This project came to SAE after the client received an RFI from Whakatane District Council on the stormwater design for three large farm buildings on a rural site in Otakiri, Bay of Plenty. There is no council stormwater network in the area, so all stormwater from the buildings and surrounding hardstand had to be managed on-site through soakage into the ground.
The site has pumice soils with very high soakage rates (800-1,720 mm/hr measured across 11 boreholes). That sounds ideal, but groundwater was only 1 metre below ground level. Under the Building Code, soakage pits need at least 200 mm clearance above groundwater, which limited pit depth to just 0.8 m. With that shallow depth, the required storage volume could not be achieved by going deeper - it had to be spread across a larger area instead.
We adopted a conservative soakage rate of 400 mm/hr - a 50% reduction from the measured field rates to account for variability across the site and seasonal changes in soil moisture. This is standard practice for field-tested soakage systems.
Four soakage trenches were designed, each 2 m wide by 50 m long and 0.8 m deep, with perforated pipes in gravel bedding to distribute flow evenly into the pumice soil. The combined effective storage across all four devices met the calculated requirement of 43,868 litres. Building floor levels were set 300 mm above ground level to manage ponding during extreme events beyond the design standard.
We completed the stormwater design and responded to the outstanding council queries. The design was approved by Whakatane District Council.
A soakage system design that works within the constraints of high-permeability pumice soils and shallow groundwater. Building consent was obtained, and the methodology has since been applied to additional buildings on the same site.
The system was designed to a 100-year event, which is not typical for rural stormwater but was warranted given the site's history of flooding. The design demonstrated that the development would not have any adverse effect on downstream properties while managing stormwater and flooding on the site itself.
In January 2026, the Bay of Plenty experienced its heaviest recorded rainfall - 198 mm fell in 12 hours in Tauranga, the wettest night on record. The existing stormwater systems on the site were overwhelmed and underwater. The new soakage system performed as designed - significant flooding surrounded the buildings but none entered them. The overland flow path design and soakage system worked together exactly as intended.
