confluence with the Severn is just south of Worcester
- Mean annual rainfall in the mountains in central Wales exceeds 2000mm, and because of orographic uplift, rainfall is often intense and prolonged
- Low rates of evaporation mean that most rainfall is converted to runoff and river flow
- Vegetation cover influences rates of interception, evapotranspiration and runoff
o Nearly 45% of land use in upper catchment compromises rough pasture and moorland
o This promotes rapid runoff and increases the flood risk
12
Q
What human factors affect flooding on the River Severn?
A
- The main human factor contributing to the flood risk is building on the Severn floodplain
- Large parts of the settlements of Upton-upon-Severn and Tewkesbury occupy the floodplain
- Without adequate flood defences, these areas are exposed to a high level of risk
- Within the lower Severn and Avon catchments there is evidence that recent urbanisation has accelerated the loss of floodplain land, reducing natural water storage and placing more properties at risk
- Rural land-use change and land-use management may also have contributed to higher peak floes
- Improved drainage of farmland speeds the movement of rainwater into rivers; so too does the conversion of pasture to arable and the clear felling of forestry plantations
- The lack of hard flood defences also exposes much of the lower Severn valley to flooding
o There are few flood embankments, and other forms of river engineering such as flood relief channels, channel widening and straightening are absent
13
Q
When did the River Severn flood badly?
A
July 2007
14
Q
What happened during the River Severn flooding, July 2007
A
- Large parts of England and Wales, including Humberside, South Yorkshire, and the lower Severn valley and the Thames valley were affected by sever flooding in the summer of 2007
- The floods in the Severn valley followed an extreme rainfall event on 20 July, when 135 mm of rain fell at Pershore in just 16 hours
- Flash floods in several small catchments such as the Isbourne in Worcestershire then fed into the major rivers, raising the Severn at Worcester nearly 6m above normal and the River Avon at Evesham to the highest level ever recorded
- The severity of flooding was due to exceptional rainfall, but also to soils already saturated by heavy rains in June
- Where flood defences were overwhelmed it was because the flow level exceeded their design
15
Q
What was the human reaction to the July 2007 floods?
A
- General management responses towards river flooding in the UK are shown in tables on next page
- Immediate responses included the erection of temporary flood barriers and flood warning issued by television, radio, the Environment Agency’s ‘Floodline’ and the internet
- The Severn Trent water authority’s response to the disruption to water supplies was the immediate distribution of 5 million litres per day of bottled water and the deployment of 1500 water bowsers
- The government provided £87 million of emergency aid for funding schools, transport and businesses hardest hit by the floods and the European Union contributed a further £31 million in compensation
- Overall spending on flood defences by the UK government doubled between 1997 and 2007 and the experience of the 2007 floods promoted the government to pledge an increase on flood defenced to £800 million by 2010-2011
- However, apart from the provision of temporary flood barriers for Upton-on-Severn, and raising some flood embankments around Gloucester, the Environment Agency (EA) has no plans for major new flood protection schemes in the lower Severn valet
- People living in the flood-prone areas will learn how to live with floods, and rely on the EA’s flood warning system and on flood insurance
- Huge investment in new river flood defences, like coastal defences, like coastal defences is seen in the long term as being unsustainable
16
Q
What were the societal impacts of the July 2007 floods?
A
- Considerable destruction to electricity and water supplies
- Flooding of a sub station near Gloucester left 50,000 households without power (some for up to 2 days)
- A water treatment plant in Tewkesbury was also flooded with the result that water supplies for 140,000 households were cut off and without water for at least 5 days
- Safe drinking water was not restored until 7 August
- Ten thousand motorists were left stranded on the M5 and surrounding roads and forced to abandon their cars
17
Q
What were the economic impacts of the July 2007 floods?
A
- The total insured loss estimated for the 20 July flood even was £1-1.5 billion
- These losses include damage to property, motor vehicles, disruption to businesses and the expense of providing temporary accommodation for those forced to leave their homes
- There were large-scale damage to property and disruption of businesses: 27,000 domestic insurance claims and 6800 business claims were made
- Many motor vehicles were damaged completely, and abandoned vehicles blocked roads and traffic flows
- The M5 was closed due to flooding and landslides on 20 July
- Crops were submerged and maize, potatoes and hay crops lost
- Where floodwater contained sewage, crops had to be destroyed
18
Q
What were the environmental impacts of the July 2007 floods?
A
- Thousands of small mammals drowned, ground nesting birds were badly affected and large numbers of mature fish were left stranded on the floodplain when water levels receded
19
Q
What non-structural measures approaches are the to river flood management in the UK?
A
- Catchment flood management plans
- Afforestation and land-use change
- Controlling development on floodplains
- Flood insurance: exposure and vulnerability
- Washlands or flood basins
- Flood warnings
- Flood risk maps
20
Q
What are catchment flood management plans?
A
- A holistic approach to environmental management of drainage basins
- The primary concern is sustainable flood management
- Other environmental issues include water abstraction, pollution, land-sue change and wildlife conservation in drainage basins
