Experimental and numerical investigation of pluvial flood flows and pollutant transport at and between system interface points

The 2007 flood events in the UK were estimated to have had an economic cost of £3.2billion and resulted in 13 deaths. The frequency and magnitude of flood events has been forecast to increase due to the impacts of climate change, urbanisation and the deterioration of wastewater infrastructure. In the last summer period alone Newcastle, Manchester, Belfast and many other UK towns and cities have experienced significant pluvial flooding events. To mitigate these effects urban flooding hydraulic models have been developed which characterise the flow in the sewer system, overland flow in the urban catchment, and the exchange of flow between these systems so as to identify those areas which are most at risk of flooding. Such models are used by local authorities and water companies to identify areas for flood mitigation work, such as prioritising investment to improve drainage infrastructure, taking measures to reduce storm water runoff and raising the awareness of residents.
However the accuracy such models is inherently difficult to verify due to the difficultly of acquiring reliable data during flood events, e.g. it is not feasible to be sure that the modelled flow paths and velocities are accurately predicted at the time of the flood event. Similarly, but more complexly, it is not currently possible to quantify the transport of potentially dangerous faecal contaminants from the sewer networks to residential areas in flood events, and hence to assess the risk to health of flood waters. Previous sampling of flood waters has shown that this health risk may be significant. The behaviour of interaction points (e.g. manholes) is critical to the transfer of such flow and pollution between sewer and surface, but the hydraulic behaviour at such interfaces is especially difficult to quantify due to the complex nature of the flow.
This proposal seeks to better understand these interactive processes within a unique scale model facility at the University of Sheffield that combines the flow in a below ground sewer system with the shallow water surface flows over the catchment, linked by a number of manholes. The overall aim of the research is to improve the verification process of urban flood models, to provide detailed measurements and a more accurate understanding of the hydraulic characteristics of interaction points, to quantify surface flow paths and to advance the modelling capability to the spread of pollutants from sewer systems within the surface flow. The outcome will be a significantly enhanced modelling capability for urban flood flows and a much improved understanding of flood risk in urban areas. This will be achieved by a detailed programme of experimental testing using the facility at Sheffield, coupled with state of the art modelling work to calibrate, improve and verify urban flood models. The proposal enjoys the support and engagement of leading developers of urban flood models in the UK (Innovyze, Microdrainage), as well as consultants, water companies and a local authority. This partnership will ensure that the research findings are incorporated into the latest modelling approaches and are utilised to improve flood risk evaluation in the UK.
As well as advancing existing models that predict flood extent and depth, it is seen as important and ambitious to extend models to predict of mass transport, which will enable quality and potential health implications of flooding to be better established. In addition, to develop these aspects further, there is also significant value in extending the work proposed to include characterising the transport of sediments from sewers to surface flow in flood conditions. This would be of scientific and practical value as contaminants that pose a significant health risk are often attached to sewer sediments. The department has therefore agreed to provide scholarship which will use the facility to study the transport of sediments, further expanding the value of the proposed work.

Pluvial flooding causes significant long term damage to residual and commercial areas, economic disruption, danger to life and social upheaval. The development of tools and techniques to predict surface water flooding is one of the key recommendations of the 2008 Pitt Review. Together with the wider public who are at risk (it is estimated that 3.2 million residents in the UK will be 'at risk' of urban flooding by 2050), all stakeholders with a responsibility for managing urban flooding would benefit from a more accurate quantification of flood risk, including local authorities, the water industry, emergency responders, the insurance industry and the Environment Agency. Hydraulic modellers and consultants involved in the work will also benefit by having access to experimental data. Developing and extending their products will enable them to become more competitive in the water modelling sector and further establish UK expertise in the sector.
The outcome of the project will be a significantly enhanced modelling capability for urban flood flows, with the development of new improved software as evidenced by the letters of support from Innovyze and Microdrainage. Specifically by making significant improvements to the capabilities of pluvial flooding models in terms of accuracy, speed and information provided.
As hydraulic models are increasingly being used to evaluate urban flood risk, the resulting improvements in modelling accuracy will see significant benefits in the targeting of asset investment (e.g. upgrading of wastewater networks or the implementation of SUDS), developing accurate flood risk mapping (leading to improved resilience of homes and higher awareness amongst residents), the development early warning systems, and the development of safe surface water management plans. In addition, it is increasing becoming recognised that it is infeasible to manage all storm water in below ground systems, partly due to the increased sewer flows forecast from climate change and changing urban hydrology. Given the unfeasible cost expanding all sewer networks to cope with increased flow, planning effective and safe management of storm water on the surface is becoming increasingly important in future drainage design. The accurate prediction of surface water flow paths, depths and velocities as well as how contaminants such as faecal matter moves though the urban environment in exceedance flow is will be critical to safe urban drainage design. An accurate characterisation of the hydraulics and transport processes in pluvial flooding conditions would enable risk mapping based on a physically based model of pollutant transport. This would allow a hazard and vulnerability assessment health risk to individual areas or properties and prioritise health checks/screening in the event of flooding by identifying those areas/properties that are potentially exposed to the highest concentrations of contaminants.
In addition, by publishing valuable experimental datasets regarding the hydraulics of urban drainage systems (in both exceedance and normal conditions), 2D flow fields in time varying shallow flows and transport of soluble material the proposal will have a significant impact on academic researchers working in the fields of urban drainage systems, water quality of flood flows, urban water management and applied fluid mechanics. This impact will be enhanced by the work of a PhD student (supported by a University Scholarship) who will extend the water quality work discussed in this proposal from the transport of solutes to fine sediments.
In summary, the results of this research will significantly benefit the UK public in terms of by enabling a more accurate quantification of flood risk. This will lead to more effective, cost efficient design and management of the UK's wastewater assets, more effective strategies for flood risk mitigation and a reduction in damage caused by pluvial floods.