To provide river bank protection to SH7 on the Waiau River, through the provision of a series of rock groynes protruding into the river over a 680m length. This work was required due to significant erosion of the right bank, moving upstream and towards the state highway (SH7), within approximately 10-15m from the road in an area of very active erosion on the outside of the bend.
CLIENT BRIEF/PROJECT CHALLENGES
WSP provided design services for the assessment of the existing channel, likely future erosion at the site, concept development and detailed design. A site overview of the river behaviours was combined with a brief study of historic river movements, used to inform the design process and avoid future outflanking of the proposed works which could have been quite likely here if only local protection works were carried out. An option assessment was followed by a review of the proposed concept design to look for efficiencies and savings of up to 50% were achieved with little impact on risk to the structure into the future.
WSP reviewed the present and past river geomorphology to understand likely future river flow patterns and erosion risks, followed by an option assessment and development of an erosion protection design. Consideration was given to the likely future maintenance needs and levels of monitoring required. Different cost vs risk balances were assessed and the favoured option was optimised for cost efficiencies while maintaining acceptable levels of risk and maintenance implications. Potential risks to upstream and downstream areas were considered, as well as along the reach and the opposite bank, with the need for removal of old railway irons that created a safety hazard for speed boats.