Post on 29-Jan-2016
Floods
Geography
How does flooding occur?
How can it be managed?
How does flooding occur?
Urban settings are often subject to sudden floods due to surface runoff.
What physical and human factors contribute to flooding?
What physical and human factors contribute
to flooding?
Human factors
Excessive, prolonged rainfall
Saturated soil Deforestation
Urbanisation
Physical factors
Snow melt
Frozen soil
Local relative rise in sea level / storm surge
Steep gradientImpermeable rock
High drainagedensity
Flooding occurs when a river exceeds its bankfull discharge
River managementShort, intense rainfall event
Flooding: storm hydrograph
Factor Effects on Hydrograph
High intensity, long duration of rainfall, or antecedent rainfall
Snow melt
Porous soils and / or permeable rock
Impermeable rock / frozen ground
Small drainage basin
Elongated drainage basin shape
Steep slopes within drainage basin
Summer vegetation
Deforestation
Steep rising limb as infiltration capacity of soil exceeded
Greatly increased discharge, especially if ground frozen
Less steep or ‘flashy’ hydrographs
Reduced lag time and steeper rising limb
Faster response, shorter lag time and steeper rising limb
Slower passage to river, so longer lag time
Faster passage to river, so shorter lag time and steeper rising limbInterception higher - slow response, peak discharge lower
Faster response and higher peak discharge
Flash Flooding
MEDC 1: Prague floods, August 2002
Intense rain fell on the 6th-7th August and then again on the 11th-13th
A one-in-500 year flood wave was triggered in the Vltava basin Highest ever discharge was recorded in Prague – 5300 cumecs Buildings swamped in 4m of water – many collapsed or left too dangerous to re-
occupy 50,000 evacuated 2/3 of these were still unable to return to their homes 12 weeks after the flood 230 million Euros of damage to underground as 13 stations flooded 3 billion Euros total damage in Czech Republic – 1/ 3 of this in Prague itself
MEDC 2: Flash flooding - Boscastle, Cornwall August 2004
Intense low pressure weather system caused localised heavy thundery downpours
200mm rain fell in 24 hours (most between midday and 5pm on the 16th) on high ground to the east.
Already saturated catchment – rapid runoff Boscastle lies in a deep valley just
downstream of the confluence of the rivers Valency and Jordan
2m rise in river levels in one hour Debris caught under narrow bridge caused 3m
high wave of water which burst down main street when bridge collapsed
70-80 cars swept away, significant structural damage, 100 people air lifted to safety but no loss of life
LEDC: Flooding in Bangladesh 288-289
The issues around flooding in an LEDC are different from those in an MEDC.
The death rate is much greater and the disasters caused by flooding increase in size with growing populations.
Spending on hazard reduction is not as great as in MEDCs.
High risk areas - Bangladesh
Major rivers converge ?
?Himalaya Mountains; (monsoonal) rainfall and snow melt
?
Storm surges, especially during cyclones / hurricanes. Also local sea level rises of 7mm/year
80% of country occupies low-lying delta < 1m above sea level
What are the human influences?• Deforestation• Agricultural practices• Densely populated• Urbanisation – Dhaka
population over 1 million• Embankments built
(road and river) – have prevented back-flow of flood water and increase siltation in drainage channels
• Low GDP and lack of investment
?
What are the physical causes of flooding?
Flooding in Bangladesh, 2004Exceptionally high rainfall totals in the monsoon of 2004 led to widespread flooding in July and August
Consequences of flooding
38% land area flooded – worst floods for 6 years
800,000 ha agricultural land flooded – small scale farmers severely affected
Capital city, Dhaka flooded.
36 million people made
homeless (nearly 29% of total population)
800 dead by mid-September
Spread of disease Flood waters mixed with
raw sewage caused diarrhoea outbreak
Infrastructure severely damaged – damage to roads, bridges, school
and hospitals estimated at $7 billion
$2.2 billion estimated cost
of damage (4% of GDP
for 2004)
time
discharge (cumecs) and rainfall (mm)
Shorter lag time as water quickly reaches the channel via surface runoff, through drains, sewers etc
Steeper rising limb due to impermeable surfaces
Urbanisation and the storm hydrograph
Higher peak flow as less water is ‘stored’; more water reaches the river
Rural
Urban
Tuesday 29th Sept
RBMPsRiver Basin Management Plans
Aims of RBMPs
Protect and enhance the ecosystem Promote sustainable use Supply of good quality water Reduce and then eliminate pollution Mitigate the effects of flooding and drought Delimit protective conservation areas
2. How can it be managed?
Hard engineering
Soft engineering
What are the arguments for and against hard engineering?
Hard engineering
FOR AGAINSTReduction in flooding and therefore protects property
Takes water away from towns more quickly
Increase in water supply e.g. on the Nile
Improved navigation e.g. Mississippi
Allows energy to be created e.g. hydroelectric power on the Colorado
Can lead to destruction of habitats along river bank
Can be visually intrusive
It can dramatically increase peak discharge, duration and timing of floods downstreamWhere meanders have been straightened, the river will try to re-establish itselfStraightening courses can lead to greater upstream erosion and downstream deposition
Flood managementHard engineering strategies involve the building of structures or alteration of the course / structure of the river
The aim is to reduce the frequency and magnitude of flood events, and therefore reduce the damage that floods cause
What are the arguments for and against hard engineering?
Hard engineering
FOR AGAINSTReduction in flooding and therefore protects property
Takes water away from towns more quickly
Increase in water supply e.g. on the Nile
Improved navigation e.g. Mississippi
Allows energy to be created e.g. hydroelectric power on the Colorado
Can lead to destruction of habitats along river bank
Can be visually intrusive
It can dramatically increase peak discharge, duration and timing of floods downstreamWhere meanders have been straightened, the river will try to re-establish itselfStraightening courses can lead to greater upstream erosion and downstream deposition
Soft engineering
Abatement strategies which aim to work with natural processes, and be more sustainable solutions to flooding• Afforestation
• Contour ploughing and strip farming to reduce runoff
• Floodplain zoning to allow (economically less valuable) areas to flood naturally
• Conservation and restoration schemes; returning rivers to their original state and protecting, e.g. bales to improve water quality
• Forecasting and early warning, e.g. Environment Agency flood watch and risk maps. Some small-scale community projects in Bangladesh have resulted in early warning systems and lives are being saved
River restorationThe River Cole near Swindon underwent a restoration project between 1994 and 1996. The aims were to change the water course back to a more natural state, improve water quality and manage bank side vegetation and habitats. The main strategies are shown below
Southern Britain, July 2007
Normal Jet stream
June – July 2007
Causes;
• Abnormal track of jet stream
• Rainfall totals for May-July highest since 1776
• Infiltration / percolation capacity minimal
• Exceptional rainfall on 20th July – event only expected once in several hundred years
Consequences;
• Flash floods across southern England; especially lower Severn and upper Thames catchments
• Drainage systems overwhelmed and transport networks severely disrupted - £25 million damage to Gloucestershire’s road system – the year’s budget!
• 45,000 households lost power; 350,000 lost running water – £1billion cost to water industry
• £3 billion damage covered by insurance. Equivalent amount uninsured loss
• 50% crops lost in affected areas – shortages and price increases
• 3 people died