Rainfall Return Periods

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  • Information on the frequency of heavy rainfalls is often required by engineers, architects and others, usually in connection with design criteria for water management or drainage schemes. In most cases it would be uneconomic to construct a system capable of coping with the most extreme rainfall possible, even if the magnitude of this is known. Instead, it is usual to design the system so that it will be capable of accommodating a rainfall likely to be exceeded only once in a specified number of years (return period).
  • A depth duration frequency model allows for the estimation of point rainfall frequencies for a range of durations for any location in Ireland. The model consists of an index (median) rainfall and a log-logistic growth curve which provides a multiplier of the index rainfall.  Rainfall station data were analysed and an index rainfall extracted. Computer applications were written to apply
    the model and produce outputs of the return period rainfalls.
  • The set of parameter values summarising the rainfall Depth-Duration-Frequency (DDF) relationship enables the production of consistent estimates of point rainfall frequencies over durations ranging from 5 minutes to 25 days. These estimates supersede those provided in Logue (1975).
  • The model enables the estimation of rainfall frequencies at any location. Rainfall Return Period tables are now freely available by submitting the form below.

Method used in the Estimation of Point Rainfall Frequencies

Download Estimation of Point Rainfall Frequencies (PDF document)

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