Stormwater detention volumes are not guesswork. They come from the difference between pre-development and post-development runoff hydrographs across multiple storm return periods. For a small infill development you might need 8,000 litres. For a 74-lot greenfield subdivision, you might need multiple tanks staged across sub-catchments. Getting the sizing right requires the correct rainfall inputs, the right runoff method, and a clear understanding of which storm event controls the design.
Step 1: Establish the rainfall inputs
Every detention calculation starts with rainfall data. In New Zealand, the standard source is HIRDS v4 (High Intensity Rainfall Design System), maintained by NIWA. HIRDS provides rainfall intensity-duration-frequency (IDF) data for any location in the country, derived from a network of rain gauges and spatial interpolation.
You need IDF data for your specific site coordinates, not a nearby town or a regional average. HIRDS v4 provides point estimates with uncertainty bounds. Use the median estimate unless the council or regional plan specifies otherwise.
Climate change uplift must be applied. Most councils now require the RCP 8.5 scenario for the 2081-2100 period, which adds approximately 16-22% to current rainfall depths depending on location and storm duration. Some councils accept the mid-century projection (2031-2050) for shorter-lived assets. Check the relevant district plan or engineering code of practice for the required projection period.
Step 2: Characterise the catchment
For each sub-catchment draining to a detention point, you need:
- Area. Measured from site plans or GIS. Include all land that drains to the detention system, not just the developed footprint. Adjacent land that currently drains through your site must also be accounted for.
- Impervious fraction. Pre-development and post-development. Pre-development is typically 0-10% for greenfield pastoral land, or 30-60% for existing urban sites being redeveloped. Post-development depends on the lot layout, building coverage, and paved areas.
- Soil type. This determines the runoff coefficient for pervious areas. Classify using the NZ Soil Classification or, for Auckland-method calculations, the SCS curve number approach.
- Time of concentration (Tc). The time it takes for runoff from the most remote point in the catchment to reach the detention inlet. For small sites (under 2 hectares), Tc is typically 5-15 minutes. For larger subdivisions, Tc may be 20-40 minutes depending on flow path length and slope.
Step 3: Choose the runoff method
The appropriate hydrological method depends on the catchment size and complexity:
- Rational Method. Suitable for catchments under 4 hectares with relatively uniform land use. Produces a peak flow estimate but not a full hydrograph. For simple detention sizing, the Rational Method combined with the Modified Rational Method for routing is adequate and widely accepted.
- Unit Hydrograph Method. Required for larger catchments or where the temporal distribution of the hydrograph matters (which it does for detention sizing). The SCS unit hydrograph is the most common approach in NZ practice.
- Hydraulic modelling. For complex sites with multiple sub-catchments, staged detention, or downstream constraints. Software such as HEC-HMS, TUFLOW, or Mike Urban produces detailed hydrographs and allows routing through pipe networks and storage structures.
For most residential subdivisions, the Rational Method or SCS unit hydrograph method is sufficient. The choice is often dictated by council preference. Napier City Council accepts the Rational Method for sites under 4 hectares. Horizons Regional Council prefers the unit hydrograph approach for greenfield subdivisions.
Step 4: Calculate pre-development and post-development peak flows
Using the chosen method, calculate peak flows for both the pre-development and post-development scenarios across the required return periods (typically 2-year, 10-year, and 50-year ARI, with some councils also requiring 100-year).
The difference between the post-development peak and the pre-development peak at each return period represents the flow that must be attenuated by the detention system. The pre-development peak becomes your target outflow rate.
A common mistake at this stage is using the wrong pre-development condition. The baseline must reflect the site's natural or original state, not its current condition. If the site is currently a sealed car park, the pre-development condition for neutrality purposes is typically pasture, not sealed surface.
Step 5: Determine the critical storm duration
This is where many simplified methods fall short. The critical storm for detention sizing is not necessarily the storm with the same duration as the time of concentration. It is the storm duration that produces the maximum difference between the inflow volume and the permissible outflow volume over the storm period.
For small catchments with short Tc values, longer-duration storms often control the detention volume because they deliver more total rainfall, even though the peak intensity is lower. The detention system must store the accumulated difference between inflow and outflow over the entire storm, not just the peak.
The standard approach is to test multiple storm durations (10 minutes, 20 minutes, 30 minutes, 1 hour, 2 hours, 6 hours, 12 hours, 24 hours) and identify which produces the maximum required storage volume. This is an iterative process, but it is critical. Skipping it and assuming the critical duration equals Tc will almost always undersize the detention.
Step 6: Size the detention volume
For each storm duration and return period, the required detention volume is the area between the inflow hydrograph and the target outflow rate, summed over the storm duration. The maximum value across all tested durations and return periods is the design detention volume.
In practice, this is calculated using a simple mass balance:
Volume = (Post-development runoff volume) - (Pre-development peak flow rate x storm duration)
This is a simplification. The actual calculation accounts for the time-varying nature of both the inflow and the outflow (which depends on the head in the detention system and the orifice size). For precise results, the inflow hydrograph is routed through the proposed storage-discharge relationship iteratively.
Real-world examples
Small infill: Barker Road, Napier
At Barker Road, the contributing catchment was 620 m² with post-development impervious coverage of 85%. The critical storm was a 2-hour event at the 10-year ARI. The required detention volume was 14,200 litres. We specified 16,000 litres (four 4,000-litre tanks) to provide a margin and accommodate construction tolerances.
Greenfield subdivision: Henderson Line, Marton
At Henderson Line, the subdivision covered multiple sub-catchments with a total developed area of several hectares. Each sub-catchment required its own detention analysis with tailored storage volumes. The critical storm duration varied between sub-catchments because of differing slope, area, and impervious coverage. Detention was provided through a combination of swales and underground storage, with the total system volume sized for the 50-year ARI event with climate change uplift.
Medium subdivision: George Street, Bulls
At George Street, the flat site and pallic soils produced high pre-development runoff coefficients (C = 0.45 for pasture), which reduced the differential between pre- and post-development peaks compared to a more permeable site. The 290-metre detention swale provided 1,020 m³ of storage, sized for the 50-year ARI event. The critical storm duration was 6 hours, reflecting the large catchment area and slow time of concentration.
Rules of thumb (use with caution)
For preliminary cost estimates and feasibility assessments, the following approximations are sometimes useful. They should never replace a proper engineering analysis.
- Small urban infill (under 1,000 m² site): 15-25 litres of detention per square metre of new impervious area.
- Medium subdivision (1,000-5,000 m²): 10-20 litres per square metre of new impervious area.
- Large greenfield (over 5,000 m²): highly variable; depends on soil type, slope, and downstream constraints. No reliable rule of thumb applies.
These figures vary significantly by location, soil type, and council standard. A site in Napier on free-draining gravel will need less detention than a site in Marton on pallic clay with the same impervious coverage, because the pre-development peak from the Marton site is already higher.
Detention sizing is a six-step process: rainfall data, catchment characterisation, method selection, flow calculation, critical storm identification, and volume determination. The critical storm duration is the step most often skipped and most likely to result in undersized storage. Test multiple durations, check multiple return periods, and always apply climate change uplift.