Designing prssure sewer systems for topographically challenging catchments: an account of the Opua pressure sewer system

This paper will present the challenges faced in designing a reticulated sewer system for Opua, and how those challenges were overcome in the design of a pressure sewer system to reticulate the township. Opua, in the Bay of Islands, New Zealand, presents significant challenges to the design of a reticulated wastewater system. The area consists primarily of bush covered steep hills and gully’s, predominantly with narrow roads running along ridge lines and conservation reserves in the gullies. The area has poor geotechnical conditions with either hard rock or thin soils on steep slopes which are prone to slipping when subjected to high rainfall, or disturbance such as that from pipe laying. This paper will outline the use of best practice design methodologies in both the hydraulic design of the pressure sewer, and the over-arching design process of optioneering and final design configuration, resulting in a successful ‘design-build’ tender. The benefits of a pressure sewer in steep and rocky terrain will be highlighted and critically compared to the attributes of a gravity sewer network with multiple pump stations. Pump station design innovations will also be show-cased. This paper will present the beneficial attributes of a pressure sewer that may be applied to other geographically challenged sites in New Zealand for which an affordable reticulated sanitary sewerage system is required.