North Shore City's infiltration/inflow reduction programme using trenchless technology

The combination of increasing population density, ageing sewer networks, and significant year round rainfall has led to increasing levels of water pollution caused by wet weather sewer overflows on to the beaches of North Shore City in Auckland. In order to improve the beach water quality, North Shore City embarked on Project CARE in 1999 with a total estimated cost of NZ $ 280 million over a 20 year period.

The reduction of stormwater infiltration and inflow (I/I) into the sewer network is a critical element of the project and requires an extensive rehabilitation programme for significant areas of the city sewer network. After the priority mini catchments are identified through flow gauging and hydraulic modelling, all public sewer lines in the prioritised mini catchments are rehabilitated using largely trenchless technology. The condition of private drainage is also assessed and where defects are found, notification is given to property owners requiring them to carry out the remedial works. A programme has also been introduced recently to mitigate localised stormwater issues as much as possible in the I/I priority mini catchments.

Some encouraging results have been observed in the reduction of Infiltration /Inflow from the post flow gauging completed so far on rehabilitated catchments.