Residual and Secondary Flood Risks in Defended Areas

The flood risk within a defended area cannot be entirely eliminated. Flood defences are designed to protect land from a specific height of flood water such as a 100 year fluvial or 200 year coastal flood event. The possibility of a flood greater than this occurring and overtopping the defences (the residual flood risk) will always remain. Another risk arises through the potential for structural collapse and breeching of the defences which could result in sudden and rapid inundation of flood water. There is also potential for back drainage systems to become overwhelmed as they are unable to discharge effectively when water levels remain high during flood conditions. In all such circumstances flood water within defended areas is likely to become trapped by the defences, resulting in longer term impacts and may require evacuation and pumping or other engineering solutions to remove.