Dorset Council: Groundwater Drainage Guidance

When a Surface Water Drainage Strategy is Required

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
• 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.

Dorset’s 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 Dorset’s requirements for testing and monitoring can be found on the council’s website.

Understanding Dorset’s Requirements

An important nuance to note in Dorset’s guidance is that in addition to ‘major developments’, ‘non-major developments’ are subject to the same advice found in their ‘Surface Water Management Schemes’ document. Therefore, no matter the scale of your project, it is important to understand the council’s requirements.

Understanding what is required from a Surface Water Drainage Strategy, such as infiltration testing or groundwater monitoring is essential for developers, as missing the all important winter monitoring window (the start of November and the end of May) can massively affect your project timescales and may mean having to wait till the next year’s monitoring window opens again.

Dorset & Groundwater

Dorset Council places special importance on the prevalence of groundwater throughout the county. For prospective developers, 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.”