Many hydrological and hydraulic design applications need to be based on reliable estimates of rainfall
quantile/design rainfall, which is the expected rainfall depth corresponding to a given duration and
average recurrence interval (ARI). The generalized design rainfall data in the form of intensity–duration–
frequency (IDF) curves are generally estimated from a regional rainfall frequency analysis (RRFA)
approach using recorded rainfall data from a large number of stations within a country, for example,
United Kingdom [60], United States [33], and Australia [40].
A review of rainfall frequency estimation methods was undertaken by Svensson and Jones [82]. They
outlined the nationwide approaches adopted for design rainfall estimation in nine countries. The difficulties in the derivation of topographically, spatially, and temporally consistent IDF information arise from the fact that many of the rainfall stations have relatively shorter record lengths and the continuous pluviometers often have poor spatial density. In fact, many countries have very little data availability on continuous rainfall records; however, daily recorded data are often widely available. This allows
estimating 24 h design rainfalls; but in many urban applications, much shorter durations are needed.
Another difficulty is that rainfall characteristics can vary sharply with distance, in particular, in mountainous terrain, which makes the spatial interpolation of design rainfall characteristics a more difficult task. Design rainfall estimation is generally made using a regional frequency analysis rather than at-site analysis, which is the fundamental difference between design rainfall and flood estimation problems; in flood estimation, at-site analysis is preferred provided there is enough data.
Design rainfall estimation involves a number of steps, which are described in the following sections
of this chapter. Various steps are illustrated in Figure 10.1 to provide an overview of the principal
steps.
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