Ashburton River Wastewater Pipeline Crossing and Wilkins Rd Pump Station

WSP worked with Ashburton District Council to assess and replace the existing Ashburton Relief Sewer to provide resilient wastewater for the township and support planned growth for decades.

Location

Canterbury, New Zealand

Client

Ashburton District Council

Project Value

$7.7m

Project Status

Completed 2020

Key insights:

At $7.7m, the Wastewater Pipeline and Pump Station was Ashburton District Council’s largest single infrastructure investment in the last decade.
The new pipeline and pump station future proofs Ashburton’s wastewater network for future generations.
The new pipeline was carefully designed to minimise risks in an active braided river bed.
The project was completed on time and under budget.

Project description

The replacement of the Ashburton Relief Sewer was critical to the future of Ashburton, and at a cost of $7.7 m, was the Ashburton District Council’s most significant infrastructure investment in the last 10 years. The existing sewer’s condition and capacity would not cater for Ashburton’s planned population growth or respond to the uncertainties produced by our changing climate. Recognising the critical nature of the asset, Ashburton District Council’s brief to WSP was for resilient infrastructure, that was easy and safe to maintain, and which could be delivered quickly. Managing stakeholders and environmental risk while building the pipeline under the braided Ashburton River was also vital to the project’s success.

WSP worked with the Council to assess the capacity of the existing Ashburton Relief Sewer, that carried wastewater from the township across the Ashburton River to the wastewater treatment plant. WSP and the Council’s chosen solution was a 9 m deep 850 l/s wastewater pump station, with a 1 m diameter wastewater pipe stretching under the Ashburton River.

The project included several complex social, environmental and engineering challenges:

Managing the depth of the pipeline to provide resilience under the dynamic braided Ashburton River
Designing a bespoke manhole drop structure upstream of the river crossing
Selecting pipe material, bedding and testing methods to facilitate rapid installation of the pipeline and minimise risks in the river
Siting the pump station in the immediate flood plain of the Ashburton River and designing the deep wet well
Mitigating the impact of construction in the river with respect to migratory eels and endangered black-billed gulls.

The Solution

WSP provided a broad range of design services including capacity assessments, options study, investigations, survey, Safety in Design and HAZOP, multidisciplinary design, tender documents and cost estimates.

Our innovative and careful design responded to the engineering requirements of the project while mitigating the environmental risks. As a result, the new pipeline was designed 3 m beneath the deepest part of the river, to manage the risk of damage from riverbed degradation and scour, and to respond to future climate change uncertainties.

The pumps were carefully selected and two rising mains provided for operational resilience and efficiency across a wide range of wastewater flows. The bespoke 7m deep polyethene drop structure upstream of the river crossing was designed to minimise turbulent flow, reduce gas generation and dissipate energy. In addition, following Safety in Design and HAZOP the design and client team chose to place the valves and manifold above-ground, reducing cost and simplifying operations.

The Benefits

The successful project increased wastewater reliability and resilience for the Ashburton community, and will support Ashburton’s planned growth for the next 30 years. In addition, the project achieved its operational and environmental goals. The completed pipeline enables greater operational flexibility and lower costs, while works were coordinated and timed to mitigate impacts on seasonal eel migration and the endangered black-billed gulls.

Canterbury Regional Council presented Ashburton District Council with an award for ‘exemplary attitude towards compliance and collaboration’, in recognition of the successful and efficient delivery.