Groundwater Flooding Guidance Update for London Boroughs
There has recently been an update to London Borough of Richmond’s Basement Impact Assessment (BIA) Guidance (March 2021), with Richmond now requesting more detail from developers on groundwater flooding at their Site. In this post, GeoSmart review the guidance and advise what can be done to meet the requirements.
If you apply for planning permission in Richmond, you may now receive a request from the Local Authority for a ‘groundwater assessment’ or similar. Other London based Local Authorities also appear to be following suit, including Hackney and Brent.
A large part of the Richmond borough is at risk of groundwater flooding to some degree – Richmond seems to specifically be focused on Sites in areas with a greater than 25% risk of groundwater flooding, or in Throughflow Catchment Areas. The ‘Throughflow Catchment Areas’, are defined in the recent investigation report ‘Further Groundwater Investigations’ (March 2021).
The Local Authority has made recommendations for basement developments and major developments which are within the extent of the areas potentially impacted by groundwater flooding. These recommendations have been incorporated into their recent Strategic Flood Risk Assessment (SFRA), which requires a screening assessment to be carried out as part of the planning application submission for all basement/cellar proposals and major applications within the throughflow and groundwater policy zones.
How can I tell if I need such an assessment?
If you’ve applied for planning permission, chances are the Local Authority would have flagged a requirement for such an assessment if your Site is in such a risk area.
If your development is at the pre-planning stage you can check whether your site is within an area requiring an assessment by consulting the mapping on the Richmond Council website.
What should be done to satisfy the Local Authority’s requirements?
Where there is such a risk of groundwater flooding, the planning applicant should address this by providing a Screening Assessment (as a minimum) that either confirms low risk of impacts (and therefore no further work being required) or advises the level of impact and the associated mitigation actions proposed.
Further detail is then required if you are proposing a basement development.
Who can produce the assessments?
Any studies and associated assessments should be prepared by a chartered professional or specialist, such as a GeoSmart environmental consultant.
What is included in such an assessment?
The following is included as part of a GeoSmart screening assessment:
- Findings from Step 1 of the Richmond screening assessment (noting that we would have already identified the requirement for a screening assessment if the Site is located within a relevant throughflow and groundwater policy zone);
- Completion of Step 2 of the Richmond screening assessment guidance (excluding Land Stability). Where the answer to any of the questions is ‘No’ then supporting evidence will be collated and presented. Where the answer to any of the questions ‘Yes’ or ‘unknown’ then an outline scope of additional work required will be presented. In order to complete Step 2 we will:
- Review readily available datasets including hydrology, geology, hydrogeology and OS mapping of the Site;
- Review available British Geological Survey Borehole logs and site investigation reports for the Site and immediate vicinity; and
- Collate details of the proposed scheme and Site layout in the context of the local water environment.
- Step 3 of the Richmond screening assessment will be completed, the Verification Form will be signed off by suitably qualified professionals (a Chartered Geologist (C.Geol) and a member of the Chartered Institution of Water and Environmental Management (CIWEM)) and;
- Presentation of a concise technical report that will outline the hydrological and hydrogeological conditions with respect to construction at the Site. This will include our interpretation of groundwater and surface water impacts in relation to the proposed development.
What is required to complete the assessment?
The following information is required to complete the assessment:
- Full details and drawings of the proposed development including elevations, plans and sections;
- Accurate elevation data to establish the Site elevation and below ground structure depth in relation to groundwater and the nearby surface water features. We can obtain this data on your behalf for an additional fee;
- Proposed and existing areas to be occupied by the development and below ground structures (m2);
- Available site investigation data (borehole/trial pit logs, groundwater monitoring data);
- Details of any adjacent or nearby existing or proposed basements; and
- Details of proposed construction methodology including foundations (depth and type).
How can I enquire about groundwater screening assessments?