Atlas of Flood Maps

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EU Flood Risk Maps Q4310 September, 2007 Examples from EU countries WL | Delft Hydraulics 4.5 Great Britain 4.5.1 England & Wales General information In England & Wales the Environment Agency has developed several mapping products to raise awareness of flood risk and support decision making. Examples of these are shown as Figure 4.24 to Figure 4.28. All are available for public or professional use; some data is published on the Environment Agency’s internet site 4 . The Flood Map is currently the Environment Agency’s main map to raise awareness of flood risk with the public and our partner organisations, such as land use/spatial planning authorities, emergency planners, emergency services, developers and drainage authorities. It has been available on the internet since 2004, although an earlier version was first published on the Internet in 2000. The Flood Map shows: � Flooding from rivers or the sea without defences – the natural flood plain area that could be affected in the event of flooding from rivers and the sea. Two shaded areas are presented, which are aligned with the Flood Zones as defined by land use planning policy for England: � Areas that could be flooded either from rivers with an annual probability of flooding greater than 1% (1 in 100) OR areas that could be flooded from the sea with an annual probability of flooding greater than 0.5% (1 in 200) � Areas other than covered by the above that would be flooded by an extreme flood with an annual probability of 0.1% (1 in 1000) from rivers and the sea � The location of flood defences – such as embankments and walls, and flood storage areas � Areas benefiting from these flood defences in a 1% fluvial flood or 0.5% coastal flood – where possible the areas that benefit from the flood defences are shown. However, not all areas that benefit from flood defences are currently shown (Figure 4.24 is an example of this). Figure 4.25 shows how areas benefiting from defences are shown where the information is available. There is ongoing work to increase the coverage of this information. On the internet the Flood Map is presented as a single layer in map form. Users search for their location of interest through a standard search tool by entering either a post code or a location name. The mapped output shown on the internet site (default scale 1:20,000) is very similar to Figure 4.24, which has been shown at a scale of about 1:45,000. The online Flood Map also has the facility to allow users to gain further information by opting to ‘learn more’ by pointing at a specific location within the map. This leads to data from the National Flood Risk Assessment, a mapped data set which provides further qualitative information on the probability of flooding taking into account the location, type 4 http://www.environment-agency.gov.uk/ 4—23
EU Flood Risk Maps Q4310 September, 2007 Examples from EU countries WL | Delft Hydraulics and condition of flood defences. This information on the actual (residual) probability of flooding is presented in three categories used by the insurance industry in the UK, as noted below: � Significant: the chance of flooding in any year is greater than 1.3% (1 in 75) � Moderate: the chance of flooding in any year is 1.3% (1 in 75) or less, but greater than 0.5% (1 in 200) � Low: the chance of flooding in any year is 0.5% (1 in 200) or less Comments on the maps A number of examples of mapped flood data are provided for England & Wales. Most relate to the city of Carlisle in Cumbria to allow direct comparison of outputs, but a further example from Burton upon Trent (Figure 4.25) is shown to illustrate information not available on the Carlisle maps. Figure 4.24 and Figure 4.25 both show extracts from the Flood Map. Flood extents ignoring the presence of defences, as described above, are shown. These extents are shaded in hues of blue, with the area of greater probability in the darker colour. The map also shows flood defences. Although these do not stand out well at the scale shown in the Atlas, they are clear on the internet version of the map shown at a scale of 1:20,000. Data on areas benefiting from defences is not available at all locations, but exists for all defences built since 1998. Further data is added to the Flood Map, as it becomes available, when modelling is updated. The Carlisle maps do not show the areas that benefit from defences, although this is available for Burton upon Trent. The map layout is clear, with the topography shown on the background without too much detail. The internet site also provides an overview map, to orientate the location within the national scale, although this is not shown on the example here. No ‘North’ indication is deemed necessary – it is customary that ‘North’ is at the top of the map in the UK. The grid with its coordinate references provides confirmation of this. Figure 4.26 is a presentation of the assessment of flood probability bands for Carlisle as produced by the National Flood Risk Assessment. It maps the ‘Low’, ‘Moderate’ and ‘Significant’ flood probability bands as defined above and takes into account the reduction in probability as a result of flood defences. The underlying information used to generate the map (the flood probabilities and depths) is also a step in the subsequent assessment of risk when combined with depth/damage information. This banding is tailored more for commercial concerns as the insurance industry in the UK has a particular interest in the 1.3% limit. Whilst this data is not available in mapped format on the internet, the information is available on the internet through the ‘Learn more’ option on the Flood Map. There may be discrepancies between this map and the areas benefiting from defences on the Flood Map. This is because the assessment used to develop the Flood Map does not take into account the presence or condition of flood defences, and so ignores the possibility of breach under different loading conditions. The areas benefiting from defences on the Flood Map may therefore show a greater area of ‘benefit’ when compared with the National Flood Risk Assessment results. Figure 4.27 shows flood hazard rating data, with 7 bands of assessed hazard rating from 0 – 30 on a non-linear scale. The hazard rating (HR) is calculated as a function of velocity (v), 4—24
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