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Flooding: What Is Normal?

February 17, 2017

By Paul Homewood


I was sent this paper which was published last March. It makes a number of points about flooding in the UK, which I have been emphasising for some time.



Flooding: what is normal?
Professor Paul Bates
School of Geographical Sciences, University of Bristol


Over the last several years widespread episodes of flooding have led to extensive media coverage and national debate that at times has come to dominate the political agenda. Flooding of the Somerset Levels during the winter of 2013-14 and as a result of storms Desmond and Eva in 2016 have led to concerns over whether the UK is sufficiently resilient to flooding, whether we have correctly identified the risks we face and whether we are spending enough money on flood defences. An answer to any of these questions first requires we understand just how often we can expect damaging floods, both in specific locations and in terms of national scale aggregated losses.

Almost by definition, river flows need to be extreme to cause to flooding at specific locations, and we therefore tend to view all episodes of flooding as somewhat unprecedented. Over many years in the UK a consensus emerged amongst politicians, risk managers and the public that communities should be protected against river floods that have a 1 in 100 (i.e. 1%) chance of occurring in any
given year. On average, one would expect such a flood to occur at a specific location only once in a century, and hence this event is known as the ‘100 year flood’. This terminology is however misleading as it implies such floods can only ever occur once in a century; it is always possible, albeit unlikely, for very rare events to occur close together simply by chance.

We estimate the magnitude of the 1% annual chance flood for particular places by analysing multidecadal series of river flow measurements and computing the statistical distribution for extreme floods. We then use this to estimate the magnitude of the 1% annual chance event in order to design defences to protect people and property against floods up to this size. Defences can always be over-topped or fail for floods larger than the design event, i.e. those with a less than 1% annual chance of occurring, and here the UK relies on the insurance system to collectivize the losses. The insurance system also deals with the losses for surface water flooding away from main rivers and for properties built within the floodplain.

Based on the above one might conclude that floods should be a rare occurrence, yet this does not seem to accord with our recent experience. Is flooding more common now, and if so why might this be? First, it should be noted that whilst particular places should only see flooding very rarely, damaging floods will occur somewhere in the UK on an annual basis. The Figure below shows the annual total number of properties flooded in the UK from 1998 to 2015. Every year sees ‘100 year floods’ somewhere on the UK river network, and whilst storm Desmond broke the UK 24 hour rainfall record in terms of national annual flood losses 2015 was depressingly normal. Flooding in 2015 was extreme where it occurred, absolutely terrible for those affected, but the national annual loss was not at all unusual. In fact consideration of these data shows that we can expect to see annual flood losses similar to 2013/4 and 2015/6 every 2-3 years in the UK. Moreover, despite the ‘unprecedented weather’ narrative of much media coverage of recent flooding, similar losses occurred in 2000, 2007 and 2012.

Annual total number of properties flooded in the UK 1998 – 2015.

Has it always been like this? Is collective memory really that short, or are there more floods now than there were?

We can answer this question by looking at the data from the very small number of relatively undisturbed sites in the UK where we have really long flood records. The next Figure shows data for the size of the largest flood in each year on the River Severn at Bewdley from the 1920s to the present day. The red line shows the changing trend in these data, and this clearly highlights the presence of ‘flood rich’ and ‘flood poor’ periods. We can clearly see a period of large floods during the 1940s and 1950s and again in the 2000s, with several decades of smaller annual maximum floods in between. Such decadal variations in the frequency of extreme floods are likely driven by large scale cyclical changes in the atmosphere and ocean, but will also be affected by long term trends such as those caused by man-made climate change. What is clear is that with the benefit of hindsight we can identify the Easter 1998 floods in the Midlands as the point atwhich we began to enter a flood rich period that is apparently still continuing.


Largest annual floods on the River Severn at Bewdley 1920 to present day (Source: UK National River
Flow Archive,


Decadal variability also makes it difficult to correctly estimate the magnitude of the 1% annual chance flood. The majority of our river flow measuring stations were installed in the ‘flood poor’ period of the 1960s and 1970s as a result of the 1963 Water Resources Act, and the data collected from this period may not be a good guide to the frequency of flooding during the ‘flood rich’ period we are experiencing today. Unfortunately, the obvious solution of using only the last 15 years of data to estimate our design flood magnitudes does not work well because with far fewer data points available large statistical errors can creep in.

There are two other reasons why flooding seems more common now than in the past. The first is that we have substantially increased our exposure to floods over the post-war period. Population increases since 1950 have resulted in a substantial expansion of housing and development, a significant proportion of which took place on floodplains as the land was flat and cheap to develop. Unfortunately the planning system has not been sufficiently robust in enforcing flood risk control, and whilst this situation is much improved inappropriate development in floodplains still continues.

Data presented to the latest UK Climate Change Risk Assessment shows that between 2001 and 2014 250,000 homes, approx. 12% of all development, were built in areas classified as having a greater than 1 in 100 annual chance of flooding. More worryingly, since 2001 approximately 23,000 homes have been built in areas having of high risk (defined as a 1-in-30 or greater annual chance flooding). With the UK population projected to increase from 64.6 million in 2014 to 74.3 million by 2039 these trends are highly likely to continue.

Secondly, over the post-war period our vulnerability to flooding has also increased. Rising incomes have increased the assets at risk and losses are now proportionately greater. As a result resilience has reduced. We now own far more than our parents did, and consequently when flooding does occur we have more possessions that can be damaged. Economic growth will exacerbate this trend too.

In fact, it is very likely that increasing exposure and vulnerability have, to date, done at least as much to increase flood risk in the UK as changes in the magnitude and frequency of flooding that we have experienced. By definition risk is the product of the scale of the threat, the number of assets that are exposed and how vulnerable these things are to damage, so all three factors need to be taken into account when we think about flood risk. As well as increasing exposure and vulnerability, the future may also bring increases in flood hazard as a result of both natural variability, catchment alterations and man-made climate change. As the River Severn flood data indicate, the natural variability in flood climatology is large, and decadal scale cyclical variations and confounding factors within the catchment, such as land use change, will mean it may be some time before we can identify conclusively the effect of man-made climate change on the frequency of flooding. However, there are very good physical reasons to believe that warmer atmospheres will also be wetter atmospheres, and this is very likely to lead to more frequent and larger floods in the future.

In conclusion, in terms of national scale annual losses we can see that, contrary to the standard media narrative, flooding during winter 2015/6 was, by recent experience, entirely normal. At present it seems we should expect annual total flooded properties to exceed 10,000 every few years. Whether this degree of resilience is acceptable needs a wider debate; personally I don’t think it is. We also need to examine openly whether the consensus of protecting against the 1% annual chance event is something we as a society are still comfortable with. At the same time scientists need to undertake further work to make sure our hazard assessments are not biased by the ‘flood rich’ and ‘flood poor’ periods in our data. Most importantly we need a more sophisticated view amongst politicians and the general public alike of how events which may be extreme in particular places can lead to levels of national scale loss that are seen much more frequently.


To summarise:

  • Historical records give testimony to cycles of flood rich and flood poor. The last flood poor period was the 1960s and 70s, which unfortunately happens to be the baseline for much planning.
  • There is nothing remotely unusual about recent years, when compared with earlier flood rich periods.
  • The media overreact to floods, as they are an emotive issue, because the collective memory is short.
  • Flooding seems more common nowadays because we have increased our exposure to them.
  • Describing flooding and other weather events as “1 in 100 year” etc is misleading as there are many areas where such an event might occur. The chance of having a “1 in 100 year” flood somewhere in the country is therefore much less greater.
23 Comments leave one →
  1. Joe Public permalink
    February 17, 2017 1:54 pm

    The takeaway sentence:

    “Every year sees ‘100 year floods’ somewhere on the UK river network, and whilst storm Desmond broke the UK 24 hour rainfall record in terms of national annual flood losses 2015 was depressingly normal.”

    • TinyCO2 permalink
      February 17, 2017 4:01 pm

      Are they depressed because come poor souls get flooded or that there isn’t clear evidence of AGW?

      • February 17, 2017 8:08 pm

        To be fair, I think he has his tongue a little bit in his cheek, as in normal means the media have no excuse to get excited.

        One additional point regarding the fact that “our parents” (which actually includes some of “us”) didn’t have the possessions that we have. They/we didn’t have nice cosy centrally heated homes and nice cosy centrally heated cars to cocoon ourselves in. It’s not solely the frequency or intensity of the flooding that we are dealing with but the extent to which we are no longer prepared to “grin and bear it”.

  2. A C Osborn permalink
    February 17, 2017 2:06 pm

    And building on Flood Plains really helps to keep the impacts down, doesn’t it.

  3. AlecM permalink
    February 17, 2017 2:07 pm

    It’s not water rise: the land is sinking!

  4. February 17, 2017 2:27 pm

    On average a NATURAL river over tops its banks about once every 1 – 2 years. And quite regularly it will go further and inundate the “flood plain”. Because the reason the flood plain is there is because it regularly floods.

    So, unless an area of flood plain was created a long time ago such as at the time of the ice-age melting, it will flood now.

    However, since the middle-ages we have been gradually canalising our rivers for boats. That lowered the river bed, removed choke points and dramatically reduced the chance of flooding along the banks. And AS A RESULT many buildings have been erected in areas that once used to flood regularly – because as far as those living there were concerned the area did not flood.

    But then with the advent of trains and then lorries, the need to move heavy goods by river disappeared. So the regularly scouring of rivers for sand and gravel (then used for building) stopped and then more recently the Environmental agency took a unilateral decision to cease all dredging (as far as I know on all rivers – but certainly ones like the Thames).

    So, we now face a situation, where rivers are gradually returning to their “natural” state. That is to say, they are returning to a state where they will start regularly flooding areas that for several hundred years have been free from flooding.

    The Environmental agency could argue that returning rivers to their “natural” state is more important than billions of pounds of householder and business property. And it would be interesting to see them making the case and seeing the public response.

    However, one cannot possibly argue that such a policy can be taken unilaterally behind closed doors, by what can only be described as the “eco-fascists” who now seem to run these organisation … who then, when the inevitable flooding comes have the audacity and hypocrisy to blame the result of their own actions on “global warming”. The blame clearly lies with the environmental agency and those government ministers who have let them take over these agencies so they can get away with this crazy policy.

    And just a note: fortunately I and hopefully all my close friends and family live well above any flood plain.

    • roger permalink
      February 17, 2017 3:15 pm

      I think you will find that not only the English EA but also the Scottish EPA imposed a moratorium on dredging many years ago in blind obedience to a EU directive.
      Certainly on the Scottish river of which I have first hand experience that was the case.

      • February 17, 2017 8:17 pm

        My understanding is that the UK went considerably further than the requirements of the EU Directive on river management.

        Possibly Phillip Bratby knows better than I do but if my memory is correct the EA took the Directive and applied it to its full extent across all rivers instead of the correct procedure which would have been to deal with each waterway appropriately. You wouldn’t expect to treat the Warwickshire Avon as you would the Severn, for example.

        Unless you were the EA, of course!

  5. February 17, 2017 2:33 pm

    A fine review of the flooding issue. A discussion over at Climate Etc. led me to post this:

    Let’s summarize the discussion about subsidence.

    Richard Muller made a small point that SLR is exaggerated as a threat from rising CO2 emissions by referring to subsidence as a factor in Florida.

    1. Sea levels have been rising for a very long time, according to tidal gauges, with scant evidence of any acceleration in the fossil fuel era.

    2. The ocean is encroaching, especially in places like Miami Beach where the land is also subsiding.

    3. People with buildings on property in such places will get no relief from efforts to reduce CO2 emissions.

    The original reference is to an article in Forbes, also worth a read:

  6. Tim Hammond permalink
    February 17, 2017 3:10 pm

    I don’t think you could call that red line on the graph a “trend line”. There is no trend surely.

    • February 17, 2017 3:29 pm

      Certainly no visible correlation with the steady increase in carbon dioxide levels.

  7. Vernon E permalink
    February 17, 2017 3:29 pm

    Of course the primary cause of excessive flooding is the failure to maintain our rivers, to keep the old stone/brick-arched low bridges (which personally I think should be replaced anyway) clear and to put and end to the nonsense about re-wilding. The water has to get to the sea. Simple.

  8. robinedwards36 permalink
    February 17, 2017 3:40 pm

    I want to look at the Bewdley data, but it seems impossible to get the numbers behind the graphics. Have tried for quite a while, but numbers (ma flow) will not appear – everything but these! Frustrating.

  9. CheshireRed permalink
    February 17, 2017 3:41 pm

    Good point against the ‘1 in 100 years’ claims. That’s yet another (annoyingly effective) PR trick by the climate con-merchants. Find 100 places (cities, towns, villages, rivers, lakes, counties, regions, flood plains etc) and the statistical average dictates there’ll be a ‘1 in 100 years’ event somewhere every year!
    Of course they don’t have to find and specifically name 100 places so they effectively have the entire UK to choose from. On that basis it’s surprising there isn’t one every day.

  10. February 17, 2017 4:27 pm

    “The chance of having a “1 in 100 year” flood somewhere in the country is therefore much less.”
    Shouldn’t that be “much greater”?

  11. Athelstan permalink
    February 17, 2017 5:22 pm

    There is some merit in this report bates hits on some good points but is unable to resist the man made CO2 chimera – dear God another mantra chanter OM, OM man made, man made OM, OM – echoing the received idiocy…………….Thus, it conveniently airbrushes over some of the major consequences of our very own man made alterations to the modern landscape of Britain.
    As has been noted above in a rather good post by Scottish Sceptic which I commend but there are too other factors affecting river system drainage and not least run-off. Particularly storm flow run off, in that when it rains in the upper catchments of rivers such as the Aire and Don because of much greater density of housing…… water in heavy rain events runs off drive ways, streets and pavements that much more quickly and thus the river rises that much faster.
    So many moorland, peatland bog areas have been allowed to be drained, this sink was a vital storage and aided the limitation of slope run off entering straight into stream courses. I go walking and have done for years, so many areas of natural bog land where I once squelchily trod are now as dry as a bone – even in spring! Multiply this sort of land use alteration even if is done on a small scale over the catchment the size of the Severn, then that’s an awful lot of the wet stuff coming into the river drainage system that much faster.

    Finally, the sort of extreme event seen in Boscastle or Lynmouth are exceptional and thank goodness not common – at all…….but still they are used as some sort of vague pointer to the you know what and that is an utterly preposterous conjecture – evidently: we are dealing with greentards and fuckwits of the lowest order.

    What needs to be done is to go back to basic river management and to kick the EA into the black bog where they all can live happily ever after with their fellow pond life, at least they’ll be on their intellectual level.

    • John F. Hultquist permalink
      February 18, 2017 3:48 am

      Well documented as the urban hydrograph:

  12. mikewaite permalink
    February 17, 2017 9:15 pm

    Although the effects of changes in river management and run-off have been rightly mentioned above , another possible factor in the flood history would be changes in precipitation ( thinking of the only thing for which Dame Slingo will ever be remembered; “warmer equals wetter”).
    So , with the Bewdley (Severn) chart in mind I looked at some of the Met Office rainfall data for Central England and SW England and Wales
    (I do not know if this is the most appropriate set of data but it seemed quite detailed)
    On cursory comparison of seasonal rainfall amounts with the flood chart I could not see any close correlation in the 1920 – present period (nor was there any obvious increase in precipitation in recent years , so if Slingo is correct , then it is not getting warmer in that area of the Atlantic from which most of our rain originates ).
    Conclusion , if flooding is more frequent it probably relates more to river management changes than to atmospheric changes .
    BTW the references in the Met office page are dominated by one PD Jones . Is that who I think that it is ?

    • Athelstan permalink
      February 18, 2017 12:00 am

      Alarmunist, mythologising prognosticator and fellow conspiracist of Mann made statistical BS c/o @ Penn State……………….phillip douglas Jones ‘the wet’ – yep.

  13. Bill Berry permalink
    February 18, 2017 11:06 am

    Flow data for Bewdley in 1770 would have been interesting – obviously influenced by the massive urbanisation of those times

    • Athelstan permalink
      February 18, 2017 1:22 pm

      Pertaining to what?

  14. Gerry, England permalink
    February 18, 2017 5:33 pm

    Not for the first time the data shows there was a drier period that led them to believe they could build on flood plains with a predictable result when the cycle gets wetter. Also gave them an excuse to cut back on river maintenance EU meddling notwithstanding.

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