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Foul Drainage, Flood Risk and Surface Water Requirements for Major Developments

These notes have been produced to accompany the 2017 Validation checklist to provide additional guidance for the successful completion of a drainage assessment

(10+ dwellings (over 0.5 hectare if number of dwellings is unspecified), Non-residential floor space greater than 1000 m² or Site area exceeds 1 hectare)

Now that the application type has been established you will need to look at the following three risk groups (Low, Med and High) to see which one applies to your specific site. The validation requirements are in a table at the end of each section. Foul drainage requirements are consistent for all risk levels.

Foul Drainage

If the development includes additional bedrooms or bathrooms then a foul drainage assessment will be required to ensure there is sufficient capacity in the existing system.

Mains Sewage Assessment

If the proposed development is within 300m of a public sewer, then the development must connect to it (If the development provides more than 10 dwellings then the distance is the No. of units x 30m). For sites that are below the sewer invert, then a pumping station will be required and this must have a 24h emergency storage capacity. Full details and calculation must be included in the assessment. Full details of the existing system should be shown on the application drawing(s).

Written permission from South West Water (SWW) will be required to confirm acceptance of the additional flows and check capacity. Enquiries to

Non-Mains Sewage Assessment

Where connection to the mains sewer is not practical, then the foul/non-mains drainage assessment will be necessary to show why the development cannot connect to the public mains sewer system and show that proposed system follows the foul drainage hierarchy as set out in the FDA1 form. A non-mains foul drainage system should be designed and built to an adoptable standard. The assessment should include full details, its location and suitability for storing, transporting and treating sewage. If the proposed development results in any changes/replacement to the existing system or the creation of a new system, scale plans of the new foul drainage arrangements will also need to be provided. This will include a location plan, cross sections/elevations and specification of the equipment used.

Foul Drainage Assessment form (FDA1) must be completed and include any supporting information and testing. The proposals should comply with the General binding rules and you need to check the EA maps to see if the site is within Source Protection Zone 1 and if an Environmental Permit is required.

Flood Risk (all forms of flooding Inc. surface water)

All applications have to acknowledge whether there is a flood risk to the development or not. Sites that are within a Flood zone or a Critical Drainage Area (CDA) are higher risk and will require a Flood risk Assessment (FRA) to ensure the risk can be managed to the appropriate level.

Requirements of a Drainage Assessment (10+ units)

The "Sustainable Drainage Systems: Guidance for Devon" document, which contains the requirements, can be downloaded from this page.

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