Jan Heijs, Greer Lees and Dean Watts.
Wastewater planning and modelling is based around assumptions such as growth and demand. Models enable asset owners to scope their projects and meet the financial planning requirements of the Local Government Act. The models can be used to test the impact of growth and demand changes tested against network performance standards too. How do you develop a strategy when there is no (or no fit-for-purpose) performance standard set? And how do you know if that performance strategy is affordable? What performance is being assessed, and what happens when there is little certainty about forecasting assumptions?
Kaitaia is a town representative of many towns in New Zealand, where little or no growth wasforecast in 2015 but has experienced population growth since. Regarding infrastructure planning, and specifically wastewater strategy, is this a blip on the horizon or a long-term trend that needs to be considered in current decision making? If the infrastructure is already at its limit, what do these demand variations do to the system? This uncertainty includes the impacts of climate variation such as increasing water leakage and pipe bursts over dryer summers; implementation of coastal retreat strategies due to extreme weather events; and changes in community willingness to pay following catastrophic events such as earthquakes.
Recent project examples are included to demonstrate that defining a wet weather performance standard for wastewater networks can grow a shared understanding of the network and justify and drive investment in wastewater infrastructure to reduce negative impacts on the community and the environment. Developing a wet-weather performance standard requires a good evidence base; enabling decisions to be confident and defendable. Collecting good data means models can be updated at regular intervals to reflect small or significant changes. With reliable network performance information, other data sets can be overlaid to demonstrate complex but important attributes of your infrastructure. The influence of flood events, receiving environment health, planning and development overlays, and social data sets can highlight trends that may affect infrastructure decisions.
This paper will also demonstrate that the way to achieve any agreed performance standard is through “adaptive wastewater strategy” including tools to ensure planning and improvement are possible in the face of uncertainty. Examples from projects carried out in the Far North, Queenstown and Tauranga will be used to show how these tools are being used to improve wet-weather network performance. With the recent finalisation of the 2018 Long-Term Plans (LTPs), this is the perfect opportunity to examine current thinking and data on wet weather performance to be in forward planning mode rather than reacting to abatement notices.