Auckland Transport

Inga Road Bridge

Background

Auckland Transport’s (AT) Inga Road Bridge, across the Wairau Creek in Milford, was initially constructed in 1926. The structure will reach the end of its economic life in 2026.

Stage 1 Study

AT engaged Weir to undertake a study to identify the existing flood hazard in terms of peak flood flows, levels and velocities at the bridge in a 100-year ARI storm event, including 3.8 degrees of climate change.

Working alongside our hydraulic modelling partners Submergent, a 2D TUFLOW model (version 2023-03-AD) was developed to determine the 2130-climate, 100-year-ARI design event maximum water levels and velocities in the vicinity of the Inga Road Bridge. Coastal inundation risk was also assessed, using the Auckland Council’s existing coastal inundation modelling.

The assessments identified the existing bridge as vulnerable to flooding from rainfall and coastal processes. Relevant peak flood flows, levels and velocities to inform the proposed bridge upgrade were then supplied to AT to inform the design process.

Stage 2 Study

AT subsequently engaged Weir to undertake a Stage 2 study to assess the effects of the preferred design (completed by AECOM) and to provide relevant updates to the hydraulic parameters necessary for detailed design.