SSO Consenting and Design Storm Selection Based on Continuous Simulation of Historical Rainfall

Purpose of the Presentation
The purpose of the presentation is to demonstrate the innovative approach to assessing sanitary sewer overflow compliance being used by Christchurch City Council (CCC), New Zealand. Continuous simulation of long-term rainfall data is used by CCC to assess overflow compliance rather than using a synthetic rainfall pattern that is intended to represent the target return period. Furthermore, collection system planning is based on a design storm that was generated based on statistical analysis of the 25-year rainfall continuous model simulation results. The presentation will show that SSO frequencies are sometimes not well represented by rainfall events that have an equivalent return period and that antecedent rainfall conditions and hydrologic and hydraulic (H&H) routing of flows unique to each collection system can help to select a better design storm to simulate overflow return periods of interest.

Benefits of the Presentation
This presentation should be selected because it will provide the following benefits to the audience and industry:
-- it will illustrate the difference in overflows predicted using a synthetic return period rainfall event and the overflow frequencies calculated based on continuous simulation of long-term rainfall.
-- it will show how the environmental regulators for Christchurch, New Zealand require continuous simulation of long-term rainfall to validate overflow compliance.
-- it will demonstrate the value in reviewing continuous simulation overflow results to develop more accurate design storms to be used for infrastructure planning.
-- it will show how a utility can use an improved approach to developing design storms to ensure investments are made in new infrastructure and infrastructure rehabilitation programs that are targeted in the relevant area of the collection system in order to meet consent conditions effectively.
-- it will illustrate how three different 15-year windows of rainfall from a 25-year period of rainfall can affect the outcome of the overflow frequency assessment but that the climatic variations do not have a significant bearing on the outcome of the overflow compliance.

Project Background
Christchurch City Council provides drinking water, stormwater, and wastewater across five council areas in the North Island, New Zealand. Christchurch has a complex wastewater network serving 373,000 people. Christchurch has a large and complex wastewater network serving approximately 373,000 people with a single wastewater treatment plant capable of treating 650 megaliters/day (MLD). Christchurch has two main river systems: the Avon River and the Heathcote River, which both flow into the Avon-Heathcote Estuary which then flows into the Pacific Ocean (see Figure 1). Christchurch City Council (CCC) holds a resource consent (permit) for the overflows to waterways. The resource consent allows an overflow frequency to each of these receiving environments which decreases over time to a 2 year ARI, based on 15 years of long time series modelling. In addition, no overflow site may overflow more than every six months on average, based on the same long time series modelling. The Christchurch Overflow Compliance Assessment project was commenced and completed in 2020. The project was completed using the latest model calibration completed in 2020 and was an update to the overflow compliance assessment completed based on the model previously calibrated in 2015. The overflow consent conditions require that continuous simulation of the most recent 15-years of rainfall be completed to assess the system performance. This project included continuous simulation of the most recent 25-years of rainfall and analysis of overflow results from three different 15-year windows to investigate whether the overflow frequencies are significantly affected by the 15-year window of rainfall selected. Overflow control measure alternatives to achieve the 2-year overflow return period target were previously assessed using a synthetic rainfall pattern. To determine whether this rainfall pattern would be appropriate for future planning studies, the long-term continuous simulation results were used to predict the location of overflows occurring more frequently than once every two years and then compared to the locations predicted using the synthetic rainfall pattern. This assessment demonstrated that the synthetic event did not provide a good representation of actual overflows and a new design storm was developed based on historic events.

Project Details
The objective of the wet weather overflow compliance assessment was to determine wastewater overflow compliance with the consent conditions stipulated by the environmental protection agency, Environment Canterbury (ECAN). This was undertaken using the calibrated hydraulic model to simulate system performance based on continuous rainfall data from the period of 1995 to 2020. To determine whether the rainfall window could have an impact on any of the consent compliance results, the assessment of overflow frequency and volume was performed for the following time periods:
1. 1995 to 2020
2. 1995 to 2010
3. 2000 to 2015
4. 2005 to 2020

The compliance assessment results summarised in Table 1 and Table 2 below, show the level of variation in overflow response according to the rainfall window chosen. While the variations are significant in some time windows, the compliance outcomes would not have been affected. Table 1: Overflow volume in each catchment for different time periods Table 2: Overflow frequency in each catchment for different time periods The objective of the design storm review was to ensure the design storm used for wastewater infrastructure planning is representative of a 2-year return period based on the 25-year continuous simulation results. The assessment was based on overflow volume, peak and spatial distribution. The 25-year continuous simulation results were reviewed to identify historical events that corresponded to an overflow return period of approximately 2 years. Due to a high degree of rainfall spatial variance in the historical events that were close to a 2-year return period, it was decided to repeat the overflow statistical analysis by receiving environment rather than system-wide. Based on the results from the overflow statistical analysis, several 2-year design storm alternatives (synthetic and historical) were shortlisted. The preferred 2-year design storm, presented in Figure 1, was the August 5, 1995 (northern basins) and April 17, 2014 (southern basins) composite event. This event provided the best representation of 2-year ARI overflow volumes, peaks and spatial distribution and comprised events of sufficiently short duration to be convenient for planning.

Conclusion
The results from this study demonstrate a novel and improved approach to developing design storms to ensure investments are made in new infrastructure and infrastructure rehabilitation programs that are targeted in the relevant area of the collection system in order to meet design criteria and/or compliance targets.