Science Interviews


Tue, 11th Dec 2012

Urban Flooding - Planet Earth Online

Rachel Dearden, British Geological Survey

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Britain’s continuing wet weather has highlighted a serious problem facing towns and cities – urban flooding.  Rather than soaking through the soil, water in built up areas is blocked by concrete, tarmac and tile and can overwhelm the drains and flood.

But there is an alternative, at least in some areas - SUDS or Sustainable Drainage Systems, with some neat ideas for urban planners.

Planet Earth Podcast presenter Richard Hollingham has been speaking to hydrogeologist Rachel Dearden at the British Geological Survey in Keyworth near Nottingham…

Rachel -   SUDS try to mimic the natural conditions that you would get in a hydrological system, so we're trying to store that water in the catchment instead of allowing it to flow quickly downhill.  So there are a few ways we can do this.  The most natural scenario is  that water infiltrates straight into the ground.  Where this can't happen, because the ground is not permeable enough, naturally that water would collect in ponds, in depressions and slowly that would then either infiltrate into the ground or it would flow in water courses through the catchments, but importantly we don't get these really intense flows usually in natural environment - this is really only an urban phenomena.

Richard -   And one of the ideas is the idea of permeable pavements, of having a hard surface but that the water can flow through.

Floods in YourkshireRachel -   Exactly. Trying to emulate what would happen naturally. So if we have an area where we want to have a hard surface then we can just make sure there are avenues for the water to dissipate through the surface into the ground.  So, importantly, we need to think about what the properties of the ground are and what sort of systems we can design to be compatible with those properties.  So, for example, just to the west of Nottinghamshire we have a sandstone bedrock, it's very permeable, and we can quite happily concentrate our water flow into a relatively small area, for example, into a soak away which is just a pit in the ground.  Then that water can dissipate quite happily into the aquifer.  Conversely to the east of Nottingham we have the Mercia Mudstone Group which we're actually standing on now and this comprises of clay - it's really quite impermeable and as we can see now and if we step around-

Richard -   We're on a bit of grass here and it's just muddy and horrible.

Rachel -   It's quite muddy and horrible and in here you would try pretty hard to focus any recharge, any rainwater, into the ground and in a place like this maybe you can install an infiltration basin that actually provides storage on the surface and therefore allowing that water to infiltrate very slowly, but actually what we choose here when we designed our new buildings - behind us is a rainwater harvesting system. So rainwater is harvested on our roofs and it is used to flush our toilets and this is a good example of a sustainable drainage system which doesn't involve the ground so we can install sustainable drainage systems absolutely anywhere.  It's just that in some places we can infiltrate to the ground and in other places we really should focus on either storing water on the surface or re-using it.

Richard -   Could you retrofit these sorts of systems?

Rachel -   Certainly retrofitting is a big area of interest.  In cities, when we regenerate areas, can we possibly put in sustainable drainage systems and there's a real space issue here.  So trying to find space for an infiltration basin is really quite difficult in urban areas.  But, still, we can do things like put in permeable pavements, for example, instead of hard road surfaces, rainwater harvesting, there's always options.

Richard -   And you've actually put together a map of where this would work in the UK and where it wouldn't work in the UK?

Rachel -   Not exactly, but almost.  We've created what we call an infiltration SUDS map and this map shows you what the properties of the ground are.  So we cannot say “here you can install a soak away” or “here you can install an infiltration basin” because it very much depends on the design of the actual system.  So, what is the surface area of the system, what is its volume.  But we can tell you have permeable the ground is, whether you're on a flood plain and whether the ground water is likely to be very shallow, whether if you put water in the ground it is going to cause the ground stability problem or you could impact ground water quality.  And so the map gives you data that tells you all about these considerations so that you can then go away and make a decision about what sort of system might be appropriate.

Richard -   But are there imperatives for people to do this, for builders, for planners, for architects, engineers to take these sorts of things into account?

Rachel -   So the case with retrofitting is less clear but certainly for new builds there's a new legislation called 'The Floods and Water Management Act' and this requires that developers must consider using sustainable drainage instead of connecting to the drainage network.  This legislation hasn't been implemented yet but when it has been it will mean that developers must prioritise the use of infiltration to the ground, so they must consider the properties of the ground and they must consider using - infiltration is the most natural and sustainable drainage system.  If that's not possible because of the properties of the ground they then must consider storing water on the surface in infiltration basins, for example, and if that's not possible then they may consider putting water into the drainage network.  But the key thing is the right for them to connect to the drainage network is not necessarily going to be there in the future and they are going to have to think about other ways to solve this problem.



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Anyone interested in SuDS should look at

SuDS have been with Town and Country planning for well over 10 years, however local councils had little power to require developers include them as part of developments, although some did to avoid other problems, or to get round otherwise insurmountbale objections to developments.  The Flood and Water Management Act (when implemented over the next two years) will change this and will give the issue more weight as a "material consideration" in the planning process.

Furthermore, in response to the increasing hardsurfacing of suburban gardens to provide parking, in 2008 the government proposed some changes to "permitted development rights" to restrict the amount of hardsurfacing a householder could create on their property  I don't think these ever came into force.

More on the implementation of the Flood and Water Management Act 2010 at

Finally, I disagree that this is a strictly urban problem whislt surface flash flooding in areas away from existing water courses is caused by poor infiltration and urban impermeable surfaces - unsustainbale changes in upland land management and other farming practice (such as large scale land drainage have exacerbated many flooding problems by reducing the length of time that the hills hold their water.  Whilst there is increasing recognition of this problem and drains (aka "grips") cut into peat bogs are being filled in and issues surrounding stock density and reforrestation are being looked into, the only possible conclusion is that we are not managing the land as well as we need to address problems such as climate change (which for the the UK at least, seems to mean increased rainfall. ) Mazurka, Mon, 7th Jan 2013

Hello Members,

Some ways to prevent urban flooding are:

* Improving Drainage
* Harvesting Rain Water
* Building Dikes and Levees
* Building Canals

Thanks and Regards,
Alex Johns AlexJohns, Tue, 8th Jan 2013

I am inclined to agree with you on this point.

Although flooding has been a problem in some of our towns & cities, the problem is certainly not confined to the areas of concrete jungle. In October 2012, the Cornish village of Clovelly saw flash floods caused by heavy rainfall. This village, which is some 800 years old, is not prone to flooding nor is it surrounded by concrete or tarmac. It is surrounded by woodland and farmland.

I wouldn't mind betting that there have been more floods in small countryside towns and villages than in London, Birmingham, Manchester etc.

Historically we had a need to build our settlements close to water and some of these settlements developed into the great metropolises. Today, with modern transport and piped water, we no longer need to be beside great rivers or even small streams, but we choose such locations on the grounds of asthetics. Nothing increases the value of property so well as the description containing the words 'sea view' or 'overlooking the river'. The view of a meandering stream from the window is most pleasent, but gentle meandering streams can turn into raging torrents and if your property does command an excellent view of a stream or river, the chances are, you are sitting in the flood plain.

The solution to flooding, is to stop building on flood plains. Don_1, Tue, 8th Jan 2013

There are certain suburbs here where you cannot get plans passed for non permeable paving, as they are areas otherwise prone to flooding. You have to have french drains and such for rainwater from the roof, preferably a tank as well. SeanB, Wed, 9th Jan 2013

Unfortunately, both Urban and Rural areas were not designed with considering 100 yr or 1000 yr flood zones in mind. 

On the West Coast of the USA, putting multiple dams on a watershed can help, although reading and seeing photos of the 1964 floods in Oregon, I do wonder if the current system gives us a false sense of security.

One thing that needs to be considered more in urban planning is asymmetrical flooding of rivers.  Often river banks are naturally steeper on one side, and flatter on another side.  If planned right, it is often fine to build a dike on one side of the river, while allowing an open flood zone on the opposite side of the river.

Problems occur with the temptation to dike both sides of rivers, and to build artificial choke points for the river flow. CliffordK, Wed, 9th Jan 2013

You can say that again!

The big problem is that unless you have somewhere to drain the water, ie the sea, building defenses in one place only moves the problem somwhere else. Don_1, Thu, 10th Jan 2013

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