Napier City Council introduced a new stormwater neutrality rule (SW-S1) in 2025, requiring developments to prove that stormwater runoff after construction does not exceed what was there before. This 6-unit residential development in Taradale was the first project submitted under the new rule.
Because it was the first, there was no existing methodology or approved template to follow. The calculation approach had to be developed from the rule text and NCC Engineering Standards from scratch.
We modelled the pre- and post-development stormwater flows for each sub-catchment on the site using HEC-HMS. Because there was no single central drainage point, the site needed four separate detention tanks - each one sized to match the area draining to it.
Tank sizing was done through storage routing in HEC-HMS, iterating the orifice diameter until the post-development outflow from each tank matched the pre-development peak flow at both the 10-year and 100-year storm events. This is not a simple calculation - a single orifice has to work at both events simultaneously, which has to be verified independently for each return period.
The result was four Tanksalot 4,000-litre underground tanks, each with a 13 mm orifice outlet. The small orifice restricts outflow to the pre-development rate while the tank fills during a storm, then drains down between events so it is ready for the next one. Total storage across the site: 16,000 litres.
The reporting format and calculation methodology we developed for this project has since become the standard template for all NCC stormwater neutrality applications we submit.
Engineering plan approval was obtained from Napier City Council. The design provided 16,000 litres of total detention storage across four tanks with orifice-controlled outlets. The methodology established on this project is now applied to all subsequent NCC infill stormwater neutrality projects, including Kennedy Road Napier and Murphy Road Taradale.
Related reading: What is stormwater neutrality? · Stormwater design service · Civil engineering in Hawke's Bay
