Christchurch wastewater overflows – consent application

In October 2011, GHD was engaged by Christchurch City Council to prepare and submit a regional consent application to repair and upgrade Christchurch's earthquake damaged wastewater infrastructure. Even though the repairs were essential for the long-term social and economic success of Christchurch, it was anticipated that the works could result in the discharge of untreated wastewater into the Avon and Heathcote Rivers, and the Avon-Heathcote Estuary. It was therefore paramount that any discharges were mitigated and/or avoided where possible, so that public health and safety was maintained.

Christchurch's wastewater network was constructed in the late 19th Century, and over the years it has naturally deteriorated. The lack of fall of the land across the city (west to east) means that Christchurch's wastewater system is gravity based, relying on over 100 pumping stations that allow wastewater to flow effectively through the city.

Following the major earthquakes of 2010 and 2011, Christchurch's already delicate wastewater network was severely damaged.

The damage to the roading, water, wastewater and stormwater infrastructure is unparalled in New Zealand's history. Hundreds of kilometres of roads, bridges and underground infrastructure require repair or replacement.

As a result of this damage, the extent of inflow and infiltration to wastewater pipes significantly increased. In turn, additional pressure was placed on the already fragile network to deliver and process a greater volume of wastewater. The chance of wet weather overflow events increased and was possible at additional sites.

To allow for essential repairs to the wastewater network during the emergency phase (immediately following the earthquakes), a temporary exemption to the resource consent requirements for discharge to water was granted by the Canterbury Regional Council.

Following the emergency phase, Christchurch City Council was required to apply for regional consent any works that may require future discharges.

Prior to discharge approval, contractors have to work through a set of conditions and alternatives that include bypass pumping to another part of the network, trucking out waste and utilising existing storage capacity within the wastewater system to contain flows for a short period. It is only when these options are exhausted (or are not feasible) that discharge can then be made directly in to rivers or estuaries.

As part of the regional consent application for the repair and upgrade of the wastewater infrastructure, a set of conditions were developed to mitigate potential discharge. To develop these conditions, GHD carried out in-depth consultation with the Christchurch City Council, Ngai Tahu Runanga, the Avon Heathcote Estuary Trust, and the Canterbury District Health Board.

The nature and scale of the repair works, combined with ongoing damage, meant there was uncertainty around the timing, location, and expected frequency or volumes of discharge into waterways. As such the consultation and conditions proposed in the consent application had to remain flexible. This proved challenging during the consultation process; while affected parties were in general agreement that the infrastructure rebuild was necessary, there was still concern regarding potential adverse environmental impacts.

To address the concerns of a community that was overwhelmed by the aftermath of the earthquakes and particularly sensitive about public health, GHD adopted an open and transparent consultation process, offering a significant number of opportunities for discussion and feedback. Affected parties recognised that repairs were paramount, so discussions were focused on repair and discharge practice, rather than on a defined volume and location of works and/or discharge.

Christchurch City Council obtained resource consent that provides flexibility to carry out the necessary repair works to the city's wastewater infrastructure.

The conditions established during the consultation detail clear criteria that must be met before discharge to local waterways can occur. The effects of any discharge are now managed in a way that not only facilitates the repair works, but also monitors and reports any potential adverse effects to the environment. The discharge procedures outlined in the conditions are now part of formal council procedure that is used by contractors.

The infrastructure repair and upgrade will result in an improved wastewater network that is significantly more resilient than the pre-earthquake network. The frequency of wet weather overflow events will reduce, resulting in improved water quality in local waterways in the long term.