Downpipes illegally connected to gully traps on residential properties are putting pressure on Council’s wastewater networks, particularly in the Tinwald catchment.

Water that comes from the roof or gutters should end up in soak pits or stormwater drains, not the wastewater network, which for Ashburton and Tinwald conveys sewage and used household water to the Wilkins Road treatment facility.

Group Manager of Infrastructure Neil McCann said that the treatment facility at Wilkins Road experienced very high flows during a big rain event recently and Council staff suspected illegal discharges into gully traps and low-lying gully traps were part of the problem.

“We asked staff to investigate areas of Tinwald and they discovered downpipes discharging into gully traps and low-lying traps, in both older and newer houses. We’ve written letters to property owners as a result and asked them to take action.

“Downpipes in some houses discharge into pipes that lead to the kerb and channel and Council’s stormwater system, and that is okay. Downpipes in other houses may discharge into soak pits in the ground and water is absorbed that way.

“However, we have found some soak pits have been paved over or need maintenance, and concerningly we found some downpipes discharging straight into the gully traps for wastewater. We also found some gully traps were set too low in the ground and surface water was able to easily run into them.”

Mr McCann said the Building Act had specific requirements for the way drainage systems were constructed, regardless of the age of a residential property.

“Council also has a drainage bylaw to project the sewerage network from stormwater infiltration, but we prefer education rather than prosecution, though that could be a step of last resort.”

The Building Act requires a gully trap be located at least 25mm above a paved surface and 100mm above an unpaved surface.

Soak pits, or soak holes, should not be in high-traffic areas so the soil above and around it is not compacted. They can get blocked with silt or vegetation over time but can be cleaned and flushed by professionals; slow drainage is a sign that your soak pit needs maintenance.

Plumbers know the legal requirements regarding stormwater and can advise the best ways for property owners to comply.

Mr McCann said high groundwater in the Tinwald area had also contributed to recent high flows in the wastewater system and Council was assessing the condition of its own pipe network to check for issues.