![]() Instead a set of accurate, consistent IFD data have been derived for the whole of Australia. An analysis of rainfall data from a single station is often unreliable not temporally or spatially consistent and should generally not be used for design purposes. The process of estimating IFDs, known as frequency analysis, is an important part of hydrological design procedures. to express the "severity" of a single rainfall event (in terms of its rarity).design of roof and stormwater drainage systems,.design and risk assessment of dams and bridges,.įor a more in-depth discussion of ARI see the following: WHY DO 100 YEAR EVENTS HAPPEN SO OFTEN?.Īs implied above, the main use of these data is in engineering design. For this reason, the results are best tabulated, or represented as a graph of intensity versus duration for 7 different frequencies (see below). In fact the number of possible combinations of duration and frequency is many - take any one of 13 standard durations from 5 minutes to 72 hours, and any one of 7 frequencies from 1 per year to 1 per 100 years, and you have 91 combinations. Just as the engineer might ask about the 1-hour rainfall exceeded on average once in 10 years, so another might ask about the 72-hour rainfall exceeded on average once every 100 years. We now have our three terms: "Intensity", "Frequency" and "Duration" used in the abbreviation "IFD". It is calculated by dividing the depth by the duration and is simply a measure of the 'heaviness' of the rainfall. This "once in ten years" is a FREQUENCY.īefore going further, we will explain that for AR&R 87 IFDs, the Bureau of Meteorology does not use depth of rainfall in this particular context but prefers to use rainfall rate (in mm per hour), known as INTENSITY. Step further, a civil engineer designing a drain intended to fail (no more frequently than) once every ten years (on average) might ask the question, "What is the rainfall depth over one hour exceeded, on average, once in ten years?". For instance, we might be interested in the highest one-hour rainfall during a calendar year at a certain location. To compare the severity of different rainfall events, we might compare several rainfalls measured over one hour. So rain is well described in terms of a depth of rainfall over a certain duration. This period of time over which the rain is measured is called the DURATION. But for completeness, we also need to specify the length of time over which the rainfall occurred: one year - in the case of annual rainfall one month (for many climate purposes) or so many days, hours or minutes. We usually think of rainfall as being of a certain DEPTH (measured in mm). In order to explain what it is, we must first define some terms about rainfall. The abbreviation IFD stands for Intensity-Frequency-Duration it refers to statistics on rainfall. Create an IFD chart or table (non-flash).National Industry Guideline for water quality metadata. ![]() National Industry Guidelines for hydrometric monitoring.Urban Water Management Information Requirements. ![]()
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