Groundwater Flood Risk

barrybarryr

Hi,

I need some help.

We are purchasing a property and have had a flood risk report done.

It has come back with a negligible risk of flooding from rivers and a low risk of pluvial flooding.

The problem is it has come back with high level of susceptibility to groundwater flooding area based on the underlying geological conditions.

I've looked on the net and I can't get my head around this.

Can anyone explain this to me and is it something we should be concerned about?

There are a few hundred houses built on the development where we are looking to buy.

Thanks

Barry

G_M

Lots of housing these days is built on what used to be (and I guess still is) flood plains. This might be the case, and if so, as global warming increases and heavy rain/flash floods become more frequent, it may be an issue.
But it includes lierally 10s of thousands of homes in Britain.

research the area. Check historic local newspaper reports of flooding. Talk to neighbours. get insurance quotes and see if insurers charge a premium. The risk level could be almost nothing, or significant.
Your choice.

barrybarryr

Thanks for your reply.

I appreciate that it is our choice but I don't understand what groundwater flooding is or it's impact.

Groundwater flooding is different to river/tidal flood plains and surface water flooding. I find it pretty confusing (because I can understand when a river bursts its banks or it rains lots but not anything to do with the water tables and geology of the area).

The property we are looking at is near a river but according to the Environment Agency it is not built on a flood plain and there isn't one within 25m.

I've looked all over the net for any history of flooding but can't find anything. The vendors have lived in the property for 8 years and have not known any flooding in it or the area.

RockOnZombies

Groundwater flooding is not mapped as it is harder to quantify the risk, and there are no specific maps for it on the Env Agency website. It is a fairly new area.

Groundwater flood risk is where the water table rises to such a point that its hydraulic head is above ground level and therefore you get springs and streams issuing from the ground itself. It has nothing to do with fluvial floodplains.

Hope that helps a little.

seabright

Groundwater flooding is something that's getting worse, so the risk could actually be increasing, despite it not having occured so far.

The more ground is built on/tarmac'd/etc the less "natural" ground there is to cope with rainwater - it just can't soak away when there are buildings/roads/patios etc, so it floods - and that's groundwater flooding.

Also, the more building there is the more the surface water sewers have to cope with and they're old and weren't planned to cope with as much water as they now have to deal with - they can't cope, flood and that's groundwater flooding too.

barrybarryr

Thanks for the replies that is really useful.

So how worried about it should I be? Is it something that would prevent someone from buying a house?

It says it is in a high susceptibility area but so must every other house in the nearby area.

It's really confusing.

RockOnZombies

There are several different flood risk sources;-
Fluvial, which is where a river overtops or bursts its banks, and is extensively mapped by the Env Agency.

Tidal, where sea levels rise to point where the tide then moves inland, again this is mapped by the Env Agency.

Pluvial or surface water flooding;- where the surface water drainage systems cannot cope with the amount of water falling from the sky. It is also where the soil is totally waterlogged and cannot absorb any more water and therefore flows over the ground. This is what Seabright is talking about not groundwater flooding.

Groundwater which is the one you are worried about is something which is where the groundwater table rises to a point where it intersects with the ground level and causes inundation of land and property. Does your report give any indication of the level of risk? Or any indication of depth? This would give you an indication of impact. It might be worth looking and the UK Groundwater Forum (cant post links but easily found on google) for some more specialised advice. Also just a thought does the house have any protective measures installed, these should be similar to the ones needed for surface water protection.

cheers

Vitruvius_2

Groundwater flooding susceptibility is not the same as groundwater flooding risk. Susceptibility maps seek to identify areas where the geological conditions and water tables indicate that groundwater may be present at shallow depths and could rise to enter basements/cellars or under certain circumstances come above the ground surface. The susceptibility maps don’t contain information on how often flooding may occur or to what depth.

As groundwater flooding is generally a relatively rare event, and not easy to separate from other types of flooding in historical records (flooding from rivers and from intense rainfall), computer models have been developed to classify areas. These models use geological maps, information from boreholes and wells and relationships between groundwater and surface water.

The susceptibility maps are mainly designed for planning; identifying areas where groundwater flooding might be an issue that needs to be taken into account by developers. Although information from the maps is often included in homebuyer reports it really shouldn’t be used in isolation as an indicator of flooding risk to individual properties. At site level variations in rock and soil type and local landscape can change the possibility that flooding will occur. Even if an area is prone to groundwater flooding, a property’s construction ((e.g. presence of a basement, level of the ground floor, construction of local drainage etc.) can determine whether or not this poses any problems to a householder.

DizzyDasher

From a practical point of view, I would talk to your solicitor and/or surveyor. They should have access to the maps that show which area is susceptible (or be able to find out how wide an area this applies to). If it applies to your entire search area, for example, then presumably other people have been able to get mortgages and insurance despite this problem, so it must be possible to overcome! But I would definitely want to figure out the insurance implications in particular before proceeding further.