Outline or Full Application- Replacement Dwelling or Change ......1 - Soakaways: Where new soakaways...

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Outline or Full Application- Replacement Dwelling or Change of Use (Development that doesn’t increase the impermeable area, or the replacement dwelling is of comparable size to the existing.)

Foul Drainage, Flood Risk and Surface Water Requirements

Introduction 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 (If the development provides more than 1 dwelling 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 [email protected]

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.


Link to page for FDA1 – Link to page for Link to Source Protection Zones

(Foul Drainage General Foul

assessment form) Bindng rules Link to Environmental permit

Flood Risk (all forms of flooding Inc. surface water) All applications have to acknowledge whether or not there is a flood risk to the development. 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.

Full and Outline Applications For lower risk small scale developments we will accept an “in principle” scheme, with the details

confirmed by condition or an informative. 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 a CDA) Low risk sites are not within flood zone 2 and 3 or a CDA and have sufficient space available to provide

a worst case drainage solution

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 2 (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.

Validation check list – Low Risk

Documents required Points covered Evidence

Surface Water Site gradient


Validation check list – Low Risk


Drainage assessment Flood history Statement in assessment

Indicative soakaway Scale plan with position of soakaway including

all required clearances for Building control.

Foul Drainage

Assessment

Private or public

system

FDA1 form for a private system or written

confirmation from SWW to connect to the

public sewer

Table 2 - Indicative Soakaway Size


Medium Risk Sites (not in flood zone 2 and 3 or a CDA) Medium risk sites are not within flood zone 2 and 3 or a 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 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.

Enquires to [email protected]


Requirements of a drainage assessment Replacement Dwelling or Change of Use

Validation check list – Medium Risk


Surface Water

Drainage assessment

Infiltration testing and

soakaway design

Testing to DG 365and designed for 1:100 year

return period + allowance for climate change

Or

‘In principle scheme’

Discharge to a water course or written

permission from SWW to discharge to a public

sewer

Foul Drainage

Assessment

Private or public

system



public sewer

High Risk Sites (in flood zone 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 – low 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

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 + an allowance for climate chan ge (currently 4 0 % )

3. C alculations 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. C onfirmation of land ownership of all land required for drainage

7. Take account of surface water/groundwater entering the development from adjacent land.

8. Foul drainage proposals.


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%)

Flood zone In accordance with Para 103 and 104 of the NPPF all developments within Flood zone 2, or 3 need to

be supported by a Flood Risk Assessment (FRA) will need to be submitted. The sequential and

exception tests are not required for Change of use 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 and, where possible,

reduces 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. See Example.

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 365will 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.

https://www.gov.uk/guidance/flood-risk-assessment-standing-advicehttps://www.gov.uk/guidance/flood-risk-assessment-standing-advicehttps://www.gov.uk/guidance/flood-risk-assessment-standing-advicehttps://www.gov.uk/guidance/flood-risk-assessment-standing-advicehttps://www.gov.uk/guidance/flood-risk-assessment-standing-advicehttps://www.gov.uk/guidance/flood-risk-assessment-standing-advicehttps://www.gov.uk/guidance/flood-risk-assessment-standing-advicehttps://www.gov.uk/guidance/flood-risk-assessment-standing-advicehttps://www.gov.uk/guidance/flood-risk-assessment-standing-advicehttps://www.gov.uk/guidance/flood-risk-assessment-standing-advice


Check List for CDA 1 1.1 Exempt Yes/No

.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, &3 or 1, 2 & 4 is Y es , then please provide a scale plan to 1:50 showing:

Proposed layout, including the size of extension

Location of soakaway (5 m 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.

Example plan

m 2.5

m 2.5

2.5 m

5 m Proposed

extension

6 m²

Soakaway 1m²

Scale 1:50

Existing

building


0

Check List for CDA 2 If the answer to any of the previous questions was no, then a full drainage assessment will be required.

Required

2.1 To Soakaway For

Please provide: validation

.1) Soakaway / infiltration testing to BRE digest 365.

.2) Soakaways designed for 1:100 + 40%, including calculations.

.3) 1:50 scale plan showing all plant and pipe routes.

Yes

Yes

Yes

If the ground is not suitable for soakaway / infiltration drainage or the water table is high then

another solution will be required.

Required

2.2 Attenuated discharge to a water course For


.1) Soakaway/Infiltration testing to DG 365to prove infiltration is not practical Yes

.2) Details of the storage system, including calculations, to make sure the scheme

can provide storage for the 1:100 + 40% condition. Yes

.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) Yes

.4) Scale plan showing all plant and pipe routes Yes

Required

2.3 Controlled discharge to public sewer For


.1) Written permission from SWW

.2) Details of any attenuation system and flow control to limit discharge to the

green field runoff rate

Yes

Yes


1

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 check list – High Risk


FRA All Flood risk Assess risk to the development or EA Standing

advice

Surface Water

Drainage assessment

Infiltration testing and

soakaway design

Testing to DG 365 and designed for 1:100 year

return period + allowance for climate change

Or

‘In principle scheme’

Discharge to a water course or written

permission from SWW to discharge to a public

sewer

Or

CDA CDA1 or CDA2 assessment

Foul Drainage

Assessment

Private or public

system



public sewer

Outline or Full Application- Replacement Dwelling or Change ......1 - Soakaways: Where new soakaways...

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