How much seawater will be in our wastewater networks?
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| Seawater infiltration is a major consideration for all coastal sewer networks. It increases the flow to pump and treat, reduces the network capacity, contributes to overflows, damages the network through corrosion and loss of soil support, and can negatively impact treatment. With a single flow gauge, we recently estimated how much of the wastewater pumped within Auckland’s Lower CBD wastewater network is seawater and how this may change over time due to sea level rise. The project analysed the relationship between tide level and wastewater flow and attempted to break down the observed dry weather flow into sewage, tide-driven infiltration and other infiltration. The study considered the diurnal and seasonal patterns in sewage flow, the mechanisms of flow entering the network, and the effect of low, high and extreme tides. Exploring and visualising the data provided an understanding of the uncertainties involved and also helped determine potential causes of seawater infiltration, ultimately informing the proposed mitigation options. From the study, a piecewise linear model was developed that gives an estimated wastewater flow for a given tide level. We used this to estimate several design flows such as peak dry weather flow in mean high tide, and average daily volumes under future extreme tides with sea level rise allowances. The project quantified the consequences of leaving seawater infiltration unchecked. Right now, seawater makes up 24% of the daily pumped volumes in Auckland’s Lower CBD, this will increase to 37% in 50 years due to sea level rise alone. This formed part of the problem definition for the catchment, which in turn informed the proposed intervention programme. The presentation will explore how data analysis techniques can help extract powerful insights from limited data, understand the uncertainties involved and provide a range of potential outcomes supporting investment decisions to make our networks more resilient to sea level rise. |