Ahaura SH7 Bridge Hydraulic Modelling and Scour Mitigation

Waka Kotahi engaged WSP to progress the design of a new two-lane (SH7) bridge to be located between the existing SH7 Ahaura bridge and the Railway Bridge.

Location

West Coast, New Zealand

Client

Waka Kotahi NZ Transport Agency

Project Value

$25m

Project Status

Start Date: 2018. Study Finish Date: October 2019. Estimated Project Completion Date: 2021.

The existing SH7 Ahaura Bridge is a narrow single lane bridge constructed in the 1930’s, located at the south-western end of the Ahaura township (located within the Grey Valley, 34 kilometres north-east of Greymouth). It is an integral and critical part of the SH7 network. WSP provided detailed design services for the replacement of the existing bridge. Design flood levels and velocities were developed for the new bridge, with scour depths assessed and mitigation measures proposed. The main flow channel is of variable width and contains a number of bends, within extensive flood plains of variable width with irregular river terraces. The effects of the proposed bridge on the existing railway bridge approximately 200m downstream were also considered.

Existing water flow

Proposed water flow

OUR SOLUTION

WSP developed a two-dimensional computational hydraulic model to analyse the existing situation, including the impact of the existing SH7 and railway bridges on the flood flow in the Ahaura River. This computational hydraulic model is calibrated against a 1988 flood extent map. The model was then used to analyse the impacts of the proposed new bridge on the river flows and water levels for a 1% AEP flood flow, for the 2018 and 2120 climate change scenarios, to assist in the design of the proposed bridge and especially the scour protection.

2d computational hydraulic model

OUTCOME/CLIENT BENEFITS

The Client was provided with detailed and accurate design parameters for the new bridge to ensure that it would stand up to extreme flood events, and with confidence that the design could proceed with minimal risk to existing structures. The model allowed a detailed understanding of the bridge hydraulics, such that design alignments could be optimised and risks reduced.