London scoping - ukcip

Final Report
71
The question naturally arises as to what extent the Thames water resource strategy (EA, 2001c)
might be affected by climate change? This can only really be answered through an integrated
regional water resource modelling exercise, that incorporates more climate change detail within
the Agency’s four socio-economic scenarios. Alternatively, research could be targeted at
critical elements in the strategy, such as modelling the reliable yield of a new reservoir, or levels
of leakage, under the full set of UKCIP02 scenarios.
5.5 Flood Risk
5.5.1 Context
Both observations (Frei and Schar, 2001; Karl and Knight, 1998; Osborn et al., 2000) and
climate models (Jones and Reid, 2001; McGuffie et al., 1999; Palmer and Räisänen, 2002)
support the view that the frequency and intensity of heavy rainfall increased during the 20 th
Century, and will continue to increase in coming decades, particularly during the non-summer
seasons. However, there have been very few credible studies of riverine flood risk in relation to
climate change. This is a reflection of the difficulties associated with adequately modelling
high-intensity precipitation (or snowmelt) events at catchment-scales, and of representing landsurface
controls of storm runoff generation (Bronstert et al., 2002).
Assessing flood risk for London is problematic, not least because of the extent of the urban
drainage system, and the localised effects of blocked culverts (open watercourses which have
been covered over i.e. at road crossings, culverts may also run under buildings) and/or
exceedance of hydraulic capacity of sewers. In addition, future flooding of the Thames estuary
will require consideration of complex interactions between sea level rise, runoff from land areas
and storminess (Holt, 1999; Lowe et al., 2001; Von Storch and Reichardt, 1997). Accordingly,
flood risk will be considered from three overlapping perspectives: 1) riverine flood risk; 2) the
design capacity of urban drainage systems and; 3) tidal surges/sea level rise.
5.5.2 Case Study
Riverine flood risk in the Thames Region
A simplistic climate change impact assessment – illustrated below – is to infer future riverine
flood risk from future changes in extreme precipitation events. For example, in a recent pilot
study, the regional climate model HadRM2 (the predecessor of the model used in UKCIP02)
predicted future increases in the magnitude of rainfall extremes of 30- and 60-day duration (as
experienced in the flooding of October/November 2000) over catchment areas influencing river
levels in Lewes, Shrewsbury and York (CEH and Meteorological Office, 2001). Other studies
have examined changes in effective rainfall (as a proxy for discharge) obtained directly from
global climate models for large river basins (e.g., Milly et al., 2002), or downscaled
meteorological variables to the scale of an experimental watershed for hydrological modelling
(e.g., Pilling and Jones, 2002).
Government estimates suggest that the value of protected land and property within the Thames
region tidal Thames flood risk area is £80 billion giving a flood damage estimate of the order of
£30 billion (DEFRA, 2001). With growing demand for new housing in London and the
preferred use of brownfield sites (often situated within the floodplain), these figures are set to
increase notwithstanding changes in climate. The river defences of central London are designed