Auckland’s Approach to the Stream Erosion Problem

Auckland’s population is projected to grow by 800,000 (50% increase) in the next 30 years and the increase is predicted to make up over 50% of NZ’s population growth in that time. Stream bank erosion is a growing concern in the region, due to development and changes in catchment hydrology. With widespread future growth and associated impervious surfaces, stream erosion is set to become an important issue affecting water quality, infrastructure, properties, ecosystem health and public safety.

Better land management is required to ensure stream erosion is minimised or prevented and waterways remain healthy. In urban areas, assets and buildings are commonly adjacent to streams and erosion of the banks are threatening their structural integrity. Sediment is a key ‘matter’ to take into account as part of the National Policy Statement for Freshwater Management (NPSFM). Regional targets, of which sediment is expected to be one, will be set and will need to be met to ensure improved freshwater quality. In the Auckland region, turbidity and sediment are major issues with significant impacts on estuaries and harbours that also require improved management as part of the New Zealand Coastal Policy Statement (NZCPS).

This paper outlines the direction Auckland Council is embarking on to proactively identify existing and future stream erosion risk and to gain an improved understanding of baseline sediment and erosion levels across the region. Identification of the erosion risk can be incorporated into future planning and development frameworks in a similar manner to floodplains and overland flow paths. It will provide an additional tool for the assessment of site suitability for land use change and for any requirements associated with managing erosion as a natural hazard. With an improved understanding of the erosion risk at the early stages of land use change, mitigation strategies can be developed and tested to understand their effectiveness for maintaining stream bank integrity and improving freshwater and coastal water quality. Modelling and monitoring of sediment delivery from stream bank erosion processes can also be undertaken to comply with the NPSFM. The paper will outline how the previous paper, titled ‘Continuous Simulation Modelling To Support Healthy Waterways’, and the following GIS stream erosion assessment work fit into Auckland Council’s wider stream erosion management framework.

The paper also outlines the development of a GIS-based stream erosion assessment screening tool, that can be applied at a region scale to efficiently identify streams that might be prone to stream erosion (due to hydraulic forces). The assessment uses data on stream gradient, channel cross-section and an estimate of the critical shear stress of the soils. The assessment has been undertaken for eight catchments in the Auckland region calculating unique channel characteristics for over 800 cross-sections extracted from LiDAR data. The study analysed and presented which parameters stream erosion (using boundary shear stress) is most sensitive to. It also determined the validity of using LiDAR derived cross-sections for the calculation of stream channel parameters and in the prediction of stream erosion. The assessment has identified specific reaches where erosion is likely to currently be occurring and validated these against site-specific assessments. Output from the erosion assessment can then be used to assist with decision making associated with future development and land use change. Further catchment or reach specific analysis can then be undertaken as appropriate to facilitate future growth and measures developed to mitigate the potential for stream erosion in these areas.