Orifice Plates for Stormwater Detention: How Small Holes Save Big Money

A 13mm hole drilled in a steel plate. That is the flow control device that made a six-unit Napier subdivision stormwater-compliant. Orifice plates are the most cost-effective way to attenuate peak stormwater flows from small developments, but sizing them incorrectly means either non-compliance or an overbuilt tank system that eats into your margin. This post walks through the hydraulic principles, the design inputs, and the sizing process used on real NZ subdivisions.

What an orifice plate does

An orifice plate is a flat plate (typically stainless steel or galvanised mild steel) with a precisely sized circular hole, installed at the outlet of a detention tank or chamber. Its purpose is to restrict the outflow rate so that the downstream discharge does not exceed a target value, usually the pre-development peak flow from the site.

As water rises in the detention tank, the head (depth of water above the orifice) increases, and so does the flow through the orifice. But because the opening is small, the outflow rate is significantly lower than the inflow rate during a storm event. The tank fills, stores the difference, and drains slowly after the storm passes. The peak outflow is controlled by the orifice diameter and the maximum head.

The hydraulic equation

Orifice flow is governed by the standard orifice equation:

Q = Cd × A × √(2gH)

Where:

• Q = flow rate (m³/s)
• Cd = discharge coefficient (typically 0.61 for a sharp-edged circular orifice)
• A = orifice area (m²)
• g = gravitational acceleration (9.81 m/s²)
• H = head of water above the centre of the orifice (m)

This equation is deterministic. Given the target outflow rate and the maximum available head (which is set by the tank depth or water level at the overflow point), you can solve directly for the required orifice area and therefore the diameter.

Design inputs you need before sizing

Before calculating the orifice diameter, you need four things:

1. Target outflow rate. This comes from the pre-development hydrological analysis. For a small residential site in Napier, this might be 2-5 L/s depending on the catchment area and soil type.
2. Maximum head. This is the vertical distance from the orifice centreline to the overflow level of the detention system. For a typical 1,200mm diameter polyethylene tank laid horizontally, the maximum head is approximately 1.0 m (accounting for the inlet pipe soffit and freeboard).
3. Detention volume. The total storage needed to attenuate post-development flows to the target rate. This is calculated from the difference between the inflow and outflow hydrographs.
4. Storm duration and return period. The critical storm event that produces the maximum detention volume. This is often not the same storm that produces the maximum peak flow.

Sizing example: Barker Road, Napier

At Barker Road, the pre-development peak flow for the 10-year ARI storm was approximately 3.2 L/s from a 620 m² contributing catchment. The post-development peak flow (with 85% impervious coverage) was 8.7 L/s. The detention system needed to attenuate the peak by 5.5 L/s.

With a maximum available head of 0.95 m (set by the tank configuration and overflow level), the orifice equation gives:

A = Q / (Cd × √(2gH)) = 0.0032 / (0.61 × √(2 × 9.81 × 0.95)) = 0.00121 m²

Converting to diameter: d = √(4A/π) = 0.0393 m, or approximately 39 mm.

However, this is the orifice size at maximum head. The design must also check that the orifice does not allow excessive flow at intermediate heads during smaller storm events. In this case, the 2-year ARI storm controlled the design, resulting in a final orifice diameter of 13 mm with the tank volume increased to 16,000 litres across four tanks in series. The smaller orifice with larger storage volume was needed to achieve compliance across all return periods simultaneously.

Why the smallest storm often controls the design

This is the detail that catches many designers. For the large storm events, you have a large detention volume and a high head, so a relatively large orifice can still restrict the peak. But for the 2-year or 5-year event, the available head is much lower (because less water enters the tank), and the allowable outflow rate is also lower (because the pre-development peak for that event is smaller). The result is that the orifice must be small enough to restrict flow under low-head conditions for frequent storms, while the tank must be large enough to hold the volume during infrequent storms.

This interaction between orifice size and tank volume is why the design is iterative. You cannot size the orifice independently of the tank, or the tank independently of the orifice. Both must be optimised together across all design storm events.

Practical considerations

Blockage risk

Small orifices are vulnerable to blockage from debris, sediment, and organic matter. A 13mm orifice will block if a single leaf or stone lodges across it. Mitigation measures include:

• Installing a coarse screen or grate upstream of the orifice (minimum 50mm clear spacing to prevent bridging)
• Placing the orifice at least 50mm above the tank invert to allow sediment to settle below the opening
• Specifying an access point for maintenance and clearing

Material and fabrication

Orifice plates for stormwater applications are typically fabricated from 3mm stainless steel (grade 316 for longevity) or 5mm galvanised mild steel. The hole must be drilled, not punched, to achieve a clean sharp edge. A punched hole creates a ragged edge that alters the discharge coefficient and can trap debris.

Multiple orifices

Some designs use two or more orifices at different heights to provide staged flow control. A lower orifice handles frequent events, while a higher orifice activates during larger storms. This approach can reduce the required tank volume compared to a single-orifice design, but adds complexity to fabrication and maintenance.

Orifice plates at George Street, Bulls

At George Street, Bulls, the detention strategy used a 290-metre swale rather than underground tanks. The swale outlet incorporated an orifice plate to control the discharge rate into the downstream council pipe network. The design principles were identical, but the maximum head was determined by the swale depth rather than a tank diameter. The larger available volume in the swale allowed a slightly larger orifice, reducing blockage risk while still achieving compliance.

When to use orifice plates vs other flow control devices

Orifice plates are the preferred option for small-to-medium developments (up to about 2 hectares of contributing catchment) where the target outflow is between 1 and 20 L/s. For larger developments, vortex flow control devices or proprietary throttle units may be more appropriate because they maintain a more consistent discharge rate across a wider range of heads and are less susceptible to blockage.

For the vast majority of residential subdivisions in Napier, the Rangitikei, and the wider Manawatu region, a simple orifice plate is the right solution. It costs under $200 to fabricate and install, has no moving parts, and provides decades of reliable flow control with minimal maintenance.