Boscastle flood damage

Forecasting flood

As the risk of flood in the UK increases, so does the need to have an effective prediction system to protect critical assets, explains E&T.

Climate change is considered by many to be the greatest challenge facing the world today. Rising global temperatures will bring changes in weather patterns, rising sea levels and increased frequency and intensity of extreme weather.

We have already seen an increase in the occurrence of severe weather events, such as floods, heatwaves and droughts, and this could change the landscape of the UK beyond all recognition in the future.

Notable floods in York in 2000, in Boscastle, Cornwall, in 2004, and widespread flooding in 2007 and last summer have made flash floods such an issue that they are now the top UK insurance risk.

While it is impossible to stop the rain falling, it is possible to evaluate the risk of flood. What happens if a river bursts its banks? Will it flood a city, town or village? These are questions that insurance companies need to know to evaluate risk and set premiums.

"High-intensity rainfall is a growing danger in the UK, but the drainage networks are not equipped to cope and the Environment Agency data is unable to reflect surface water flood risk," says Justin Butler, managing director of Ambiental Technical Solutions, the specialist environmental risk mapping and modelling consultancy. "Companies must take a new approach to flood risk assessment and critical assets must be protected - but time is running out."

Water industry regulator Ofwat has insisted that water companies review the risk flooding poses to all sites and identify whether more protection is needed. "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 2010-15," he explains.

Butler says that water companies are taking this very seriously and remarks, "I think that will continue after their business planning phase.

"All the utilities companies are putting together their PR09 five-year business plans and that requires them to look at protection of critical infrastructure from flood risk. Certainly the more proactive ones have already embarked on this and I'm sure that this will continue given the huge amount of sites at risk throughout the UK. I think the first task is to screen out those at the highest risk and deal with them first - that is something else we are assisting them with."

Determining risk

One of the ways flood risk can be determined is by using very accurate 3D digital maps, captured from aircraft-mounted lasers. Ambiental is using this technique to assess the risk of flooding at critical utility sites across the UK. The project, undertaken on behalf of one of the UK's largest water companies, uses LiDAR (Light Detection and Ranging) data supplied by aerial survey specialist Bluesky to gauge the likelihood, extent and depth of potential flooding as part of a review of flood defence measures at each of the five sites.

The Bluesky data is part of a 3D terrain map, available online, that covers most of England and Wales including all major urban centres, coastal areas and flood plains. The high-resolution LiDAR data, supplied by Bluesky, was used to map each risk in 3D to ascertain the spatial distribution of different flood depths. The results of these assessments were used for business planning purposes, specifically within cost-benefit analysis for flood defence measures at each site.

"LiDar data is very popular at the moment and has been so in the last couple of years. It has not just been requested by Ambiental, but lots of other companies as well," explains David Reid, sales executive at Bluesky. "The data can also be used in civil engineering projects because it is useful when you are looking at quite large areas. Topographic surveys can be quite expensive for large areas, and LiDar data can act as a cost-effective substitute."

Ambiental used a combination of existing datasets, models and gauged data in combination with its proprietary modelling and mapping techniques to assess and quantify each flood source in turn. Potential sources of flooding considered during the analysis included fluvial (river), tidal, surface water, sewer and groundwater.

Likely flood risks

"The first stage is really to scope out which specific flood sources are likely to have an impact on a given area - if the site is not close to the sea then coastal and tidal will be ruled out, so it would be irrelevant to look at those sources," explains Butler. "We would then narrow down the sources on the flood list to fluvial and pluvial (rain water) and then run them independently, but we can also look at the interaction between the different flood sources.

"So, for example, although a flood defence may reduce flood risk at a particular site from fluvial flood risk, sometimes that can have the opposite or deleterious effect whereby ponding of surface water from heavy rainfall can actually be held back by flood defences."

This requires the sources to then be looked at in combination. "Whether or not we run a multi-source model together really depends on the situation," says Butler. "In the first situation we can look at the sources independently and how they interact."

When it comes down to which defence system to adopt Butler says: "We can run an analysis on the various flood barrier options available, but what we normally do is provide an interactive flood depth map for different scenarios or return periods.

"They [the client] can then do a cost benefit analysis on what the costs would be for say a two-metre high flood wall around a site to protect against a one in a 100-year level, including the impacts of climate change. Then they could almost draw a curve of cost against standard of protection and decide upon the standard of defence that is most appropriate and cost effective in that particular situation," he explains.

Being able to predict the likelihood of flooding is one thing, but Ambiental is also able to assess the likely insurance costs of an event as well. "We have done post-event loss estimation using new forms of satellite imagery, says Butler. "What we can do is collect flood outline and flood extent information and combine that with other sources of information, such as built environment and insurance claim information, where that is available, and work out what the loss is likely to be from that event. This is usually done pretty soon after the event has taken place.

"However, we didn't actually do that task in Gloucestershire. Most of the work that we did there has been validation and calibration of our models against insurance claims data," he explains. "It is something that can certainly be done using something like cloud-piercing radar data for looking at the flood extent."

Insurers that have invested in flood risk assessment technologies have saved tens of millions of pounds by careful risk selection. Ambiental is able to provide detailed flood risk information at the postcode or individual building level for all sources of flood risk.

The problems of flooding are only going to get worse with climate change. "The latest IPCC [Intergovernmental Panel on Climate Change] figures are talking about very rapid sea level rise and certainly the likelihood of increased intensity rainfall in winter months. They are not quite sure yet what the summers are going to look like - probably more droughts and higher probability of extreme drought but at the same time tempered with extreme rainfall potentially as well. So, it is really the intense rainfall storms and more of them that are problematic for the UK, given our limited sewer capacities."

Butler explains: "Another contributing factor to flood is that something like 20 per cent of land surface has decreased in permeability in the last 30 years or so - a characteristic that is ubiquitous throughout major towns and cities, so that increases the flashiness of the rainfall run-off regime."

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