There are two types of wastewater overflows:
Dry weather overflows are usually caused by something blocking the wastewater mains which cause wastewater to back-up behind the blockage and fill the pipes and overflow out of the manholes upstream of the blockage.
The wastewater then makes its way to the streams and rivers via the stormwater pipe network. These types of overflows are usually small because members of the public call the Council quickly to report them and the maintenance contractor responds very promptly.
Blockages can be caused by a number of things such as collapsed pipes, tree root intrusion, inappropriately disposed items and fat accumulation.
The Council monitors and maintains the types of pipe that are more likely to block to help prevent dry weather overflows, such as flushing lines prone to blocking with fat.
Wet weather overflows are caused by the wastewater system becoming inundated with stormwater and groundwater during wet weather events which can cause its capacity to be exceeded.
Stormwater can enter the wastewater system directly through things such as low lying house gully traps, stormwater down pipes accidentally connected to the wastewater system, and manhole vents. Groundwater can enter through cracks in underground private or public pipes.
To minimise the risk to human health, constructed overflow points to streams and rivers prevent wastewater from entering streets and private properties. When the system gets overloaded with stormwater inflow and groundwater infiltration, it fills up it exits through these outfalls instead of backing up onto properties.
Wet weather overflows are a mix of wastewater, rainwater, and groundwater, with the majority being rainwater. The volume can be large if the storm that causes them is significant. The Council has a continuing programme of work to reduce wastewater overflows, by reducing the amount of inflow and infiltration into its wastewater network as well as increasing the capacity of the network.
All dry and wet weather overflows are reported on the recent and historic wastewater overflows webpage. Signs are put out in the area surrounding the overflow warning of pollution and testing is carried out on the receiving body of water (stream or river) until the amount of pollution has reduced enough to remove the signs.