Using global tide gauge data to validate and improve the representation of extreme sea levels in flood impact studies

The largest collection of tide gauge records assembled to date, called GESLA-2, has been used to provide reliable extreme sea level parameters at 655 locations around the world. This has enabled a rigorous assessment of the European Union-funded DINAS-COAST (D-C) data set of extreme sea level information for the global coastline that has been used in many published flood impact studies. We find the D-C extreme levels to be generally both too high, compared to those from GESLA-2, and too flat, when plotted as a function of return period. This leads to an over-estimation of the probability of extreme sea levels in the present day for most locations around the world, and also to an over-estimation of the probability of extreme sea levels in the future as sea level rises. A detailed impact study is conducted for the world's largest coastal cities following the approach of Hallegatte et al. (2013), resulting in similar conclusions for these particular locations. We suggest that most previous studies that have relied upon D-C information should be re-assessed in the light of these findings, using more recent modelling-based estimates of extreme sea level information.