WAKA KOTAHI WELLINGTON STATE HIGHWAY STORMWATER MONITORING PLAN – CHALLENGES AND PRIORITIES

C.Conwell (SLR Consulting), I.Idris & M.Zheng (Wellington Transport Alliance), C.Horrox (Waka Kotahi)

ABSTRACT

There are ~808 km of state highway across the Wellington Region. This includes five operational State Highways and a mix of both old and newly operational sections of highways. The Greater Wellington Regional Council (GWRC) introduced a two-stage consenting regime for the discharge of stormwater from the state highway network. The regime requires a ‘global’ approach for stormwater discharges from state highways.

In a first for New Zealand, GWRC granted a global discharge consent (October 2022) to Waka Kotahi that authorizes the discharge of stormwater from routine operational use of the state highway network across the Wellington region. This excludes discharges from construction and maintenance activities, and associated with slips and accidents).

Stage one (5-year duration) is largely an information gathering phase to fill knowledge gaps. It requires the monitoring of the quality of stormwater discharging from the state highway network to fresh and/or coastal waters. It does not consider hydrological impacts.

Stage two of the consenting regime requires the development of a long-term strategy for managing the effects of operational discharges, taking account of wider policy drivers set out in the PNRP. The aim is to develop catchment specific stormwater management plans that address the management of cumulative effects (on aquatic ecosystem health, mahinga kai, contact recreation, and Māori customary use).

The intention behind the two-stage consenting regime is that monitoring undertaken during Stage one will inform the development of a prioritised programme for improvements in the Stormwater Management Strategy (SMS) required by Stage two.

The paper describes the method for selecting priority sites for monitoring operational stormwater discharges from Wellington Region’s state highway network. It considers the logistics of site access since the target zones are subject to vehicles travelling at high speed. It has applied information from a range of databases to identify priority catchments and monitoring sites. This includes state highway stormwater assets from the Waka Kotahi, current monitoring and consents across the catchments, relevant schedules for ecological and cultural significance and sensitivities, hydrological processes of the sub-catchments, and a simple contaminant load model (using vehicle telemetry data).

Data were arranged in a spatial webmap to identify key interactions, and to assist in developing a prioritization framework for selecting representative monitoring sites.

More difficult and complex issues needed to be integrated into the SMP. These included the opening of the new sections of the SH1 network e.g., Transmission Gully Motorway and the Mackays to Peka Peka and Peka Peka to Ōtaki Expressways. With the older sections of SH1 pending revocation, the SMP has incorporated both the new and old sections of the SH network.

The paper discusses the range of monitoring techniques that have been identified across priority sites, and an adaptive management and monitoring pathway that will be applied across the Stage one consent over the next five years.