Swale trial seeks better solutions

What is the most effective method for establishing stormwater wetland vegetation?

Extensive planted swales or linear wetlands are necessary at the Ruakura North urban growth area in Hamilton to ensure sustainable stormwater management. However, the widespread planting required could result in significant capital costs for developer, Chedworth Properties Ltd, over the course of the development, and long term maintenance costs for Hamilton City Council when they take over control of these assets.

Boffa Miskell was engaged to investigate the most successful methods for swale plant establishment and management with a specific focus on rapid and dense plant cover. Our ecologists proposed a trial swale experiment to determine which of three different planting methods and two different maintenance approaches gives the best outcomes. The goal is to demonstrate the most suitable method for maximising wetland performance and functionality in the establishment phase, in order to minimise the ongoing maintenance costs.

The trial wetland was established in autumn 2015, comprising two 10m wide swales and two 5m wide swales excavated to different depths. Two different weed control regimes are being trialled and three plant establishment methods; broadcast seeding, conventional planting, and natural re-colonisation. By assessing plant density, growth rate and long-term plant survival, our ecologists will discover which combination of methods produces the best outcomes for each swale type and how best to maintain them. The trial will run for two years and be monitored at six-month intervals. It will also be a notable test of natural seed germination in stormwater systems, as the seed used for the project was partly eco-sourced from naturally re-colonised wetland plants in the Rotokauri stormwater wetlands.

The Ruakura North swale trial has the support of both the city and regional councils, and is being watched with interest by stormwater designers involved in Ruakura development. The results are expected to be incorporated into Hamilton City Council’s Infrastructure Technical Specifications Manual to guide developers in establishing swales and stormwater treatment wetlands in future.