Wellington Water provides drinking water, storm water, and sewer across five regional council areas in the south eastern tip of the North Island, New Zealand. Wellington Water engaged WCS Engineering to evaluate, optimize and prioritize wastewater network improvement alternatives for the Porirua wastewater catchment and produce a preliminary master plan strategy.

Optimization scenarios were performed for a range of overflow frequency containment targets to help select the desired level of service that would effectively balance cost and benefits to the community and environment. This included considering hybrid containment targets such as 1-year ARI design storm containment for manhole flooding and 6-month ARI design storm containment for controlled overflow structure discharge.

To identify design storms that provide a good representation of each return period to be considered, Wellington Water engaged WCS to run a continuous model simulation of historic rainfall and perform statistical analyses of the overflow results. The historical rainfall from Jan 2008 to Jan 2018 was used in the continuous model simulation. Detailed statistical analysis of the InfoWorks model results was completed based on controlled overflow structure discharge and manhole flooding. Due to the significant difference observed in system response between the upstream and downstream networks, it was necessary to complete separate analyses and select different design storms for the upstream and downstream networks.

The preliminary optimization results identified significant opportunities for cost savings through effective placement of wet weather storage facilities and asset rehabilitation to reduce inflow and infiltration (I/I).