London scoping - ukcip

Final Report
72
to withstand floods of a 0.1% probability (i.e., a 1 in 1000 year event), however the standards of
protection to some limited Thames-side areas and on many of the tributary rivers are lower. The
effects of climate change are likely to reduce the standard of protection of existing defences
through rising sea levels, rising groundwater, and/or increased storm magnitudes. For example,
under the UKCIP98 High scenario, the 100 year return period (naturalised) daily flow at
Kingston on the Thames is predicted to increase by 13% by the 2020s (Davis, 2001). Flood risk
maps in catchments with significant groundwater contributions will also need to be re-evaluated
in the light of enhanced winter recharge and antecedent baseflows (Wade et al., 2001).
For the purposes of this current study the statistical downscaling model SDSM was calibrated
using 1961 to 1990 areal average daily precipitation totals for the Thames Region (see Section
3.3), and climate variables for the SE grid-box (Figure 4.5). Significant correlations were found
between the daily precipitation amounts and several regional climate indices (near surface
humidity, zonal airflow strength, vorticity, mean sea level pressure, and 850 hPa geopotential
heights – a measure of the thickness of the atmosphere). Once calibrated, SDSM was then used
to produce future estimates of the daily precipitation under Medium-High Emissions and
Medium-Low Emissions scenarios.
Under both scenarios there is a slight increase in the number of winter rainfalls exceeding 12.5
mm/d by the 2080s, suggestive of greater flood risk by this time (Figure 5.4). Far more
remarkable is the strong decline in the summer incidence of these events (which on average
occurred just over twice per summer in the period 1961 to 1990). Under this scenario, the
frequency of pollution events associated with the ‘flushing’ of combined sewer outflows (CSOs)
would be expected to decline by the 2080s. However, this trend could be countered by an
intensified heat island triggering more convective instability and localised thunder storms under
marginal conditions (Atkinson, 1968). Furthermore, even with fewer events, the polluting
potential from the flushing of CSOs could still be greater due to a combination of less diluted
stronger sewage (due to lower summer infiltration to sewerage systems) and/or lower flows in
the receiving water course(s). Wash-off pollutants, accumulated by impermeable surfaces such
as roads during extended dry periods, could also adversely affect water quality.