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Post-event measures: Harmonize flood hazard maps,standardize use and interpretation,increase public availability

Flood hazard maps are very powerful tools. They highlight flood hazards and provide a basis for discussion when topics include vulnerability, and the people and assets at risk. Floods are a spatial hazard with high resolution: detailed studies are needed to identify and model flood hazards. A great variety of flood maps are available, but flood maps are currently not standardized. This creates confusion. What is shown on one flood map might be very different from another. This makes maps hard to interpret. As part of an effort to inform and advise on risks, flood hazard maps should be publicly available, easily accessible, and harmonized across watersheds. Ideally, this should be done on an international scale. If every flood map looked the same, these maps would be a much more powerful tool, allowing for greater understanding and use of such maps. Flood return periods and assumptions about how flood zones are derived (e.g. with or without levee protection) need to be harmonized, too. While current efforts, such as the European Union’s Floods Directive, are going in the right direction by providing flood maps Europe-wide, these still fail to provide clear guidance. For example, while a 100-year return period flood is usually chosen for maps as one of the flood probabilities shown, return periods for ‘frequent flooding’ and ‘extreme flooding’ are subject to interpretation by EU members. The choice then comes down to the level of individual states in these countries. The result is that a large number of different flood maps are available on a variety of flood web sites and internet portals. Flood maps should also account for the dynamic of flood risk. As seen in the example of the Inn (page 34), new river sections may need to be analyzed in more detail. Flood maps need to be constantly adjusted and communicated as hazards change – we suggest a regular update of flood maps every couple of years at least. We advocate flood maps that show clear return periods: for example, 10, 50, 100 and 500 years. Hazard maps and risk maps should not be combined into one map. ‘Hazards’ need to be shown separate from ‘event severities,’ and both need to be separate from the ‘vulnerability of assets’ in the flood zones

PERC Reports