Comparative Environmental Risk Assessment of Auckland City's Drainage System

Abstract

A Comparative Environmental Risk Assessment (CERA) was completed of the operation of Auckland City's drainage system. The CERA was carried out to support the Assessment of Environmental Effects required for resource consents filed by Auckland City Council (ACC) and Metro Water Limited to legally operate the drainage system under the Resource Management Act. The Australian/New Zealand 4360:1999 Risk Management guideline set the framework for the assessment. The CERA was evaluated based upon a mixture of qualitative and semi-quantitative methods. Because of the present lack of detailed understanding of the drainage system, quality of discharges and the receiving environment, no attempt was made to measure or model environmental consequence. Instead, a comparative rating was evaluated based on the receiving environment activity / use and exposure to impaired stormwater discharge in each of the 38 drainage management areas (catchments), ranked as high, medium or low. Results are tabulated in a hierarchy: (1) catchment activity / use, (2) catchment sensitivity / vulnerability, (3) catchment (unit) risk, (4) catchment environmental impact and (5) catchment environment risk. The output of the process is a comparative assessment of the 38 catchments: 13 catchments are ranked high for activity / use and six for environmental risk. No weightings were utilised, as it was felt that these and/or additional factors should be derived via public consultation. At such a stage a re-evaluation of the risk assessment can take place. A sensitivity analysis was performed to reveal if any subfactors unduly influenced the assessment, as well as to analyse human activity / use, cultural and ecological factors separately. The present application can be updated as improved data becomes available. Whilst simple, the methodology is objective, repeatable, and rigorous; it permitted a rapid assessment of the comparative environmental risk of the operation of the drainage system.