Flood Risk and Surface Water Drainage in Enfield

The latest NAFRA2 report states “Over one quarter of properties in areas at high flood risk from surface water flooding are in London – nearly a third of a million”

The London Borough of Enfield has several bespoke planning policies and numerous Critical Drainage Areas (CDAs) to navigate. Enfield can seem at first glance a problematic region to gain planning approval for your project. However, have no fear! Over the last 3 years, Aegaea has been involved with over 100 different projects minor through to major applications within the area, covering both flood risk and drainage.

In line with the Enfield Council’s Validation Checklist (updated in October 2024), you’ll need to provide a Flood Risk Assessment if the development involves any of the following constraints (this includes Minor Developments);

• Surface water flood risk (Or in a CDA)
• Flood Zone 2, 3a, 3b.
• Greater than 1ha
• Proposals including new buildings, basements, extensions to basements, extensions within 8m of a watercourse

Most if not all developments in Enfield are now required to demonstrate how surface water will be managed as part of the development.

To make sure that your development complies with Policy CP28 of the Enfield Core Strategy (2010), the relevant policies of the London Plan (SI12 and SI13), as well as the NPPF, must be demonstrated through relevant calculations and assessments that the disposal of surface water is by the means of a suitably designed sustainable drainage system (SUDS) – for all developments.

This means adhering to the London Plan Drainage Hierarchy – aiming to achieve greenfield (i.e. pre-development) runoff rates whilst assessing the following disposal solutions in order (note our previously published blog on the London Plan Drainage Hierarchy provides further information on the below):

1 store rainwater for later use

2 use infiltration techniques, such as porous surfaces in non-clay areas

3 attenuate rainwater in ponds or open water features for gradual release

4 attenuate rainwater by storing in tanks or sealed water features for gradual release

5 discharge rainwater direct to a watercourse

6 discharge rainwater to a surface water sewer/drain

7 discharge rainwater to the combined sewer.

Therefore as a minimum, the following information will likely  be required to supplement the surface water drainage strategy:

1. Infiltration testing (in accordance with BRE 365 digest). To provide the key evidence Enfield Council need to understand if there is potential for an infiltration-led solution. We work with contractors who can undertake this and are happy to source a quotation for the testing if desired during the project
2. Thames Water asset plans will need to be acquired in order to understand if there are any public sewerage assets within the vicinity, which could be utilised for discharge as a fall back option provided infiltration is not viable for your site. This will in most cases need to be supplemented by a CCTV survey to understand both the existing drainage network on-site, and existing invert levels to inform the proposed sustainable drainage design. Again, Aegaea are more than happy to source a suitable contractor to undertake this.
3. If a discharge to a public sewer is deemed the best disposal option in line with the hierarchy, a formal capacity check is usually required in order to confirm if Thames have space within their assets to support the development. Aegaea can undertake this when required to save you time!

Previously, In line with the Enfield Council’s Validation Checklist you’ll need to provide a Surface Water Drainage Strategy if the development involves any of the following;

• Major developments
• Commercial/community/industrial changes of use or new buildings or extensions to basements or ground level extensions
• New residential buildings or extensions to basements or ground level extensions (this is an important requirement, as not all Councils request Surface Water Drainage Strategies (SuDS) for extensions. 

Every submitted Surface Water Drainage Strategy for the developments listed above must have included a SuDS Proforma checklist to accompany the submission as standard. 

For the case of minor developments, there is some discrepancy in what the council requires in terms of a planning application. Previously, the level of detail required was similar to that outlined for the above applications, However, recent updates and correspondence with the Council at the end of 2024 have now indicated that minor developments (i.e. developments below 250m2 and all Householder developments) may only require their specific SuDS proforma to be completed, which should include:

1. Drainage (and other) Drawings showing the location of the proposed SuDS features
2. Specifications and cross sections of the proposed SuDS features
3. Management Plan for future maintenance of the proposed SuDS features

This means that although Enfield Council do require information on the proposed SuDS features for minor developments, it may not be as extensive as previously required, potentially saving on both time (and cost) in getting your development over the line!  Future conditions could be imposed to provide further detail.