NZ Transport Agency Stormwater Workshop


The NZ Transport Agency would like to invite you to a Transport and Stormwater workshop to learn more about and discuss two key projects that the Agency has recently commissioned. The first project is a piece of research looking at the environmental risk of stormwater runoff from roads and the second project relates to a draft State Highway Stormwater Specification that has been developed. We are keen to hear your feedback on both projects at the workshop. A brief description about the two projects is below.

The workshop is being held in Auckland on 2 December at NIWA. See the full agenda and venue details here.

Please RSVP by Tuesday November 24th to:

Session One: Risk assessment of road stormwater runoff research presentation

Laurie Gardiner (MWH) and Jonathan Moores (NIWA)

09:00 – 10:20

Road runoff can have potentially significant adverse effects on aquatic receiving environments. Contaminants in stormwater discharges from the road network are complex and include fuels, additives, oil, grease and brake and tyre residues containing a variety of toxic and ecotoxic components, including heavy metals and organic compounds. Receiving environments of stormwater from roads include streams, rivers, lakes, wetlands, estuaries, harbours and the open coastline. The characteristics of each of these different types of water body influence the fate of contaminant inputs, how they are assimilated and therefore their sensitivity.

Reflecting the need for cost-effective ways of prioritising the management of stormwater in relation to the risk of adverse effects, the NZ Transport Agency commissioned MWH in association with NIWA to develop a screening model that addressed the following research question:

Under what conditions is stormwater run-off likely to cause adverse environmental effects?

Building on earlier research, the aim of this 1-year study was to revise and enhance the Transport Agency’s Vehicle Kilometres Travelled (VKT) screening tool for road runoff to allow its wider application to rivers/streams and coasts/estuaries, and with provision for factoring in the effects of pathway attenuation, traffic congestion and non-road pollution sources. While the principal intention was to develop an improved screening method for road networks, a secondary objective was to describe ways in which the method could be extended to provide for an absolute risk assessment in relation to established effects thresholds.

The Road Stormwater Screening (RSS) Model developed in this study provides a robust, consistent method for establishing the relative risk of adverse effects from road runoff that can be applied anywhere in New Zealand using existing datasets.

The RSS model should be of assistance to the NZ Transport Agency and territorial authorities in screening and prioritising areas of the road network for improved management of road runoff. The risk-based approach used in assessing the impact of stormwater discharges on receiving environments aligns with current development by regional councils to move towards a risk-based stormwater consenting regime in support of the National Policy Statement for Freshwater Management.

Laurie Gardiner and Jonathan Moores will present the findings of this research, including the Porirua Harbour case study. They would welcome any feedback on the model and its potential application from an end user perspective.

Session Two: NZTA State Highway Stormwater Specification presentation

Tim Fisher (Tonkin + Taylor)

10:50 – 11:15

The NZ Transport Agency’s Environment and Urban Design team has released the draft State Highway Stormwater Specification for internal and external consultation. The Transport Agency has drafted this specification, together with input from a group of internal and external people with expertise in stormwater management on state highways.

The scope of the specification is intended to set out the design and construction stormwater requirements for all state highway improvement projects. This includes:

  • stormwater surface drainage
  • collection and conveyance
  • stormwater treatment
  • cross drainage
  • erosion and scour protection
  • existing stormwater systems
  • operation and maintenance

This specification does not cover temporary works or sub-soil drainage, and relies on other specifications for bridge waterways, landscaping, pavements, erosion and sediment control and ecology.

It is expected that this stormwater specification will be used as the starting point for the development of project specific requirements and will need to be adapted as appropriate to address local issues and the scope of any given project.

Feedback on the draft is being sought before this specification is finalised and rolled out to staff and suppliers. Information about the specification is also available online as well as a quick survey through the link below. You are also welcome to send feedback to

Tim Fisher will present the draft stormwater specification and early feedback. The session will allow for time for discussion about the draft stormwater specification, which will be used as part of the feedback process by the NZ Transport Agency.