Christchurch has a large and complex wastewater network serving 384,000 people. Past efforts to reduce wet weather overflows were based on traditional trial-and-error modelling, and the system upgrades stemming from those modelling efforts have not been as effective as hoped, despite costing more than $150 million.  Even with those upgrades in place, there remains an overflow volume of 38,000 m³ to waterways and a further 40,000 m³ overflows from 165 manholes during a 3-year ARI (average recurrence interval) storm.

This project’s objective was to identify the most cost-effective suite of projects to prevent system overflows for three different return period storms (6-month, 1-year and 2-year ARI).

To best evaluate a broad range of alternatives in the improvement plan, Christchurch City Council (CCC) chose to use genetic algorithm (GA) optimization, Optimatics’ Optimizer WCSTM software, and the experienced optimization team of WCS Engineering.

Improvement alternatives were gradually added in order of complexity (starting from increasing conveyance capacity along existing alignments, then flow controls and diversion options, then storage facilities, and finally I/I reduction options). This strategy enabled the team and CCC to review optimized solutions incrementally and gain appreciation of the potential cost savings associated with various alternatives.

This project achieved capital cost savings of up to 32% to achieve an aspirational target of no overflows in a 3-year annual recurrence interval storm. The optimization process also gave CCC certainty that the recommended projects were the most cost-effective suite of projects to achieve its target.

Figure 1 – Preliminary optimized solution to achieve 2-year ARI overflow frequency

The preliminary prioritization assessment results are shown in Figure 2 and Figure 3. Figure 2 shows the outfall discharge volume and cost to abate overflows at each outfall for the 2-year and 1-year ARI design storms. The data are shown from left to right in order of return on investment. That is, outfalls with relatively low cost to eliminate large outfall volumes are shown toward the left on the figure.

Figure 2: Outfall volume and capital cost to abate overflow

Figure 3: Prioritized return on investment profile for overflow abatement based on capital cost (assuming projects to abate manhole overflows are already implemented)