Henderson Line, Marton was a 74-lot residential subdivision requiring full infrastructure design across all three waters and roading. The existing drainage network was undersized for the post-development catchment. Due to low cover under the road, we initially designed a 1,500×1,000mm precast box culvert with concrete surround. When pricing came back from the manufacturer at approximately $700,000 in material costs, it was clear an alternative was needed.
The project ran from 2021 to 2025, from preliminary design through to construction completion and multi-stage S224c certification.
The project also required coordination between two council jurisdictions: Rangitikei District Council under consent RM210083 for the subdivision, and Horizons Regional Council for stormwater discharge rules. Meeting both sets of requirements in a single design required careful alignment of stormwater modelling methodology, report format, and engineering plan content throughout the design development and consent process.
We started by assessing the existing site and its constraints: flooding risk, stormwater capacity, access, wastewater, and existing pipe infrastructure. One of the more unusual findings was a pipe clash where the existing wastewater pipe was running through the inside of a stormwater manhole. All of these issues were worked through in consultation with Rangitikei District Council and Horizons Regional Council.
The new design increased the capacity of both the stormwater and wastewater networks and removed the pipe clash entirely. For the critical stormwater crossing, we reanalysed the node and found that dual 1,050mm reinforced concrete pipes, run in parallel, could deliver the same hydraulic capacity as the box culvert while handling the traffic load at the shallow cover depth. The material cost dropped from $700,000 to approximately $100,000 - a $600,000 saving for the client on a single line item.
Detention tanks were designed for individual lots to achieve stormwater neutrality per Rangitikei DC requirements. Stormwater modelling was carried out in HEC-HMS with storage routing for detention sizing.
Construction supervision commenced in 2021 and continued through to 2025. Site visits covered pipe installation, compaction testing, kerb and channel, and road formation. As-built surveys were conducted using GNSS/GPS, with as-built drawings compiled in Civil 3D. S224c certification was issued for Stages 1 and 2 in 2025 following council acceptance of all as-built documentation and producer statements.
The entire subdivision was modelled in Civil 3D - roading, stormwater, wastewater, and water supply pipe networks. The 3D model was used directly by GPS-controlled diggers on site, meaning what was designed is exactly what was built. No interpretation, no guesswork on the ground.
Engineering plans and reports were prepared to support the subdivision consent through Rangitikei District Council and the discharge consent through Horizons Regional Council. Section 92 requests and council queries were managed throughout the consent process.
The engineering design included stormwater modelling in HEC-HMS with storage routing for detention sizing, dual 1,050mm reinforced concrete pipes replacing a $700,000 box culvert for $100,000, individual lot detention tanks for stormwater neutrality, and full three waters network design to NZS 4404:2010.
On-site construction supervision ran from 2021 through to completion in 2025, covering pipe installation, compaction testing, kerb and channel, and road formation - all monitored and verified against the design.
All completed works were surveyed using GNSS, with as-built drawings compiled in Civil 3D. S224c certification was issued for Stages 1 and 2, and the full package was accepted by Rangitikei DC, allowing titles to be issued.
