WILL WATER SENSITIVE URBAN DESIGN RESULT IN MEASURABLE IMPROVEMENTS IN ECOLOGICAL HEALTH OF URBAN STREAMS?

E. Graham & L. McKergow (NIWA Hamilton), K. Borne & S. Yalden (NIWA Auckland)

ABSTRACT

Urban land use results in degradation of waterways including changes to habitat (channelisation, reinforced banks, reduced riparian vegetation), altered hydrology, increased contaminant concentrations, poor water quality, and reduced ecological diversity; the combination of these effects leads to the “urban stream syndrome.” Across New Zealand, there is increasing demand (and legislative requirement, e.g., through the National Policy Statement for Freshwater Management 2020), to improve the condition of streams in urban areas, and to reduce the impact of future urban development.

Methods to improve aquatic environments include implementation of stormwater treatment systems, including green infrastructure, or nature-based solutions (GI/NBS; systems that use plants and soil to manage stormwater quantity and quality) such as wetlands, raingardens and tree pits, which are components of Water Sensitive Urban Design (WSUD).

In consultation with decision makers involved in urban water management, we have identified three key science needs around the use of green infrastructure:

1. Improved understanding of the processes that influence the performance of stormwater treatment/mitigation systems, and the ability to cost-effectively measure and predict mitigation performance;
2. Methods to predict the effects of urban land use on stream water quality and ecosystems, with and without mitigation – ideally in ways that can be used for catchment-accounting and/or limit-setting (i.e., beyond average annual load predictions);
3. Methods or tools to understand the cumulative effects of multiple stressors acting on urban stream ecosystems, to enable informed decision-making.

We are currently in the first year of a five-year project aimed at addressing these questions by developing a model for predicting stream ecological response from the characteristics of urban development (with and without mitigation). The operational model will be a spatial version of the “effective imperviousness model” created by Chris Walsh and collaborators at the University of Melbourne for the Little Stringybark Creek catchment (Walsh et al. 2022).

The four major tasks required to develop this model also constitute stand-alone goals:

• Task 1 Build and assess the performance of models of selected stormwater treatment devices (raingardens, wetlands, Stockholm tree pits).
• Task 2 Develop and/or implement catchment-scale models for impervious surface identification, and for predicting hydrology and water quality changes.
• Task 3 Undertake spatial surveys of urban streams across the spectrum of effective imperviousness (i.e. catchments with various imperviousness and degrees of GI implementation).
• Task 4 Undertake a case study to assess the change in ecological condition over time as a catchment develops from rural land use to urban, incorporating WSUD features/GI interventions.

By presenting an overview of the project and progress at this early stage, we seek feedback on the research design, access to available data, and potential collaboration opportunities.

Walsh CJ, Burns MJ, Fletcher TD, Bos DG, Poelsma P, et al. (2022) Linking stormwater control performance to stream ecosystem outcomes: Incorporating a performance metric into effective imperviousness. PLOS Water 1(2): e0000004.