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