Stormwater Neutrality in Rangitikei: Pallic Soils, Horizons Regional Council, and Why TP108 Doesn't Always Apply

Rangitikei District sits within the Horizons Regional Council jurisdiction, and its stormwater rules are different from what Auckland or Napier developers are used to. Pallic soils with low permeability dominate much of the Marton and Bulls landscape, which rules out soakage as a neutrality strategy. Horizons RC resource consent is required for any discharge to watercourses, and their engineering standards reference TP108. But the application of that document to Whanganui-region catchments requires careful interpretation.

The regulatory framework

Stormwater management in the Rangitikei involves two regulatory bodies. Rangitikei District Council (RDC) administers the district plan and sets subdivision consent conditions. Horizons Regional Council administers the One Plan and controls discharges to water, including stormwater.

For any subdivision that discharges stormwater to a watercourse (which is most of them, given the soil conditions), you need both a subdivision consent from RDC and a discharge consent from Horizons. The two processes run in parallel but have different requirements, different processing officers, and different timeframes. Coordinating them is one of the less obvious challenges of developing in this district.

Why soakage does not work here

In Auckland, soakage is a standard stormwater management tool. Many Auckland soils (particularly the volcanic basalt soils of the isthmus) have infiltration rates of 20-50 mm/hr or more, which allows stormwater to be directed into soakage pits, permeable paving, or rain gardens that discharge to ground.

Rangitikei pallic soils are a different proposition entirely. These soils, derived from loess and tephra deposits, typically have saturated hydraulic conductivities of 1-5 mm/hr. In winter, when the soil profile is wet, effective infiltration can drop below 1 mm/hr. Soakage testing on Marton and Bulls sites routinely returns results that make ground disposal impractical for any meaningful stormwater volume.

The consequence is that all stormwater from developed sites must be managed through surface detention and controlled discharge. There is no option to "lose" water into the ground. Every litre that falls on an impervious surface must be detained and released at a controlled rate to a downstream watercourse or council network.

The TP108 question

Auckland Council's TP108 (Technical Publication 108: Guidelines for Stormwater Runoff Modelling in the Auckland Region) is widely referenced as a design standard across New Zealand. Horizons Regional Council's engineering guidelines reference TP108 as an acceptable methodology for stormwater detention design. But there is an important caveat.

TP108 was developed for Auckland conditions. Its default parameters, catchment response assumptions, and design charts are calibrated to Auckland rainfall patterns, Auckland soils, and Auckland catchment characteristics. Applying TP108 directly to Rangitikei catchments without adjusting the inputs can produce incorrect results.

The key differences are:

• Rainfall intensity. HIRDS v4 data for Marton and Bulls shows different rainfall intensity-duration-frequency relationships compared to Auckland. The shape of the design storm hyetograph differs, which affects the timing and magnitude of peak flows.
• Catchment response time. The flat topography and low soil permeability of Rangitikei catchments produce longer times of concentration than the steeper, more permeable Auckland catchments that TP108 was calibrated against.
• Runoff coefficients. The low permeability of pallic soils means that even "pervious" surfaces generate significant runoff. Pre-development runoff coefficients of 0.4-0.5 are common for pastoral land in the Rangitikei, compared to 0.2-0.3 for equivalent land use in Auckland.

The practical implication is that TP108 can be used as a methodology framework, but the rainfall inputs must come from local HIRDS data, the runoff coefficients must reflect local soil conditions, and the results should be verified against first-principles hydrological analysis rather than relying on TP108's pre-calculated charts.

What Horizons RC actually wants to see

From our experience on projects including Henderson Line, Marton, George Street, Bulls, and Hereford Heights, Marton, Horizons RC discharge consent applications for stormwater require:

1. Pre- and post-development hydrological analysis using site-specific rainfall data from HIRDS v4 with climate change uplift factors.
2. Demonstration of stormwater neutrality for the 2-year, 10-year, and 50-year ARI events (some sites also require 100-year analysis).
3. Water quality treatment assessment. Horizons expects evidence that the first flush (typically the first 25mm of rainfall) receives some form of treatment before discharge. Vegetated swales, constructed wetlands, or proprietary treatment devices are acceptable.
4. Erosion and sediment control plan for the construction phase.
5. Long-term maintenance plan for the stormwater system, including responsibilities and funding mechanisms.

Practical implications for developers

Developing in the Rangitikei is not harder than developing in Auckland or Napier. It is different. The soils dictate the approach (surface detention, not soakage). The dual consent process (RDC plus Horizons) requires coordination. And the engineering analysis must use local data, not Auckland defaults.

The cost of getting this wrong is significant. We have reviewed projects where an engineer unfamiliar with Rangitikei conditions designed a soakage system that had to be completely redesigned after soakage testing failed. We have also seen TP108 calculations submitted with Auckland default parameters, which were returned by Horizons with a request for site-specific analysis. Both scenarios add months and thousands of dollars to the consent process.

The solution is straightforward: engage an engineer who has completed stormwater designs in this jurisdiction and understands the local soil conditions, regulatory framework, and council expectations from the outset.