Sewer Symphony: An Investigation of Hydraulic Phenomena in a Falling Main Through Acoustics

Joe Allen (Jacobs New Zealand Limited), Matt Sheppard (Jacobs New Zealand Limited), Jess Carruthers (Christchurch City Council), Patrick Cantillon (Christchurch City Council).

The Christchurch City Councils (CCC’s) goal is to create a livable city with strong communities and healthy environments. In 2007 CCC engaged with the Lyttelton Harbour/ Te Whakaraupō communities on the future of wastewater management in the harbour. The response was clear – stop pumping treated wastewater intoour harbour! By 2015 the concept design for what would become the Lyttelton Wastewater Scheme was underway. By 2018 consents were approved, Jacobs New Zealand Limited (Jacobs) had completed the detailed design, and Fulton Hogan were engaged as the constructor.

This project was a transformational change for the Lyttelton Harbour communities, ending the practice of disposing wastewater into the harbour that had taken place for nearly 200 years. The scheme comprised conversion of the existing Diamond Harbour, Governors Bay, and Lyttelton Treatment Plants into transfer pump stations, as well as construction of a new terminal pump station, and a combination pumped/gravity wastewater transmission pipeline. The transmission pipeline conveyed the wastewater to PS15 in Woolston and on to the Christchurch Wastewater Treatment Plant at Bromley. For the purposes of this paper, we refer to the gravity section of this pipeline, from the tunnel’s northern portal through to PS15 as the Heathcote Gravity Main. The Heathcote Gravity Main is a 4.6 km DN450/350 pressurized gravity sewer. 

During initial commissioning of the terminal pump station, surges and backflows occurred in the Heathcote Gravity Main under certain flow conditions, leading to overflows at the transition chamber where the pressure main transitions to the gravity main. The hydraulic design and behaviour of the pipeline had well considered during the design process, however understanding of the cause of the surging overflows required moving beyond conventional theoretical assessment into developing and testing of hydraulic hypotheses. 

Innovative techniques to ground-truth engineering theory were applied such as the use of a ground microphone to track the filling front of the pipeline to enable the project team to “see” inside a buried pipe that was otherwise inaccessible. After identifying the hydraulic phenomena causing the surging backflows, Jacobs were responsible for establishing and implementing a solution that enabled the safe operation of the scheme without any physical alteration to the pipeline. This led to the successful completion of commissioning of this asset and hand over to CCC. 

This paper details the unexpected overflows from the top of the gravity sewer transmission main and the series of investigations that were undertaken, both in the field via physical testing and using engineering hydraulic principles, to identify the causes and characterise the hydraulic behaviour in the pipeline under various conditions. This work brought together both in house and independent, local, and international hydraulics experts who, combined with the teamwork of the CCC operations and management staff, Fulton Hogan Site staff, and projectmanagement team, were able to come together as a high performing team and resolve this commissioning challenge.