Geoscience Reference
In-Depth Information
Urban development on flood plains
NEW DEVELOPMENTS
Large tracts of urban land in central and southern Britain were subject to repeated episodes of flooding during the
winters of 1998-99, 2000-01, to a lesser extent in 2002 and in summer 2007, leading to little loss of life but well in
excess of £2.1 billion of damage and economic losses. It was claimed initially that 'freak weather' conditions and
early impacts of global climate change were responsible. Blame was attached to the Environment Agency for poor
flood warning, although extra time could only have mitigated impacts and not prevented the flooding. People were
'reassured' that forecast return periods of more than once in a hundred years meant they were unlikely to recur
during their lifetimes, even though climate is changing! There is some truth in each claim but the convergence of at
least four separate elements is driving up flood hazard and risk:
Modern urban development places people and infrastructure in the flood path - often out of choice.
Poor maintenance of drains, sewers and flood protection schemes.
Poor information exchange and co-ordination between planning and environmental authorities.
Global climate change.
Hydrological models have a place in flood avoidance by forecasting the probable maximum precipitationfor any given
geographical area. This, and other hydrometeorological data, is transformed into the probable maximum flood by
factoring in the hydrogeological character of the catchment. Updated with predictions of global climate change from
AOGCMs and IPCC reports, forecasts shouldimprove future flood adjustment or abatement schemes. It is not fully
recognized, however, that some climatic shifts could considerably shorten recurrence intervals. The UK Environment
Agency estimates that some 5 million people live in flood-risk areas. Flood trends clearly challenge the wisdom of
urban development on flood plains, calling for changes in planning policy, removal of confusion in flood perception
and enhanced flood protection or proofing. Faster evacuation of some potential flood waters from urban areas through
channelization provides a false sense of security. It overlooks the fact that river channels and the flood plain itself
are natural stores which reduce flood risk elsewhere, and that protection is only as good as the maintenance standard
of sewers, drains and defences. Economic pressures on building land, and the attractions of waterside living and
recreation, draw urban development increasingly into the hazard zone. The time has come to consider stricter
regulation and zoning on future building ( Figure 14.20 ). This can be less expensive than hard protection measures
such as embankment, channelization, flood relief routing or rebuilding above a threshold level storage reservoirs,
diversionary channels and other flood-routing schemes, the emplacement of control structures upstream (dams, etc.)
and the socio-economic cost of failure. Integrated catchment management policies are essential for flood abatement,
by assessing activity in every part of the catchment for its contribution to flood risk. Current trends towards planning
zonation, dechannelization, meander restoration, etc., are more sustainable options.
Very little changed after those millennium floods, as the UK 2007 summer floods revealed. An abnormally warm and
dry spring may have created a false sense of security, rudely shaken during intense cyclonic and convective rainfall
in June and July. The Polar Front jetstream track failed to migrate as far northwards as usual, steering depressions
across mainland north-western Europe instead of through the Nordic Sea. Blocked by northern Eurasian anticyclones,
they also tended to track more slowly south q north, rather than south-west q north-east, spending longer over
land and dumping widespread, heavy and prolonged rain. Between 14 and 25 June, Birmingham, the Humber
catchments in west and south Yorkshire, Lincolnshire, the mid-Severn catchment and Ulster experienced flash floods
and some overbank flooding. One month later between 19 and 22 July - after much intervening precipitation - a
similar slow-moving, north-tracking cyclone ( Figure 14.21 ) was dumping 50-100 mm of rainfall over a wide area and,
locally, driving forty-eight-hour rainfall totals of 130-160 mm in the south Midlands. This triggered immediate, short-
lived surface flooding through overland flow and rapid throughflow from saturated ground, triggering small but
widespread debris flows and landslides. Within a week, major and prolonged flooding occurred along the middle
to lower Thames, Severn ( Plate 14.12 ) and Humber rivers. Up to 450,000 people were flooded out of their homes
and/or lost water and power supplies and both the Environment Agency and Association of British Insurers estimated
losses at over £3.5 billion, likely to rise steeply. These events were triggered by the highest UK summer rainfall in
 
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