FLOOD CHALLENGES AND COLLABORATIVE SOLUTIONS: COMPREHENSIVE STORMWATER MODELLING ON EASTERN BUSWAY

T. Newman (AECOM) & P. May (Jacobs)

ABSTRACT

The Eastern Busway Alliance will deliver Auckland's first urban busway, providing sustainable transportation options to link the suburbs of Botany and Pakuranga to Auckland's city centre. The busway is anticipated to accommodate 18,000 passengers daily by 2028 through 5 km of dedicated bus lanes while providing 12 km of separate walking and cycling paths.

Without the adoption of mitigation measures, the busway could result in detrimental environmental effects such as stream channel erosion and flooding. Leveraging flood modelling as a robust analytical tool, a comprehensive assessment of stormwater impacts to maintain flood neutrality was undertaken using ICM software.

The existing Pakuranga stormwater network was designed to convey the 5-year Annual Exceedance Probability (AEP) storm event without consideration of climate change, as was Manukau City Council’s requirement in the 1960s. This resulted in the stormwater network being significantly under-capacity for the 10-year AEP with allowance for climate change as required in the Auckland Council Code of Practice, as well as the additional runoff from increased urbanisation. Consequently, the 10-year AEP flood modelling indicated significant overland flow paths through the project area.

To address the flooding issues that existed before the busway development, the pre-development flood model was used to determine the necessary stormwater network upgrades needed to accommodate the 10-year AEP with climate change runoff and additional urbanisation runoff. This approach identified existing pipes for upgrades and integration with the design pipe infrastructure. The pipe selection promoted a collaborative approach with Healthy Waters to enhance flood conditions in the area.

The busway design criteria required that busway lanes do not experience more than 10 mm during a 10-year AEP and less than 100 mm during a 100-year AEP storm event, maintaining an operational transport link. Design scenarios were modelled to inform design requirements to prevent detrimental flood impacts and to determine the busway's operational continuity during future extreme stormwater events.

A 0.25 m² mesh zone was applied across the road corridor providing a higher-than-typical resolution to represent the proposed road design for the crest, crossfalls, kerbs and medians. Raingardens and passive drainage channels were represented using mesh level zones adjusting the underlying ground model. Grated drains were also modelled using mesh level zones, providing stable inflow into the stormwater network along the length of the grated drain.

Flood modelling of pipe blockage scenarios and Representative Concentration Pathway (RCP) 8.5 temperature increases offered valuable insights into project resilience. Flood risk from overland flow paths was assessed by reducing the pipe conveyance capacity. Modelling of the RCP 8.5 global warming temperature increase scenario, which is greater than the current design standard, provided further understanding of the potential effect of future storm events on the Eastern Busway.

This paper demonstrates the importance of detailed hydraulic modelling in project-wide assessment of flood mitigation measures and their performance using design scenarios to identify appropriate infrastructure measures to be applied. This is critical to accurately predict flood risk and ensure infrastructure can perform to the required level of service now and into the future.