New Zealand Transport Agency: SH16 Causeway Upgrade Project

The New Zealand Transport Agency (NZTA) is upgrading and elevating the State Highway 16 (SH16) Causeway near Auckland to reduce roadway flooding during extreme tidal conditions under projected future sea-level rise. The SH16 Causeway Upgrade Project involves raising 4.8 kilometers (3 miles) of both roadway and bike path along SH16, also known as the Northwestern Motorway, by 1.5 meters (5 feet). For this project, NZTA utilized sea-level rise planning recommendations produced by the NZ Ministry for the Environment. The Causeway represents a critical portion of a future alternate transportation route around the Auckland region, which will help improve transportation capacity, efficiency, and economic growth.

The old SH16 causeway was built in the 1950s on soft marine mud; as a result, land subsidence has caused the causeway to gradually sink, rendering it vulnerable to flooding during high tides. In recognition of the impacts of climate change and sea-level rise, NZTA has designed the new Causeway at a higher elevation so that it will be less prone to flooding over its lifetime. In designing the Project, NZTA incorporated sea-level rise planning guidelines produced by the New Zealand Ministry for the Environment in its manual, “Coastal Hazards and Climate Change: A Guidance Manual for Local Government in New Zealand.” The Manual recommends designing to accommodate a magnitude of sea-level rise based upon the acceptability of potential risk, by considering three factors:

  • The possibility of particular sea levels being reached within the design life,
  • The associated consequences and potential adaptation costs, and
  • How any residual risks would be managed for consequences beyond an accepted sea-level rise threshold or if the design underestimates sea-level rise.

To provide further guidance on the sea-level rise risk assessment, the Manual recommends, at a minimum, accommodating a base value of 0.5 meters sea-level rise (relative to the 1980-1999 average) for planning and decision timeframes out to 2090, and assessing the consequences of greater sea-level rise values (0.8 meters or more). For timeframes beyond 2090, the Manual recommends allowing for an additional 10 millimeters sea-level rise per year beyond the year 2100.

The Causeway is a key part of the future Western Ring Route, which upon completion will provide an alternative route around Auckland, allowing for improved network efficiency and regional growth. The Western Ring Route is one of New Zealand’s seven “roads of national significance,” a program that is part of the country’s National Infrastructure Plan and is designed to build or upgrade critical roads around the country’s five largest population centers, thereby enabling economic growth. The entire Western Ring Route, the largest project ever undertaken by NZTA, is anticipated to cost NZ$2.4 billion and be completed by 2021. The SH16 Causeway portion is estimated to cost NZ$220 million and be completed by early 2017, when other new links within the Western Ring Route are also expected to open.

In addition to raising the roadway and bike path, the Causeway Upgrade Project will also widen and improve these portions of SH16 to help add capacity to the transportation system. The Project also involves using innovative construction techniques and design refinements to minimize risks to the Motu Manawa Pollen Island Marine Reserve, through which the SH16 Causeway runs. One of the initiatives to protect the reserve involves treating stormwater before discharging it into the harbor. Stormwater treatments in the project design include a minimized pipe network, conveyance channels, and vegetated treatment swales. The selected design for stormwater treatment reduces the overall footprint of the Causeway, which will result in fewer environmental impacts to the Marine Reserve.  

 

This Adaptation Clearinghouse entry was prepared with support from the Federal Highway Administration. This entry was last updated on March 23, 2016.

 

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