Environmental Engineering Reference
In-Depth Information
Figure 5.7
The return period for precipitation totals at
Sheffield.
Source: Based on methods in Flood Studies Report 2 (NERC 1976).
normally to produce something that will cope with all but the most extreme events. The
frequency of heavy rainfall events may also be of significance in affecting slope
processes and landslips. It is important to remember, however, that the figures are only
probabilities, derived from average conditions over a specific period. It is quite possible
for two storms with an average recurrence interval of fifty years to occur on successive
days!
Another way of expressing information on rainfall variation is to plot annual rainfall
totals on similar graphs. Thus in Figure 5.8 the frequencies of annual rainfall for
Sheffield and Timimoun are shown. We can see that there is a 50 per cent probability of
at least 820 mm of rainfall occurring in any year at Sheffield, while at Timimoun the
equivalent total is 19 mm. It is also apparent from the graphs that the variability at
Sheffield is fairly small compared with that at Timimoun, although the latter is, on
average, much drier.
Again this type of data may be very useful. For example, a particular crop may grow
satisfactorily only if the annual rainfall exceeds 600 mm. From information on annual
rainfall frequencies it is possible to determine the likelihood of receiving that amount of
precipitation. If the probability is, say, 90 per cent, the farmer may well think it
