5.5 Floods Flashcards
What is of the one meteorological cause of floods
One of the meteorological causes of floods in the UK is the jet stream and low pressure weather.
- Mid latitude zones e,g the UK are boundaries between the polar and tropical zones
- the temperature differences between these zones combined with the earths rotation create a jet stream( a wind ribbon in the upper atmosphere moving weather systems) , which carries low pressure to the UK which can then cause flooding
What is another meteorological cause of flood
- Monsoons in places like India are another meteorological cause of flood
- they are caused by a seasonal reversal in prevailing wind direction due to a contrast of continental air and more moist, thermostable oceanic air
- This causes the winds to create orographic rainfall on a seasonal wet/ dry basis
- Around 70% of average annual rainfall occurs within 100 days in monsoon season
What is a meteorological cause of flood that affects some areas
Ice melt in areas such as Siberia is a meteorological cause of flooding .
- Snow and ice melt in mountains in spring causes extensive flooding in continental interiors
- This melt is due to quick seasonal transition creating rapid snowmelt as well as frozen ground preventing infiltration
- for example the great north Siberian rivers such as the Ob and Yenisei, cause vast annual flooding in the plains of Siberia
What are the key facts about the Cumbrian floods of 2015
- Storm Desmond brought the highest levels of discharge ever recorded for some of the rivers in Cumbria
- Honister in Cumbria recorded 341.1 millimetres of rain between December 4 and 5
- 405 mm of rain fell at Thirlmere in 38 hours
What were the physical primary causes of the Cumbrian floods of 2015
Orographic rainfall -Due to the Cumbrian fells proximity to the coast orographic rainfall was created when the humid hot air met the mountains. It caused precipitation on the windward side
Low pressure- low pressure -946 millibars- was brought directly over Cumbria from the deep Atlantic low pressure system . This caused exceptionally prolonged rainfall .
Atmospheric river- there was also a system carried extratropical cyclone (a plume of moist air) from the Caribbean to the UK by the atmospheric river
-The jet stream - Rossby waves from undulation of the jet stream formed and developed low pressure troughs which affected Cumbria as they were higher than usual so did not stop low pressure moving up
-Antecedent conditions - there were already saturated ground conditions across Northwest England - as November 2015 had been the second wettest on record
What were the secondary physical causes of the Cumbrian floods
- The ground was already saturated as November 2015 was second wettest on record so excess rainfall simply flowed off as surface run off
- The steep gradients of the mountains in the Lake District (up to 978m tall mountains) led to rapid surface run of increase
- The Derwent and Cocker rivers were already swollen with previous rainfall
- The impermeable rock type of the Lake District( from the windmere group of upper and lower volcanic) impeded infiltration
- Many locations e.g cockermouth were built on river confluences increasing susceptibility to flooding
What are the human factors leading to flooding
Urbanisation-
- the creation of impermeable surfaces (such as that which goes on in London with around 60% of residents paving over their front garden ) impedes infiltration and allows rapid run off into rivers
- The building of artificial conduits also speeds up water drainage which reduces lag time
- Urbanisation also may include floodplain drainage which reduces the natural storage capacity of the floodplain thus making the area even more susceptible to flooding
- Urbanisation may also include the straightening of river channels e.g on the Mississippi to increase the flow which in turn increases the likelihood of downstream flooding
Population growth - population growth leads to increased urbanisation and agricultural land use change. It also leads to the building of dams, wells and reservoirs to supply water to the increased population which then increased flood risk
-land use change assocaited with agricultural development causes deforestation which reduces interception and evapotranspiration which increases surface run off. Deforestation and intensive crop growing also expose the soil to greater levels of erosion, increasing River sediment load which reduced the capacity of the river to carry water and increased the likelihood of flooding e.g deforestation in Nepal is known to be increasing the magnitude of flood in Bangladesh from the Ganges
River management - straightening of channels ( realignment) to increase river flow can cause downstream flooding e,g river Mississippi realignment increased affects of 1993 flood . Resectioning by dredging can have severe impacts
What were the human activities that exacerbated flood frequency and magnitude in Cumbria
- The drains and sewers in Keswick were inadequate and not designed to cope with high levels of rainwater
- The urbanisation of the town has created more impermeable surfaces -> last flash card
- There was river mismanagement based on the incorrect flood return period as Keswick flood defences were built to allow the river to rise 5m but in 2015 the river rose to 5.9 metres and overtopped new defences
- Political and economic factors also had a part to play- Flood damage coincided with reduced flood defence budgets due to conservative austerity and not seeing Keswick as valuable for protection. Green EU priorities put environmental concerns before maintenance ( Dredging was reduced due to its environmental impacts but dredging was needed as the riverbed had been raised considerably )
- Overgrazing in rural areas- sheep-wrecking- the intensive pastoral farming in uk upland areas- reduced vegetation and therefore interception and increased surface run off in Cumbria ~ making flooding worse
- Conflict over land management - the views of farmers over the reversion of farmland into wetland meant wetlands- which would have reduced the effects of the floods- could not be put in place
What are the deciding factors on how bad the impacts of floods were
- The degree of threat posed by a flood depends on the depth and velocity of the water, the duration of the flood and the quality of water ( e.g is it heavily polluted)
- Research on the impact of river depth and velocity suggests water 0.5m deep can wash cars away and water flowing at 2m per sec can cause the foundations of buildings to collapse
What are the socio-economic impacts on floods
- Floods are likely to have great economic impacts in developed counties. Whilst the fatalities are usually below 30 for each flood event, the cost are very high. For example, a cost of £1.3 billion has been quotes for the most recent series of flood events in the Uk in December 2015 and in Australia average annual flood losses are $372 million
- floods also have social impacts. Between 1990-2010 the Emergency events database recorded over 3000 flood disasters worldwide and these were collectively responsible for 200,000 deaths and 3 billion people were adversely affected by them. The worst social impacts coming in developing and emerging countries, as shown by the fact 90% of flood deaths occur in Asia, this is due to the fact posy flood morbidity is likely to be higher in these countries as water borne diseases are more prevalent
- Flood can affect livelihoods in many ways. There can be direct infrastructure damage which can then cost money and cause problems when reselling the house ( as it has to be listed as flood prone)
- Crops, livestock and agricultural infrastructure is likely to be damaged by floods. Where framing is subsistence flooding is likely to lead to a direct loss of food which means famines can occur
- Floods can kill the tourism industry in affected areas which reduces income of the area
- Loss of livelihood occurs when services and businesses are flooded
What are the environmental impacts of flooding
Positive impacts of flooding
- Floods play an important role in maintains key ecosystems functions and biodiversity by linking the river with its land surroundings
- Floods recharge groundwater systems, fill wetlands and move sediment and nutrients around the landscape into marine environments
Negatives
-In environments degraded by human activities, the impacts of flooding become more negative. Intense flooding, caused by excess overland flow can lead to oversupplies of sediments and nutrients, with possible eutrophication (excessive richness of nutrients causing dense plant growth and then animal death due to lack of oxygen). They can also introduce pollutants into habitats
-animals can be killed by this introduction of pollutants
What were the social impacts of the 2012 flood in Cumbria specifically
- 5200 homes were flooded
- Services were closed including the Mcvities biscuit factory in Carlisle leading to the temporary loss of over 1000 jobs
- About 40 schools were closed in Cumbria and hospital appointments were cancelled
- Footpaths and walls are washed away as was vegetation
- the A591 and M6 had partial closure as did railways
What were the economic impacts of the Cumbrian floods
- transport and infrastructure was damaged e.g bridges in cockermouth and Keswick
- House prices fell
- tourists were deterred ~ taking away a major source of income
- £400-500 million cost
- insurance claims will hit between £900-£1.2bn
What were the environmental impacts of the Cumbrian floods
- Many river banks were eroded which added to future flood risk
- Rivers were choked with debris and contaminated with polluted water
- the saturated ground led to the decomposition of plants and animals - giving off noxious gasses such as hydrogen sulphide
- The saturated ground led to landslides e.g at Glenn Riding Beck in the Lake District
- Thousands of trees that had once lined the river banks were ripped from the environment
