ARF 1 bDa97

where D is duration in hours and the coefficents a and b are functions of catchment area given in Table 9.4 (Keers and Wescott, 1977). A graphical representation of these reduction factors is shown in Fig. 9.13.

This type of analysis can be extended to combine considerations of areal rainfall depths over a range of areas and varying durations of heavy falls. The technique of depth-area-duration analysis (DAD) determines primarily the maximum falls for different durations over a range of areas. Analysing the depth-area plots for different

Table 9.4 Flood Estimation Handbook (loH, 1999) area reduction factor coefficients

Area A (km2)

A

B

A < 20

0.40

- 0.0208ln(4.6 -

lnA)

0.0394A0354

20 < A < 100

0.40

- 0.00382ln(4.6

- lnA)2

0.0394A0354

100 < A < 500

0.40

- 0.00382ln(4.6

- lnA)2

0.0627A0254

500 < A < 1000

0.40

- 0.0208ln(lnA -

4.6)

0.0627A0254

1000 < A

0.40

- 0.0208ln(lnA -

4.6)

0.I050A0180

Duration (hours)

Fig. 9.13 Area reduction factors (after Flood Estimation Handbook, Vol. 2, loH, 1999, Copyright NERC CEH).

Duration (hours)

Fig. 9.13 Area reduction factors (after Flood Estimation Handbook, Vol. 2, loH, 1999, Copyright NERC CEH).

durations provides data to compile the highest maximum DAD values from envelope curves drawn for each duration.

9.5 Temporal variation in rainfalls 9.5.1 Checking for consistency at a site

Rainfall amounts are very variable from year to year, even in a humid environment like the UK (e.g. Fig. 9.14). Monthly totals can also show high variability from year to year, totals for individual days even more so. This means that it can be quite difficult to detect inconsistencies in rainfall measurements, for example, where it has been necessary to change the position of a rain gauge. One technique of checking for consistency over time is to use a method called double mass curves. To plot a double mass curve, accumulated rainfall totals from one or more rain gauges are plotted against the accumulated total for the gauge being checked. Fig. 9.15a shows the accumulated rainfalls for three sites in the upper River Eden catchment, the Mallerstang valley. The Castlethwaite site was being closed down by the Environment Agency, with a new site being established at Aisgill at higher elevation, close to the southernmost divide of the catchment. The two sites were run together during the period 1999/2000. Fig. 9.15b shows the double mass plots for both sites plotted against the Scalebeck site for the

Fig. 9.14 Daily and annual rainfall variability at Hazelrigg, Lancaster: (a) 10-min 0.2 mm tipping bucket rainfalls during part of February 2007; (b) daily rainfalls observed during part of 2007; (c) annual rainfalls observed for the water years (1 October to 30 September) from 1966-7 to 2006-7, mean annual rainfall 1072.7 mm, standard deviation 170.6 mm, mean daily rainfall 2.94 mm day-1.

Fig. 9.14 Daily and annual rainfall variability at Hazelrigg, Lancaster: (a) 10-min 0.2 mm tipping bucket rainfalls during part of February 2007; (b) daily rainfalls observed during part of 2007; (c) annual rainfalls observed for the water years (1 October to 30 September) from 1966-7 to 2006-7, mean annual rainfall 1072.7 mm, standard deviation 170.6 mm, mean daily rainfall 2.94 mm day-1.

1000

0 0

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