Geoscience Reference
In-Depth Information
In many river reaches, the design limit of flood alleviation schemes was exceeded;
similarly the design capacities of urban drainage systems were exceeded in many
areas. An unusual, and very significant, feature of the summer flooding was the high
proportion of damage not attributable to fluvial flooding. Around two-thirds of the
properties affected (more than 8000 in Hull alone) were inundated as drains and
sewers were overwhelmed following the summer storms. More than 55 000 homes
and 6000 businesses were flooded and related insurance claims were approaching £3
billion by the end of 2007 (Marsh and Hannaford, 2008).
From a meteorological perspective, the cause of the 2007 floods was jet-stream
displacement that affected weather patterns (the jet stream is a fast, eastward-moving
ribbon of air at an altitude of around 8-12 km). This displacement was associated
with a commonplace but anomalously strong Scandinavian atmospheric circulation
pattern (a Rossby-wavelike train of tropospheric anomalies, with a cyclone over the
British Isles and a strong anticyclone over Scandinavia). From a climate change
perspective, global warming affects jet-stream location (more of which shortly), in
addition to increasing ocean evaporation and increasing the atmosphere's ability to
hold water: approximately 6-7% more is carried per degree Celscius of warming near
the Earth's surface, as determined by the Clausius-Clapeyron equation.
The following year also saw more increased rainfall. August 2008 saw parts of
Britain receive rainfall that was well above average. North-west Wales, Cumbria and
parts of the Chilterns saw 50% more rain above the long-term August average; south
Cornwall, mid-Devon and west Wales had 100% more rain; parts of Northern Ireland
150% and south-east Scotland 200% more rain. This affected the harvest. In addition
to some crops being lost, grain needs to have less than 10-15% moisture content to
be harvested. Otherwise the crop needs to be dried within 24 hours of harvesting, and
this necessitates the use of costly energy. Both losses and extra energy costs raised
food prices. Some market garden crops, which thrive in a warm, damp summer, did
well.
Then, in September 2009, there were a number of rainbursts. In the first few days
of the month there were 94 flood watches in operation in England and Wales and one
in Scotland. Some places received more than a month's worth of rainfall in a single
day. With soil still wet from August's rain, there was considerable run-off and homes
were flooded in parts of the West Midlands. Flash floods hit Yorkshire, Shropshire,
Herefordshire and Worcestershire. Northumberland was particularly badly affected
with an estimated 1000 properties flooded in Morpeth. In Upton-upon-Severn, which
was cut off by floods the previous year, the river levels rose to more than 5 m. At the
height of the 2007 floods they were 5.93 m high.
Nearly two million properties in floodplains along rivers, estuaries and coasts in the
UK are potentially at risk of river or coastal flooding. Eighty thousand properties are
at risk in towns and cities from flooding caused by heavy downpours that overwhelm
urban drains: so-called 'intra-urban' flooding. In England and Wales alone more than
4 million people and properties valued at over £200 billion are at risk. Flooding
and flood management cost the UK around £2.2 billion each year: the 2004 spend
was £800 million on flood and coastal defences, and in 2005-7 the UK experienced
an average of £1400 million of damage a year. The UK Foresight Programme's
estimation of future flood risks is fraught due to future uncertainties. However, it
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