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Destination dry land

Mapping is becoming ever more important in the fight to understand where flooding is most likely to occur. Margo Cole reports on the latest developments.

The flooding that has hit much of the UK during the last two months has thrown a spotlight on the accuracy of predicting where floods are likely to occur and the tools available to make those predictions. The Environment Agency estimates that around 5M properties in England alone are at risk of flooding, but of course the important issue is knowing which properties these are - not just so that their owners or occupants can be prepared, but also so that flood defence measures can be targeted more accurately and people not at risk do not pay unnecessarily high insurance premiums.

Just before Christmas, the Agency published new maps that showed that fewer households are actually at risk of surface water flooding than previously assumed - a reduction from 3.8M to 3M.

This revelation came just two months after specialist consultant ESI published another set of maps that indicated that the risk of groundwater flooding is also not as widespread as previously thought.

This apparent downgrading of the flood risk for some households is mainly due to the increased sophistication of the new models compared with their predecessors.

As the models become more refined, it is easier to distinguish which properties are most at risk rather than simply identifying a general area of high risk.

ESI managing director Mark Fermor says we can expect even more refinement of the models in the future. “I think there’s going to be significant improvement in how representative these things are over the next five years - every year you will see an improvement,” he says.

The Agency agrees. In a statement accompanying the launch of the new surface water flood maps the Agency said: “Future versions of the maps will be more detailed and accurate as the modelling ­technology develops.”

Ultimate goal

Fermor accepts that the ultimate goal is to be able to pull together all the different models to produce accurate maps that can identify the likelihood of a property flooding - whatever its cause.

“When you look at the overall risk of flooding, it’s not about river flooding, or groundwater flooding - it’s just flooding,” he says.

However, responsibility for managing the various forms of flooding - particularly in England and Wales - has historically been devolved to different organisations, so each type of flooding is mapped separately.

The current situation, which has been in place since the introduction of the 2010 Flood Management & Water Act, is that local authorities are responsible for dealing with groundwater and surface water flood risk, while the Environment Agency handles the effects of river flooding.

Surface water flooding occurs when rainfall is too heavy for the existing drainage systems to cope. Groundwater flooding occurs when subsurface water emerges from the ground because aquifers are full, or because high river levels or tides drive water though near-surface deposits.

Some properties may be at risk from both forms of flooding, and both may occur as a result of high rainfall, but Fermor describes groundwater flooding as “the flooding that can happen when the sun’s shining”.

He says that, compared with other causes of flooding, groundwater flooding is more complex and less well understood.

“You have to understand how the groundwater interacts with the surface, and how it flows,” he explains. “It is a combination of how likely the water table is to reach the surface, and the amplitude of fluctuation.
“Some shallow ground can get wet, but because of the permeability of the ground, you will not have a lot of flow,” he adds.

As a result, Fermor says, groundwater flooding is far more of a localised issue than other forms of flooding, so if they are going to be of any value, flood risk maps have to be accurate to a high resolution.

The model ESI has developed drills down to give a classification of risk for an area of 50m by 50m.

This compares with the previous best dataset that indicated simply where geological conditions meant groundwater flooding might occur at a scale of 1:50,000.

According to Fermor, this increased accuracy should result in fewer “false positives” - properties labelled as being at risk of groundwater flooding when in fact they are not.

Costly issue

“Groundwater flooding is not a widespread problem, but where it does occur it can be a costly issue,” he says.

Unlike fluvial and surface water flooding, groundwater flooding is less likely to put lives at risk, but it can be disastrous for properties.

As a result, Fermor believes the more accurate maps that ESI has developed will be particularly beneficial for people involved in the property sector, such as developers, surveyors, land agents, architects, lawyers, planning consultants and lenders. They will also help insurers to set premiums appropriately.

But a big market for ESI is local authorities, which can use the data to better prepare for flooding and direct resources to the most at-risk locations. As a result, the consultant is making the maps available free to all local authorities in return for them giving feedback on groundwater flooding events.

This will in turn be used to verify and refine the model.

“We’re creating a national database of groundwater flooding,” says Fermor.

The maps give an indication of groundwater flood risk based on a 200-year return period, with four levels of risk: negligible, low, moderate and high.

Negligible risk means that, for a one in 200 year return period, no further investigation should be necessary unless the site is unusually sensitive, whereas in low risk areas there is a “remote possibility” of groundwater flooding that could damage basements or subsurface infrastructure.

Moderate risk areas are those where there is a “significant possibility” of flooding from shallow pools or streams.

And in high risk areas, flooding is likely, and may result in damage to property and road or rail closures, as well as ­putting lives at risk.

The maps currently cover the whole of England and Wales, and are set to cover Scotland later this year. Fermor says they can be ­considered as “a 200 year high water position across the country”.

“If you are planning to build a structure in a particular location, and you know you are going to have a basement at a certain level, we can provide an instant report that looks at the risk of groundwater flooding at that location and depth,” he explains, adding that this can help designers come up with appropriate dewatering ­strategies.

He acknowledges that the model has its limitations - for example, a property or site might appear to be at risk of groundwater flooding, but the water will not reach at-risk basements because it instead finds its way into granular material used to backfill nearby utility trenches.

Likewise, local conditions can change, for example if dewatering associated with a historical industrial site, such as a coal mine, is switched off.

But he believes the new maps are a significant improvement on anything that has previously been available, and will get even more ­accurate as time goes on.

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