What is an overflow?
A wastewater overflow occurs when wastewater (sewage) spills out from gully traps, manholes, engineered overflow points or pump stations and flows into public or private property. It can flow into waterways and the sea.
There are three types of overflows:
- Type one: These are from pump stations. These may occur due to power outages or equipment failures, in addition to wet weather and blockages.
- Type two: These are from engineered overflow points. They are designed to overflow during heavy storms.
- Type three: These are from points in the network, such as manholes and private gully traps. We rely on people reporting these overflows.
Wastewater pump station
Overflowing manhole
Engineered overflow point during an overflow
Why do overflows occur?
Overflows can occur anytime, anywhere. The causes may be lack of capacity in the network or blockages or a combination of both. The blockages may be caused by fat, rubbish and tree roots clumping together and restricting the flow. When this happens, wastewater can spill through private gully traps, manholes, engineered overflow points or pump stations.
Overflows can also occur on rainy days when stormwater enters our wastewater network and overwhelms it.
Did you know? In heavy rain, the amount of stormwater that drains from an average roof can be equivalent to the wastewater flows from more than 40 households. That’s why it is essential for houses to have rainwater plumbed into approved stormwater outlets, rather than allow it to flow into the wastewater network.
Rain falling from the roof
Child’s play? No way! Our staff have pulled numerous toys out of the wastewater network over the years. Other objects include false teeth, ropes, bricks and even wine bottles. But they say cleaning up overflows is no child’s play. They respond very quickly, clear blockages and thoroughly clean the affected area. Our focus is on minimising the risk to public and environmental health.
Some of the toys pulled from the wastewater network
What happens when an overflow occurs?
When we are notified of an overflow, our crews respond as quickly as possible. They water blast the sewer to clear any blockages. Then they clean and disinfect the area, removing debris and ensuring the area is safe.
Our operations and planning teams work closely together. The role of the planning team is to investigate the cause of an overflow. They may use a hydraulic model to work out if an overflow was caused by a lack of capacity in the network, a severe storm, or by blockages. They may also arrange CCTV inspections or smoke testing. CCTV checks the condition of the sewer and smoke testing helps to identify cracked drainage as well as incorrect stormwater connections. Sometimes, stormwater drainage is connected to the wastewater network by accident.
If the planning team find that the cause of an overflow is a lack of capacity, they will scope upgrade work. This involves securing funding as well as carrying out site and design investigations. Depending on the location and complexity of the upgrade, they may need to coordinate a response with the Local Board, Auckland Transport, Auckland Council, mana whenua, local interest groups and the local community.
Our engineered overflow points are designed to operate during extreme weather events. These overflows – which consist of wastewater that is heavily diluted with stormwater – flow along waterways to the sea. Auckland Council has a website called SafeSwim that lets the public know if a beach is safe to swim at from a health and safety perspective. If there is a direct wastewater overflow onto a beach, it is signalled on the website with a black flag.
A screenshot of the Safeswim website