How can we predict the future when we don’t fully understand the past?

archer By David Archer  (Visiting Fellow Newcastle university and JBA Trust)
27th October 2016

Over the last four years, I have been compiling chronologies of flash floods and the associated causes from intense rainfall, associated occurrence of hail, and results in terms of drowning, deaths by lightning, destruction of houses and bridges, erosion of hillsides and valleys and flooding of property. The main focus of SINATRA has been on Northeast England, Cumbria and Southwest England but chronologies are now almost complete for Lancashire and Yorkshire; an additional less comprehensive chronology has been prepared for the rest of Britain. The source material has been mainly the online British Newspaper archive, with its 15 million searchable pages, but a wide range of documentary sources has also been used. Given the rapid growth of published newspapers in the mid nineteenth century, the records can be considered comprehensive since at least 1850.

In compiling this chronology, event by event, I was struck by the variability of occurrence by year and by decade, which did not fit with the concept of more intense rainfall in a world warming with climate change (Kendon et al. 2014). The most frequent and really damaging flash floods tended to concentrate in the late nineteenth and early twentieth centuries and there were fewer events in many of the later decades of the twentieth century. Figure 1 shows the decadal chronology for Northeast and Southwest England (Archer et al, 2016)

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Figure 1 Time series of flash floods by decade from 1800 to 2010 divided by severity for (a) Northeast England and (b) Southwest England (insets show mapped areas covered by time series).

My first reaction to these findings was: Can I explain them away? Are these patterns of change the result of variable reporting of such events or have they been the result of changing catchment conditions? In response to the first, I am convinced that, except for WWII when such reporting was prohibited, such severe events would be reported and described in the press. With respect to catchment changes, the assessment of the relative magnitude of historical pluvial floods is the most problematic. Urban growth has increased impermeable area (likely to increase flood risk) but sub surface drainage has been improved (likely to decrease flood risk). However, in extreme events such as described, where the rainfall intensity is far in excess of the design capacity of drainage systems, sewers are surcharged and surface flows exceeded gulley capacity in both historical and recent events. A fuller discussion can be found in Archer et al (2016).

The chronology has also assembled a time series showing the decadal variability of large hail in Southwest England and Northeast England (Fig. 2) which shows a similar time distribution to flash floods. It is probable that the less frequent reporting in recent decades of hail causing serious breakage of glass is due to the increased glass strength of standard glass panes but the decline in other reports of large hail must reflect a real decline in occurrence. A similar pattern is reported for the whole of England with decadal declines from a maximum around the turn of the 19th/20th century and a minimum occurrence in the 1970s (Webb et al. 2009).

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Figure 2 Number of occurrences of large hail with and without reported extensive glass breakage for Southwest and Northeast England.


Chronologies of historical flash floods and occurrence of large hail for Northeast and Southwest England indicate strong natural variability, with the second half of the twentieth century showing the lowest frequency of such events. Unless we can explain the sources of such variability and incorporate them in models to project future incidence we run the risk is of serious underestimation even without the expected increase in risk due to rising temperatures.

Figure 2 Number of occurrences of large hail with and without reported extensive glass breakage for Southwest and Northeast England.

References

Archer (in press) Hail – historical evidence for influence on flooding, Circulation

Archer, D.R., Parkin, G. and Fowler, H.J. ( In press 2016) Assessing long term flash flooding frequency using historical information, Hydrology Research. doi: 10.2166/nh.2016.031

Kendon, E.J., Roberts, N.M., Fowler, H.J., Roberts, M.J., Chan, S.C. and Senior, C.A. (2014) Heavier summer downpours with climate change revealed by weather forecast resolution model, Nature Climate Change  4, 570–576 doi:10.1038/nclimate2258.

Webb, J.D.C., Elsom, D.M. and Meaden, G.T. (2009) Severe hailstorms in Britain and Ireland, a climatological survey and hazard assessment, Atmospheric Research 93,  587–606.

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