1. Floods Geography How does flooding occur? How can it be managed?

2. How does flooding occur? Urban settings are often subject to sudden floods due to surface runoff. What physical and human factors contribute to flooding?

3. 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 gradient Impermeable rock High drainage density Flooding occurs when a river exceeds its bankfull discharge River management Short, intense rainfall event

4. Flooding: storm hydrograph FactorEffects 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 limb Interception higher - slow response, peak discharge lower Faster response and higher peak discharge

5. Flash Flooding

6. MEDC 1: Prague floods, August 2002 Intense rain fell on the 6 th -7 th August and then again on the 11 th -13 th 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

7. 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 16 th ) 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

8. 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.

9. 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?

10. Flooding in Bangladesh, 2004 Exceptionally 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)

13. 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

14. Tuesday 29th Sept

15. RBMPs River 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

16. 2. How can it be managed? Hard engineering Soft engineering

17. What are the arguments for and against hard engineering? Hard engineering FORAGAINST Reduction 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 downstream Where meanders have been straightened, the river will try to re- establish itself Straightening courses can lead to greater upstream erosion and downstream deposition

18. Flood management Hard 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

25. What are the arguments for and against hard engineering? Hard engineering FORAGAINST Reduction 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 downstream Where meanders have been straightened, the river will try to re- establish itself Straightening courses can lead to greater upstream erosion and downstream deposition

26. 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

27. River restoration The 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

28. 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 20 th 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