In line with Dorset’s Validation Checklist, you’ll need to provide a Surface Water Drainage Strategy if;

• You are applying for a major development (where relevant to the development proposed);
• Your proposals seek to increase the footprint of an existing building, and/or the development of car parking or any other hard standing/impermeable surface that may increase flood risk downstream, and;
• If the application could be influenced by surface water or foul water drainage.

If you are subject to the provision of a Surface Water Drainage Strategy – Dorset Council requires that Infiltration Testing (BRE 365 Digest) and Groundwater Monitoring are conducted in order to determine what the most appropriate discharge location would be for surface water for both outline and full planning.

Dorsets guidance on Groundwater Monitoring states that (ideally) 12 months of readings should be taken with 2 readings per month taken. However, if there are time constraints, Dorset is willing to accept monitoring undertaken between the start of November and the end of May (Winter Monitoring). Further guidance on Dorsets requirements for testing and monitoring can be found on the councils website – but if you have any questions about the process, then Aegaea are here to help!

Dorset Council places special importance on the prevalence of groundwater throughout the county. The map below outlines those areas within the Ground Water Susceptibility to Flooding layer, as shown on Dorset Council’s ‘Dorset Explorer’ tool. 

Areas within the Ground Water Susceptibility to Flooding layer.

For prospective developers, this mapping, and understanding whether your site falls within one of the areas noted to be Susceptible to Groundwater Flooding will be relevant for several reasons, including informing the likely SuDS Drainage Strategy for your project.

If your site lies within an area which is Susceptible to Groundwater Flooding this could mean that water table levels are high at that location and therefore the use of infiltrating SuDS features such as soakaways or infiltration basins may not be appropriate. Dorset’s Guidance states that; ‘Groundwater monitoring should demonstrate 1m of clearance between the base of the infiltration device and the maximum groundwater level as per BRE Digest 365 and Ciria SuDS Manual.’ Therefore, proving this clearance is possible for areas where there are high water table levels could prove challenging – in such cases exploring alternative options such as discharging surface water to a waterbody or sewer may be necessary.