Flood planning just got easier

Last month, flood risk maps were produced to a level of detail possible because of sophisticated new tools using laser technology. And developers need to take notice because the Environment Agency has more weight than ever in objecting to planning applications in risk areas. John Haven reports

Last summer was one to forget. The endless downpours brought a level of destruction that caught everyone by surprise. Thousands of homes and businesses were flooded, 13 people were killed and around 7,000 people were rescued from the flood waters by the emergency services.

Unsurprisingly, June’s Pitt Review concluded Britain was ill-prepared for the scale of the damage.

It was the wettest May to July period for 250 years. One year on, thousands of people are still out of their homes. And the Association for British Insurers estimates the total cost of the June and July floods to be around £3B.

It has been a time for tough questions. Workable solutions must better prepare the country for the emerging threat of flash flooding from tropical-style downpours.

At the forefront of much of this work is the Environment Agency (EA). Its activities are watched keenly by local planning authorities and developers. And they should be watching because, in England, local planning authorities must consult the EA on most development proposals at risk from flooding.

The latest initiative to arouse considerable interest is the EA’s eagerly awaited new flood risk maps, published last month.

The agency produced its first flood maps in 2000 and updated them in 2004. But the 2008 maps are different. What sets them apart is the level of accuracy – the result of months of painstaking work by the EA’s mapping England’s landscape like never before.

At first glance, the new maps look a bit like what is already available. The big difference now is in the detail. They have been produced using Lidar units and J Flow. These are sophisticated tools which use laser technology to produce a detailed series of three-dimensional maps.

Features of the landscape such as small water courses, which would never have shown up in the old maps because of their size, are now clearly visible thanks to these new modelling techniques.

All this is likely to have big implications for developers – as they discovered in June this year when the practice guide for Planning Policy Statement 25 was published setting out what planners and developers in England must do to reduce flood risk.

It is this detail which will serve as a vital guide to planners and developers, says Laura Errington, an environmental scientist with Aardvark EM. “The EA now has a much bigger say in whether planning applications should be passed or objected to on grounds of flood risk. And, with more accurate mapping, this will mean it requires a much more comprehensive flood risk assessment. The new maps are already having an impact on how flood zone areas are categorised. Because details such as watercourses are now clearly visible, this may well have the effect of pushing an area into a higher flood risk zone than had previously been the case, and this will have an immediate impact on developments.”

This was precisely the problem Errington and her team at Aardvark encountered while carrying out a flood risk assessment for a development in Devon.

On the old maps, the site was situated within flood zones 1 and 2. But the new maps indicated an ordinary watercourse, which immediately put the site within the higher-risk flood zone 2 and 3 – part of which is a small functional flood plain.

Important implications

This has had important implications for the developer, which is now in negotiations with the EA regarding mitigation measures. It has been agreed a small amount of land raising can take place providing there is no overall loss of storage to the flood plain.

Errington also believes that, because the EA used such advanced data-gathering techniques to create its new flood maps, and covered a wider area than with previous maps, developers will have less opportunity to challenge. Those inclined to do so would have to use expensive hydraulic modelling, and in all likelihood would end up producing the same results as the EA’s own system.

The EA would also expect that, if the model clearly showed the site to be within

flood zones 2 or 3, then sequential tests would have to be done by the local planning authority and passed as part of a Flood Risk Assessment. The purpose of sequential testing is to help steer new development to the lowest-flood-risk zone appropriate to the proposed use.

What is clear is that, only a month since their introduction, the new flood maps are already having ramifications for developers. And this serves to highlight why a real understanding of flood risk is so important in the planning stages of new developments.

Last year’s floods drew attention to the problems of development within flood plains. Government planning policy on development and flood risk, under PPS25, aims “to ensure that flood risk is taken into account at all stages in the planning process to avoid inappropriate development in areas at risk of flooding, and to direct development away from areas at highest risk”. “Where new development is, exceptionally, necessary in such areas, policy aims to make it safe without increasing flood risk elsewhere and where possible, reducing flood risk overall.”

In reality, this now means an increasing number of planning applications require an accompanying flood risk assessment before some local planning authorities will even look at them.

Furthermore, flood-resistant techniques such as land raising and the use of a flood board are no longer considered the adequate prevention measures they once were.

“Land cannot simply be raised to remove it out of, for example, flood zone 3, nor can flood defences be built as both these of have the affect of removing flood plain storage and therefore creating a further risk of flooding downstream,” warns Errington.

FRA’s also show up the risk from surface water flooding – something the new EA maps, for all their sophistication, do not. “The new EA maps show flooding from rivers and seas, but not flooding from surface water when a lot of rapid rainfall causes drains to back up,” adds Errington.

“That’s a limitation which only an FRA will highlight because you must demonstrate how you’re going to deal with surface water produced by a development.

“It can’t simply go into a drain. What the EA prefers to see is the use of sustainable-urban-drainage systems.”

Developers’ options

So what options are open to developers hoping to develop in areas at risk of flooding?

The answer lies in the thoroughness of the flood risk assessments. These will suggest what mitigation measures should be pursued to ensure that a development is sustainable and safe in terms of flood risk. The Aardvark teams recently advised on mitigation measures for a new development being built just 50m from the River Thames and sited in the functional floodplain.

The solution included ensuring finished floor levels were at a height of 300mm, which took them above flood zone 3. Other flood-resistant techniques used included raising the height of electrical sockets, putting in concrete floors with waterproof membranes, and including voids in the design of the building to ensure flood flow routes or storage were not impeded.

But, for developers worried about getting bogged down in the detail now required, Errington has some encouragement. “Delays in the planning process can be frustrating for everyone,” she says.

“Our advice, particularly to developers who will have a long-term view of land earmarked for development, is to get the flood risk assessment completed at the very earliest stage. This will help prevent things being held up further down the line.”

Action inspires action. Stay ahead of the curve with sustainability and energy newsletters from edie