Recurrence intervals and prediction
flood risk analysis: environment agency is responsible for flood risk analysis and for
issuing warnings. important as tells homeowners/tenants what risk a property
faces. works out flood risk at a place by using ‘magnitude and frequency analysis’.
scatter graph produced using historical flood data. magnitude plotted against
recurrence interval on semi-log graph and line of best fit can be drawn. further
years can be calculated. useful when planning defences and when drawing flood
risk maps.
Flood risk maps: reassures and informs people who live in that area and shows
whether buildings and agricultural land/animals are at risk.
rainfall radar plot approach of depressions and other rain-bearing weather
systems. data is fed into agency’s computer system.
tipping rain gauges throughout the drainage basin monitor the actual amount of
rain falling and feed this info into the system
along the river, automatic river discharge gauges monitor the rising river levels,
and this is added to the system data systems database includes a model of the way the drainage basin behaves at
different times of the year and with different rainfall inputs. based on past flood
events
as a result, the computer system can continuously compare incoming data with
past events and produce predictions of river levels at different points along the
river. it can predict how high the flood peak will be and when it will reach different
places.
on the basis of these predictions the Environment Agency issues detailed flood
warning. people are then given to prepare for the flooding.
Causes impacts and management
Causes: heavy, persistent and/or intense rainfall. rapid snowmelt. impermeable soil
and bedrock. lack of vegetation in the drainage basin. cold temperature which
reduce evapotranspiration. Deforestation. Urbanization. mechanised farming. acid
rain. global climate change.
Impacts: death of people and animals. damage of buildings, infrastructure and
farmland. disruption to people’s lives. can be social or economic. impacts vary from
country to country. in HICs the economic cost of flooding is higher than in LICs. in
LICs the death toll is higher than in HICs.
How well the hazard is dealt with depends on: level of economic development of
the place, influences factors such as emergency service provision, infrastructure
and ability to recover from the flood. willingness of the local people and their gov
to spend money on flood preparation and alleviation. accuracy and length of
warnings given.
Forecasts and warning: make sure people understand the flood forecasting and
warning service. time to move animals, furniture and evacuate. have an
emergency plan so people know what to do once warnings are given and once
flooding takes place. each house and whole town needs a plan. organise at-risk
houses so they have tiled floors, moveable mats, floor drains, cupboards and pull
sockets above flood level, wide stairs and storage space upstairs. take out
insurance, no flood, insurance company keeps money. if there is a flood, insurance
gives money to repair the damage. amount paid depends on flood risk level.
Hard engineering: dams and reservoirs built upstream, as long as not already full,
they can store some floodwater. platforms built on floodplain before any buildings
built, raises buildings above flood level but reduces room for water on the
floodplain. build embankments along the river, effective but expensive and visual
pollution, temp barriers are a solution, but predatory work is permanent. dredge
the river, lowers the bed and makes more room for water in the channel, expensive
and not permanent as river deposits fresh silt over time. straighten the river,
allows the water to flow faster and prevents build-up of flood waters, causes bigger
floods further downstream. retention basins built, areas surrounded by an
embankment into which floodwaters are diverted at times of crisis, land-use within
the retention basin is restricted. flood relief channels, artificial channels can be
built around a town to take away excess water and prevent town flooding, causes
worse flooding downstream.
Soft engineering: floodplain zoning, only use floodplain land for things that aren’t
affected by floods (sports pitches) don’t build houses. Rely on wash lands,
upstream land allowed to flood. acts as a safety valve and protects the town. plant
trees in the upper drainage basin, tress encourage interception, evapotranspiration
and infiltration, forested areas have fewer floods. wetland and riverbank
conservation schemes, protecting the natural floodplain areas which still remain,
gives floodwaters a place to go and protects disappearing habitats, enhancing
species diversity. river restoration schemes, returns river and its floodplain to
natural state providing sustainable environmental gain, economic cost involved.
Chang Jiang, China: flooding
china is one of the largest countries in the world. threatened by many natural
hazards which include earthquakes, typhoons and flooding. has many large rivers
with the Chang Jiang as the 3rd longest in the world (6380km). every year the
summer monsoon rains, combined with snowmelt from Himalayas, cause the river
to rise to very high levels. despite embankments, flooding is common. flooding has
caused many problems. valley is home to 400 million people. floods annually but in
1931, 1934, 1954 and 1998 the floods were catastrophic. 300 000 died in 20th
century floods. floods are getting worst and 1998 set new records.
Physical causes: flooding is an annual event caused by snowmelt and monsoon
rains. 1998 rains lasted a month longer than usual and the ‘El Niño’ effect was
blamed for this.
Human causes: urban population growing. many cities beside river. people and
property is at risk than in the past. counteract the pressure for rural-urban
migration, factories been built in the village to provide jobs for the country folk.
many factories are on the floodplain and aren’t protected well so at risk. more
buildings on floodplain mean less room for floodwater so levels rise higher.
deforestation in the headwaters in 60’s means there’s less interception,
evapotranspiration, and infiltration. results in more surface run off and bigger flood
peaks. mismanagement of the deforested land led to soil erosion and more silt
being washed into channel so less room for water. canalisation to improve
transport straightened it, sped up the flow, reduced lag time and increased flood
peak. embankments have constrained the river, so it can hold more water but
when bank break, the flooding is rapid and deadly, less time to escape. Dongting
and Poyang Lakes near Wuhan acted as safety valves. floodwater was diverted into
the lakes, reducing the food peak. farmers have reclaimed polders (area of land
reclaimed from sea, usually below sea level and has to be drained artificially using
pumps) from lakes to create farmland. so, the lakes are much smaller and less
room in lakes for floodwaters, flood peak level increased. global warming results in
more rainfall in China.
Impacts: monsoon rains weren’t more intense than normal but lasted for a month
longer. river was 45m high at one point. 240m people in 7 provinces were affected
by floods in some way. 4000 people died. 1000’s of farm animals died. huge crop
areas were destroyed; rice and cotton. to protect Wuhan (largest city in area)
many of rivers embankments had to be deliberately breached. lots of countryside
were flooded as a result, destroying houses and factories. 14m people were
homeless for months until houses could be repaired or replaced. people were out
of work for months while factories were repaired. great thickness of sticky clay
were deposited onto fertile fields. clay had to be removed before farming could
recommence. total cost to china’s economy was huge.
Management: several schemes have been developed to reduce flooding, includes
both ‘hard’ and ‘soft’ engineering projects.
Three gorges dam: hard engineering
downstream of Chongqing, Chiang jiang flows through a deep narrow section of its
valley known as three gorges. ideal site for a dam as reservoir is big but contained
and doesn’t spread. dam built at sandouping and reservoir is 660km long and 1km
wide, extending upstream as far as Chongqing. largest hard engineering project
ever on a river. construction started in 1994 and completed in 2006. project
finished in 2009 when reservoir filled up. dam is 2.3km long and 200m high. series
of huge ship locks has been built at NE end of dam and an 18,000-megawatt HEP
station built inside the SW end. cost in excess of US$38b to build.
Benefits: 50m people protected from flooding like 1998, includes residents of
megacities. millions of ha farmland protected and provided with irrigation, raise
grain and oil seed production. secure water supplied to 13m people in shanghai.
generates 10% of china’s electricity, equivalent to 15 nuclear power station. clean
HEP and production won’t affect environment. power will boost economic growth in
central and east china. navigable by ships unto 10,000 tonnes, 500% increase in
river traffic and will boost economic growth.
Disadvantages: reservoir flooded 150 towns/cities and 1300 villages. 1.2m people
resettled in new settlements close to old homes but compensation didn’t cover
costs. reservoir is polluted by toxins from flooded mines and factories. damaged
the river’s fragile ecosystem and species. landscape can be a problem. water seeps
into rocks of steep valley sides, landslides are expected. it’s an earthquake region
and if the dam broke the resulting flood is huge. Chiang jiang is laden with silt.
deposited in reservoir and reducing its capacity. will be dredged or flushed out
regularly. past, downstream farmland was fertilised with a thin layer of silt each
year, been lost and more chemical fertilisers needed. 1000 cultural and
archaeological sites been flooded.
Afforestation: soft engineering
upstream of Chongqing, Chiang jiang runs through hills on border on Yunnan and
Sichuan provinces. was heavily forested but in 1960s cut down for fuel, timber and
farmland. new fields used to grow crops, but terraces are poor and soil erosion
happens. animals grazed here too. big problem to get locals to replant the trees
wherever possible.
Advantages: trees encourage interception, evapotranspiration and infiltration. will
reduce flooding both locally and downstream. forested areas have less soil erosion.
good for local area and will reduce amount of silt further down river. reduces
flooding and reduce rate of sedimentation in three gorges reservoir. trees
harvested for food fodder fuel and timber
Disadvantages: local farmers losing arable and grazing fields. many trees plants
are fast growing conifers which are less useful than the natural forest and don’t
stimulate biodiversity