Standard 3 - Management of extreme rainfall and flooding

Unless it is explicitly stated otherwise, climate change allowances must be applied to all design storm events. The allowance must be the correct upper end climate change allowance for the lifetime of the development, as per the appropriate guidance published by the Environment Agency.

The SuDS Manual suggests that minimum discharge rates to avoid assessing the impact of discharge volumes is QBAR or 2l/s/ha. The national standards for SuDS suggest that QBAR or a higher rate of 3l/s/ha may be used for the same purpose.

We have followed the advice of the NSfS and have accepted this higher discharge rate, with immediate effect from publication on 19 June 2025.

The NSfS do not suggest a minimum discharge rate. However, the proposed rate must be considered against the specification of the flow control and balanced against the risk of blockage. Where half drain times are longer than 24 hours additional storage may be required. Each application is assessed on its own merit, however, we have approved applications with flow rates as low as 0.5l/s with anti-blockage measures.

Please see our detailed guidance note relating to previously developed sites and the change in guidance for these.

We will use the term “annual exceedance probability” (AEP) to describe each design event. This terminology is a more accurate representation than the previously used “1 in x year event”. The latter descriptions sometimes led to misconceptions regarding the likelihood or timing of a design event.

The AEP description is clearer that it represents the probability of that event occurring in any one year, over the lifetime of the development.

The most used AEPs are shown below with their equivalent “1 in x year” descriptors to avoid confusion.

• 0.1% AEP = 1 in 1000 year
• 0.5% AEP = 1 in 200 year
• 1% AEP = 1 in 100 year
• 3.33% AEP = 1 in 30 year
• 10% AEP = 1 in 10 year
• 50% AEP = 1 in 2 year (approximated for QBAR)

All infiltration designs must now half empty their runoff volume for the 3.3% AEP event within a maximum of 24 hours. This represents a stricter and requirement than that of The SuDS Manual and BRE DG 365, which allowed for the potential support of a design which only emptied half its runoff volume within 24 hours for the 10% AEP event.

Where the scale and layout of a development have been decided by a full or reserved matters planning application prior to 19 June 2025, we will expect the design to attempt to meet the requirements of the NSfS. However, if it can be demonstrated that this cannot be achieved and the site can empty half its volume within 24 hours for the 10% AEP event then a departure from the NSfS would be suggested. Additional storage may be required to demonstrate that the site has the resilience to drain subsequent storms.

The NSfS prescribe that greenfield runoff estimates shall be calculated using the FEH statistical method. This is the Environment Agency's recommended method for assessing greenfield runoff rates for small catchments.

We will accept greenfield runoff estimates using the IH124 method where the scale and layout of a development has been decided by a full or reserved matters planning application prior to 19 June 2025.

Where the IH124 method has been used on a supporting outline, reserved matters or full planning application, we have no objection to the designer using the FEH statistical method for the detailed design submission, even if this results in an increased discharge rate and reduced storage.

To demonstrate that the inputs are correct in the FEH statistical method, we expect the FEH 22 point data files to be submitted in accordance with clause 5.1.7 of the FEH Web Service terms of use. These must be submitted directly to land.drainage@arun.gov.uk rather than to the planning portal (via the planning officer), this is to ensure that the data remains confidential in accordance with those terms.

It is necessary to demonstrate that the critical storm duration, considering both summer and winter events, has been modelled.

There is no prescriptive national guidance to specify the range of storms that must be modelled, but we suggest that best practice represents durations of 15 minutes to 7 days (10080 minutes).

If the critical event is also the longest duration that has been modelled, we will request that the simulation is re-run with longer duration storms to demonstrate that the critical event has been captured.

Please see our detailed guidance note relating to SuDS in Flood Areas.

The onus is on the applicant to demonstrate the source of any areas of surface water flood risk. If it is believed that flooding would only be generated by runoff on the site itself, then the SuDS will be designed to address that surface water risk.

However, if topographic levels show that runoff can enter the site from elsewhere then this should be accounted for in the SuDS design and appropriate flow paths considered. It is not sufficient to simply claim that the surface water is generated on site only, this must be accurately demonstrated with an analysis of the topography of the area.

Vortex flow controls are generally preferred as these allow for a larger clear opening which significantly reduces the risk of blockage.

However, we will accept orifice sizes of less than 50mm where they are robustly protected from that risk. Upstream components that provide filtration through permeable granular material, membranes, or purpose made prefabricated filters can assist in this regard.

The use of small orifices, and the associated blockage risk mitigation should be detailed within the SuDS management and maintenance manual and considered an important element to regularly maintain and inspect.

It is expected that the submitted modelling to support the design includes analysis of the effects of a surcharged outfall where infiltration is not proposed. We will use this modelling as the basis of our assessment for the worst-case design events unless it can be accurately demonstrated that it is not required or likely.

The surcharge level should be based upon modelled flood levels. However, it is acknowledged that this data is often not available for ordinary watercourses and all piped networks.

For ordinary watercourses, the surcharge level is expected to be the top of the bank. This represents the level at which water is no longer fully contained within the watercourse and flooding would occur. The top of bank does not need to be adjacent to the outfall, but the onus is on the designer to justify the level that they have modelled and demonstrate that it is suitably conservative and realistic.

For piped networks the designer is expected to suggest a reasonable surcharge level. This may be based on existing modelling data, or where modelling is unavailable, engineering judgement is to be used with due consideration for the hydraulics of the existing network. The justification for the surcharge level should be submitted for approval.

The drainage design should show flow routes through the proposed development, demonstrating where surface water will be conveyed for three types of flow:

1. Low flow routes

These are designed regular flow pathways on the site. Examples of low flow routes include permeable pavements, swales, open channels and filter drains. Regular runoff should travel in low flow channels through the development in a controlled way contributing to landscape quality and biodiversity.

2. Overflows

In the event of local blockages or surcharge a simple overflow arrangement should allow water to bypass the obstruction and return to the management train sequence until conditions return to normal.

3. Exceedance routes

When SuDS are overwhelmed by exceptional rainfall, then exceedance routes are required to protect people and property. These provide unobstructed overland flow routes from the development and should be considered for all drainage schemes.  Exceedance routes should also be protected from future changes in land use.