Jan Heijs, Greer Lees, Dean Watts, Tim Lockie and Barry Somers.
The Far North District Council (FNDC) has experienced a very high frequency of wet weather wastewater overflows resulting in community concerns and an abatement notice from the Northland Regional Council. Initial estimates to improve the situation also raised community concerns around the ability to fund improvements. FNDC commissioned a study to find the best value solutions to improve the network and reduce wet weather overflow frequencies to one spill per year or better.
The investigations adopted two significant changes to a traditional approach when seeking solutions to network performance issues. Firstly the point of diminishing returns was established using a cost benefit approach. This included consideration of a wide range of options to identify the best value solutions. Secondly a long term rainfall series was used, rather than the use of design storms, which do not reliably reflect the performance of the system. A previously built and calibrated model was made available for this study. A verification process was undertaken to assess the reliability of the model and its limitations.
This paper discusses the background to why these alternative approaches are preferred, as well as the outcomes. The paper also discusses the results of the different improvement types that are possible, given the interesting hydraulic attributes of the network.
Significant cost efficiencies have been identified and put to FNDC for consideration.