Several projects for Thames Water and Defra, 2005 to 2015: This project consisted of an original cost-benefit analysis of investment options addressing the combined sewage overflows (CSOs) to the tidal Thames in 2005 as well as updates of the analysis in 2014 and 2015. CSOs happen when, the sewage system cannot hold the volume of rainwater and sewage due to heavy rainfall. In such instances, the excess volume is released to the water bodies in untreated form. The objective of this study was to assess the environmental impact of CSOs and to develop possible solutions. A consistent and comprehensive framework was designed for the comparison of the quantifiable costs and benefits of the proposed engineering solutions. This framework was then used to undertake a cost-benefit analysis in order to identify the ‘preferred option’. Benefits of the proposed investments involved improvements to the environmental and aesthetic quality of tidal Thames. These benefits were estimated using stated preference studies involving residents from within and outside London given the importance of the River Thames as a national asset.