Flooding

Aerial views of flooding in Oxford.
  • How can the impacts of flooding on society, the economy and the environment be better understood?
  • How can communication of flood potential to decision makers be improved?

Environmental change is likely to result in greater flood risk, both from surface flooding and from groundwater flooding. Rising groundwater was a significant causative factor for the summer 2007 floods in Oxford. Adopting an integrated catchment approach to flooding could lead to more accurate flood predictions and better informed policy on flood risk management within the Thames basin.

Geological and hydrogeological factors

To fully understand the potential impacts of flooding on society, the economy and the environment, the role played by geological and hydrogeological factors needs to be recognised. Equally, the function of human activity must be considered, including land use, human interaction with the subsurface and wider policies on surface water management.

In addition to the immediate impact of flooding, broader issues should be taken into account, including the potential for flood events to remobilise contaminants, induce geological hazards and for flood potential to sterilise natural resources. To ensure that the results of flood modelling provide an effective resource for decision making and planning in the region, appropriate methodologies for the communication of uncertainty must be employed.

More specific questions that we are helping to answer:


What are the implications of flood and subsidence hazards for the insurance industry?

This project investigates the correlation between flood hazards and subsidence hazards and the effect that these two sources of risk will have on insurance losses in the Thames Gateway. In the UK, household buildings insurers require a reasoned view on the likely scale of losses that they may face from natural hazards in order to assist in strategic planning, reinsurance structuring, regulatory returns and general risk management. The summer flood of 2007 highlighted a need for insurers and reinsurers to better understand how events may correlate in time and space, and how to most effectively use the computational models of extreme events. Consideration of the potential future risk of losses due to the combination of hazards such as subsidence and flooding is increasingly important in those areas where aggregations of large numbers of new properties are planned, such as in the Thames Gateway. Subsidence and flooding are generally considered independently of one another in risk assessment; however, the potential of one event to influence the magnitude and likelihood of the other should be taken into account when determining risk level. This research suggests that flood events can lead to increased subsidence risk due to the weight of additional water and sediment, or rehydration of sediment under flood water. The first part of the study identified flood risk areas that may be affected by subsidence and explored potential hydrological and geophysical drivers and links between flood and subsidence events within the Thames Gateway. The second stage of the project will look for links on developed sites and consider how climate change will affect risk to developments in the context of subsidence and flooding.

How can we assess shallow groundwater levels? How do shallow groundwater levels affect ground conditions, engineering works, flooding and sustainable drainage systems (SuDS)?

Groundwater flooding risk assessment can be difficult to carry out due to the complexities of groundwater flooding and lack of data. There is also more understanding required about how groundwater flooding affects underground structures such as cellars, basements, building foundations and tunnels. Groundwater flooding occurs due to water table rise, which is characterised by one or more of the following:

  • extreme high intensity and/or long duration rainfall
  • groundwater flow in alluvial deposits bypassing river channel flood defences
  • cessation of groundwater abstraction for public water supply or mine dewatering
  • underground structures creating barriers to groundwater flow.

Groundwater flooding can result in surface water ponding, intermittent stream flow or the anomalous activation of springs.

How can river and groundwater quality be managed to achieve sustainability?

Groundwater abstractions for public supply in London come mainly from the Cretaceous Chalk Group aquifer. The River Thames catchment area draws this water from a resource base of about 35 per cent groundwater and 65 per cent river water. Due to water resource pressures on the catchment, some groundwater abstractions are at risk of licence reductions while the development of new abstractions and management of operational abstractions can require complicated hydrogeological analysis. Thames Water therefore develops and uses regional groundwater models to aid communication with its stakeholders such as the Environment Agency (EA). This study shows the importance of these models as a context for sub-catchment scale analysis of groundwater source development and a range of local systems and interactions. The models are updated regularly, which enhances conceptual understanding and calibration and the models cost less than one per cent of the capital cost of new water source schemes. These models are active decision support tools and promote the clarity and consistency when stakeholder-specific issues can be analysed within an agreed regional model framework.

The Thames basin consists of multiple and separate aquifer systems that are used for public and private supply and also provide baseflow to the River Thames itself and its tributaries. The most important aquifer is the Chalk, which is being intensively studied to understand its hydraulic characteristics. A distributed numerical model can be built to simulate the groundwater flows within this aquifer. In this study open modelling interface (OpenMI) standard has been used to allow appropriate models to be linked. The composition consists of a fully distributed groundwater flow model of the Chalk, a simplified groundwater model of the Jurassic limestones and a river model. OpenMI allows the different types of models to communicate during run-time. This approach allows an appropriate representation of the flow processes within the Thames basin and consequently improves the simulation of groundwater flows, enabling better management of groundwater and surface water resources under future climate scenarios.

How can climate change models (temperature and precipitation) be used to inform and model the fundamental processes and impacts of climate change and flooding?

What role can sustainable drainage systems play in reducing flood risk?

Groundwater flooding has become more important after it became part of national and European legislation. This study uses the city of Oxford on the River Thames floodplain to examine the mechanisms for groundwater flooding in urbanised floodplain settings. After a major flood event in 2007, an extensive dataset was gathered that shows groundwater flooding of a significant number of properties is shown to occur in areas isolated from fluvial flooding due to high ground created historically to protect property and the transport network from flood inundation. Options for mitigating this type of flooding, including measures to increase the rate of conveyance of flood waters through Oxford, which are designed to reduce fluvial flood risk have also been recognised as a means for reducing groundwater flood risk within the city.

The Floods and Water Management Act was enacted in 2010 and includes the provision for sustainable drainage systems (SuDS), which aim to tackle this problem as well as improve water quality in rivers. A key sustainable drainage technique is infiltration to the ground: these systems utilise the storage capacity of the subsurface to attenuate rainfall and include, for example, soakaways, infiltration basins, infiltration trenches and free-draining permeable pavements. In the UK, the subsurface in many areas provides effective and practical opportunities for the attenuation of rainwater; however, a number of considerations must be taken into account during the planning, design and installation process. To promote these techniques, BGS is developing a national dataset that incorporates a wealth of subsurface data necessary for preliminary desk-based site assessment. The prototype national dataset provides:

  1. information on ground conditions that is useful when planning to install infiltration-based SuDS schemes
  2. subsurface information that warns the user of potential subsurface considerations that may limit the use of infiltration

The dataset guides the user through the necessary ground considerations and hence minimises the possible environmental impact of infiltration systems on the subsurface, ground stability and water quality.

Is flooding in Oxford likely to worsen as a result of climate change?

Within the River Thames floodplain in Oxford, data is helping us to understand the interaction between the natural environment and human settlements. Upstream of Oxford, a state-of-the-art, high density, wireless network of underground sensors is being installed to help understand how nutrient input, water accessibility and the flood regime within an ecological site determines the health of a sensitive plant community. This will have relevance to the design of flood mitigation measures being considered to protect urbanised floodplains located downstream.

What is the resilience to drought within the Thames basin and how would water resources stand up to, for example, a 'three-winter drought' or similar?

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