The Potential Impact of Climate Change Upon Sewerage

This paper outlines the potential threats upon sewerage systems arising from any increase in extreme rainfall events. The paper then describes the methodology and outcome of research undertaken by the Met Office on behalf of Severn Trent Water as the preliminary stage of an extensive project investigating changes in short period rainfall as a result of increasing greenhouse gases. Results and conclusions are presented, together with a discussion regarding their relevance to sewerage design philosophy and criteria. Introduction The potential problems arising from the impacts of climate change as a result of global warming is a topic of increasing interest and concern to many. It is widely accepted that the global climate is changing, and the focus has now changed from “if” climate change will occur to “when”. One particular area of concern for the wastewater planner relates to the hypothesis that global warming may lead to an increase in the frequency and intensity of the short-duration extreme rainfall events which can give rise to sewer flooding. To date, there has been very little quantitative research into the long term effects of climate change on the magnitude and frequency of such storm events. However, concern has been raised within Severn Trent Water that, should this hypothesis prove to be correct, one would find that the increased rainfall would create a consequential increase in urban run-off which could not be accommodated in the storm sewer systems leading to wide-scale flooding. Thus, the following question was posed: “Can the Company’s nineteenth century infrastructure cope with twenty-first century storm events?” Severn Trent Water covers a landlocked area of some 8000 square miles of central England, serving eight million customers from the Humber estuary to the Bristol Channel and from mid Wales to the East Midlands. For some time, Severn Trent Water has recognised the potential threats to be addressed by water companies arising from climate change and, as such, the Company has played a lead role in the instigation of research into climate change research in the United Kingdom. The Severn Trent sewerage network comprises more than 52,000 km of pipework. It is therefore clear that any change in the climate, giving rise to increased levels of precipitation, would have a significant impact upon the Company, in terms of storm sewerage. The existing network has been designed to current accepted standards; standards which may or may not meet the demands of any future climate change. Furthermore, Severn Trent, like other UK water companies, has a major ongoing capital works programme, of which investment in sewerage is a significant factor. Any new or replacement sewers will also be designed to current standards. Therefore, it is vital to the industry that the potential for any increases in rainfall is investigated and quantified as a matter of some urgency in order to ensure that appropriate standards are adopted for future sewers. Sewerage Design Philosophy A detailed analysis and discussion of the design of sewerage systems, and how that might change in the future, is outside the aegis of this report. It is important, however, to consider the overall philosophy which underpins the design procedure.