BEST FOR AUCKLAND STORMWATER OUTCOMES FOR EASTERN BUSWAY

P. May (Jacobs)

ABSTRACT

The Eastern Busway Alliance (EBA) is delivering an integrated, multi-modal transport system to support population and economic growth in southeast Auckland and decarbonise transportation. Eastern Busway will enable investment in intensification along the busway corridor.

This paper focuses on the development of the design philosophy, project outcomes and meeting Connection Requirements in Schedule 4 of the Auckland Regional Network Discharge Consent (NDC) framework. Aligned with the project objectives, the stormwater design philosophy adopts a maintenance-led approach, prioritising the best for Auckland outcomes through collaboration with Auckland Transport, Auckland Council, mana whenua. The design therefore incorporates Auckland Council Healthy Water Betterment objectives, the EBA objectives and mana whenua aspirations.

To balance maintenance considerations, treatment outcomes, and project constraints, a best practicable option (BPO) approach for stormwater treatment was adopted within the NDC framework. This approach evaluates options based on mana whenua aspirations, whole-life costs, and contaminant load reduction outcomes for outfalls. Mana whenua challenged the EBA to practice tiakitanga by treating stormwater better, fix flooding and generally do it once and do it right while keeping operations and maintenance costs affordable for the community.

The preferred BPO option for stormwater treatment involves green infrastructure (e.g., raingardens, passive irrigation channels, and swales) where feasible, with gross pollutant traps (GPT) in other areas. The GPTs are sized to achieve a 50% total suspended solids removal. Discretionary treatment targeting high contaminant generating roads and parking areas outside the project works was also incorporated and betterment including treatment of existing stormwater networks. The project is predicted to achieve overall reductions in contaminant loads to the environment from roads in the order of 40% for total suspended solids and 20-30% for total zinc, copper, and total petroleum hydrocarbons.

Upgrading existing stormwater networks from the former Manukau City Council standards is a crucial aspect, as these networks currently have a capacity approximately equal to a 50% Annual Exceedance Probability (AEP) event with climate change, leading to significant overland flows and flooding around the project corridor for 10% and 1% AEP events.

The agreed philosophy incorporating betterment involves combining and upgrading the project and existing networks across the busway corridor, upgrading outfalls, and diverting existing networks into the upstream end of the new stormwater systems to reduce flooding, overland flows, and enhance infrastructure resilience. The flooding and overland flow management design elements are predicted to eliminate flooding across the road corridor during the 10% AEP event and significantly reduce flooding crossing the busway in the 1% AEP event. No new or increased flooding is predicted on private property as a result of the project with significant improvements for most properties that experienced flooding in the existing case.

Incorporating the Auckland Council Healthy Water Betterment works into the Eastern Busway Alliance's scope will result in best-for-Auckland solutions that meet current standards, improve resilience, provide safe and affordable maintenance activities, and reduce overall whole-of-life outcomes. The improved resilience of the road corridor to flooding helps enhance the public transport customer experience and decarbonise transportation in East Auckland.