View all publications

Appendix 4: An example of the preliminary scenario method for extreme rainfall

This Appendix illustrates the application of the preliminary scenario method for extreme rainfall using information from Table 2.2 and Table 5.2. The example is for Christchurch Airport. 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.

Table A4.1 shows design rainfall depth frequency estimates for Christchurch Airport data from the period 1971–2000.86

Table A4.1: Base data for design rainfall depth (in mm) for 1971–2000, Christchurch Airport. ARI is average recurrence interval (ie, return period).

ARI (years)

Duration

2 5 10 20 50 100

10 minutes

4.4

6.2

7.9

9.9

13.3

16.6

30 minutes

7.9

11.2

14.2

17.9

23.9

29.8

1 hour

11.4

16.3

20.6

25.9

34.7

43.2

2 hours

16.1

22.4

27.9

34.5

45.3

55.5

6 hours

27.8

37.2

45.1

54.3

69.0

82.5

12 hours

39.3

51.2

61.1

72.3

90.0

106.0

24 hours

55.5

70.5

82.7

96.4

117.4

136.2

48 hours

68.4

87.0

102.1

118.9

144.9

168.1

72 hours

77.4

98.4

115.4

134.5

163.9

190.1

From Table 2.2 of this Guidance Manual, projected annual mean temperature changes for Christchurch for 1990–2040 are 0.2 (low), 0.9 (medium) and 1.9°C (high).

The percentage increases in extreme rainfall for the preliminary screening analyses are obtained by multiplying entries from Table 5.6 by these temperature changes, resulting in the following two tables (Table A4.2 and A4.3).

Table A4.2: Increase (in %) in extreme rainfalls, 2040, Christchurch Airport, low scenario. ARI is average recurrence interval (ie, return period).

ARI (years)

Duration

2 5 10 20 50 100

10 minutes

1.6

1.6

1.6

1.6

1.6

1.6

30 minutes

1.4

1.5

1.5

1.6

1.6

1.6

1 hour

1.3

1.4

1.5

1.5

1.6

1.6

2 hours

1.2

1.3

1.4

1.5

1.6

1.6

6 hours

1.1

1.2

1.4

1.5

1.6

1.6

12 hours

1.0

1.2

1.3

1.5

1.6

1.6

24 hours

0.9

1.1

1.2

1.4

1.6

1.6

48 hours

0.8

1.0

1.2

1.4

1.6

1.6

72 hours

0.7

1.0

1.2

1.4

1.6

1.6

Table A4.3: Increase (in %) in extreme rainfalls, 2040, Christchurch Airport, high scenario. ARI is average recurrence interval (ie, return period).

ARI (years)

Duration

2

5

10

20

50

100

10 minutes

15.2

15.2

15.2

15.2

15.2

15.2

30 minutes

13.7

14.1

14.4

14.8

15.2

15.2

1 hour

12.7

13.5

14.1

14.6

15.2

15.2

2 hours

11.8

12.7

13.7

14.4

15.2

15.2

6 hours

10.1

11.6

12.9

14.1

15.2

15.2

12 hours

9 1

11.0

12.4

13.9

15.2

15.2

24 hours

8.2

10.3

12.0

13.7

15.2

15.2

48 hours

7.2

9.5

11.6

13.5

15.2

15.2

72 hours

6.7

9.1

11.2

13.3

15.2

15.2

Actual low and high scenarios for 2040 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:

Table A4.4: Extreme rainfall rate scenario for 2040, Christchurch Airport, low temperature change scenario. ARI is average recurrence interval (ie, return period).

ARI (years)

Duration

2 5 10 20 50 100

10 minutes

4.5

6.3

8.0

10.1

13.5

16.9

30 minutes

8.0

11.4

14.4

18.2

24.3

30.3

1 hour

11.5

16.5

20.9

26.3

35.3

43.9

2 hour

16.3

22.7

28.3

35.0

46.0

56.4

6 hours

28.1

37.6

45.7

55.1

70.1

83.8

12 hours

39.7

51.8

61.9

73.4

91.4

107.7

24 hours

56.0

71.3

83.8

97.7

119.3

138.4

48 hours

68.9

87.9

103.3

120.6

147.2

170.8

72 hours

77.9

99.4

116.8

136.4

166.5

193.1

Table A4.5: Extreme rainfall rate scenario for 2040, Christchurch Airport, high temperature change scenario. ARI is average recurrence interval (ie, return period).

ARI (years)

Duration

2 5 10 20 50 100

10 minutes

5.1

7.1

9.1

11.4

15.3

19.1

30 minutes

9.0

12.8

16.2

20.5

27.5

34.3

1 hour

12.8

18.5

23.5

29.7

40.0

49.8

2 hours

18.0

25.2

31.7

39.5

52.2

63.9

6 hours

30.6

41.5

50.9

62.0

79.5

95.0

12 hours

42.9

56.8

68.6

82.3

103.7

122.1

24 hours

60.1

77.8

92.6

109.6

135.2

156.9

48 hours

73.3

95.3

113.9

135.0

166.9

193.7

72 hours

82.5

107.4

128.3

152.4

188.8

219.0


86  Calculated using a beta-test version of HIRDS3.