The Integrated Catchment Study of Auckland City (New Zealand): Overview

The NZ$23.5 million Integrated Catchment Study (ICS) was a four-year study whose objective was to develop a comprehensive understanding of the Auckland Isthmus drainage systems and receiving environments, and to develop decision-making tools to enable the following to be achieved: (1) prioritised investment decisions based on triple bottom line (economic, environmental and social), (2) management decisions and planning now and in the future to 2050 and (3) support for Auckland City Council and Metro Water Limited's drainage network resource consent applications to Auckland Regional Council. The ICS was a computer modelling focused study to assist Auckland City and Metrowater to develop robust understanding and solutions to address: (1) wet weather combined and wastewater overflows that can create short-term public health issues, (2) habitable floor flooding that can produce economic loss and public health issues, (3) stormwater quality that can produce long-term environmental health issues and (4) provision for growth, which can exacerbate drainage issues. The study was performed in three stages: (1) data capture, (2) catchment analysis (model development and system performance) and (3) options analysis and reporting. Computer models were developed for the (1) integrated wastewater, combined, and stormwater surface and piped networks, (2) groundwater aquifers and (3) coastal receiving environments. Key outcomes of the ICS and related studies include identification of (1) habitable floors subject to flooding, (2) contaminant loads, (3) elevated public health risks at beaches up to 48 hours after rain events, (4) a combination of subcatchment sewer separation and storage / maintenance of combined systems as preferred drainage solutions, (5) trunk wastewater solutions in the Central City subject to continued Joint Planning with Watercare (regional provide) to resolve trunk sewer issues, (6) zinc as the principal contaminant of concern, with zinc coated roof source control as the preferred solution, (7) groundwater aquifer capacity to accept more than double current stormwater disposal, (8) additional stormwater disposal to aquifers that is not expected to adversely affect other groundwater uses.