DEVELOPMENT OF THE NATIONAL STORMWATER MODELLING GUIDE

Michael Arthur (Metis Consultants), Rathika Jebamony (Awa Environmental) and Nicci Wood (Water New Zealand)

ABSTRACT

The purpose of this project is to develop a national stormwater modelling guide. The guide covers urban stormwater flood modelling, including urban streams and watercourses, but excludes major rivers except as they impact as boundary conditions on urban networks. The scope of the guide is to provide a nationally consistent and robust urban stormwater modelling process to aid in understanding flood risk and management. It will help address differences, inconsistencies and gaps in flood risk information including modelling, flood hazard maps, selection of freeboard and variations between councils' approach, design standards and policies related to flooding. Having a holistic and consistent approach and understanding of the flood hazard in different areas provides certainty for planners, developers and engineers about the best ways to manage risk.

The national stormwater modelling guide provides a consistent framework for model planning, delivery and use for application by a diverse range of stakeholders. This paper will provide valuable insights to those who want to know more about the development process and the background thinking that informed the final guide.

Guide development was started by Water New Zealand Modelling Special Interest Group volunteers in 2019. The initiative was then allocated funding by Department of Internal Affairs in late 2022, Water New Zealand hired as project managers and consultants engaged to complete the guidance in early 2023. The process of reviewing current practices (local and international) began in March 2023 with a range of industry workshops and surveys delivered throughout the rest of 2023. A literature review, gap analysis and initial guide structure was published in October 2023. A final draft was completed in December 2023 and circulated to ~180 industry volunteers for feedback. The first edition of the guide is planned to be published in March 2024 for industry-wide use.

The work is being supported by an Advisory Group of volunteers from industry. Members include consultants, developers, local government staff, and representatives from the Ministry for the Environment and NIWA. The role of the Advisory Group is to provide feedback on work as the project progresses and ensure that the final guide is, accessible practical and cohesive for ongoing use in the wider industry.

This paper will provide an overview of the first edition guide and a discussion of:

• How workshops were used to understand industry needs, ‘test’ key content and influence the final document.
• The importance of model planning – why this ended up being the longest and most complex section of the guide.
• Lessons learned in developing a system to describe stormwater model types that linked clearly with purpose and end use of results.
• The challenge of creating an accessible guide (and not a detailed specification).