Urban Flow Design Level of Service - Setting the Standard

This paper discusses the question: what is an appropriate Design Standard (Level of Service/Level of Protection) for primary and secondary stormwater systems in urban areas? Naturally the best answer varies for existing and developing areas across the country. Currently there is a lack of national guidance to support the definition of these levels and consequently there is a range of standards in use.

Some existing New Zealand local government requirements are summarised and these are compared with the Building Code and NZS4404:2010. To add context, multiple related issues are considered including cost, asset renewal cycles, land ownership, value at risk, access, easements, community expectations, land use, climate variability and safety.

The complex challenge of achieving cost-effective stormwater management lends itself to processes such as multi-criteria analysis. However, additional steps are required to effectively incorporate community aspirations and ecological considerations. Therefore, a process methodology is proposed to assist setting appropriate design values for existing and new urban areas.

The paper suggests short and long term means to advance the application of improved levels of protection and apply precautionary risk reduction.