Notes

This report describes a GIS-based road stormwater screening (RSS) model developed to upgrade and widen application of the NZ Transport Agency’s 2007 vehicle kilometres travelled screening tool. The RSS model provides a robust, consistent method for assessing relative risks to receiving waterbodies using estimates of copper and zinc from road traffic and non-road (urban) sources. Risk levels are evaluated using contaminant strength and receiving environment sensitivity scores with streams/rivers assessed by sub-catchment reach and coasts/estuaries at their catchment outlets. The model uses nationally consistent datasets and takes account of traffic congestion, load attenuation in the road corridor and land use type. Results of a case study risk assessment of Te Awarua-o-Porirua Harbour catchment (Pauatahanui Inlet and Onepoto Arm) are described including risk profiling, sensitivity analysis, validation against field data and example applications. The spatial output supports a global consenting approach for road networks appropriate to receiving environment risk and is consistent with the water quality accounting system under the National Policy Statement on Freshwater Management. The model should assist road controlling authorities and network operators screen new developments, prioritise areas of the existing network for improved management of road runoff and develop supporting catchment management plans for consenting purposes.