How do I check flood risk on site?

E

EarthAngel

Guest
I am interested in buying a site to build a house. It is about 300 yards from a river with agricultural land inbetween.

What do I need to check for flood risk and the site generally?
What qualifications should I be looking for in the surveyor?

Thanks
 
Hi EarthAngel,

The Minister for the Environment published Draft guidelines in relation to the Planning Process and Flood Risk Analysis in 2008:

http://www.environ.ie/en/DevelopmentandHousing/PlanningDevelopment/NationalSpatialStrategy/News/MainBody,18440,en.htm

They have since been passed into law:
http://www.environ.ie/en/Legislation/DevelopmentandHousing/Planning/FileDownLoad,21081,en.pdf

These may have resulted to Variations or Amendments to the County Development Plan or Local Area Plan, or the inclusion of areas of exclusion or special Flood Risk zoning in the Plans.

The simplest course of action would be to talk to the area planning officer or area engineer, but either or both may not want to talk to you if you don't actually own the land.

Read the Self Build FAQ on this forum, a work in progress. There you will find links to some basic information on flood risk. You will see how important it is to perform a detailed review of the current Development plan, not just to look for flood risk amendments.

An amateur you might not review the Plans competently or fully, including recent variations. You might miss a section dealing with a flood risk issue or another relevant issue - for example if the land is zoned as part of an area of architectural consevation.

My best advice is to retain a competent building professional to engage with the local authority on your behalf and report back to you. He should be retained to design the house in compliance with the Building Regulations.

If an engineer is required to perform a Flood Risk Assessment he will advise you of this and you will need him to also take account of any resulting special flood risk design considerations for the house.

FWIW

ONQ.

[broken link removed]
 
I would suggest you chat up some local people especially senior people to enquire do they ever remember the area being flooded. Of course you should seek all the professional help you can but sometimes local knowledge can be very helpful.
 
dewdrop said:
I would suggest you chat up some local people especially senior people to enquire do they ever remember the area being flooded. Of course you should seek all the professional help you can but sometimes local knowledge can be very helpful.
Click to expand...


+1

Anyone who has lived in an area for the last 50 years will give you far better information than any map.

(check the maps too)
 
Having been in this situation in the past few months I would urge a lot of investigation. These are just my thoughts.
Speaking to locals is a good idea although be aware that even if your site has never flooded that does not mean it never will. This years floods had not happened in my area in 80 years (as far as anyone can remember) so if it didn't flood this year, it is business as usual for all. I would not be so confident.
Also be aware that while your site may not currently be in a flood risk area, this can be amended on the new Local Area Plan (as happened to us) - all plans may now be updated however due to the legistlation mentioned below.
Also check that you will be able to access your property at all times. I was speaking to a girl yesterday who has had to leave her property twice in the past 3 years due to no access other than by boat, but their house was never flooded.
I have also heard of someone who was in their house 2 months and they were flooded. They had applied for planning to raise the house by two blocks and were refused. They went ahead and built at the lower level and are now suffering for it.
I am not sure if this is the case, but would the insurance companies have more information following recent events? Would it be worth talking to someone in the business to ensure that you could get insurance?
As far as I am aware, distance from river does not count but gradient. Possibly a levels survey on the agriculatural land might be an idea - with owners consent obviously.
I am on a steep learning curve and some of my information may be incorrect and apologies if so.
Best of luck
Tommygirl
 
tommygirl said:
Speaking to locals is a good idea although be aware that even if your site has never flooded that does not mean it never will.
Click to expand...

+1 Ask local people.
300 yards sounds quite close. Flood plains are usually very flat and smooth with no hills, lumps or bumps. They can be quite wide, depending on the river and they are usually a lot lower than the normal fields. The fields tend to slope towards the flood plane - all the slope on these fields can flood too.
 
With respect to all the posters, how many "local people" have you seen interviewed on the news in the past year expressing their wonderment at how high the water has been or the fact that it hasn't flooded here before?

Think of Bandon where I visited around Christmas 1997 IIRC andsaw the river plain flooded to a depth of 1200mm in places. I didn't think it would ever reach main street. You don't - until it happens.

Also how many "local people" will tell you the facts about flooding in their area - possibly devaluing their property in the process - when they are NOT waist deep in water looking for the help of the government?

Local knowledge may be good for certain things, and I have advocated it in the past myself, but the recent flooding has given greater raw data than anything ever recorded before and thus is the primary source for information at the moment.

The reason that flood risk assssment has become law now is to ensure that speculation is prevented, old wives tales get ignored, and professional evaluation of the risk takes place.

The fact is that events upstream caused by a new development today may end up causing flooding downstream in a week't time. Many new developments have occured in regions not affected by the recent flooding and no one has formally assessed their effects yet.

So in another twist, even where local knowledge may be forthcoming it may be unreliable.

And even the much lauded flood risk assessment is based on rainfall parameters that may be exceeded in years to come.

ONQ.
 
Hi,

As someone involved in undertaking Flood Risk Assessments, I might be able to offer some pointers. Generally engineers will undertake detailed surveys, calculations/models to determine if there is a flood risk to a site. However, there are a few tell tail signs we also look for:

-As RKQ suggested, with reasonable sized rivers in well defined valleys the historical floodplain will be clearly visible as the flat portion of land adjacent to the river. Sometimes geological and soil maps can also show this area as alluvial deposits.
-Flood levels are often dictated by constrictions in the river (particularly bridges). Search for constrictions well upstream and downstream of your site. An upstream constriction may cause water to follow alternative flow paths (roads, old channels, hollows etc). Downstream constrictions may cause flood levels to be much higher.
-Flooding can also occur from different sources: groundwater (particularly sites in hollows), overland flow (where higher ground falls down to your site) and coastal.
-Look at the upstream catchment, urbanisation can dramatically increase flood flows
-Ask around: as the other posters mentioned, the Area Engineer and neighbours may know of previous problems.

I hope that helps. While I have a conflict of interest, I'm sure most will agree that it would be advisable to get an engineer to put their reputation (and PI Insurance) on the line and do a flood risk assessment.

Scott

Water Resources Engineering Ltd (wre.ie)
 
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