Overflow consenting and design storm selection based on historical rainfall

Joel Wilson (WCS Engineering), Andrew Rickert (WCS Engineering), Bridget O’Brien (Christchurch City Council), Tyler McMillan (Christchurch City Council)

Christchurch City Council (CCC) has a large wastewater network serving approximately 373,000 people with a single wastewater treatment plant capable of treating 650 megalitres/day. This paper outlines the innovative approach to assessing overflow compliance used by CCC. Continuous simulation of long-term rainfall data is utilised to assess overflow compliance rather than a synthetic rainfall pattern representing the target return period. Network planning is based on a design storm generated using statistical analysis of the 25-year rainfall continuous model simulation results. The paper demonstrates that overflow frequencies are sometimes not well represented by rainfall events that have an equivalent return period and that antecedent rainfall conditions and hydrologic and hydraulic routing of flows unique to each collection system can help to select a better design storm to simulate overflow return periods of interest.

Christchurch has two main river systems: Avon and Heathcote. CCC holds a resource consent that 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 timeseries modelling. Additionally, no site may overflow more than every six months on average, based on the same long timeseries modelling.

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 the consent compliance results, the assessment of overflow frequency and volume was performed for four time periods: 1995-2020, 1995-2010, 2000-2015 and 2005 2020. The compliance assessment results showed a significant level of variation in overflow response dependent on the rainfall window chosen; however, the compliance outcomes were not affected.

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 continuous simulation results as the previous synthetic event did not provide a good representation of actual overflows. The assessment was based on overflow volume, peak and spatial distribution. Due to a high degree of rainfall spatial variance in the historical events close to a 2-year return period, it was decided to complete 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 were shortlisted. The preferred 2-year design storm was a composite event made up of August 5, 1995 (northern basins) and April 17, 2014 (southern basins). 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.

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