Sump sizing

When is a sump not a sump? When it doesn’t comply to code. APD outlines the requirements for Type 1, Type 2 and bubble-up surface water sumps.

AUTHOR: ROSS MILLER

Anon-compliant sump is a pain for everyone: the person’s property on which it is installed, the tradesperson who has to replace it, and on occasion, the insurance agent who has to pay out on unnecessary damage.

The Building Code is very specific and clear on sumps. The fact that you can get a cheaper product that ‘will do’, or ‘the inspector won’t notice’ isn’t an excuse, and often ends up making more work for everyone and a bad name for the installer.

The requirements to comply are very clear in NZBC E1:

TYPE 1

A Type 1 surface water sump must be 375mm dia wide with 300mm minimum from the base to syphon trap to ensure the silt has had time to drop out of solution. Often a 300mm dia sump is being found installed on sites, greatly reducing the capacity needed.

TYPE 2

A Type 2 surface water sump needs to be 650mm x 460mm or 600mm dia, with 380mm to the base of the syphon trap. Often a concrete 450mm x 450mm sump is installed, creating a much bigger medium-term problem with faster silt build-up, as it is used for greater surface areas up to 40,000l/m2 than the Type 1.

Access caps and syphon traps

The requirements for the use of removable access caps and properly-designed syphon traps are to ensure the long-term viability of the stormwater system installed. Being able to easily clean out sumps and associated pipework makes for ease of maintenance and collection of unwanted silts and debris in the system. The basic requirements outlined apply to both Type 1 and Type 2 sumps.

Rainfall requirements

The Type 1 sump is deemed suitable for drainage of up to 4,500l/m2 and the Type 2 for an area up to 40,000l/m2, where the litre figure is the rainfall intensity for a storm with a 10% probability of occurring annually.

ABOVE: Figure 8 of NZBC Acceptable Solution E1/AS1 shows a Type 1 surface water sump, suitable for an area of up to 4,500l/m2 .

Plumbers should know the basic rainfall exposure they work in and select sumps accordingly. Note that some areas are prone to downpours, which ups the requirements for sumps. To a certain extent, the geography of the section and how it funnels surface water can be a factor.

ABOVE AND BELOW: Figures 6 and 7 of E1/AS1 show requirements for bubble-up chamber systems.

TYPE 1 & TYPE 2: IN BRIEF

Type 1 requirements are:

Maximum 1000mm height 375mm dia

Removable access cap for cleaning

Submerged or trapped outlet 300mm minimum at base for silt settlement

Minimum 90mm outlet pipe. Type 2 requirements are: 650mm x 460mm or 600mm dia

Removable access cap for cleaning

Submerged or trapped outlet 380mm minimum at base for silt settlement

Minimum 150mm outlet pipe.

Bubble ups

The bubble-up sump is designed to work along the lines of a lock on a river. It allows for the situation where the collection point is slightly lower than the main system but can still be connected without a huge amount of infrastructure. The bubble-up chamber allows water to be discharged through pipes laid at the allowable minimum gradients, and for the convenient collection and removal of any silt or debris that might enter the system. However, caveats are: The ground level adjacent to any downpipe discharging to the bubble-up chamber is at least 150mm higher than the level of the top of the chamber outlet The connections between the drain and downpipes are sealed The total chamber depth does not exceed 1m.

The bubble-up chamber allows the water to be discharged through pipes laid at the allowable minimum gradients, and for the convenient collection and removal of any silts or debris which might enter the system.

About APD: APD Ltd is New Zealand’s largest manufacturer of buried HDPE stormwater attenuation systems. For enquiries, please contact Sales & Marketing Manager Duane Rice on 0272 033910; duane@apd.co.nz; www.apd.co.nz