Glimpse into future of flooding

Ofwat has demanded flood risk assessment be included in April's five-year exercise. Justin Butler, managing director of Ambiental Technical Solutions, explains the assessment and protection measures utilities need to take

With climate change set to increase high-intensity rainfall, flooding is a growing danger in the UK. But the drainage network is not equipped to cope and Environment Agency (EA) data is unable to reflect surface water flood risk.

Companies must take a new approach to flood risk assessment and critical assets must be protected – but time is running out. Industry regulator Ofwat has insisted that water companies review the risk flooding poses to all sites, and identify whether more protection is needed, by April 2009.

Each company must demonstrate it has accurately identified flood risks to all sites and present an action plan in its report on price limits for 2010-2015 next April.

Companies must also prove the measures they propose will deliver cost benefits to the public. So how can companies meet Ofwat guidelines as well as protect assets and prevent service disruption?

For water companies, the problem of flooding is compounded by the concentration of assets in high-risk locations. It has been estimated that around 750 water and sewage treatment works are at risk of flooding in the UK.

Further investment

Last summer, Britain experienced some of the worst flooding it had seen for many years and the water industry was badly affected. For example, Severn Trent Water was severely impacted in Gloucestershire – Mythe Water Treatment Plant in Tewkesbury was inundated leaving 140,000 homes without running water.

Improved understanding of the precise nature, and multiple sources, of flood risk is required if we are to reduce the impact of significant flood events in future. Further investment is also needed to map, model and assess flood risk, while continued pressure should be put on government to control development on floodplains, increase flood defence spending and improve drainage infrastructure.

As part of its response to 2007’s serious flooding, Ofwat commissioned Halcrow to write a report, Asset Resilience to Flood Hazards: Development of an Analytical Framework. The document outlines a process for dealing with flood risk.

Precise details of the Service Risk Framework (SRF) can be found at Ofwat’s website, but basically the method falls into three categories:

  • Risk screening
  • Risk assessment
  • Risk management

Risk screening is the starting point to identify which assets may be at risk. EA flood hazard mapping may be used for this process, but this is only suitable as a preliminary test.

At the same time, companies also need to judge the consequences if each particular asset were to fail. Would service be disrupted? How significantly, and to how many properties? This should result in an evaluation of the costs that would be accrued if the asset flooded.

These two procedures are necessary to work out how much it is reasonable to spend on protecting individual assets. This cost-benefit analysis (CBA) is a required part of Ofwat’s new recommendations for April 2009.

Once low-risk assets are set aside each company must then move onto a thorough risk assessment. Ofwat has drawn attention to the deficiencies in EA data and said that additional sources of flood risk information may be needed to achieve a quantitative analysis of risk. In particular, the authority has advised companies to source more accurate data on pluvial (surface water) and groundwater flood hazards.

Risks to individual sites and hot spots within urban catchments need to be thoroughly understood so risk management activities can ensure flood defences are effective, and infrastructure upgrades are correctly designed and specified. So how can companies ensure their risk assessment covers everything accurately, including pluvial flood risks?

Pluvial flooding is caused by extremely heavy rain in urban catchments. In many cases this poses the greatest risk to a site.

Sewer capacities

High-intensity, long-duration rainfall events, coupled with insufficient sewer capacities, can combine to trigger complex flow paths within urban areas. These flow paths converge at topographic low points and are unable to drain away due to high levels of impermeability.

A recent study conducted by Ambiental on behalf of Thames Water showed that the coverage of impermeable surfaces within selected London boroughs has increased, on average, by nearly 20% over the past 30 years. Although rates of change in impermeability were found to be variable between boroughs, this is of particular concern given the link between reduced infiltration – for example, through the conversion of green spaces into industrial estates or paved driveways – and increased urban flood risk. The ageing UK sewer and drainage network can no longer cope with the monsoon-like rainfall that has recently become more common.

Ambiental has developed a modelling approach that accounts for pluvial flood risks, as well as all other sources of flooding. Flowroute is a three-dimensional, flood risk modelling platform, developed in conjunction with flood scientists at Cambridge University, which can be used to evaluate risk and impacts:

  • To existing buildings/infrastructure
  • To new buildings and future development
  • Resulting from future climate change

Flowroute incorporates topographic information at the sub-metre level, which includes buildings, structures and flood defences as well as the impact that these can have on patterns of flood risk over large urban areas. Simulated water flows are routed along streets and around buildings.

The model provides information, including depth, duration and extent of flood risk down to the individual building level. These digital flood risk maps are being used by Thames Water as part of the logging up process to lobby for increased funding to enable sewer upgrades.

This data can also be used to assess the damage that might be caused by a burst water main or heavy rainfall. This is more accurate than conventional modelling approaches – they cannot show how flooding evolves over time down to the level of individual buildings.

The topographic data used to model outcomes for Thames Water comes principally from light detection and ranging providers. This is the optical equivalent of sonar, whereby a laser scanner attached to an aircraft sends out a signal.

The time taken for the reflected pulse to be received back at the aircraft is recorded, providing accurate information about distance to the target. This information is then used to build up a detailed, digital three-dimensional model of the topography.

Network planning

Andrew Hagger – network modelling manager at Thames Water, who has been using Flowroute to assist with network planning and to inform capital investment programmes – said:

“Although we know we need to invest money in a particular area, we can now be much more precise in the location, size and specification of the solution that needs to be implemented. These flood maps and models have been very useful and the wastewater operations director was most enthusiastic about the application of the technology.”

The implications of flooding to water treatment plants and distribution networks are many and varied.

For example, floodwater can enter freshwater supplies causing contamination, exacerbating the polluting effect of floodwaters.

Damage to water infrastructure reduces its ability to operate as normal, and this has implications for insurance, business continuity and continuity of service provision as well as capital replacement and repair costs. Overflowing drains or sewers are a public nuisance, and water companies have been taken to court for breach of human rights.

Assets at the highest level of risk can often be smaller installations where the risk and impact of flooding may not have been assessed as comprehensively as would be the case for larger installations.

With this in mind, water companies should also consider evacuation routes and continuity of business relocation sites as part of the wider business continuity management planning function.

Those building any new infrastructure will have to adhere to PPS25 guidance, as with any other developer. If a potential site is identified as being at risk of flooding, this regulation means the risk must also be appropriately mitigated.

Small-scale sites not classified as being at risk of flooding from fluvial or tidal sources can potentially slip through the net as they may not be required to conform. This can then expose them to other risks, notably pluvial and sewer-related intra-urban flood risk.

Flooding, especially in the UK, is likely to be at the forefront of the political and risk- management agenda for a long time to come. It is therefore extremely important for water companies to recognise the importance of their role in helping to better understand and reduce flood risk to key installations.

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