What is Compensatory Flood Storage?
Planning policy requires that new developments cannot cause detrimental flooding impacts to areas upstream or downstream of a site.
Developments within a floodplain may remove areas where floodwater is stored during a flood, and can displace floodwaters. There is thus the potential that flood levels surrounding the site could be increased without careful design consideration.
To ensure there is not a detrimental flood risk impact to neighbouring areas, any development resulting in a loss of floodplain storage may be required to provide compensatory storage to negate these potential impacts.
The amount of compensation required is dependent on a) the footprint of any development within the floodplain, and b) the potential depth of flooding in these areas. Together these factors indicate the potential volume of floodwater that could be displaced by development.
Assessment of a Compensatory Flood Storage
An assessment of flood storage compensation is often completed as part of a Flood Risk Assessment, required to be submitted with the planning application.
The Flood Risk Assessment will need to demonstrate that a development has no detrimental flood risk impacts, which will include an assessment of flood storage compensation if required.
Direct and Indirect Compensatory Flood Storage:
Compensatory flood storage can be divided into two categories: direct and indirect.
Direct or ‘level for level’ compensation is the reorientation of the land by the lowering of ground levels. This ensures that the same volume of flood storage is available at all levels of flooding.
In general, level for level compensation should only be applied in areas where flood water is stored; flood flow routes should be protected. There may sometimes be benefits in altering routes or increasing flood flow capacity, however it should only be carried out after careful assessment of the downstream impacts.
Indirect compensation methods rely on displace floodwater entering a storage area, which then releases the water at a controlled rate. Generally, direct schemes are more favourable, unless indirect methods are the only option available.
What Information Should be Provided for Compensatory Flood Storage?
In order to demonstrate that level for level flood storage compensation can be achieved at a site, the following information is likely to be required:
- A topographic site survey;
- The 1 in 100 plus climate change flood level for the site. This is the flood level that the National Planning Policy Framework requires new development to be designed to.
- A plan showing where flood waters are being displaced by the development;
- A plan showing where the flood compensation area is located; and,
- A cut and fill table demonstrating that floodplain compensation can be provided on a level for level basis.
If you’re proposing a development within or close to the floodplain, it is likely that flood storage compensation will be required to demonstrate the proposals do not increase flooding to neighbouring areas.
FPS Environmental can assist at all stages of the planning application process, ensuring that the proposals appropriately address flood risks and are policy compliant.