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Foul Drainage, Flood Risk and Surface Water Requirements for Outline and Full applications (Small Minor Development)

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

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 30m of a public sewer, or 60m for 2 dwellings, then the development must connect to it. 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 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 assessment should include a full assessment of the site, 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 and 3 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.

Full and Outline Applications

For lower risk small scale developments we will accept an "in principle" scheme, with the details confirmed by condition. Higher risk sites will need to evidence how the risk is reduced or managed. This means that the same level of information is required for both full and outline applications.


Low Risk Sites (not in flood zone 2 and 3 or CDA)

Low risk sites are not within flood zone 2 and 3 or CDA and have sufficient space available to provide the worst case drainage solution

Providing the site can meet the set criteria below then we would accept an in principle scheme design and recommend that the final design is conditioned if permission is granted.

  • Is the site slope / gradient less than 1:10, i.e. 1m rise / fall over 10m 
  • If there is no historical flooding at the site
  • If a soakaway from Table 1 (based on impermeable area) can be accommodated on site and achieve the minimum distance of 5m from any dwelling or highway.

Note: The soakaway size given in Table 1 is based on an assumed infiltration rate and should only be used for indicative planning purposes. The condition would be prior to commencement or included in a reserved matters application and would in no way relieve the applicant from their responsibilities and duties to prevent flooding of third party property.

Validation Checklist - Low Risk

Documents RequiredPoints CoveredEvidence
Surface water drainage assessment

Site gradient

Flood History

Indicative 
soakaway

Levels on a plan or photographic evidence

Statement in assessment

Scale plan with position of soakaway including all required clearance for Building control

Foul drainage assessmentPrivate or
public system
FDA1 form for a private system or written confirmation from SWW to connect to the public sewer

 

Table 1 - Indicative Soakaway Size
Note: Exact size must be used as the shape affects performance.
Based on infiltration rate of 1 x 10 -6 

Impermeable Area
Roof and hard surfacing (m2)

Crate Soakaway
(L x W x D)
Alternate Crate Size
(L x W x D)
51 x 1 x 0.4m 
101 x 1 x 0.8m
151 x 1 x 1.2m
201 x 1.5 x 1.2m
252 x 1.5 x 1.2m
302 x 2 x 1.2m3 x 1 x 1.2m
353 x 2 x 1.2m3 x 1.5 x 1.2m
403 x 2 x 1.2m4 x 1.5 x 1.2m
453 x 2.5 x 1.2m4 x 2 x 1.2m
503 x 3.5 x 1.2m4 x 2.5 x 1.2m
554 x 3.5 x 1.2m5 x 3 x 1.2m
604 x 3.5 x 1.2m5 x 3 x 1.2m
654 x 4 x 1.2m5 x 3 x 1.2m
705 x 3.5 x 1.2m6 x 3 x 1.2m
756 x 3.5 x 1.2m7 x 3 x 1.2m
806 x 4 x 1.2m7 x 3 x 1.2m
856 x 4.5 x 1.2m8 x 2.5 x 1.2m
906 x 5 x 1.2m8 x 3 x 1.2m
956 x 5.5 x 1.2m8 x 3.5 x 1.2m
1007 x 5.5 x 1.2m9 x 3.5 x 1.2m
1057 x 6 x 1.2m9 x 4 x 1.2m
1107 x 6.5 x 1.2m10 x 3.5 x 1.2m
1157 x 7 x 1.2m10 x 4 x 1.2m
1208 x 7 x 1.2m11 x 4 x 1.2m
1258 x 7.5 x 1.2m11 x 4.5 x 1.2m
1308 x 8 x 1.2m11 x 5 x 1.2m
1359 x 7.5 x 1.2m12 x 4.5 x 1.2m
1409 x 8 x 1.2m12 x 5 x 1.2m
1459 x 9 x 1.2m13 x 5 x 1.2m
15010 x 8.5 x 1.2m13 x 5.5 x 1.2m
16010 x 9.5 x 1.2m14 x 5.5 x 1.2m
17011 x 10 x 1.2m15 x 6 x 1.2m
18011 x 11 x 1.2m15 x 7 x 1.2m
19012 x 11 x 1.2m16 x 7.5 x 1.2m
20012 x 12.5 x 1.2m17 x 7.5 x 1.2m

 


Medium Risk Sites(not in flood zone 2 and 3 or CDA)

Medium risk sites are not within flood zone 2 and 3 or CDA but may not have sufficient space available to provide an indicative drainage solution, or the site is steeper than 1:10. Discharging to a water course or public sewer could be offered as an 'in principle' scheme. Any discharge to a public sewer will also require written permission form SWW.

A drainage assessment will be required to identify a practical drainage solution and give reasons why that particular option has been selected. In accordance with Development Policy DP4 and where space permits, SuDS should be considered which address water quality and offer low maintenance for the life of the development.

Drainage assessment - 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 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 watercourse, 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.

Requirements of a Drainage Assessment
(Small Scale Minor Developments; 1-2 units)

  1. 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
  2. Infiltration or attenuation designed to contain the critical site design storm 1:100 year + an allowance for climate change (currently 40%)
  3. Calculations showing the drain down time is less than 24hours
  4. Indication of overland flow routes and safeguarding of properties from flooding
  5. Evidence that the peak runoff post-development is no worse than previous
  6. Confirmation of land ownership of all land required for drainage
  7. Take account of surface water/groundwater entering the development from adjacent land
  8. Proposals, where relevant, for integrating the drainage system into the landscape or required publicly accessible open space and providing habitat and social enhancement
  9. Foul drainage proposals.

Points to consider

  • 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 - Medium Risk
Documents RequiredPoints CoveredEvidence
Surface water drainage assessment

Infiltration testing and soakaway design

OR

'In Principle Scheme'

Testing to DG 365 and designed for 1:100 year return period + allowance for climate change

Discharge to a watercourse or written permission from SWW to discharge to a public sewer

Foul drainage assessmentPrivate or public systemFDA1 form for a private system or written confirmation from SWW to connect to the public sewer

High Risk Sites (in flood zone 2 or 3 or CDA)

High risk sites are located within flood zone 2 or 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 and 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 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 46 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 2 or 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 FRA for proposals 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. 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.

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%).

Minimum Requirements for Drainage Assessment
(Small Scale Minor; 1-2 Units)

  1. 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.
  2. Infiltration or attenuation designed to contain the critical site design storm 1:100 year + 40% climate change
  3. Calculations showing the drain down time is less than 24hours
  4. Indication of overland flow routes and safeguarding of properties from flooding
  5. Calculations showing the pre- and post- development peak runoff flow rate for the critical rainfall event
  6. Confirmation of land ownership of all land required for drainage
  7. Proposals, where relevant, for integrating the drainage system into the landscape or required publicly accessible open space and providing habitat and social enhancement
  8. 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

Infiltration testing and soakaway design

OR

'In Principle Scheme'

Testing to DG 365 and SuDS designed for 1:100 year return period + allowance for climate change

Discharge to a watercourse or written permission from SWW to discharge to a public sewer

Foul drainage assessmentPrivate or public systemFDA1 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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