Guidelines for rainfall analysis as utilised for the Dunedin City Council 3 waters strategy

As part of the 3 Waters Strategy project for the Dunedin City Council, URS New Zealand Ltd undertook a rainfall analysis for incorporation into storm water, wastewater and potable water models being prepared for Dunedin. This paper outlines the derivation of the rainfall information that was used to provide a robust rainfall analysis, critical to achieving better quality and site specific storm water modelling. Although it is based on the Dunedin 3 Waters Strategy, this methodology could be applied to any urban area in New Zealand, taking local requirements into consideration.

This paper provides a guide to the derivation of rainfall data for use in urban storm water modelling and is aimed primarily at younger practitioners. It will also assist decision makers regarding outcomes that should be achieved by a rainfall analysis.

The paper focuses on the basic steps for rainfall analysis including:

• Consideration of intended use, evaluation of available data
• Development of Depth-Duration-Frequency (DDF) curves
• Development of a design storm hyetograph
• Consideration of storm shape and peak intensities
• Analysis of the spatial variation
• The effects of climate change and climate variability on rainfall data
• Determination of Areal Reduction Factor (ARF)