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Appendix 4: An Example of the Preliminary Scenario Method for Extreme Rainfall

[Assistance is acknowledged from Mike Harkness of OPUS.]

This Appendix illustrates the application of the preliminary scenario method for extreme rainfall described in Table 5.2. The example is for a site near Timaru. A table showing current design rainfall rates for various durations and return periods was first prepared. Further tables were then developed for percentage increases to the design rainfall rates for low and high temperature change scenarios for 2030. The percentage increase tables were then applied to the first table, to provide low and high scenario tables for actual rainfall rates in 2030. These could be used in, for example, a preliminary examination of the resilience of existing stormwater drainage systems to plausible 2030 extreme rainfall rates.

This appendix does not address uncertainties in the base period design rainfall depth estimates. As mentioned in Section 5,2 it is good practice to consider such uncertainties as part of the assessment of the likely impacts of changes in heavy rainfall.

The following table shows design rainfall depth frequency estimates for Gleniti developed by Opus using data from the period 1977-98. Rainfall is given in millimetres.

Base data, Gleniti

ARI (years) Duration 2.33 5 10 20 50 100
5 minutes 4 6 12 14 15 23
10 minutes 5 7 13 18 23 25
30 minutes 9 12 16 31 41 60
1 hour 13 18 26 37 47 61
2 hours 18 27 35 43 54 62
6 hours 29 43 56 67 82 94
12 hours 39 55 72 89 112 129
24 hours 53 77 95 112 135 152

From Figure 2.2 of the Guidance Manual, projected annual mean temperature changes for Timaru from the 1990s to the 2030s are 0.2 (low), 0.6 (medium) and 1.3°C (high).

The percentage increases in extreme rainfall for the preliminary screening analyses are obtained by multiplying entries from Table 5.2 by these temperature changes, resulting in the following two tables. [The column headed 2.33 is obtained by interpolating between two- and five-year values from Table 5.2.]

Percent increase in extreme rainfalls, 2030, Gleniti, low scenario

ARI (years) Duration 2.33 5 10 20 50 100
5 minutes 1.6 1.6 1.6 1.6 1.6 1.6
10 minutes 1.6 1.6 1.6 1.6 1.6 1.6
30 minutes 1.5 1.5 1.5 1.5 1.5 1.5
1 hour 1.4 1.4 1.4 1.5 1.5 1.5
2 hours 1.3 1.3 1.4 1.4 1.4 1.5
6 hours 1.3 1.3 1.3 1.4 1.4 1.4
12 hours 1.2 1.2 1.2 1.3 1.3 1.4
24 hours 1.1 1.1 1.2 1.2 1.3 1.3

Percent increase in extreme rainfalls, 2030, Gleniti, high scenario

ARI (years) Duration 2.33 5 10 20 50 100
5 minutes 10.4 10.4 10.4 10.4 10.4 10.4
10 minutes 10.4 10.4 10.4 10.4 10.4 10.4
30 minutes 9.7 9.7 9.8 9.9 9.9 10.0
1 hour 9.2 9.2 9.4 9.6 9.7 9.7
2 hours 8.7 8.7 9.0 9.2 9.4 9.5
6 hours 8.2 8.2 8.6 8.9 9.0 9.2
12 hours 7.5 7.6 8.1 8.4 8.6 8.8
24 hours 7.0 7.1 7.7 8.1 8.3 8.6

Actual low and high scenarios for 2030 depth frequency values can now be estimated, as given in the tables below, by adjusting values from the first table in this appendix by the values in the percentage change tables, as follows:

Extreme rainfall rate scenario for 2030, Gleniti, low temperature change scenario

ARI (years) Duration 2.33 5 10 20 50 100
5 minutes 4 6 12 14 15 23
10 minutes 5 7 13 18 23 25
30 minutes 9 12 16 31 42 61
1 hour 13 18 26 38 48 62
2 hours 18 27 35 44 55 63
6 hours 29 44 57 68 83 95
12 hours 39 56 73 90 114 131
24 hours 54 78 96 113 137 154

Extreme rainfall rate scenario for 2030, Gleniti, high temperature change scenario

ARI (years) Duration 2.33 5 10 20 50 100
5 minutes 4 7 13 15 17 25
10 minutes 6 8 14 20 25 28
30 minutes 10 13 18 34 45 66
1 hour 14 20 28 41 52 67
2 hours 20 29 38 47 59 68
6 hours 31 47 61 73 90 103
12 hours 42 59 78 97 122 141
24 hours 57 83 103 121 147 165
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