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Stormwater treatment and attenuation systems manage the quality and quantity of runoff from developed land. Attenuation temporarily detains stormwater - reducing peak flows to pre-development levels and protecting downstream infrastructure from being overwhelmed. Treatment removes sediment, hydrocarbons, and contaminants before runoff reaches the receiving environment. Both are commonly required by councils as conditions of resource consent for new developments.
Councils across the Wellington region increasingly require attenuation where development would otherwise increase peak stormwater flows to council networks. Untreated runoff from roads and carparks carries oil, heavy metals, and fine sediment - contaminants that accumulate in streams and estuaries over time. Designing systems that satisfy both objectives - attenuation and treatment - while fitting within a development's layout requires engineering that balances hydraulic performance, available footprint, and long-term maintenance.
We size attenuation volumes and outlet structures using catchment hydrology and council-specified performance criteria. We design below-ground tanks, surface wetlands, and raingardens depending on what the site allows and what the council's design guidelines require. For treatment, we specify devices appropriate to the runoff characteristics - gross pollutant traps, proprietary treatment units, or constructed wetlands - and integrate them into the overall stormwater network. All systems are designed for maintainability: devices that cannot be cleaned or inspected fail over time, creating compliance problems for future landowners or body corporates.
