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

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

(Minor Development = 3-9 dwellings (unless floor space exceeds 1000m²), Non-residential floor space up to 999 m² or Site area less than 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 90m of a public sewer, then the development must connect to it (If the development provides more than 3 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. Enquires to DeveloperServicesPlanning@southwestwater.co.uk

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 a 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. The main plant will need to be 7m away from the dwelling and the drainage field should be 15m from a dwelling.

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 Flood zone 2,3 or a Critical Drainage Area (CDA) are a higher risk and will require a Flood risk Assessment (FRA) to ensure the risk can be managed to the appropriate level.

Full and Outline Applications

Details of a practical SuDS scheme will be required for all full applications. For Outline applications there needs to be an "in principle" scheme which needs to be achievable, only the fine details or alternative scheme can be agreed by condition. For small scale minor development the requirements for low and medium risk site will require similar information. Medium risk sites will need to demonstrate that a suitable drainage scheme can be accommodated within the site constraints.

Low Risk SiteMedium Risk Site
Low risk sites are not within flood zone 2, 3 or a CDA and have sufficient space available to provide a workable drainage solution.Medium risk sites are small or steep and will need further justification or analysis to support the proposed method of surface water.
 

 

Drainage assessment (Low and medium risk)

The drainage assessment needs to evidence how roof water and surface water shall be managed for the development. For these smaller developments full SuDS schemes can be impractical so a practical approach can be taken if the following drainage hierarchy for sustainable development is achievable.

  1. Soakaways/infiltration systems
  2. Controlled/attenuated discharge to watercourse
  3. Storm water/combined sewers

1 - Soakaways:

Where new soakaways are proposed, percolation tests should be undertaken in accordance with the testing method set down in BRE DIGEST 365. The results of such tests should be included in the Drainage Assessment. Soakaways/infiltrations systems must be designed with sufficient volume to cope with 1:100 year return period plus an allowance for climate change (currently 40%). Supporting calculations should be included in the Drainage Assessment and form part of the planning application.

Sites steeper than 1:10 with require a ground assessment to ensure the water doesn't cause problems down slope. Small sites will need to show that soakaways etc. can be accommodated and still meet the minimum 5m distance to dwellings or the highway.

2 - Discharge to watercourse:

If soakaways are not feasible then the next option is to discharge to a water course, if one is available on site. The discharge will have to mimic the existing Greenfield run off rate and attenuation systems should be designed for a 1:100 year return period plus an allowance for climate change (currently 40). Written consent, in principle, must be gained from the Environment Agency if the point of discharge is to a main river. Supporting calculations should be included in the Drainage Assessment.

3 - Discharge to combined sewer:

The final option is to discharge to a public sewer. It should be noted that in most circumstances surface water is not allowed to be connected to the public foul sewers. Only where there is no other possible option will this be considered and where it can be proved that all other options have been explored. Please include evidence which confirms the outcome of the investigations undertaken and reasons why discharge to the sewer is the only possible option. Written evidence from SWW Ltd or the owner of the sewer that confirms that the proposed development can be connected to the water sewer network. Confirmation of the agreed discharge rate must be supplied. Attenuation prior to discharge may be required in some circumstances.

SuDS

The proposed scheme should be fully Suds compliant as per CIRIA SuDS Manual C753. SuDS schemes should reduce the area of impermeable surfacing and manage the surface water at source. Traditional soakaways do meet some of the criteria but do not address water quality and can have long-term maintenance issues. For these reasons low cost surface features such as ponds, swales and detention basins are preferred as they improve quality by sediment removal and are low maintenance.

SuDS schemes will be assessed against the three main criteria:

  1. Reduction of surface water runoff and local recharge of ground water.
  2. Improvement in water quality, biodiversity and integration with amenity space.
  3. Low maintenance for the life of the development.

If it is proven that SuDS are not appropriate for the site then the drainage assessment will need to justify why the proposed method has been chosen and show that it gives the best available environmental protection.

Management and Maintenance:

The drainage assessment should include details of the proposed management, maintenance of the drainage system and identify the parties responsible


Requirements of a drainage assessment Minor Development (3-9 Units)

  1. 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.
  2. A statement of SuDS to be incorporated and final discharge point(s) where relevant. This should include how the drainage design satisfies SuDS techniques, both in terms of water quality and attenuation of water quantity, in accordance with best practice and design
  3. A drainage plan identifying the type(s) of SuDS to be incorporated and SuDS land take
  4. Proposals, where relevant, for integrating the drainage system into the landscape or required publicly accessible open space and providing habitat and social enhancement
  5. Calculations showing the pre- and post- development peak runoff flow rate for the critical rainfall event
  6. 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.
  7. Infiltration drainage or attenuation designed to contain the critical site design storm 1:100 year + 40% climate change
  8. Calculations showing the half drain down time is less than 24 hours
  9. Indication of overland flow routes and safeguarding of properties from flooding
  10. Details of which body will be responsible for vesting and maintenance for individual aspects of the drainage proposals
  11. Confirmation of land ownership of all land required for drainage
  12. A method statement detailing how surface water arising during construction will be dealt with and, if temporary or sacrificial SuDS will be established.
  13. Foul drainage details

Validation checklist - Low and Medium Risk

Documents RequiredPoints CoveredEvidence
Surface Water Drainage assessment

Fully designed scheme for full applications

OR

'In principle scheme' for outline applications

Inc. Testing to DG 365 and designed for 1:100 year return period + allowance for climate change. Statement of SUDS

i.e. Discharge to a water course or written permission from SWW to discharge to a public sewer

Foul Drainage AssessmentPrivate or public sewerFDA1 form for a private system or written confirmation from SWW to connect to the public sewer

High Risk Sites (in flood zone2, 3 or CDA)

High risk sites are located within flood zone 2, 3, CDA, or both and have been identified as being at risk of flooding or having the potential to increase flood risk elsewhere.

Flood zone

If the site is within Flood zone 2 or 3 as identified by the EA, or in the SFRA, then a Flood Risk Assessment (FRA) will need to be submitted. The sequential and exception test should be carried out and demonstrate that the development is not at risk from flooding, or will cause a flood risk to other properties.

FRA

This also applies if the main development is in zone 1 but the access lies in zone 3

The FRA should address the issues of flood to both property and people. The FRA should identify and assess the risks of all forms of flooding to and from the development. It must also demonstrate how these flood risks will be managed, taking climate change into account.

The sequential test should be applied, and only once this has been passed will the exceptions test be considered. The FRA should identify opportunities to reduce the probability and consequences of flooding. The FRA should include the design of surface water management systems including Sustainable Drainage Systems (SUDs) and address the requirement for safe access to and from the development in areas at risk of flooding

The submitted FRA should demonstrate whether the proposed development (including its access and egress) will be safe over its lifetime without increasing flood risk elsewhere. Where possible, it should demonstrate how it will reduce flood risk overall.

The Environment Agency have standing advice for certain types of development, further advice can be found at https://www.gov.uk/guidance/flood-risk-assessment-standing-advice

Sequential test

All applications (except minor applications or change of use - NB excluding change of use to a caravan, camping or chalet site) within flood zones 2, 3a and 3b. In terms of the application of the sequential test, minor development is defined in paragraph 046 of the Flood Risk and Coastal Change section of the PPG:

"Minor development means: minor non-residential extensions: industrial/commercial/leisure etc. extensions with a footprint less than 250 square metres. Alterations: development that does not increase the size of buildings e.g. alterations to external appearance. Householder development: For example; sheds, garages, games rooms etc. within the curtilage of the existing dwelling, in addition to physical extensions to the existing dwelling itself. This definition excludes any proposed development that would create a separate dwelling within the curtilage of the existing dwelling e.g. subdivision of houses into flats".

The sequential test will also apply in situations where the development itself lies within Flood Zone 1 but the access lies within Flood Zone 3. Evidence must provide evidence to demonstrate that a sequential approach to site selection has been undertaken.

NB. The applicant should be aware that new development will only normally be acceptable in Flood Zones 2 and 3 if it has been demonstrated that there are no other reasonably available sites within Flood Zone 1. In the cases where the LPA determines that the sequential test can be satisfied, or that the failure of the test can be outweighed, the exception test will then need to be applied.

The submission of an acceptable FRA is required to meet the second part of the exception test. (Table 3 in paragraph 067 of the Flood Risk section of the PPG explains when the exception test is required). Nonetheless, failure of the sequential test is sufficient reason for the Council to refuse a planning application, regardless of the submission of an acceptable FRA.

CDA

A CDA is an area with critical drainage problems (which has been formally notified to the LPA by the Environment Agency). Within CDAs proposed development may present risks of flooding on-site and/or off-site if the surface water runoff is not effectively managed.

The drive behind the Critical Drainage Area (CDA) allocation is to reduce downstream flooding by controlling the accumulative impact of surface water runoff from multiple development sites in sensitive catchment areas. This means that any site, discharging surface water to a watercourse or public sewer, must attenuate the flow to mimic the green field runoff for a 1:10 year rain fall event. Where the surface water can be managed within the site for the "1:100 + 40%" condition, there is no change to the standard surface water drainage requirement.

The drainage assessment will normally form part of the Flood Risk Assessment for development proposals which are over 1ha within Flood Zone 1, where the main consideration will be appropriate management of surface water. It should be appropriate to the scale and nature of the proposed development.

"All off site surface water discharges from development should mimic "Greenfield" performance up to a maximum 1 in 10 year discharge. On-site all surface water should be safely managed up to the "1 in 100+climate change" conditions. To satisfy the above will require additional water storage areas to be created within the site compared to the normal SUDS design thereby contributing to a reduction in flooding downstream." Environment Agency 2015


Drainage assessment High risk

The drainage assessment can be included in the FRA and needs to evidence how roof water and surface water shall be managed for the development. For these smaller developments full SuDS schemes can be impractical so a practical approach can be taken if the following drainage hierarchy for sustainable development is achievable.

  1. 1. Soakaways/infiltration systems
  2. 2. Controlled/attenuated discharge to watercourse
  3. 3. Storm water/combined sewers

1 - Soakaways:

Where new soakaways are proposed, percolation tests should be undertaken in accordance with the testing method set down in DG 365. The results of such tests should be included in the Drainage Assessment. Soakaways/infiltrations systems must be designed with sufficient volume to cope with 1:100 year return period plus an allowance for climate change (currently 40%). Supporting calculations should be included in the Drainage Assessment and form part of the planning application.

2 - Discharge to watercourse:

If soakaways are not feasible then the next option is to discharge to a water course, if one is available on site. The discharge will have to be limited to the 1:10 year Greenfield run off rate and attenuation systems should be designed for a 1:100 year return period plus an allowance for climate change (currently 40%). Written consent, in principle, must be gained from the Environment Agency if the point of discharge is to a main river. Supporting calculations should be included in the Drainage Assessment.

3 - Discharge to combined sewer:

The final option is to discharge to a public sewer. It should be noted that in most circumstances surface water is not allowed to be connected to the public foul sewers. Only where there is no other possible option will this be considered and where it can be proved that all other options have been explored. Please include evidence which confirms the outcome of the investigations undertaken and reasons why discharge to the sewer is the only possible option. Written evidence from SWW Ltd or the owner of the sewer that confirms that the proposed development can be connected to the water sewer network. The discharge will have to be limited to the 1:10 year Greenfield run off rate and attenuation systems should be designed for a 1:100 year return period plus an allowance for climate change (currently 40%)

Requirements of a drainage assessment (Minor Development: 3-9 Units)

  1. 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.
  2. A statement of SuDS to be incorporated and final discharge point(s) where relevant. It should include how the drainage design satisfies SuDS techniques, both in terms of water quality and attenuation of water quantity, in accordance with best practice and design
  3. A drainage plan identifying the type(s) of SuDS to be incorporated and SuDS land take
  4. 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.
  5. Infiltration drainage or attenuation designed to contain the critical site design storm 1:100 year + 40% climate change
  6. Calculations showing the half drain down time is less than 24hours
  7. Indication of overland flow routes and safeguarding of properties from flooding
  8. Details of which body will be responsible for vesting and maintenance for individual aspects of the drainage proposals
  9. Confirmation of land ownership of all land required for drainage
  10. A method statement detailing how surface water arising during construction will be dealt with and, if temporary or sacrificial SuDS will be established.
  11. Foul drainage details

Points to consider

  • If the development is in a critical drainage area and an offsite discharge is the only possible option then the flow must be attenuated to the 1:100 year + 40% cc and the discharge must be limited to the 1:10 year Greenfield run off rate.
  • Take into account possible future development
  • Consider impact of development on pollution risk to groundwater, and/or mobilisation of groundwater contamination
  • Possible additional level of treatment for discharge to sensitive receiving waters
  • Demonstration of good ecological practice including habitat enhancement and de-culverting
  • Take account of surface water/groundwater entering the development from adjacent land.

Validation checklist - High Risk

Documents RequiredPoints CoveredEvidence
FRAAll flood riskAssess risk to the development, sequential and exception test
 
Surface Water Drainage assessment

Fully designed scheme for full applications

OR

'In principle scheme' for outline applications

Inc. Testing to DG 365 and designed for 1:100 year return period + allowance for climate change. Statement of SUDS

i.e. Discharge to a water course or written permission from SWW to discharge to a public sewer

Foul Drainage AssessmentPrivate or public sewerFDA1 form for a private system or written confirmation from SWW to connect to the public sewer


Email enquiries to: DM.Drainage@swdevon.gov.uk

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