Both the CFB_Extreme_Sea_Levels_2018.shp and CFB_Extreme_Sea_Levels_Estuary_2018.shp shapefiles contain 16 extreme sea levels (prefixed ‘T’). They also include 16 lower (2.5%) and upper (97.5%) confidence levels (prefixed ‘C_T1’ and ‘C_T2’ respectively).
Each extreme sea level value represents the peak total sea level that occurs during a tidal cycle associated with particular extreme probability. Predicted tidal levels combine with storm surge to produce the extreme peak sea level. Information on how to generate a design time series tide curve using CFB extreme sea level and surge information is provided in the supporting documentation: Coastal flood boundary conditions for the UK: update 2018, User report.
The confidence levels provided account for statistical uncertainty in the skew surge joint probability method. They do not take account of uncertainty due to interpolation using model data or predicted tidal variation between gauge sites. Locations of gauge sites used in the statistical analysis are given in CFB_Gauge_Data_2018.shp.
The shapefiles also include information on base year and Highest Astronomical Tide (HAT) and Mean High Water Spring (MHWS) predicted tide levels (not suitable for navigation). The location and chainage of each point is given.
Location chainage information included in the shapefiles is unchanged from CFB 2011 where the same location has been included in CFB 2018.