Surface Water Flooding Risk

[westv]

Hello,
I'm curious to know any others on here that are living in houses marked as "high risk" for surface water flooding? I'm not sure about other areas but here it seems to be a mish mash of properties marked high, medium and low. The house to our right is high risk too but the one to our left is medium risk. Those four doors away in either direction are low risk. The other side of the road (20m) away is medium risk.
The area is as flat as a pancake.
Just to clarify. This is surface water flooding risk not river or sea flooding.

 

[Mcr Warrior]

Who actually makes the low / medium / high risk assessment? Is this the Environment Agency, or someone else?

 

[westv]

Isn't it some sort of computer model? I don't know who runs that.

 

[gg1]

Maybe the different properties were assessed at different times and the perceived risk level has varied over time.

 

[westv]

As far as I'm aware the mapping was done at the same time for the whole of England and Wales.

 

[brad465]

Isn't it some sort of computer model? I don't know who runs that.

Who actually makes the low / medium / high risk assessment? Is this the Environment Agency, or someone else?

The Environment Agency has a lot of flood modelling/flood risk information regarding fluvial, coastal and groundwater flooding, but surface water flooding is not currently its domain. Usually surface water flooding is the responsibility of the council.

 

[westv]

The Environment Agency has a lot of flood modelling/flood risk information regarding fluvial, coastal and groundwater flooding, but surface water flooding is not currently its domain. Usually surface water flooding is the responsibility of the council.

I will try and Google it but are you perhaps confusing dealing with sw flooding rather than assessing the risk of sw flooding??

 

[bspahh]

Hello,
I'm curious to know any others on here that are living in houses marked as "high risk" for surface water flooding? I'm not sure about other areas but here it seems to be a mish mash of properties marked high, medium and low. The house to our right is high risk too but the one to our left is medium risk. Those four doors away in either direction are low risk. The other side of the road (20m) away is medium risk.
The area is as flat as a pancake.
Just to clarify. This is surface water flooding risk not river or sea flooding.

How old are the houses? Are you near any water courses? These could be ditches that are normally dry.

This is a PDF file from Cambridgeshire County Council on who is responsible for maintaining a water course.

https://www.cambridgeshire.gov.uk/asset-library/Watercourse-Guidance-2022-Rights-and-Responsibilities-for-Watercourse-Maintenance.pdf

 

[westv]

How old are the houses? Are you near any water courses? These could be ditches that are normally dry.

This is a PDF file from Cambridgeshire County Council on who is responsible for maintaining a water course.

https://www.cambridgeshire.gov.uk/asset-library/Watercourse-Guidance-2022-Rights-and-Responsibilities-for-Watercourse-Maintenance.pdf

All built in the 70s.
Nowhere near a watercourse but sw flooding doesn't relate to water courses, rivers etc.

 

[bspahh]

There is a description from 2013 of how to use the flood risk maps here

https://assets.publishing.service.gov.uk/media/5a7c1bdee5274a1f5cc75d70/LIT_8986_eff63d.pdf

When I search at https://check-long-term-flood-risk.service.gov.uk/map near my house, the houses with a high risk of flooding from surface water are ones where they have paved their front garden for extra parking.

 

[westv]

When I search at https://check-long-term-flood-risk.service.gov.uk/map near my house, the houses with a high risk of flooding from surface water are ones where they have paved their front garden for extra parking.

That's interesting. I will have a quick check here..

Edit: Quick check.
To the left paved parking - medium risk
Next house along paved parking - low risk
To the right grass front and back - high risk.

 

[bspahh]

Hello,
I'm curious to know any others on here that are living in houses marked as "high risk" for surface water flooding? I'm not sure about other areas but here it seems to be a mish mash of properties marked high, medium and low. The house to our right is high risk too but the one to our left is medium risk. Those four doors away in either direction are low risk. The other side of the road (20m) away is medium risk.
The area is as flat as a pancake.
Just to clarify. This is surface water flooding risk not river or sea flooding.

I know someone who works in flood risk management. I asked them about this and they commented:

The flood map for surface water isn't designed for use at an individual property level

 

[westv]

I know someone who works in flood risk management. I asked them about this and they commented:

I see. I think insurance companies seem to use it to identify individual houses or maybe they use their own software.

 

[brad465]

I will try and Google it but are you perhaps confusing dealing with sw flooding rather than assessing the risk of sw flooding??

Apologies yes I mixed that up.

When I search at https://check-long-term-flood-risk.service.gov.uk/map near my house, the houses with a high risk of flooding from surface water are ones where they have paved their front garden for extra parking.

In reality the only thing one individual property can do to reduce surface water flooding risk to themselves is to put in sandbags or temporary flood barriers when needed, similar to with fluvial flooding impacts. While paving over drives/gardens doesn't help, the road drainage network, the roads and the effects of several properties stacking up impermeable surface area are all too great for one property owner to influence. What's really needed is better policies to reduce the risk, but that's another matter entirely.

 

[The Ham]

Apologies yes I mixed that up.


In reality the only thing one individual property can do to reduce surface water flooding risk to themselves is to put in sandbags or temporary flood barriers when needed, similar to with fluvial flooding impacts. While paving over drives/gardens doesn't help, the road drainage network, the roads and the effects of several properties stacking up impermeable surface area are all too great for one property owner to influence. What's really needed is better policies to reduce the risk, but that's another matter entirely.

There are some other things that they can do:
- build a land drain (perforated pipe in gravel which connects to a drain)
- change the levels in their gardens to direct water away from their house (doesn't need to be by very much)
- depending on the soil types, building a soakaway to take some of the water (double effect, some of the water is stored underground rather than risking the property but also gets rid of some of the water)
- replace a driveway with a permeable paving (water can more easily be taken off the surface, shows down the speed at which water gets into the sewers and provides some storage of water)
- add more drainage (including gulleys or slot drains) so water can more easily get into the drains and/or so there's more storage for the water

Global warming is likely to result in more extreme rainfall (whilst this may not show up on the annual rainfall, this is because there's less wet days overall).

Depending on the circumstances there's generally two types of storms which could cause the problems.
- short (up to 30 minutes) which have very heavy intense storms which overload entry points to sewers
- long (several hours in length) which is an issue because of the risk volume of water the systems are dealing with

The first is easier to mitigate against as the volume of water is less.

 

[Snow1964]

Hello,
I'm curious to know any others on here that are living in houses marked as "high risk" for surface water flooding? I'm not sure about other areas but here it seems to be a mish mash of properties marked high, medium and low. The house to our right is high risk too but the one to our left is medium risk. Those four doors away in either direction are low risk. The other side of the road (20m) away is medium risk.
The area is as flat as a pancake.
Just to clarify. This is surface water flooding risk not river or sea flooding.

The maps are based on digitised versions of Ordnance Survey data of ground levels. They don't work brilliantly at local level because the maps aren't accurate.

Flood maps work on basis of if river rises to Xcm above current level, then tries to shade in what it thinks is below that level. But if the original surveyors only put in contour lines every foot (30cm) it is going to do lots of guessing.

If your area is as flat as a pancake, then it is going to struggle with exact heights because contours could be long way apart. It could be OS hasn't actually done full ground survey for decades and just updated old maps using info from aerial photos etc.

If you want to research it, get old detailed OS map of your area before estate was built, there is likely to be one (or planning application map) with your title deeds as they were often in physical bundle of title documents 50 years ago. Alternatively local library or local history society might have copies of old maps. Then see if the builders changed the levels. It should be fairly obvious if house few doors down was on land that was originally foot lower, but was built at same height as yours etc.

 

[bspahh]

There are some other things that they can do:
- build a land drain (perforated pipe in gravel which connects to a drain)
- change the levels in their gardens to direct water away from their house (doesn't need to be by very much)
- depending on the soil types, building a soakaway to take some of the water (double effect, some of the water is stored underground rather than risking the property but also gets rid of some of the water)
- replace a driveway with a permeable paving (water can more easily be taken off the surface, shows down the speed at which water gets into the sewers and provides some storage of water)
- add more drainage (including gulleys or slot drains) so water can more easily get into the drains and/or so there's more storage for the water

Paving Expert has a good article on permeable paving https://www.pavingexpert.com/permeable-modular-paving

Sand bags don't work well as a flood barrier. There are much more effective solutions available if you are at high risk of flooding.

If your house is going to get flooded in any case, then you can make it easier to recover if you move electrical sockets higher up the wall, choose water resistant floor coverings etc.

 

[westv]

The maps are based on digitised versions of Ordnance Survey data of ground levels. They don't work brilliantly at local level because the maps aren't accurate.

Flood maps work on basis of if river rises to Xcm above current level, then tries to shade in what it thinks is below that level. But if the original surveyors only put in contour lines every foot (30cm) it is going to do lots of guessing.

If your area is as flat as a pancake, then it is going to struggle with exact heights because contours could be long way apart. It could be OS hasn't actually done full ground survey for decades and just updated old maps using info from aerial photos etc.

If you want to research it, get old detailed OS map of your area before estate was built, there is likely to be one (or planning application map) with your title deeds as they were often in physical bundle of title documents 50 years ago. Alternatively local library or local history society might have copies of old maps. Then see if the builders changed the levels. It should be fairly obvious if house few doors down was on land that was originally foot lower, but was built at same height as yours etc.

We are around a mile from the nearest river.

Paving Expert has a good article on permeable paving https://www.pavingexpert.com/permeable-modular-paving

Sand bags don't work well as a flood barrier. There are much more effective solutions available if you are at high risk of flooding.

If your house is going to get flooded in any case, then you can make it easier to recover if you move electrical sockets higher up the wall, choose water resistant floor coverings etc.

Whether it will flood is debatable - even if the experts say it could.
In 2007 large parts of Hull were affected by some very serious surface water flooding due to very heavy rain and blocked drains but there was no flooding where we were. I know of course that could change but it could change in a lot of places.

 

[Jamiescott1]

The area i live in has 3 locations where Thames water tankers are constantly being filled from pumping stations. Its 24 hours a day constant tankers.
Not sure if sewage or ground water.
We are classed as a high risk area

 

[The Ham]

The area i live in has 3 locations where Thames water tankers are constantly being filled from pumping stations. Its 24 hours a day constant tankers.
Not sure if sewage or ground water.
We are classed as a high risk area

Tankers generally shouldn't be taking stuff out of pump stations unless the pumps have failed or their doing something to the rising main - even then it's generally for a limited time (i.e. long enough to fix the pumps).

Although works to the rising main could be quite a long (possibly months) period is time.

If it's ongoing then it's likely that there's issues with the sewer that it connects to.

I suspect that if it were surface water you'd see the frequency of tankers change depending on the weather.

 

[westv]

I know someone who works in flood risk management. I asked them about this and they commented:

The flood map for surface water isn't designed for use at an individual property level

I've also thought as well, if it isn't designed for use at individual property level, why does the Environment Agency do so?