A national perspective on the probability of extreme river flows

The Government’s 2016 National Flood Resilience Review[1] (NFFR) found that “while the probability of an extreme river flow that could result in a severe flood at any given location is very small, such flows are not unusual when considering the whole country”. This statement was one of the conclusions of the Scientific Advisory Group (SAG) formed to examine the meteorological and hydrological evidence underpinning the review.

(Click on image for presentation)

This presentation, given at the Flood and Coast conference in 2017, explains the scientific analysis that led to this advice. It outlines the statistical modelling methodology, detailing how data from 916 river flow gauges (Figure 1) were combined within a national-scale joint probability analysis. The statistical analysis assessed the likelihood (annually and over the next 10 years) of experiencing extreme flows, likely to cause flooding, at any location in the country.

Figure 1. Location of 916 river flow gauges over which the joint probability of extreme events was assessed

The work draws on analysis carried out to support the development of national level flood risk scenarios for the 2016 update of the National Risk Assessment[2], adopting recently-developed joint probability methodology[3]^,[4] to assess the combined likelihood of extreme events being observed to occur together throughout the river gauge network, taking account both of the marginal probability at each location and of the dependence between extreme events between locations.

Figure 2. Results from initial empirical analysis and joint probability model showing the chance of one or more an extreme river flows being experienced somewhere in England and Wales, in any given year

The presentation shows the results that stand behind the headline conclusion, presenting further analysis to examine the robustness of the results and comparisons with an alternative, higher-level assessment based on a simpler empirical analysis of a smaller river flow data set. Both analyses (Figure 2) show that there is a non-negligible chance of extreme river flows being experienced somewhere in England and Wales in any one year.

[1] https://www.gov.uk/government/publications/national-flood-resilience-review

[2] https://www.e3s-conferences.org/articles/e3sconf/abs/2016/02/e3sconf_flood2016_01003/e3sconf_flood2016_01003.html

[3] Lamb, R, Keef, C, Tawn, J, Laeger, S, Meadowcroft, I, Surendran, S, Dunning, P & Batstone, C 2010, ‘A new method to assess the risk of local and widespread flooding on rivers and coasts’ Journal of Flood Risk Management, vol 3, no. 4, pp. 323-336. DOI: 10.1111/j.1753-318X.2010.01081.x

[4] Keef, C, Tawn, JA & Lamb, R 2013, ‘Estimating the probability of widespread flood events’ Environmetrics, vol 24, no. 1, pp. 13-21. DOI: 10.1002/env.2190