How Wide is the Stream?

Under the Resource Management Act 1991, a 20m wide esplanade reserve is required to be set aside if land is subdivided to allotments of less than 4 hectares and is adjacent to a stream of 3m or greater in width. The definition in the RMA requires the stream bed to be more than 3m wide “at its annual fullest flow without overtopping its banks”. There is currently no standardised method for determining stream width. Historically stream width has been assessed by Surveyors based on a site visit and professional judgement based on cross section shape. Their ability to determine the stream width at the “annual fullest flow” has at times been questioned.

This paper presents a methodology which has been developed to address this problem by providing a set procedure for determining a stream’s width for the purposes of setting aside esplanade reserve under the RMA. The procedure proposed is easily replicated, is transferable and will provide consistent results. The methodology relies on a hydrological modelling approach to determine the annual fullest flow, and hydraulic modelling with surveyed cross sections to determine the stream width. This together with a site assessment provides a more robust assessment technique which has recently been tested and upheld by the Environment Court.