Flood maps have been created to support PPS 25 and TAN 15. The maps published by the Environment Agency in 2004 were based on Digital Terrain Modelling (DTM) to a 5 metre grid pattern. Flooding was modelled using JFlow for rivers and HYDROF for the sea. Structures such as bridges and embankments, which may affect the flood plain locally, have not generally been taken into account in producing the flood maps.
The EA is committed to improving the flood maps data and refining the outlines as better data becomes available. They will also consider challenges to the maps submitted by others and make amendments as appropriate.
External challenges can be based on any of the following:
- New modelling information
- New flood event information
- New topographic information
In order to change the Flood Zone outlines, revised flood extents must be "Good Quality" information which involves more detailed modelling/ assessment (in accordance with EA guidance), provided it is:
- Based on flood extents without defences
- Based on surveyed ground levels (in accordance with EA guidance)
- No allowance is made for blockages
- No allowance is made for climate change
The flood map challenge includes the following:
- Relationship of site levels to potential flood levels (AOD)
- Historical data research on past flooding incidents
- Obtaining data from the EA for river levels
- Agreeing with the EA the extent and details of the model
- Extensive survey information for the hydraulic model
- Modelling using MIKE FLOOD or HECRAS as appropriate
- Reviewing existing flood defences
- Breach Assessment if requires
- Calculating 1 in 100 and 1 in 1000 year flood levels
- Mapping 1 in 100 and 1000 year flood extents (zones)
The EA update the published flood map quarterly. The Development Advice Maps for Wales will not be updated until mid-2007. The EA's advice to Planning Authorities in England and Wales will be based on their latest information.
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