TP108 in Auckland: How to Apply Auckland Council's Stormwater Detention Standard

Technical Publication 108 (TP108) is Auckland Council's primary reference for stormwater management in subdivisions, and it remains the standard against which most engineering assessments are measured. But TP108 was written for a range of Auckland catchment types, from highly permeable volcanic soils in the east to impermeable clays in the south, and its application to a specific site requires careful judgement about which design storm to use, which pre-development land cover to assume, and how to size attenuation storage when infiltration is not viable.

What TP108 Actually Is

TP108, formally titled "Guidelines for Stormwater Runoff Modelling in the Auckland Region," was published by the Auckland Regional Council (now part of Auckland Council) and provides the hydrological methodology for calculating stormwater runoff from development sites across the Auckland region. It is not a design code in the strict sense. It is a methodology document that sets out how to calculate pre-development and post-development runoff, which rainfall data to use, and what the performance standard is for stormwater management.

The core requirement of TP108 is stormwater neutrality: the post-development peak discharge from a site must not exceed the pre-development peak discharge for a range of design storm events, typically the 2-year, 10-year, and 100-year ARI storms. Where the post-development discharge exceeds the pre-development rate, the difference must be attenuated through detention storage.

Key Design Decisions Under TP108

Pre-development land cover. The pre-development runoff calculation depends on what land cover is assumed before development. For a greenfield site, this is typically pasture with a corresponding runoff coefficient or curve number. For a brownfield redevelopment, the question becomes more complex: does pre-development mean the existing impervious surface, or the original undeveloped condition? Auckland Council's current position, reflected in the Auckland Unitary Plan and GD01 guidelines, generally requires the pre-development baseline to be the undeveloped (pasture or bush) condition, not the existing developed state. This is significant because it means a redevelopment of an existing impervious site must still provide detention to meet the undeveloped baseline.

Design storm selection. TP108 provides rainfall intensity-duration-frequency (IDF) data for multiple Auckland sub-regions. The correct IDF curve depends on the site's location within Auckland. Using the wrong sub-regional IDF data will produce incorrect design rainfall depths and therefore incorrect detention volumes. The engineer must confirm which TP108 rainfall zone applies to the specific site.

Time of concentration. The time of concentration (Tc) is the time it takes for runoff to travel from the most remote point of the catchment to the point of discharge. TP108 provides methods for estimating Tc, but the result is sensitive to the assumed flow path length, slope, and surface roughness. For small residential sites (under 2 hectares), the Tc calculation often dominates the design: a shorter Tc produces higher peak flows and larger detention requirements.

Curve number or rational method. TP108 supports both the SCS curve number method and the rational method for runoff estimation. The choice of method affects the calculated peak flows and, therefore, the required detention volume. For sites where the SCS method is used, the curve number must reflect the actual soil type and land cover. Auckland's soil types vary significantly: volcanic basalt soils in areas like Mt Eden have high infiltration capacity, while Waitemata Group clay soils in areas like Henderson have very low infiltration rates. The curve number must match the site-specific soil conditions.

Sizing Detention Storage

Once the pre-development and post-development hydrographs are calculated, the detention volume is determined by routing the post-development hydrograph through the proposed storage with a controlled outlet. The outlet is sized to limit the discharge to the pre-development peak flow rate, and the storage volume is the maximum difference between the inflow and outflow volumes during the design storm.

The detention system must be tested across multiple storm durations and return periods, not just the critical duration storm. A system that works for the 10-year, 1-hour storm may overtop during the 10-year, 6-hour storm if the storage is too shallow relative to the sustained inflow volume. TP108 requires the designer to check a range of storm durations to identify the critical combination.

At Glendale Road in Henderson, the stormwater design required detention storage on a site with Waitemata Group clay soils where infiltration was negligible. The detention system was sized as a tank with a controlled orifice outlet, designed to attenuate post-development flows to pre-development rates across the 2-year through 100-year ARI storms. The clay soil conditions meant that no infiltration credit could be applied, so the full volume differential between pre-development and post-development runoff had to be stored and released at a controlled rate.

Common TP108 Mistakes

Using the wrong rainfall zone. Auckland spans multiple TP108 rainfall zones. Using a generic Auckland IDF curve rather than the site-specific zone produces incorrect design rainfall, which cascades through the entire calculation.

Assuming infiltration on clay soils. Some designs claim infiltration as a stormwater management mechanism on Waitemata Group clays, where saturated hydraulic conductivity is typically below 1 mm/hr. Unless site-specific soakage testing demonstrates viable infiltration rates, detention (not infiltration) is the appropriate mechanism on these soils.

Not checking multiple storm durations. Designing for a single critical duration storm and assuming the detention works for all durations is a common error. The critical duration varies with catchment size and storage geometry, and the only way to confirm adequacy is to test the full range.

Ignoring the tailwater condition. If the detention outlet discharges to a watercourse that has a high tailwater level during the design storm, the outlet capacity is reduced and the required storage volume increases. This interaction is often overlooked in desktop assessments.

TP108 and the Wider Consent Framework

TP108 does not operate in isolation. For Auckland resource consents, it sits alongside GD01 (Stormwater Management Devices in the Auckland Region), which provides design guidance for treatment and detention devices, and the Auckland Unitary Plan provisions for stormwater management areas. The engineering assessment must address all three documents to satisfy council requirements.

For subdivisions managed through development managers such as HAL Projects, the TP108 assessment is typically subject to independent CPEng peer review before council accepts the engineering design. The peer reviewer will check the methodology, design storm selection, curve numbers, and detention sizing. Errors in any of these elements will be identified and must be corrected before the S224c process can proceed.