Appendix 4 – Flooding at Cavendish Ave-Gray St, Perth

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Appendix 4 – Flooding at Cavendish Avenue/Gray Street, Perth 
Advantages
• Works would protect properties from flooding up to the 1 in 30 year rainfall event; 
• Removal of surface water from combined sewer system has wider benefits – reduced water treatment
energy use/CSO spills;
Disadvantages
• Limited standard of protection – risk of flooding remains during events above design standard (30 year
storm event) although the option would reduce the frequency and total volume of flooding;
• No allowance for increase in future flood risk due to the effects of climate change;
• High Capital cost (£8.6m) – refinement of option estimates indicate that this will increase further;
• Benefit cost ratio (0.045) is well below 1 .0 – the estimated cost of the works therefore greatly exceeds
the potential benefits over time. The works are therefore not economically viable.