Adapting for a changing climate
Climate change projections suggest a future increase in average rainfall in the winter and a decrease in the summer for the UK. But what are the potential impacts of these changes and possible solutions for combined sewer systems?In 2009, Patrick Cooney enrolled as one of nine research engineers in the newly-formed Stream Industrial Doctorate Centre (www.stream-idc.net; part-funded by the Engineering and Physical Sciences Research Council).
After studying civil engineering at the University of Bristol he received an MSc in urban water engineering and management from the University of Sheffield. Patrick reapplied to the University of Sheffield a year later to study climate change impacts on buried infrastructure under the supervision of Professor Adrian Saul, and now works as a research engineer for Yorkshire Water as part of the STREAM Industrial Doctorate Centre.
Yorkshire Water owns around 20,000 miles of sewer pipes across the Yorkshire region (with nearly an additional 14,000 miles of currently private sewers and lateral drains due to come under company ownership from October), which are servicing close to 2M homes and delivering to over 600 waste water treatment works. Yorkshire Water is spending £250M on waste water treatment works to comply with environmental guidelines such as the Water Framework Directive's requirements for all inland and coastal waters to reach 'good status' by 2015. This is not only a major driver for improving the quality of river bound output from treatment works throughout the Yorkshire and Humberside region, but also a driver for improving the understanding of combined sewer overflows (CSOs) in a changing climate.
As part of Yorkshire Water's vast sewerage network there is a requirement for close to 2,000 CSOs. These are designed to discharge excess stormwater to the environment during times of rainfall, when the hydraulic capacity of the sewerage system is exceeded.
Discharge frequency and duration can vary greatly from one CSO to the next within the same combined sewer system due to the spatial variation of rainfall and the variation in capacity and complexity of the sewerage system itself. Most CSO outfalls flow into a watercourse only during or following very high return-period rainfall events, whilst others discharge more frequently. CSOs are designed to only spill a dilute flow of wastewater after rainfall events in order to limit the environmental impact on the receiving watercourse.
The concentrations of pollutants in an overflow volume are often dependent on the antecedent climate conditions as well as the duration/intensity of the contributing rainfall event(s). A long dry period followed by an intense rainfall event may lead to a spike of accumulated pollutants in the urban environment being washed into the sewerage system and subsequently into the receiving watercourse via CSO structures.
UKCP09 Climate Change Scenarios conceive that across the Yorkshire and Humberside region there could be an average 11.1% increase in winter rainfall and an 18.5% decrease in summer rainfall by the 2050s.
This is projected to increase further to a 14.9% rise in winter rainfall and a 22.9% fall in summer rainfall by the 2080s. These figures are taken at a medium relative emissions scenario for a 50% confidence level where projections are equally likely to be higher or lower than these values. Though there is uncertainty still associated with these projections, there is likely to be an increase in overflow volumes from CSOs in the future if steps are not taken to deal with the increase in stormwater flows.
Patrick has started an investigation into the impact UKCP09 projections have for acute and cumulative rainfall events for two case study drainage zones. This research will improve understanding of how Yorkshire Water's drainage systems will be able to cope with an increase in a given design storm size, as well as an accumulation of rainfall outputs given by a future weather generator.
Sewer systems have, in general, historically been designed to cope with a given design storm, but the magnitude of such a storm is likely to become more frequent in the future. In theory, this could mean overflows from CSOs will become more dilute in pollutants, aiding the quality of the receiving waters, but to the detriment of treatment works performance where the treatment of dilute effluents becomes cost ineffective.
However, studies have shown that there is often a 'first flush' or spike in concentration of pollutants flowing through a CSO during or soon after a heavy rainfall event. The impact of the first flush on overflow volume is very dependent on the available capacity in the sewerage system as well as the causal rainfall event, the antecedent weather conditions and the assimilative capacity of the receiving water at the time of the spill event.
To measure the magnitude and impact of the first flush it is necessary to use drainage modelling software to look at each CSO on an individual basis, thereby enabling a level of risk to be assigned to each CSO and subsequently to entire sub-catchments or drainage zones. For high risk areas, mitigation steps such as sewer separation, SUDS conveyance or conventional storage increase can then be appraised and prioritised.
Patrick's research into the changing conditions under which CSOs will operate in the future will provide an opportunity to assess the most sustainable methods for the mitigation of CSO climate change impacts across the Yorkshire Water business. This will assist Yorkshire Water in managing the ongoing performance of its sewerage network, and ensuring that climate change does not lead to future deterioration of receiving watercourses.