The Flower Street site in Bulls presented a design challenge that is deceptively common in provincial Rangitikei subdivisions: the soil simply does not drain. S-Map data confirmed the site soils belong to the Flaxtonford series, Pallic soil order - a classification that indicates poor natural drainage and low permeability. In practical terms, this means the first instinct of many designers - a soakage pit - had to be properly tested before it could be accepted or ruled out.
At the same time, the kerb-discharge alternative is not straightforward either. To drain a detention tank to the road kerb, the outlet orifice must overcome siphon losses in the outlet pipe and there must be adequate elevation difference between the tank outlet invert and the kerb channel invert. If the available head is insufficient, the orifice cannot drain at the required controlled rate and the tank effectively becomes a stagnant pool rather than an attenuation device. The geometry of each lot had to be individually checked before any outlet configuration could be recommended.
A further obligation was the flood risk review. Rangitikei District sits within Horizons Regional Council's jurisdiction, and flood extent data for the Bulls township area required review to confirm the site sits outside the 1:200-year flood extent. Not every site in Bulls is clear of this boundary, so independent verification against Horizons RC flood mapping was a necessary step for the resource consent package.
SAE Ltd evaluated both discharge options in parallel. The soakage assessment used S-Map soil data to derive an indicative infiltration rate for the Flaxtonford series and sized a soakage pit against the required drawdown period. For the soakage option, each lot in Lots 2 and 3 would require a 6,000-litre tank with a 12 mm orifice to achieve the necessary attenuation. The soakage option was feasible in volume terms but carries a long-term maintenance burden given that Pallic soils can become even less permeable over time with compaction and surface sealing during construction.
The kerb-discharge option was assessed using LiDAR-derived elevation data to measure the available head from each proposed tank location to the road kerb. Siphon losses were calculated for the outlet pipe length and bend geometry on each lot. Once available head was confirmed to be adequate on both lots, the orifice diameters were sized using the orifice flow equation to release the detained volume at or below the pre-development peak flow rate. The design storm rainfall intensities were sourced from NIWA HIRDS V4 with an RCP 6.0 climate change factor applied, consistent with Rangitikei DC requirements for subdivision consents lodged in 2024.
Detention volumes and orifice sizes were calculated using the Rational Method and TP108/SCS methodology. The S92 information request from Rangitikei District Council was responded to with a technical memorandum that addressed each RFI point individually, including a plan showing the lot-specific tank locations, outlet pipe alignments, and kerb discharge points. Horizons RC flood mapping was overlaid on the site plan to demonstrate, with reference to the council's own data, that the site sits outside the 1:200-year flood extent.
Two detention configurations were designed and documented. Under the soakage option, Lots 2 and 3 each require a 6,000-litre tank controlled by a 12 mm orifice discharging to a subsoil soakage pit. Under the kerb-discharge option - which was the preferred and recommended option - Lot 2 requires a 10,000-litre tank with an 11 mm orifice, and Lot 3 requires a 10,000-litre tank with a 14 mm orifice. The different orifice diameters reflect the different lot areas and roofed impervious areas contributing runoff from each lot. Combined, the kerb-discharge design provides 11.5 m³ of detention against a calculated requirement of 11.2 m³, giving a modest positive surplus of 0.3 m³. Rainfall inputs were derived from NIWA HIRDS V4 for the Bulls grid cell, with the RCP 6.0 climate change intensity factor applied to the 10-year ARI design storm in accordance with Horizons RC guidance.
The flood risk assessment referenced Horizon Regional Council's floodplain mapping for the Bulls urban area. The site's lowest ground level was compared against the mapped 1:200-year flood extent boundary and confirmed to sit outside the inundation envelope. This finding was documented in the S92 response with a reproduced flood map extract annotated to show the site boundary. The S92 response, submitted as Revision 3 in September 2024, addressed the council's information requests in full and enabled the consent process to advance without further technical holds.
The completed stormwater neutrality report provided Rangitikei District Council with a fully documented design for the 3-lot subdivision at Flower Street. Both discharge options were presented with calculations, but the kerb-discharge option was recommended and adopted as the consent basis. The detention volumes - 10,000 litres per lot on Lots 2 and 3 - satisfy the stormwater neutrality requirement under the Rangitikei DC subdivision code and Horizons RC stormwater rules. The design complies with NZBC B1.4 for structural performance of the tanks and associated pipework.
The S92 RFI response demonstrated SAE's capability to engage constructively with council technical staff under a live consent process. Rather than resubmitting a revised report, the response addressed each RFI point with targeted technical evidence - flood map extracts, revised orifice calculations, and elevation survey data - allowing the council to close out the information request without a further round of queries. The project illustrates how a methodical, soil-first approach to stormwater design avoids the risk of specifying a soakage system on land that cannot absorb the required volume within any reasonable timeframe.
When S-Map data flags Pallic soils, soakage cannot be assumed - the design must evaluate kerb discharge as a primary option and verify available head through LiDAR elevation analysis before specifying orifice sizes. On this Bulls site, that process produced a 10,000-litre kerb-discharge detention system per lot with individually sized orifices (11 mm and 14 mm), meeting the stormwater neutrality requirement and satisfying Rangitikei DC's S92 information request with targeted technical evidence.
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