Recommended trade-off set

The recommended Pareto optimal set of plans obtained by simulating the four Pareto optimal sets from the four bootstrapped ensembles against combined bootstrapped 440 scenario ensemble and non-dominated sorting is shown in Figure 5-10. The cardinal axes represent the total discounted energy cost requirements (fEnergy), total discounted

capital cost requirements (fCapex), and undiscounted supply resilience (fSupRes) to reflect

the worst 5 year period duration of occurred failure experienced by a plan. The arrows point towards the direction of preference; the ideal solution would lie in the lower left hand side corner of the cube. Such solution is not possible to achieve due to the trade- off between the financial and engineering performance. Improving the resilience of the system requires higher capital investment. However, plans of the same capital cost requirements and resilience levels differ in their energy requirements. The colour

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exhibit the worst eco-deficit while the blue plans the lowest eco-deficit. The

environmental performance seems correlated with the engineering performance of the system but some plans of the same resilience levels and capital cost requirements with higher energy cost requirements achieve lower eco-deficit than plans of lower energy cost. The size of the points refers to the aggregated robustness indicator, which shows the average robustness of the two considered metrics fLoS3 and fLoS4. The indicator is

strongly correlated with the resilience; the bigger the point the more robust the plan is. The two distinct fronts visible in this particular many-dimensional view are created by implementation of the RO/nonRO Deephams reuse scheme. Plans from the right hand side front implement the RO DRS which requires much higher energy to operate than the nonRO DRS scheme. In contrast, the majority of plans implement only nonRO Beckton reuse scheme.

Figure 5-10. Recommended set of Pareto optimal plans and their performance trade-offs. The cardinal axes reflect the total discounted capital cost, energy cost and undiscounted supply resilience. The undiscounted eco-deficit is shown by colour; blue plans exhibit lowest deficit whilst red plans the highest deficit. The size of the points refers to the aggregated robustness metric for Demand Level 3 and Level 4 LTCD violations; the bigger the point the more robust the plan is. The arrows point towards the direction of preference.

The variety of plan schedules within the recommended Pareto optimal plans is

illustrated in Figure 5-11. The horizontal axis shows the individual interventions whilst the vertical axis shows the planning periods. The lowest label on the vertical axis refers to no implementation, i.e., where an intervention is not implemented within the

141 and the size of the points refers to the number of plans where a particular intervention occurs in a particular planning period. The colour of the lines shows the Upper Thames Reservoir capacity where plan schedules shown in blue build the smallest reservoir present in the Pareto optimal plans (90 ML) whilst the red lines the biggest reservoir (150ML).

Figure 5-11a illustrates schedules of all Pareto optimal plans from the recommended set. It can be seen that majority of schedules does not include the RST transfer supply intervention and about half of the plans do not implement the UTR reservoir and ESD desalination plant. Plans that do implement ESD built it from 2040 onwards. All plans include the implementation of Meters and ALC demand management interventions and the majority do so at the beginning of the planning time horizon. Figure 5-11b illustrates the diversity of plans where the most controversial intervention, the UTR reservoir, is implemented (coloured lines in Figure 5-11b). Most of these plans build the reservoir bigger than 130 ML. It is worth noting that all these plans always implement all demand management interventions in various planning periods.

Figure 5-11. Schedules within the recommended Pareto optimal plans. The horizontal axis shows interventions whilst the vertical axis refers to the planning periods. A single line then illustrates the schedule of a single plan. The size of the points refers to the number of plans implementing an intervention at a particular period; the bigger the point the more plans the intervention occurs in in the particular planning period. Panel a) illustrates schedules of all Pareto-approximate plans from the recommended set. Panel b) highlights schedules of plans that implement the UTR reservoir. Panels c) and d) then “brush” the schedules further where the highlighted plans build UTR in 2020 and 2035, respectively. The colour of the lines in panels b, c, and d refers to the reservoir capacity.

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The schedules can be “brushed” further to explore plans that implement UTR in particular period. For instance, Figure 5-11c shows all plan schedules where UTR is built at the beginning of the planning horizon. The colour of the lines shows that all of these plans build the reservoir of the highest capacity (140 – 150 ML). Most of these plans also schedule the Meters, ALC, Mains and the BRS reuse scheme for the

beginning of the planning horizon. Figure 5-11d shows schedules where UTR is built 15 years later, in 2035. There is a greater variety of schedules than when UTR is built in 2020 (Figure 5-11c) as well as the implemented reservoir capacity (shown by the colour scale).