Sizing Graded Stormwater Pipes

Stormwater drainage is not covered under the Plumbing and Drainage Act 2019 as it is considered building work. Plumbers are increasingly being asked by building certifiers to provide a Form 43 for work relating to the installation of stormwater drainage for a Class 1a building they have installed. A Form 43 is used for the purposes of sections 68, 69, and 70(1)(a) of the Building Regulation 2021, to state the aspect work for a single detached class 1a building and class 10 building and structure is compliant with the building development approval.

The National Construction Code (NCC) references AS/NZS 3500.3:2021 as a means of satisfying the Deemed-to-Satisfy Provisions for stormwater drainage. Previously we discussed a method of estimating the number of downpipes and calculating the design flows in gutters and downpipes for a Class 1a building.

The example used an Annual Exceedance Probability of 5% (*5% is equal to 20 years ARI), and a 5 minute duration rainfall intensity of not more than 255mm per hour in accordance with the NCC. We selected a rainfall intensity of 235mm/hr for the chosen location and calculated a total catchment (roofed area) flow of 19.90 l/s or 1.697 l/s/ 90 mm down pipe.

The Standard specifies a minimum gradient of 1 in 100 (1%) for pipe sizes of DN 90, 100, and 150. The piping material to be used is U-PVC which has a Colebrook-White roughness coefficient, k (mm) of 0.003 - 0.015 as referenced in AS 2200 for thermosetting plastics. This simply relates to the ‘smoothness’ of the inside of the pipe. The lower value 0.003 represents the expected value for clean, new pipes laid straight. Where multiple pipes are to be joined they shall be connected via the use of a junction fitting or through a storm water pit. For simplicity we have selected to use junctions with the pipes laid straight. For in-ground stormwater the code recommends limiting the velocity to not more than 2m/s so as to limit frictional losses through fittings and other associated items.

AS/NZS 3500.3:2021 does provide a method of sizing graded stormwater pipes in Section 5. However, for this example we have chosen to use the table provided by Vinidex Pty Ltd. This provides a simple means of determining the pipe size for a given flow at the specified gradient using the k value of 0.003. At the minimum gradient of 1.00% a DN 100 pipe can carry 9 l/s at a velocity of 1.1 m/s. Using the data previously gathered the total discharge from the roofed area is 19.90 l/s, therefore we would need to increase our pipe size to DN150 that has a capacity of 26.2 l/s at a velocity of 1.4 m/s.

Table from Vinidex Pty Ltd

Alternatively, we could install multiple DN 100 pipes to carry the required flow. The latter may be necessary where the minimum cover of 100mm as required by the Standard cannot be achieved or where stormwater pipes are to discharge to the kerb. By way of comparison, if the pipe gradient was to be increased to 2.00% (1:50) a DN 150 pipe would still be needed to carry the required discharge of 19.90 l/s as a DN 100 pipe at 2.00% has a flow capacity of 13.2 l/s approximately, at a velocity of 1.6 m/s

This example has considered the roofed area only that has a run-off coefficient of 1.0. For other areas such as paved or turfed surfaces the run-off coefficient will vary depending on the surface type. These catchments should be drained via inlet pits suitably located to intercept surface flows. The discharge from these areas can be calculated using the rain fall intensity by the area in m² by the run-off coefficient for the given surface. They can then be added to the total expected flow.