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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