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Flooding

Flooding and flood water management in Hampshire

Environment Agency flood risk maps

The Environment Agency have reviewed their maps showing national flood risk. They are available from Environment Agency’s website.

Why has the Environment Agency produced these maps?

The purpose of publishing this information on the Environment Agency’s website is to inform people about flood risk and help householders, businesses, local authorities and others to be prepared for flooding.

The Risk of Flooding from Rivers and Sea map now takes flood defences into account and the Environment Agency’s Flood Map for Planning (Rivers and Sea) continues to be updated. For the first time they are also publishing maps showing the risk of flooding from surface water.

The Environment Agency continue to provide flood risk information for reservoirs on their website. These maps show the extent of flooding but do not include information about depth or velocity due to concerns over national security.

Managing the risk and responding to surface water flooding is a role for Lead Local Flood Authorities . Hampshire County Council has this new responsibility as a lead local authority. The Environment Agency have produced the new surface water maps as they have the tools and experience to do so and have worked with Lead Local Flood Authorities to incorporate their surface water flood risk information.

What do the maps show?

Risk of Flooding from Rivers and Sea map

This shows the areas at risk from flooding by rivers and the sea (both along the open coast and tidal estuaries). The new map of rivers and the sea is an improvement on the previous flood map as it takes flood defences into account and so describes the actual chance of flooding, rather than the chance if there were no defences present.

While flood defences reduce the level of risk they don’t completely remove it. They can be overtopped or fail in extreme weather conditions, or if they are in poor condition. This is why the maps may show areas behind defences which still have some flood risk.

Risk of Flooding from Surface Water map

This shows the flood risk from surface water from the impact of heavy rainfall. This is the first time surface water flood maps have been published. Although the maps allow for water soaking into the ground and being taken away by drainage systems, surface water flooding is very sensitive to the local conditions. In some cases the assumptions that have had to be made, do mean that the maps may over-or under-estimate the flood risk in an area. When you click on the map you will also be able to see more information showing depth and velocity of flood water for the areas at each level of flood risk.

Risk of Flooding from Reservoirs map

This shows the extent of flooding if a reservoir were to fail and release the water it holds. It shows the worst case scenario; it is very unlikely that this would happen and that any flood would be this large.

Flood Map for Planning is the original Flood Map, containing Flood Zones and shows the areas that would flood if there were no flood defences or buildings in the “natural” floodplain. This map is still available to provide important information for land-use planning purposes.

The Flood Risk Maps are summary PDFs that show what is affected by flooding, ie. people, infrastructure and environmental areas of importance. These maps are at a River Basin District showing the impact of flooding from river, sea, surface water and reservoirs.

Hampshire County Council’s Role

Managing the risk and responding to surface water flooding is a role for Lead Local Flood Authorities. For further information on the management of local flood risk in Hampshire please see: