Managing and prioritising wet-weather overflows to deliver better, more cost-effective outcomes
C Port, F Garofalow, M Cassidy, N Abulafia, T Chen, C Cantrell
Publication Date (Web): 13 July 2016
DOI: https://doi.org/10.21139/wej.2016.030

Overflow points in Sydney’s wastewater system allow excess wastewater to flow to stormwater drains or waterways in heavy rain to avoid flooding of homes and businesses. Sydney Water is regulated for wet-weather overflows by Environment Protection Licences (EPLs) from the NSW Environment Protection Authority (NSW EPA). Sydney Water has been working to reduce the frequency of wet-weather overflows, mainly by building large storage tanks, tunnels or bigger pipes and pumps, as well as reducing the amount of stormwater entering the sewer system.

Overflow frequency is traditionally used as the measure of system performance for wet-weather overflows. It does not take into account the volume or location of the overflow, the sensitivity of the environment or community needs, and expectations for the waterway and its surrounds. It also does not consider the risk imposed by other pollution sources such as urban stormwater runoff. The high cost of meeting containment standards is also a problem shared by water utilities worldwide. The significant additional infrastructure required for Sydney’s four major coastal wastewater systems to meet overflow frequency targets is estimated to cost at least $5.5 billion (in 2012 dollars).

Sydney Water has actively engaged with the NSW EPA over the last two years to develop a risk-based approach to manage wet-weather overflows and a regulatory measure to include in environment protection licences to deliver better, cost-effective outcomes. The risk assessment process is focused on three waterway values, representing the risks from wet-weather overflows to the environment and community, based on established waterway objectives and guidelines, and community input. The three values are:

  • Waterway ecosystem health: The potential for wet-weather overflows to impact aquatic ecosystems and riparian vegetation health.
  • Public health: The potential for wet-weather overflows to impact the public health of waterway users (for example, swimming and boating).
  • Aesthetics: The potential for wet-weather overflows to impact the community’s enjoyment of the environment, in and around waterways (such as in parks), through visual pollution and/or odours.

The risk-based approach provides a robust way of prioritising investment to improve wet-weather overflows, where the environmental and social benefits have traditionally been difficult to measure. A risk profile can be developed to form a baseline and gauge progress and improvement over time. It can also be used as the basis of a means of licensing and regulation. This approach provides a superior way to manage and regulate wet-weather overflows because it:

  • Identifies and targets improvement to areas of greatest risk and where greatest benefit can be gained;
  • Aligns regulatory performance with environmental and community outcomes;
  • Is adaptive to ensure outcomes are aligned with community uses and aspirations for waterways and the environment;
  • Allows solutions to match problems, for cost-effective delivery of outcomes;
  • Allows for proactive, constructive, consistent management and incorporates best practice approaches.

Under this approach, Sydney Water can achieve environmental and community outcomes more cost-effectively than under the traditional frequency based approach. Being an adaptive management process, Sydney Water can progressively reduce risks from wet-weather overflows to ensure overflows do not inhibit waterways from achieving the shared outcomes that councils, government and other stakeholders are working towards.

 Risk profile for each waterway value


Video: Wastewater overflow

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