THE RSS MODEL: FLUSHING OUT STORMWATER RISKS

Increasing regulatory focus on stormwater discharges under the National Policy Statement on Freshwater Management is placing a spotlight on the management of stormwater runoff from roads. This paper describes a GIS-based road stormwater screening (RSS) model designed to assist road controlling authorities and network operators manage stormwater runoff and develop supporting catchment and asset management plans.

The RSS model provides a robust, conservative method for screening the risk level to receiving waterbodies from road stormwater runoff, based on the copper and zinc contaminant load from road traffic and non-road (urban) sources. Risk is evaluated using a rating for contaminant strength and receiving environment sensitivity with streams/rivers assessed at the sub-catchment level and coasts/estuaries at their stormwater catchment outlets.

The method uses nationally consistent datasets and takes account of variations in traffic volumes and congestion, load attenuation by road drainage and land use. The model can disaggregate contributions from local roads and state highways and has a `drill down’ facility to identify `hot spots’ where traffic loads are highest. The model’s spatial analysis supports a global consenting approach for road networks appropriate to the risk to the receiving environment.

Results from a case study evaluation of the model are presented for the Porirua Harbour catchment (Pauatahanui Inlet and Onepoto Arm) including risk profiling, sensitivity analysis, validation against field data and example applications.