Malvern Street Sewer Renewal and Protection is a finalist in the Excellence in a Water Project Category in the 2022 IPWEA NZ Asset Management Awards.

The Malvern Street Sewer Renewal and Protection project required mutual trust and a shared commitment to meet all the original project objectives, most notably providing security to the sewer pipeline and surrounding land against damage and contamination during flood events.

The Malvern Street sewer was laid in the 1940’s upstream of a natural weir and downstream of an existing Heritage Listed bridge. A concrete apron was added to protect the cast iron sewer pipe from boulder impact when the river is in flood.

During rainstorms, the river produced a scour hole and eroded the true right bank, leading to exposure of the cast iron sewer. On several occasions between 2014 and 2018 the cast iron pipe was struck by boulders causing discharge of effluent into the Water of Leith.

The scope of the project was to implement a permanent repair to the existing sewer and future proof the site from further environmental impacts.

While the project was initiated in 2018, it gradually evolved in scope until it took its final form in 2021 following workshops with the Principal and appointed Contractor. Concept and detailed design of the final solution took place during 2021 before construction began in November 2021. In-stream works were completed in April 2022.

This project demonstrates asset management excellence by providing protection to the existing assets and extending their useful life in a pragmatic and cost-effective manner by utilising the sunk cost of existing assets. For example, it was investigated and recognised that CIPP lining of the sewer pipe was still able to meet the projected demand for the area without the significant expense and construction-related risk associated with complete replacement. Furthermore, strategic investment in erosion protection structures has secured the pipeline for the foreseeable future, hence saving the cost of reactive mitigation and negative environmental impacts of accidental effluent discharge.

Asset Management Principles

Future Proofing and Growth

The upstream catchment for the sewer pipe was assessed for current and future growth within the area. This analysis indicated that the current pipe size has sufficient capacity for the predicted growth over the next 50 years.

The existing cast in-situ reinforced concrete manholes were inspected and found to be in good condition, hence it was decided to extend their useful life as opposed to completely replacing these assets.  Similarly, rather than replace the cast iron pipe, the project team decided that the optimal solution was to structurally reline the pipe.

The original structure of the weir and groyne was causing scour within the riverbed and undermining the true right bank of the river, threatening private assets. To mitigate these impacts the new structure was designed to contain a Q100 event and dissipate the energy of the river, thereby reducing the risk of future erosion of the riverbed and banks.

Extending the structural apron upstream and using this as a prop for the retaining walls has decreased the amount of excavation required for the whole project and has provided further protection for the foundations of the bridge, thus extending the useful life of the bridge and increasing the return on investment to the community.

Sustainability and Climate change

Using the available hydrometric data, the design flows were derived, and various design events were analysed for their annual exceedance probability.

The design flood estimates for the project used the Ministry for the Environment (MfE) climate change predictions for New Zealand based on the IPCC 5th Assessment (MfE, 2016). The method developed in 2010 for determining the projected increase in rainfall because of climate change in New Zealand was adopted. For the IPCC 5th Assessment, a new set of four forcing scenarios was developed, known as representative concentration pathways (RCPs), for the years 2040, 2090, and 2110.

Using the design flood estimates for climate change scenario RCP 6.0 and an AEP of 1% the new weir structure was designed with combination of HEC-RAS and computational fluid dynamic modelling to optimise the height, width, and length of the weir, stilling basin, and energy dissipation structure. This reduced the amount of excavation required to a minimum and optimised the size of the structure as required to mitigate the design flood.

Regulatory Environment and Environmental Impact Management

The existing sewer has previously been breached during storm events and effluent has spilled out into the river. The design structurally repairs and strengthens the existing sewer pipe, and the apron provides further protection to sewer pipe. This significantly reduced the risk of further effluent spills into the river and protects the environment from the impact of the effluent.

The original weir configuration was not specifically designed to allow for the passage of fish. The new structure with a stilling basin, fish passage, and baffles enables fish to pass the new weir and travel upstream. Local residents have reported seeing fish migrating upstream as well as jumping up the weir from the stilling basin.

To minimise the impact on the environment, excavation at the site was minimised through the design process. Any excavated material was screened with that material used as backfill to the retaining wall, the screened cobbles and boulders used as rip rap protection, and placed downstream of the structure to provide further protection of the river bed and banks.

Project sustainability

The wider benefits to the community are an enhanced stream environment, protection of the existing assets thus extending their life, and the opportunity to view migrating and indigenous fish to travel further upstream.

The new design provides resilience to the local environment from the impact of future storm events and mitigates the risk of severe environmental impact, by protecting both public and private assets. Utilising the existing structures which were in good condition, and either providing further protection or increasing their structural capacity, maximised the return on investment of the existing assets.