Rainfall Input for Master Urban Drainage Planning: The Integrated Catchment Study of Auckland City (New Zealand)

Rainfall is a principal input in hydrologic and hydraulic computer simulations that form part of a master urban drainage planning study. The selected rainfall intensity, storm patterns and duration can affect simulated drainage performance. Rainfall input used in the Integrated Catchment Study (ICS) of Auckland City is presented, including reasoning, methods employed, and outputs generated. The NZ$23.5 million ICS was a four-year study to develop a comprehensive understanding of the Auckland Isthmus drainage systems and receiving environments. It was a computer modelling focused study to assist Auckland City Council and Metro Water Limited to develop robust understanding and solutions to address: (a) wet weather combined and wastewater overflows that can create short-term public health issues, (b) stormwater quality that can produce long-term environmental health issues, (c) habitable floor flooding that can produce economic loss and public health issues and (d) provision for growth, which can exacerbate drainage issues. To address these issues the ICS undertook: (1) long term time series rainfall simulations to depict typical drainage performance and (2) design storm simulations to identify extreme event performance. Long term time series consisted of five-minute interval rainfall for (i) current climate and (ii) most probable future (2050) climate scenarios. For current climate, historic and stochastic time series were used. Design storms were considered from a number of different sources; a 24-hour duration nested (alternating block) storm was adopted for use. This approach allowed problematic drainage and receiving environment issues to be identified and initial solutions developed in priority areas.