A return period is the average length of time in years between events such as the flooding of a particular level. A 1 in 50 year return level is where there is a 1 in 50 chance of that level being exceeded in a year. We used the current national standard guidance in exceedance probabilities to assign return periods to high waters. The Environment Agecny commissioned study (McMillian et al., 2011; Batstone et al., 2013) that produced these, is the latest in a number of related UK investigations from the last six decades (see Batstone et al. 2001 and Haigh et al. 2010 for a summary) that have contributed significantly to developing and refining appropriate methods for accurate and spatially coherent estimation of extreme water levels.
Batstone, C., Lawless, M., Tawn, J., Horsburgh, K., Blackman, D., McMillan, A., Worth, D., Laeger, S., and Hunt, T. A UK best-practice approach for extreme sea-level analysis along complex topographic coastlines, Ocean Eng. 71, 28–39 (2013).
Haigh, I.D., Nicholls, R.J., Wells, N.C. A comparison of the main methods for estimating probabilities of extreme still water levels. Coastal Engineering 57(9), 838-849 (2010).
McMillan, A., Batstone, C., Worth, D., Tawn, J. A., Horsburgh, K., and Lawless, M. Coastal flood boundary conditions for UK mainland and islands. Project: SC060064/TR2: Design sea levels, Environment Agency, Bristol, UK (2011).