Project Hobson tunnel design
Annual Conference
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Project Hobson is Watercare’s project to replace the Hobson Bay above ground sewer pipeline which serves a large part of Auckland City but is reaching the end of its life.
The replacement sewer is a 3 km long, 3.7 m internal diameter sewer tunnel running from Parnell to a new pump station in the Orakei Domain. The sewer tunnel will provide storage to capture wet weather flows and practically eliminate overflows from Watercare’s wastewater system into Hobson Bay.
The tunnel was sized based on modeling which took account of the level of service, capacity of the downstream sewer and the upstream catchment characteristics. Alternative hydraulic system options were considered to develop the optimum, sustainable solution. 30 m deep vortex drop shafts have been included in the design to deliver the flow down to the tunnel and reduce erosion, odour and noise by controlling the flow within a vortex drop pipe.
A ventilation system is provided to ensure the new sewer does not cause odour complaints. The strategy involves the operation of a fan at the furthest downstream shaft (PS64 Wet Well) pulling air down the Hobson Bay sewer. The extracted air will be processed through a bark biofilter before venting to atmosphere.