Potential impact of climate change on NSCC's $300M wastewater capital works programme
Annual Conference
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This paper discusses the application of the North Shore City Council’s (NSCC) calibrated trunk sewer MOUSE model to assess the potential impact of NIWA’s forecast climate change rainfall to the NSCC trunk wastewater network.
As part of a national program forecast climate change rainfall was developed by NIWA to estimate the potential climate change impacts on infrastructure across NZ. 30 years of climate change rainfall was generated from Regional Downscaling of the Global Circulation Model to produce a spatial and temporal forecast rainfall.
This analysis estimates the change in wastewater network performance and associated cost implication as a result of the forecast climate change rainfall as applied to the NSCC trunk sewer system.
A change of performance due to climate change is of particular importance, as consent conditions detailing the containment standard for wastewater networks are becoming more common. A containment standard is usually expressed as the number of times per annum a network is allowed to spill to the environment. The containment standard forms the basis for scoping and design of capital works.
The NSCC wastewater network targeted containment standard is an average of two overflows per annum by 2021, This containment standard is a core component of the NSCC wastewater network discharge consent. To achieve the Level Of Service (LOS), NSCC have developed a calibrated hydrological and hydraulic model of the trunk wastewater network, this model is utilised to assess and size upgrade works to meet the targeted LOS.
As a result of climate change the consented containment standards based on historical rainfall may not be appropriate. The cost implication of climate change on wastewater network capital budgets is currently unknown. Assessing the potential impact of climate change is important particularly as wastewater typically has the largest capital works budget of the 3 waters for cities in NZ. NSCC has a capital works budget for wastewater improvements of ~$300M before 2021 – it is the effect climate change may have on this budget which is being assessed.
This paper briefly outlines the development of the Project CARE strategic wastewater model, details the methodology applied to assess network performance from historical and forecast climate change rainfall, and summarises system performance impacts and the cost implication of the forecast climate change.