Making Space for Water
Mike Woolgar, Atkins managing director of water management, looks at combating flood risk.
It is just over a year since the UK experienced unprecedented levels of summer flooding, resulting in loss of life and livelihood and impacting 49,000 homes, 7,000 businesses and critical infrastructure.
In areas affected by flooding, notably Tewkesbury, Gloucester, areas upstream of Oxford and South Yorkshire, some homeowners are still living in temporary accommodation.
The floods occurred as a result of heavy rainfall overwhelming drains, rivers and streams, and from high levels of runoff from hard surfaces.
As flood specialists are only too aware, flooding can be just as much about land management as it is about management of water infrastructure. With this in mind there are some forward-looking projects working with landowners to examine how space can be made available for water during critical periods.
Building on the flood plain has exacerbated flooding and it is clear that, despite the focus that flooding is now attracting, such building cannot be prevented entirely.
The insurance industry has long had concerns over the impact of climate change and resulting predictions of weather volatility and, as a result has been working with organisations such as the UK Climate Impacts Programme to improve understanding of flood hazard.
Meanwhile many homeowners are coming to terms with high insurance premiums and buildings that could be considered uninsurable. The recent agreement between ABI and the Government about levels of protection from some sources of flooding will no doubt be welcome news to property owners at risk of flooding.
The Pitt Final report called for urgent action from private and public sectors, with better co-ordination between the various stakeholders in the flood management arena. In addition, the Environment, Food and Rural Affairs Committee found that work was required to address flooding in a holistic manner.
A comprehensive understanding of the complexities of flood mechanisms in conurbations is required to ensure that future investment is planned for wisely; flooding from drainage, surface water flow and river or coastal flooding can all, in certain circumstances, interact and make the situation worse than expected.
What is certain is that implementation of schemes to eliminate flooding risk everywhere would be unacceptable socially, in terms of monetary expense and cost to the environment. What is not known in sufficient detail is where all the risks lie, how these can be mitigated in the most efficient way and what are the priorities.
It is vital that future planning for homes, business and services, such as water and wastewater treatment plants and power stations, are informed by a comprehensive situational understanding of flood risk.
Equally, detailed information is required to ensure that funding spent on flood defence is utilised effectively. These drivers have placed integrated modelling at the forefront of flood management.
The Atkins Water & Environment team has been pioneering this area of flood risk management and prediction. Through the coupling of Infoworks RS and CS models using OpenMI, which allows different types of environmental models to talk to each other, Atkins is delivering huge advances in integrated modelling.
Havant Flood Mapping Study
Atkins was commissioned by the Environment Agency to carry out a flood mapping study in Havant, in Hampshire, to predict the extent of flooding in the town under a range of fluvial conditions.
The Havant catchment comprises two main streams, with the upper two-thirds of the catchment being largely rural and underlain by permeable geology. The lower reaches of the catchment are on clay, where the streams run through the densely populated town of Havant. As a result of heavy urbanisation, there are many surface water storm drains discharging into the river network throughout the urban reaches of the rivers.
There are two main flooding mechanisms in operation in the Havant catchment. The first is controlled by long duration or multipeaked rainfall events, which fill up groundwater stores, resulting in high baseflow in the streams for prolonged periods, and could, lead to overbank flow.
The second is due to short duration high intensity rainfall, which results in rapid runoff from the urban areas, which could eventually overwhelm the urban drainage network, resulting in drainage excess flooding, or localised ponding in the urban areas.
When these two flood mechanisms act in combination, the risk of flooding is exacerbated, as high river levels prevent the surface water drains from discharging to the river. With high baseflow levels lasting for up to three months, the risk of combined urban and fluvial flooding is crucial in this catchment.
The catchment hydrology was modelled using continuous simulation, in order to represent both the long duration multipeak events as well as the short duration events both of which characterise the high-risk flood events in the catchment. This is a departure from the event-based flood hydrological modelling normally used in such studies, and it provides more flexible and more realistic storms.
The river network was developed in InfoWorks River Systems (RS) hydrodynamic model, which provides a one-dimensional representation of the river network using river channel survey data, and pseudo-two-dimensional representation of the flood plain flow, utilises LiDAR data of the flood plain topography.
The surface water sewer network was developed using an existing Southern Water InfoWorks Collection Systems (CS) model, which was based on detailed pipe survey data belonging to Southern Water. The models were configured to ensure that the outfalls of surface water drains from the CS model were inflow points to the RS model.
The models were coupled using OpenMI which enabled the models to ‘talk’ to each other, such that outflow from the CS model outfalls provided inflows to the RS model, and the RS model water levels served as downstream boundary conditions for the CS model. This coupled modelling system has many advantages.
Firstly, it allows the two different modelling environments to exchange data at the timestep level, thus enabling a truly linked system with accurate representation of the feedback between the two. This represents a step change in how urban catchments have been traditionally represented in river models and provides a more realistic representation of the urban-fluvial interface.
In addition, it provided efficiency savings to the modelling process. Secondly, it provides a framework within which several different sources of flooding and their interactions can be explicitly represented and assessed.
More importantly, is could provide the framework within which to develop holistic strategies for dealing with the flood risk from the various sources. Thirdly, this integrated approach allows the basis for communication between those responsible for the various sources of flooding. Given the plethora of responsible agencies, an integrated modelling and decision support tool such as this could prove invaluable.
To be effective, flooding needs to be managed in a more integrated way to enable risk- based assessment of urban flooding to come together.
At the moment responsibility for urban flooding is fragmented, divided between the water companies, drainage authorities, the Highways Authority, local authorities, private landowners and the Environment Agency.
More integration with clarity of responsibilities for flooding from fluvial, coastal and pluvial sources, and for management responses, including modification to surface and subsurface infrastructure, should improve the picture.
Indeed, many influential figures within the water and civil engineering industries have called for the establishment of a single flood authority, whether as part of the Environment Agency, or as a standalone body.
Such an organisation could be empowered to establish a coherent and integrated approach to the complexities of flood prevention and management, not least in resolving the apparent conflicts of interest that exist between those currently responsible for the issue.
The creation of such a unitary organisation would also help in addressing the obligation that the EU’s Water Framework Directive (WFD) and the EU Flood Directive places on Member States to take measures for flood management and land-drainage schemes to ensure compatibility with the new WFD environmental standards. The Pitt report has not gone as far as that in its recommendations, but the roles of the Environment Agency and of local authority engineers are proposed to be clarified and strengthened.
There have been a number of recent consultations such as the Government’s strategy for flood and coastal risk management called Making space for water. The first report of March 2005 said that the Environment Agency should adopt a strategic overview of all flood risk management issues. As a result of PPS25, it now plays a key role in the control of new developments in flood plains. As noted above the Pitt report is supportive of this.
In January 2007, fifteen integrated urban drainage pilot studies were launched, looking at ways to better prepare urban areas to cope with flood risk from overwhelmed drains and sewers. Atkins currently supports Denbighshire County Council with its pilot scheme in Prestatyn in Wales.
And more recently, an extensive report on groundwater has been completed and the programme board is considering the results, with further studies to investigate how to map the level of risk from all sources of flooding.
These are steps in the right direction. However, the key innovation lies in the development of risk-based methods, capable of exploring the performance of multiple flood management strategies within a single framework. Achieving this remains a significant challenge. Understanding how the system and management options behave over time will also need to be considered for developing strategic management choices.
Improved flood management will have significant environmental and social benefits. The techniques that Atkins is helping to pioneer enable the performance of urban flood systems to be evaluated better and flood risk modelled more precisely.
The total UK exposure to flood risks from all sources is difficult to calculate, but is known to be substantial. This lack of base information can hinder effective policy making and prioritisation of expenditure and can lead to financial inefficiency within water companies and government.
Innovations in integrated modelling will help water companies to make more informed investment decisions, make debate on the issues and priorities more soundly founded and assist the government and its agencies in delivering efficient and effective flood risk mitigation measures. It will also support the delivery of both national and international policy, and places the UK at the cutting edge of flood management technology.
Some of the challenges laid down to develop detailed and reliable flood modelling tools have been met. But research is essential to the continued development of flood management. Whilst modelling does not hold all the answers, and flooding and erosion can never be prevented entirely, effective modelling is an essential tool to help facilitate flood risk management and prediction.
Without it, procedure, methods and tools for exploring the effectiveness of future flood management policies could be hindered, targeted and integrated private and public sector funding would be more difficult to evaluate, and insight into uncertainties regarding the behaviour of the urban flooding system, would rely on less sophisticated techniques.
If the delivery of an integrated approach to urban flood management and its practicalities is the ultimate find, demanded by the Water Framework Directive, then advances in integrated modelling provides a collaborative tool that will enable stakeholders to begin to speak the same language.
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