London scoping - ukcip

Final Report
74
Figure 5.5 The 60-day duration autumn-winter maximum precipitation for the Thames Region
(Medium-High Emissions, and Medium-Low Emissions, downscaled), with respect to
the 1961 to 1990 average.
Table 5.6 Percentage change in Thames Region 30- and 60-day duration autumn/winter
maximum precipitation totals under the Medium-High Emissions scenario
(downscaled)
Scenario
2020s
2050s
2080s
Precipitation anomaly (%)
100
80
60
40
20
0
-20
-40
-60
-80
1961 1981 2001 2021 2041 2061 2081
Probability level (30-day event) Probability level (60-day event)
0.5 0.10 0.05 0.5 0.10 0.05
-5
-5
+4
-6
-4
+3
+1
+10
+8
5.5.3 Urban Drainage Systems
Urban expansion over the last 200 years has resulted in the loss of several open rivers within
central London such as the Fleet, Tyburn and Effra that now flow underground. A survey of the
landscape status of London’s river channels between 1992 and 1996 revealed that 29 per cent
were natural, 56 per cent were artificially surfaced, and 15 per cent were culverted (EA, 2001b).
This is of consequence, not only for the recreational assets of the City, but also for the rate and
volume of runoff following excessive rainfall or snow melt (DETR, 2000a). In fact, a
significant proportion of insurance claims are from non-riverine floods arising from intense
rainfall events overwhelming urban drainage systems (ABI, 2002). The changes in future
rainfall patterns shown in Figure 5.4, therefore, point to an increased likelihood of such flooding
by the 2080s.
Research is currently underway to evaluate the performance of existing sewerage systems in
relation to past patterns and future changes in rainfall event sequences and storm event profiles
(UKWIR, 2002a). The project will also report on the significance of secondary factors in sewer
system performance arising from groundwater infiltration, soil moisture deficits, and changes in
water levels in receiving waters affecting sewer outfalls – all of which are potentially climate
sensitive. For example, in catchments with significant groundwater contributions, enhanced
winter re-charge and antecedent base flows will have implications for sewerage networks.
Higher groundwater levels will mean that there will be both an increase of infiltration into sewer
Precipitation anomaly (%)
100
80
60
40
20
0
-20
-40
-60
-80
1961 1981 2001 2021 2041 2061 2081
-4
-1
-1
+1
+11
+10
0
+15
+16