Drainage (Surface Water) Assessment - Non-Major Applications

Drainage Assessment - Non Major Applications

Required for all non-major application Types:

1. Householder

2. Outline

3. Full

4. Reserved Matters

5. Discharge of condition

All non-major applications will be assessed by the Council using standing advice or comments from an Engineering Specialist

Note: Sites that are within Flood Zone 2/3, or a Critical Drainage Area (CDA), will require a Flood Risk Assessment (FRA) to ensure the risk can be managed to the appropriate level. See FRA section for more details.

CDAs - where stricter drainage standards will be required include:

• Ivybridge
• Kingsbridge
• Modbury
• Okehampton
• Tavistock
• Totnes (Bridgetown)
• Totnes (Warlands)
• Areas on the edge of Plymouth

All applications that increase the impermeable surface area will require an appropriate Drainage Assessment. Given that non-major applications range from a simple extension up to 9 new dwellings the requirement is split into 3 risk categories: Low, Medium and High.

Low Risk

• Householder
• 1-2 dwellings
• Agricultural dwellings
• Not within FZ2/2 or a CDA and can demonstrate that soakaways are an option or there is access to a water course or public sewer.

Due to the varied size and nature of these applications the specific requirements can only be established from the guidance.

Medium Risk

• Householder
• 1-2 dwellings
• Agricultural dwellings
• Not within FZ2/2 or a CDA but unable to demonstrate that soakaways are an option or there is no access to a water course or public sewer.

Due to the varied size and nature of these applications the specific requirements can only be established from the guidance.

In most cases a drainage assessment will be required to show that the site can be adequately drained and doesn't pose a flood risk.

High Risk

• Householder (within CDA or FZ3/3)
• 1-2 dwellings (within CDA or FZ3/3)
• Agricultural dwellings (within CDA or FZ3/3)

Requirements of a drainage assessment - Medium / Low Risk

• Evidence of soil infiltration tests at the location of any intended infiltration device and the proximity of the winter water table, testing to BRE digest 365
• Infiltration or attenuation designed to contain the critical site design storm 1:100 year + an allowance for climate change (currently 40%)
• Calculations showing the drain downtime is less than 24 hours
• Indication of overland flow routes and safeguarding of properties from flooding
• Evidence that the peak runoff post-development is no worse than previous
• Confirmation of land ownership of all land required for drainage
• Take account of surface water/groundwater entering the development from adjacent land
• Proposals, where relevant, for integrating the drainage system into the landscape or required publicly accessible open space and providing habitat and social enhancement

• Foul drainage proposals

Requirements of a drainage assessment - High Risk

• Plan showing existing drainage arrangements including watercourses, ditches, culverts, sewers and general land drainage, both within and adjoining the site. This is to ensure that drainage proposals integrate with and do not compromise the function of natural drainage systems.
• A statement of SuDS to be incorporated and final discharge point(s) where relevant. This should include how the design satisfies SuDS techniques, both in terms of water quality and attenuation of water quantity, in accordance with best practice and design.
• A drainage plan identifying the type(s) of SuDS to be incorporated and SuDS land take.
• Proposals, where relevant, for integrating the drainage system into the landscape or required publicly accessible open space and providing habitat and social enhancement.
• Calculations showing the pre- and post- development peak runoff flow rate for the critical rainfall event
• Evidence of soil infiltration tests at the location of any intended infiltration device and the proximity of the winter water table, testing to DG 365.
• Infiltration drainage or attenuation designed to contain the critical site design storm 1:100 year + 40% climate change
• Calculations showing the half drain downtime is less than 24 hours
• Indication of overland flow routes and safeguarding of properties from flooding
• Details of which body will be responsible for vesting and maintenance for individual aspects of the drainage proposals
• Confirmation of land ownership of all land required for drainage

• A method statement detailing how surface water arising during construction will be dealt with and, if temporary or sacrificial SuDS will be established.