Pallic Soils and Stormwater: Why Soakage Doesn't Work in Much of the Rangitikei

Pallic soils, the low-permeability, weathered loessial soils common across the Manawatu-Whanganui region, are largely unsuitable for stormwater soakage. Where Auckland developers might design an infiltration trench to achieve stormwater neutrality, a Marton developer faces soils that drain at rates measured in millimetres per day rather than metres per day. That means your stormwater solution will involve detention, not infiltration, and your engineer needs to design for surface discharge to the nearest council drain, not a sump.

What Are Pallic Soils?

Pallic soils are classified under the New Zealand Soil Classification (NZSC) as soils with pale-coloured subsoil horizons that reflect poor drainage and seasonal waterlogging. In the Rangitikei district, these soils are predominantly derived from loess (wind-blown silt deposited during glacial periods) overlying older terrace gravels or mudstone. The loessial layer is typically 1 to 3 metres thick, and its hydraulic conductivity is very low.

Manaaki Whenua Landcare Research's S-map database classifies much of the Marton, Bulls, and Hunterville area as Kairanga, Ohakea, or Tokomaru soil series, all of which fall within the pallic soil order. Field-measured infiltration rates on these soils are commonly in the range of 1 to 5 mm/hour under saturated conditions. For comparison, the volcanic soils in Auckland's North Shore typically infiltrate at 50 to 200 mm/hour.

Why Soakage Fails

Stormwater soakage systems (infiltration trenches, soakage pits, rain gardens with infiltration bases) work by directing stormwater into the ground faster than it arrives. The design relies on the soil's hydraulic conductivity being high enough to drain the storage volume between rainfall events. If the soil cannot drain fast enough, the system fills during the first event and provides no attenuation for subsequent events until it drains, which can take days or weeks in pallic soils.

At 2 mm/hour infiltration rate, a 1 metre deep soakage pit takes over 20 days to drain fully. A design storm in the Rangitikei can deliver 30 to 50 mm of rainfall in an hour. The mismatch between inflow rate and outflow rate is two orders of magnitude. No amount of storage volume can compensate for that disparity in a cost-effective residential stormwater system.

There is a secondary problem. Pallic soils develop a perched water table during winter and spring, when the subsoil is saturated from seasonal rainfall. A soakage system that is surrounded by saturated soil has zero effective infiltration capacity. The system is full before the storm arrives.

What Works Instead

In the Rangitikei, stormwater management for subdivision means detention and controlled surface discharge. The design approach has three components:

Collection: Roof and hardstand runoff is collected via conventional guttering, sumps, and piped reticulation, exactly as in any other subdivision. The difference is that this water cannot be directed into the ground.

Detention: The collected stormwater is directed to a detention facility, typically a detention basin (open pond), an underground tank, or an oversized pipe system. The detention facility stores the difference between the post-development peak flow and the allowable discharge rate, then releases it at a controlled rate over several hours.

Discharge: The attenuated flow is discharged via a piped outfall to the nearest Horizons-managed watercourse (Tutaenui Drain, Folly Stream, or another receiving environment). The discharge requires a Horizons resource consent, and the allowable discharge rate is set to match pre-development peak flows (stormwater neutrality).

This approach is well established in the Rangitikei. SAE Ltd has designed detention-based stormwater systems for subdivisions at Henderson Line (Marton), George Street (Bulls), and Hereford Heights (Marton). Each of these projects is on pallic soils, and each uses detention with surface discharge rather than soakage.

Implications for Your Project

If you are developing in the Rangitikei, soakage is almost certainly not an option. The implications are straightforward:

• You will need a detention system. This takes up space on the site (above ground) or adds cost (underground tanks). Budget for it from the outset.
• You will need a Horizons discharge consent. Because the stormwater discharges to a watercourse rather than soaking into the ground, a discharge permit under s15 of the RMA is required.
• Geotechnical investigation confirms the soil type. Do not assume soakage will work based on a neighbouring site or a desktop assessment. A simple soakage test (falling head permeameter or double ring infiltrometer) will confirm the infiltration rate and determine the design approach within the first week of the project.

The cost difference between soakage and detention is not necessarily large for a well-designed system. What costs money is designing for soakage, discovering it does not work during construction, and then redesigning for detention under time pressure. Getting the soil type confirmed early avoids that scenario entirely.