Rainfall Intensity

Amounts of rainfall are measured in terms of the depth of the layer created by spreading the water on a horizontal surface. It is now expressed in millimetres depth, and the rate (or intensity) of precipitation during a given period is the total collected divided by the duration, usually expressed in millimetres per hour. It can be measured for periods longer than a few minutes by means of a pluviograph, an instrument for recording the times between refillings of a small cup into which the collected rain flows.

Rainfall intensities fluctuate during a storm. For example, the amounts in six successive five-minute periods in a shower might be 0.5 mm, 2.5 mm, 1.0 mm, 1.0 mm, 0.8 mm and 0.2 mm, implying a maximum intensity on a five-minute basis of 30 mm/h (i.e. 2.5x60/5), but only 21 mm/h over ten minutes, and 12 mm/h over thirty minutes. So the maximum intensity of rainfall is less for greater periods of averaging. Amounts of rain collected in the course of a year, for instance, are commonly plotted on maps, where places with equal amounts are linked by lines called isohyets.

'Light rain' means less than 1 mm/h and 'heavy' rain means more. A rate of over 60 mm/ h for at least five minutes is called a cloudburst. The intensity may peak at 120 mm/h for a minute or two in a normal storm, though over 500 mm/ h was measured at one spot in Sydney during five minutes on 2 April 1992. age intensity. The same has been found for Sydney record rainfalls, though they are only about a seventh of global record values.

Extreme Rainfalls

The world-record rainfall by 1986 during one minute (at Barot, Guadeloupe) was equivalent to 2,300 mm/h; over twenty minutes the highest rainfall rate had been 1,200 mm/h (Curtea-de-Arges in Argentina); over an hour 430 mm/ h (Holt, Missouri); a day 76 mm/h (Foc Foc at Reunion, a volcanic island at 21°S in the western part of the Indian Ocean); a week 26 mm/h

(Commerson, Reunion); a month 12 mm/h (Cherrapunji, Assam); and over a year 3 mm/h (Cherrapunji). Such figures suggest that a fourfold increase of duration halves the record aver

The record daily rainfall in Australia was at Bellenden Ker Top Station near Cairns in Queensland on 4 January 1979, when 1,150 mm fell, equivalent to 48 mm/h. A world record was set there in the same month—3,847 mm during eight days.

The intensity is related to the drop size (Note 9.D) and soil erosion (Note 10.F). Figure 10.1

indicates that the highest intensities occur especially at low latitudes. The diagram compares rainfalls at Darwin and Hobart, in terms of the average time (or 'recurrence interval' or return period) between rainfalls of a particular intensity and duration. The Hobart curves are lower, so, for instance, 13 mm/h over one hour is likely to be exceeded once a year, compared with 50 mm/h at Darwin. Darwin rates are especially high for recurrence intervals of only an hour or so, because of the sporadic

/

Darwin 10( ^Darwin 1

5 yrs yr

V

Hobart 100 yrs f / Hobart 1yr '

time: hours

time: hours

Figure 10.1 Rainfall intensity, duration and frequency diagrams for Darwin (dashed pair of curves) and Hobart (solid pair). The lower of a pair displays the one-year recurrence interval, the upper one gives the 100-year interval. For instance, the wettest hour each year yields 10 mm at Hobart but 50 mm at Darwin, on average. Also, the rainfall on average exceeds 100 mm/h for 45 minutes once each hundred years at Darwin, and 5 mm/h for 5 hours once a year at Hobart.

intensity of the convective rainfall there, whereas rain at Hobart is chiefly frontal, i.e. prolonged but gentle.

The average annual intensity derived from many years of records at Alice Springs is about 250 mm/a, falling within about forty days, i.e. about 6 mm per rainday. Such a figure is a useful index of what a wet day is like. It is about 20 mm/rainday at the equator, but less at higher latitudes (Figure 10.2).

Runoff

Urban drainage channels are commonly designed to cope with the maximum rainfall to be expected over an hour, whereas extreme river flows are more related to the 'maximum rainfall' over a day. This 'maximum rainfall' is typically chosen as that with a twenty years' recurrence interval, but if overflowing would have particularly serious consequences the drain or river is designed to cope with rains exceeded only once a century, on average. The design is a compromise between (i) the greater expense of building a larger channel to cope with heavier rainfalls, and (ii) greater costs from flood damage

Figure 10.2 Rainfall per rainday in Australia.

due to more frequent overflowing of a smaller channel. Unfortunately, such a procedure for optimising the benefit-cost ratio of a channel ignores changes of climate and runoff ratio over periods as long as a century, so any estimate of the best design is only approximate and must have a margin added for safety.

The daily rainfall at a place, likely to be exceeded only once in some specified period, such as a century, can be estimated roughly from measurements over a shorter time (Note 10.G).

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