Coastal Flood Model
The Coastal Flood model is based on the Environment Agency’s authoritative Flood Map for England and Wales, launched in September 2004, which makes use of the Intermap digital terrain model and National Flood and Coastal Defence Database. The Environment Agency supply regular updates which illustrate the continuous improvements in terms of resolution, accuracy and coverage. The Coastal Flood model covers all areas in England and Wales known to be at risk from tidal flooding. The limits of the floodplain are indicative of the area that could be affected by specific flood events, overtopping or breaching of flood defence structures. They are based on the areas which are potentially at risk of coastal flooding from an event with a 0.5% annual probability of exceedence (200 year average return period) for coastal areas, under present conditions. The floodplain extent is based on the best available information at the time of survey.
River Flood Model
ISL’s Fluvial Flood model is based on data from the Environment Agency Flood Map, the Centre for Ecology and Hydrology (CEH) and SEPA (Scottish Environmental Protection Agency).
The limits of the floodplain are indicative of the area which could be affected by flood events, overtopping or breaching of flood defence structures. They are based on the areas which are potentially at risk of river flooding from an event with a 1% annual probability of exceedence (100 year average return period) under present conditions. Where a known flood covers a greater area, then this has been used to define the limits.
The CEH also produced a model of flood risk for Scotland considering 1:100 year events using data supplied by SEPA. This model maps risk on a spatial grid of 50 metre squares and is available in the insurance market exclusively through ISL. It is the only model available of this quality and resolution for Scotland giving complete coverage of flood risk in Great Britain.
The Windstorm model is derived from storm claims information and gust wind speed information from a weather data company ‘Weathernet’. The current rating model has been built from a firm foundation of research and refinement going back over many years. An outline below of some of the stages involved should give an indication as to the depth and extent of the work involved in building the model.
The feature within UK windstorms that tends to cause structural damage is the level of gusting, together with windspeed. For rating, therefore, the windstorm hazard definition:
- starts with the variation of peak gust windspeed (this is the determining loading in terms of structural damage) and its spatial variation across the area of interest;
- then the impact on the windspeed of macrozonation factors are addressed in terms of the variation of topography and terrain across the country of interest;
- the impact of microzonation (below 500m resolution) can not be addressed adequately independent of the property under consideration, thus is addressed as part of the vulnerability development.
- at each definable specific location in terms of macrozonation the resultant peak gust windspeed and associated probability of occurrence are established.
Brief introduction to Wind Vulnerability Functions
The layperson might assume that the building standard would give an indication as to the vulnerability of the particular property to windstorm damage. However the building code does not give you vulnerability functions, it really is just what a structure designed to standard should be able to “withstand” with a high degree of confidence. Typically “withstand” means no significant structural damage, that is it is still habitable subject to minor repair, not there is no damage and hence loss below the standard design wind hazard levels.
ISL have utilised the wealth of research undertaken in recent years both in the UK and overseas to more fully understand windstorm risk.