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Flood Risk Management

Protecting communities and increasing resilience

Joint Position Statement on Sustainable Drainage Systems in Fluvial Flood Zones

Last Updated 28/09/2021 12:00am


Aim

The aim of this statement is to clarify Devon’s stance in relation to the siting of above ground sustainable drainage systems (SuDS) in fluvial Flood Zones.

Joint Position

Generally, there should be no siting of above ground sustainable drainage systems in fluvial Flood Zones 2 or 3 as this would result with the displacement of flood water and alter/block conveyance routes and flow paths.  The basins could be full of fluvial flood water prior to the runoff entering the basin or vice versa thereby completely defeating the purpose of the basins.

Exceptions

In certain circumstances, above ground SuDS could potentially be located in fluvial Flood Zones 2 and 3 for example:

  • When the Flood Zone has been modelled to show the area is not operational during the 1 in 100 year plus climate change event.  The modelling needs to be agreed with the EA and appropriate climate change figures utilised within the modelling.
  • SuDS are allowed in tidal Flood Zones where the risk is only from tidal sources and there is no other flood risk for example from fluvial or surface water.  This would apply in instances where sites are located immediately adjacent to the coast. Please note this does not apply to tidal watercourses.
  • SuDS are allowed in fluvial Flood Zones in areas that are significantly protected by existing EA or Local Authority Flood Schemes provided they are not in an area at risk of surface water flooding.
  • Conveyance SuDS can be sited in flood zones as ultimately they need to discharge into a watercourse.
  • If the area being proposed for the above ground sustainable drainage feature forms part of a wider project, for example a river restoration scheme or similar, then this may be acceptable too.

Further Information

Where SuDS are proposed in Flood Zones they would require erosion protection measures as well as enhanced maintenance regimes to ensure the features are ‘fit for purpose’ after flood events.  Any control structures should be suitable for submerged conditions.  Consideration should also be given to the impact on the banks as well as third party land.

No SuDS should be sited in flood storage areas as these areas are essentially designated functional floodplain.

As a last resort, sealed underground attenuation tanks are suitable in fluvial Flood Zones as long as they can be operational during flood events and are maintainable.

Conclusion

As a general rule, above ground SuDS are not appropriate in fluvial Flood Zones as they would lead to an increase in flood risk. However, this Position Statement documents instances where they may be acceptable.

 

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