I’m finding most of the debate about this on the TV and radio a bit simplistic, as although the flood plain is a real place, it’s actual size is variable.
There is a lot of confusion about ‘the flood plain’ as some parts will flood regularly e.g. 3 times a year, others once every few years and others once a century or less. This is called the return period. i.e. a 1:100 year flood or 1% flood is likely to occur once a century (however, this is based on risk which means you may get three 1:00 year floods in 3 consecutive years and then not have one for 200 years). That’s why you constantly hear people saying ‘flooding is definitely getting worse, it’s not flooded this badly before and I’ve been here 50 years’. A single person’s experience is not a good indicator of flood risk increasing or decreasing as they are not likely to have experienced all the different flood / drought level expected to occur in the area.
The debate about building on the flood plain is all about risk. Before we start saying that there should be no development on the flood plain we should say what we mean by the flood plain i.e. do you mean the land covered by a water in a 1:1 year event (annual flood) which is very little land, 1:5 (one flood every 5 years) which is a lot more land, 1:20, 1:50, 1:100? You get the idea, what risk do you want to take? We defend certain structures e.g. the Channel Tunnel Rail Link to a 1:1000 year event.
This isn’t necessarily my point of view but it may be sensible to have all infrastructure out of the 1:200 year floodplain, allow housing on the land which floods between a 1:100 and 1:50 year event (making them flood resilient e.g. no accommodation on the ground floor) and no development except sports fields etc in the less than 1:50 year flood plain. This is further complicated by the topography, in some places a 1:5 flood and 1:100 flood will cover virtually the same land and so lots of land would be sterilised from development, while in others e.g. steep valleys the opposite applies i.e. only land right next to the river would be sterilised.
As for the ‘clear out the ditches brigade’ – Vegetation removal only affects the lower order floods i.e. normally only 1:5 year event or less, where you definitely shouldn’t be building anyway, but it has no effect on large order events like the ones we’ve just seen. Seeing the aerial views of the flooding, does any thinking person really believe that removing watercrowfoot, common reed or some river gravel would make the slightest bit of difference? Butchering the river’s biodiversity to reduce the flood risk very marginally cannot be justified except where human life is threatened.
What causes the problem is not the ecology, it’s the field drains, lack of buffer zones, straightened channels which are shorter and higher gradient than natural channels, the silt from the fields blocking the channels, the constrictions such as bridges and the concrete banks in urban areas i.e. we’ve short circuited this part of the water cycle and then tried to constrain it in small steep and frictionless channels – we’ve made most of the problem and then build in the high risk areas – seems a bit of a no brainer to me. In the end it's a choice of if we want to carry on building in high risk areas then we need to accept the economic and human cost as there's no way we will ever keep that much water in a single or multiple channel system unless the channels are so big they destroy the buidings you're trying to protect - not to mention the catastrophic environmental destruction it would cause. Lets only build stuff in the floodplain we're prepared to get flooded based on the risk we're prepared to take, stop trying to put in larger defences which make the problem worse and use whats left of the functional flood plain.
If anyone is interested in the government’s present guidance to planning authorities on building in the flood plain, it’s contained in Planning Policy Statement 25 (PPS25) which came out in December last year.
Planning Policy Statement 25: Development and Flood Risk - Communities and Local Government
Cheers, Chris