Sustainable Drainage Systems (SuDS): guidance for developers

Last updated: 12 December 2022

What to provide as part of an outline planning application

Outline planning application

You must provide a surface water drainage strategy.

You may have to provide a flood risk assessment.

The list is not exhaustive, the Lead Local Flood Authority may request additional information depending on site-specific conditions.

Flood risk assessment

You will need to submit a site-specific flood risk assessment if the criteria in Footnote 55 of the NPPF are met.

In a flood risk assessment you will need to:

a. Include surface water, groundwater, fluvial, reservoir, sewer and any artificial waterbodies (e.g. canals).

b. Consider how flood risk may change in the future such as climate change.

c. Apply Sequential Test and Exception Test (if criteria met).

d. Demonstrate a sequential approach to locating a development.

e. Consider mitigation measures.

Surface water drainage strategy

Find out if you need to submit a surface water drainage strategy and how to do it.

In order to submit a surface water drainage strategy for a site, you will have to consider a number of things.

Existing drainage arrangements

When submitting a surface water drainage strategy, you will need to:

a. Detail how the site currently drains.

b. Carry out a topographical survey to understand any sub-catchments on site and any existing drainage infrastructure.

Indicative surface water drainage layout

When submitting a surface water drainage strategy, you will need to provide details such as:

a. Location of drainage components.

b. Storage volumes.

c. Outfall location.

d. Exceedance routes.

e. Topographical survey details.

Method of surface water disposal

When submitting a surface water drainage strategy, you will need to consider how you will dispose of the surface water.

If you are planning to use the method of infiltration, you will need to provide:

a. Site specific infiltration rate testing in accordance with BRE 365.

b. Groundwater monitoring over the winter period (from the beginning of November to the end of April) a minimum of once a month.

Ground investigations must also include:

c. Summary of ground conditions, soakage potential, constraints (e.g. stability issues), groundwater context.

d. Trial pit logs.

e. Observed data from infiltration rate tests.

f. Infiltration rate graphs.

g. Groundwater monitoring logs.

If you are planning to use the watercourse method, you will need to:

a. Provide details of the location of watercourse shown on an annotated map.

b. Provide confirmation of riparian right to connect or where it lies outside of the ownership boundary then in-principle third party permission.

c. Invert level of watercourse to demonstrate a gravity-fed connection is achievable.

d. Demonstrate downstream connectivity to wider watercourse network (incl. survey information).

Sustainable drainage

You will need to demonstrate that you have considered Sustainable drainage systems (SuDS) as part of your development by providing certain details such as:

1. How you plan to incorporate multifunctional sustainable drainage systems.
2. Clear justified reasons why certain SuDS components cannot be used .
3. Where cost is a reason for not including sustainable drainage systems, provide information to enable comparison with the lifetime costs of a conventional public sewer connection.

Calculations

You will need to include certain calculations such as:

a. Greenfield runoff rates for Qbar, 1-year, 30-year and 100-year return period.

b. Brownfield runoff rates for 1-year, 30-year and 100-year return period if required

c. Proposed discharge rate (this must be as close to the greenfield rate as possible).

d. Runoff volume calculation for existing and proposed scenarios 1 in 100 year return period.

e. Infiltration rates in accordance with BRE 365 methodology (no infiltration for the base of soakaways).

f. Half drain down times of infiltration based SuDS.

a. half drain down time for 1 in 30 year return period must be within 24 hours.

b. 1 in 100 year return period must be tested with a consecutive 10 in 100 year return period

g. Storage calculations to show 1 in 100 year return period plus climate change is safely contained on the site.

h. Climate change allowances are set to 35% for 1 in 30 year return period and 40% for 1 in 100 year return period.

i. Simulation for a submerged outfall where drainage system discharges to a watercourse.

Maintenance

You will need to provide details of how you plan to maintain the drainage systems such as:

a. Indicative plans for the maintenance of the proposed surface water drainage strategy including the type of activity and its frequency and who will be responsible for carrying out these tasks.

b. Consideration of maintenance buffers around SuDS components to allow for access for maintenance.