ALL DRESSED UP AND NO PLACE TO FLOW, A $25 MILLION OUTFALL

A major stormwater upgrade to the pipe network passing through the Ports of Auckland land was required to reduce upstream flooding and to replace aging infrastructure. Without the upgrade, drainage improvements to the upstream network (which have already been constructed) would increase downstream flood risk. Indicative capital costs of the upgrade are approximately $25 million.

Due to construction complexity and hydraulic limitations, a range of design options were considered by an Early Contractor Involvement (ECI) group that would reduce disruption to the port whilst providing improved flood resilience. A risk based approach was used to establish the costs and benefits of the options, so that a realistic hydraulic performance objective could be established by Auckland Council for the ECI group.

The risk based approach considered the effects of a range of design storms, tailwater levels, and sea level rise for the different options. The outcomes of the assessment were used to create a business case that needed to provide both value to existing ratepayers, resilience to future changes in climate and consider the effects of disruption to the Ports of Auckland.

This paper will focus on the risk based assessment and business case development that formed the recommendation to the ECI group. It will include discussion on the quantitative risk assessment including the flood damage assessment, and the economic and qualitative viewpoints encountered along the way.