Maximum observed rainfalls

The world's highest recorded rainfalls for a wide range of durations were assembled by Jennings (1950), and these formed the basis for what has become a well-known log-log plot of rainfall against duration from 1 min to 24 months. The records in the higher durations stemmed from Cherrapunji, India, with a maximum 2-year total of 41 000 mm in 1860-61. New records have recently been set by measured falls from tropical cyclones over the volcanic crater of La Reunion, an island in the Indian Ocean (Quetelard et al., 2008). A revised graph with the values in millimetres is given in Fig. 9.1 The points define an envelope curve for a relationship between the maximum values, R, and duration, D. Higher falls may have occurred, of course, but not been observed. However, because this represents a sample over a period of more than 100 years and over many thousands of gauges, it is expected that the envelope curve would be exceeded only very rarely.

100000

10000

10000

100 1000 10000 Duration (min)

100000

100 1000 10000 Duration (min)

100000

1000000

1000

♦ Convective □ Frontal a Orographic

100 1000 Duration (min)

10000

Fig. 9.1 (a) Extreme rainfalls and straight-line envelope curves for the UK and world rainfalls with durations from 1 min to 1 year (data from Tables 9.1 and 9.2). (b) Extreme event rainfalls in the UK classified by event type (data from Hand et al, 2004).

A similar envelope curve can also be constructed for the UK where there are a total of more than 6000 gauges, mostly recording daily catches. Maximum rainfall totals for different durations have been abstracted by the UK Met Office and are also shown in Fig. 9.1a. The UK shows a similar pattern of increasing totals with increasing duration but at much lower values (note the logarithmic scale for the rainfall totals). These extremes are also compared in Table 9.1 for the world and Table 9.2 for the UK.

A useful study of heavy daily falls in the UK was made by Bleasdale (1963) for the years 1893-1960. He listed the 142 occasions on which more than 127mm

Table 9.1 World rainfall extremes for different durations

Duration

Rainfall (mm)

Place

Date

1 min

38

Barot, Guadeloupe

26 Nov 1970

20 min

206

Curtea-de-Arges, Romania

7 Jul 1889

42 min

305

Holt, USA

22 Jun 1947

60 min

401

Shangdi, Nei Monggol, China

3 Jul 1975

360 min

840

Muduocaidang, China

1 Aug 1977

1 day

1 825

Foc Foc, La Réunion

7-8 Jan 1966

2 days

2 467

Aurere, La Réunion

7-9 Apr 1958

3 days

3 929

Cratère Commerson, La Réunion

24-27 Feb 2007

4 days

4 869

Cratère Commerson, La Réunion

24-28 Feb 2008

10 days

6 028

Cratère Commerson, La Réunion

18-27 Jan 1980

1 month

9 300

Cherrapunji, India

1-31 Jul 1861

1 year

26 461

Cherrapunji, India

Aug 1860-Jul 1861

2 years

40 768

Cherrapunji, India

1860-1861

Table 92

UK rainfall extremes for different durations (data from Hydrology

in Practice, 3rd Edition

and Hand et al., 2004)

Duration

Rainfall (mm)

Place

Date

5 min

32

Preston, Lancashire

10 August 1893

12 min

51

Wisbech

27 June 1970

15 min

56

Bolton

18 July 194

18 min

93

Hindolveston (Norfolk)

11 July 1959

30 min

80

Eskdalemuir, Dumfries and Galloway

26 June 1953

45 min

97

Orra Beg (Antrim)

1 August 1980

60 min

92

Maidenhead, Somerset

12 July 1901

75 min

102

Wisley (Surrey)

16 Julyl947

90 min

117

Dunsop Valley, Lancashire

8 August 1967

120 min

193

Walshaw Dean Lodge, West Yorkshire

19 May 1989

155 min

169

Hampstead, Greater London

l4 August l975

180 min

178

Horncastle., Lincolnshire

7 October l960

24 h

279

Martinstown, near Dorchester, Dorset

l8 July l955

24 h

314

Seathwaite, Cumbriaa

l8 November 2009

1 month

1 436

Llyn Lydaw, Gwynedd

October l909

1 year

6 527

Sprinkling Tarn, Cumbria

l954

aThis event occurred soon after this manuscript was delivered to the publishers. Initial information suggests that this winter synoptic event has now replaced the summer convective event at Martinstown 1955 (see Fig. 9.12) as the highest recorded 24-h rainfall in the UK.

aThis event occurred soon after this manuscript was delivered to the publishers. Initial information suggests that this winter synoptic event has now replaced the summer convective event at Martinstown 1955 (see Fig. 9.12) as the highest recorded 24-h rainfall in the UK.

(5 in) had been recorded, and noted that all the falls greater than 102 mm (4 in) numbered over 450. From this study, Bleasdale concluded that no part of the UK could be completely immune from a daily fall of at least 102 mm. Rodda (1973) analysed 121 records with more than 50 years of data and showed how daily rainfall extremes could be related to mean annual average rainfalls across the country. Hand et al. (2004) also analysed a variety of extreme rainfall events in the UK and concluded that different types of extreme event (convective, convective with frontal forcing, orographic and frontal) occupied different parts of the rainfall-duration

1

11

i i

i i LIi Jl

III. 1. lLJill.

t'T 1 T

1870 1880 1890 1900 1910 1920 1930 1940 1950 1960

Year

1870 1880 1890 1900 1910 1920 1930 1940 1950 1960

Year

Fig. 9.2 Maximum annual daily rainfalls recorded across all sites in the UK as recorded in British Rainfall 1866 to 1968 (from Rodda et al., 2009, with kind permission of John Wiley & Sons).

plot (Fig. 9.1b). Rodda et al. (2009) have produced a digital archive of all the UK rainfall extremes reported in the publications of British Rainfall Organisation from 1866 to 1968 (see e.g. the graph of the annual recorded extreme in Fig. 9.2). In the Flood Estimation Handbook (Institute of Hydrology; IoH, 1999), the occurrence of daily totals for given return periods can be derived from the values of the parameters in the depth-duration-frequency (DDF) distribution described below in Section 9.6.1, which are available across the UK on a 5-km grid.

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