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

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, 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. 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 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 that increase the impermeable area by more than 15m² have to acknowledge whether there is a flood risk to the development or not. This includes:

  • Roof extensions
  • New Garages
  • Bitmac driveways 
  • Areas of hard paving

Sites that are within a Flood Zones 2 and 3 or a Critical Drainage Area (CDA) are higher risk and will require compliance with Environment Agency standing advice or a Flood risk Assessment (FRA) to ensure the risk can be managed to an appropriate level.


Low Risk Sites (not in Flood Zones 2 and 3 or a CDA)

Low risk sites are not within flood zones 2 and 3 or a CDA and have sufficient space available to provide the worst case drainage solution. For Householder applications this applies to development that increases the impermeable area by more than 15m²

Providing the site can meet the set criteria below then no further information will be required.

  • 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: Sites that are steeper than 1:10 are likely to be too steep for the use of soakaways and will require testing and design by a suitably qualified professional if that that is the only option. The soakaway size given in Table 1 is for a crated system based on an indicative infiltration rate and should only be used for planning purposes. This in no way relieves the applicant from their responsibilities and duties to prevent flooding of third party property.

Table 1 - Indicative Soakaway Size

Impermeable area
Roof & hard surfacing (m²)
Crate Soakaway
(L x W x D)
Alternate crate size
(L x W x D)
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


Validation Checklist - Low Risk

Documents RequiredPoints CoveredEvidence
Impermeable area greater than 15m2
Surface water drainage assessment
  1. Site gradient < 1:10
  2. Flood history
  3. Indicative soakaway

1. Levels on a plan or photographic evidence

2. Statement in assessment

3. Scale plan with position of soakaway including all required clearances for Building control.

If additional bedrooms or bathrooms are proposed
Foul drainage assessment

Private System

OR

Public System

FDA1 form for private system

 

Written confirmation from SWW to connect to the public sewer


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

Medium risk sites are not within flood zones 2 and 3 or a CDA but may not have sufficient space available to provide the worst case drainage solution, or the site is steeper than 1:10 which means that soakaways might not be suitable for the site. Discharging to a watercourse or public sewer could be offered as an 'in principle' scheme to obtain a conditional approval. Any discharge to a public sewer will require written permission from SWW.

For Householder applications that increase the impermeable area by more than 15m² a drainage assessment will be required to identify a workable drainage solution. It will need to 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 (Householder Development)

  1. If soakaways other than those from table 1 are proposed then evidence of soil infiltration testing to DG 365 will need to be submitted to support the proposal. Tests should be at the location of any intended infiltration device and the proximity of the winter water table noted
  2. Infiltration or attenuation designed to contain the critical site design storm 1:100 year + an allowance for climate change (currently 40%). Calculations showing the half drain down time is less than 24 hours
  3. Discharge to a watercourse within the site boundary or written permission from SWW to discharge to the public sewer
  4. Indication of overland flow routes and safeguarding of properties from flooding
  5. Confirmation of land ownership of all land required for drainage

Validation Checklist - Medium Risk

Documents RequiredPoints CoveredEvidence
Impermeable area greater than 15m2
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

If additional bedrooms or bathrooms are proposed
Foul drainage assessment

Private System

OR

Public System

FDA1 form for private system

 

Written confirmation from SWW to connect to the public sewer


High Risk Sites (in flood zones 2 and 3 or a CDA)

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

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 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 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%)

Flood zone

In accordance with Para 103 and 104 of the NPPF all developments within Flood zone 2, or 3 then a Flood Risk Assessment (FRA) will need to be submitted. The sequential and exception tests are not required for Householder developments.

FRA

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.

It 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

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.

All application types are required to meet with CDA guidelines and most developments will be able to utilise soakaways and will be a low risk. These will be classed as CDA1 and the applicant will only need to prove that they can meet the minimum requirements. All other applications will be CDA2 and will need to provide a full drainage assessment.

CDA1:

It is acknowledged that small developments, which are away from a watercourse and have sufficient space for a soakaway in the garden, will pose a much lower risk. For sites that meet the following criteria a more practical approach will be taken if:

  1. Area of impermeable surfacing less than 15m² (including roof, parking areas etc.)
  2. Not in flood zone 2 or 3
  3. Plot size is adequate to fit a 1m³ Soakaway (approx. 8.5m long by 6m wide)
  4. Written permission from SWW to discharge to a storm water sewer has been granted in writing, if (3) cannot be achieved

This information will need to be evidenced and a scale plan submitted showing the soakaway 5m from buildings and the highway and 2.5m from other boundaries

CDA2:

All developments that are in flood zone 2 or 3, or have limited space for a soakaway solution or do not meet the requirements of CDA1 require a drainage assessment. If soakaways are proposed then infiltration testing to DG 365 will be required to support the drainage scheme.

If the infiltration rate is low or a suitably sized soakaway cannot be accommodated on site then permission must be sought to discharge into the watercourse or a public sewer. Any discharge must be at a controlled rate and if discharging to public sewer then written permission from SWW will be required. Design requirements are:

  • Attenuation/storage for 1:100 + 40% return period.
  • Discharge limited to 1:10 year green field runoff rate.

Checklist for CDA1

1.1 Exempt

  1. Is the planned roof size or area of impermeable surfacing less than 15m²?
  2. Is the application site outside of Flood zone 2 or 3?
  3. Can a soakaway of 1mᶟ be accommodated on site (8.5m long by 6m wide)?
  4. Do you have written permission from SWW to discharge to a public sewer if (3) cannot be achieved?

If the answer to 1, 2, and 3 or 1, 2  and  4 is Yes, then please provide a scale plan to 1:50 showing:

  • Proposed layout, including the size of extension
  • Location of soakaway (5m from buildings/highway and 2.5m from boundaries)
  • Dimensions of key points

If the answer to any of the above is No then proceed to the next section.

Checklist for CDA2

2.1 To soakaway

Please provide:

  1. Soakaway / infiltration testing to DG 365. 
  2. Soakaways designed for 1:100 + 40%, including calculations. 
  3. 1:50 scale plan showing all plant and pipe routes

Note: If the ground is not suitable for soakaway / infiltration drainage or the water table is high then another solution will be required.

2.2 Attenuated discharge to a watercourse

Please provide

  1. Soakaway/Infiltration testing to DG 365 to prove infiltration is not practical
  2. Details of the storage system, including calculations, to make sure the scheme can provide storage for the 1:100 + 40% condition.
  3. Details of the flow control system to limit the discharge to the 1:10 year green field run off rate. (i.e. Orifice plate or hydro-brake)
  4. Scale plan showing all plant and pipe routes

2.3 Controlled discharge to public sewer

  1. Written permission from SWW 
  2. Details of any attenuation system and flow control to limit discharge to the green field runoff rate

All the details in one of the above sections will need to be submitted with the application to allow it to be validated. This will make sure a quick assessment of the application and reduce the possibility of delays during the decision process.

Validation Checklist - High Risk

Documents RequiredPoints CoveredEvidence
Impermeable area greater than 15m2
EA standing adviceFences or small structuresEvidence to show that the works will not affect the flood flow routes
FRAAll flood riskAssess risk to the development or EA Standing advice
 
Surface water drainage assessment

Infiltration testing and soakaway design

OR

'In principle scheme'

 

OR

CDA

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

 

CDA1 or CDA2 assessment

If additional bedrooms or bathrooms are proposed
Foul drainage assessment

Private System

OR

Public System

FDA1 form for private system

 

Written confirmation from SWW to connect to the public sewer

 

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

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