Flood risk - refining risk level; mitigations

[Drellingore]

How can I prove that the risk of surface and groundwater flooding is lower than desktop surveys suggest? If I can't, what sort of mitigations might one expect to be required?

 

Our desktop survey flagged high risk of surface water and groundwater flooding. The GOV.UK website says that groundwater flooding is "unlikely." The site has not flooded in the last 80 years, according to someone who was born there.

 

The desktop surveys probably highlight a risk based on geology. There's an underground chalk river about 100m north of the site. For this reason the local water supply is extracted 300m away. As a result, it's almost certain that there'll never be any groundwater flooding, because it's being pumped out for drinking water on an industrial scale. In the distant past that underwater river would occasionally break ground level, roughly once a decade. It's not been seen here since the pumping station was installed.

The architect is quite rightly pointing out that under their Principal Designer obligations that they need to be seen to be addressing the highlighted risk.

I need to either:

1. Prove that the risk is lower than the desktop survey suggests
2. Figure out what mitigations might be required, and how expensive they'll be

 

Any thoughts on the above? We'll leave out of the equation for now that apparently we're now in a very high radon risk area...

[nod]

Our planners demanded a sequential study  As we where deemed just inside the flood zone The cut a long story short We where advised that apart from costing a fortune The sequential report would not go in our favor So we consulted a  expert in this field who re drew the flood lines Putting us just outside of the flood zone 

[ProDave]

The official flood risk map shows a very high risk of surface flooding immediately to the south of our plot.

 

Correct.

 

In very heavy prolonged rain, the water table rises to the point the field floods.  But it is no risk to us as the flooded part of the field runs off into the burn, and the ground level on our side of the burn is higher.

 

It's things like the local topology that matter, so if your house is even slightly higher than the surrounding land, any flooding will just run off somewhere else.

 

I guess your biggest risk is if the pumping station failed and the industrial scale water extraction stopped.

[nod]

8 hours ago, ProDave said:

The official flood risk map shows a very high risk of surface flooding immediately to the south of our plot.

 

Correct.

 

In very heavy prolonged rain, the water table rises to the point the field floods.  But it is no risk to us as the flooded part of the field runs off into the burn, and the ground level on our side of the burn is higher.

 

It's things like the local topology that matter, so if your house is even slightly higher than the surrounding land, any flooding will just run off somewhere else.

 

I guess your biggest risk is if the pumping station failed and the industrial scale water extraction stopped.

I went to tile a swimming pool last Monday and the tank that collects surface water has two pumps that work off a float One failed and the other couldn’t cope 

2000 mil of water in what was a bone dry pool area the day before 

[saveasteading]

11 hours ago, Drellingore said:

How can I prove

It depends who is asking.

 

I read it that it is only your Architect.

Thus a reasoned report by a Civil Engineer should suffice and have the credibility the architect requires.

You could do a lot of the prep yourself and present it, because you have the local knowledge and the desire to prove it, and thus minimise the fee.

The reports of neighbours can help, as can checking other nearby planning applications.

Your own observations will be useful, as you have had this property for a while and some major rain.

The Engineer can do tech stuff like the topography, geology.

Being appropriately skilled and qualified, that should satisfy the architect and later the planners And bco.

Plus, you could get an upgraded design against any residual risk If necessary.

 

If the planners were to require an EA computer modelling then you have a problem. It's stupidly expensive and designed for major events, not your project.

 

[Drellingore]

9 hours ago, ProDave said:

I guess your biggest risk is if the pumping station failed and the industrial scale water extraction stopped.

 

That's a fair point. I'm not sure what the cut-off of likelihood is though. One might think that if the local water supply was taken offline for long enough for aquifer to fill then we'd have bigger problems than flooding! 😄

[BadgerBodger]

Our area has a flood risk assessment with a predefined FFL which all new properties are required to comply with. I’m not sure we could have argued otherwise if we liked!

 

We just accepted it for what it was and our internal FFL is 450mm above the external level. 

 

Is this something that is available/defined for your area? 

[Mr Punter]

Raising your ffl above the flood level can be a requirement.  I think @Russell griffiths did this.

[crispy_wafer]

as above, we did, 600mm above an open internal drainage board drain that runs through the land.  Having multiple floors for refuge in case of that 1 in a 100 year event also helps.

[Gus Potter]

On 01/10/2025 at 20:33, Drellingore said:

How can I prove that the risk of surface and groundwater flooding is lower than desktop surveys suggest?

Hiya.

 

I loved your pragmatic adventure with Grand designs so thanks for that. 

 

The desk top study you have will show a big fat line and shaded flood risk zones. 

 

Call up the folk that produced the flood risk map, explain what you need, give them your exact geographical coodinates and the actual height of your site above ordinance datum ( it may be on a hillock!), pay some money and hopefully thet will help make the problem will go away. 

 

If it turns out you are prone to flood let us know and you'll get lots of suggestions on BH.

 

How come you are in chalk and have a radon risk? @saveasteading knows a lot about chalk and building on it so hopefully he will solve this quandry. 

 

At the end of the day at some point you or your kids may want to sell the house so preserving the asset value is worth consideration.