Natural Channel Design in the Tasman Region

The Nelson Tasman region is experiencing unprecedented growth with the development of greenfield areas. Stream channels within these growth areas are being modified and Tasman District Council (TDC) has perceived a lost opportunity to provide for enhanced ecological and fluvial-geomorphological functions or processes.

In the absence of any specific guidance material available at national or regional levels, the current approach is resulting in mixed and sometimes undesirable outcomes. Stream channels are often over-armoured with large rock or erosion is being exacerbated by unsuitable channel design. These changes can lead to the cumulative loss of habitat heterogeneity for aquatic biota, altered stream behaviour, increased sediment loading, risk to infrastructure from erosion, and increased future management and maintenance costs.

The principles of stream dynamics and function have been understood by the scientific and engineering communities both internationally and within New Zealand for some time. International literature and case studies regarding stream design and management have also become available in recent years. However, comprehensive stream design guidelines in New Zealand are effectively absent. TDC acknowledged the need for authorities to provide greenfield developers and resource consent applicants a practical design philosophy in order to improve on the current design of stream channels. As a result, the Natural Channel Design Guidelines project was developed to integrate ecological, fluvial-geomorphological and engineering principles as well as good practice.

TDC has developed Natural Channel Design Guideline Part One as a ‘vision’ for Natural Channel Design (NCD)

The Part One Guideline contains local stream type definitions that the guidelines apply to, an overview of ‘type specific’ stream channel processes and potential risks to constructed stream channels, an ideal project plan, and some high level guidance on the four stages of investigation and design. While the guideline does not provide a step-by-step process of how to design a stream channel at this point, it does provide the guiding vision and intent of each of the steps in a stream design processes, from project scoping, preliminary assessments, concept design, detailed design and ongoing monitoring. It also aligns with the Water Sensitive Design (WSD) principles of the Nelson Tasman Land Development Manual.

The Part One Guideline will aid land development project teams to scope stream design projects, and as a last resort, design stream diversions through their developments. It is envisaged that the Guideline will also help developers, practitioners, resource consent applicants and TDC to meet the statutory requirements of the Resource Management Act (RMA), the Tasman Regional Policy Statement and Tasman Resource Management Plan. The Guideline incorporates cross-discipline principles to meet local and national objectives, leaving streams in a better state for the next generation.

TDC intend to develop Part Two of the guideline, which will be a step-by-step methodology for all eight steps in the stream design process.