GeoSmart SD50 FAQs

Government policy across the UK is to introduce sustainable drainage systems (SuDS) to control site rainfall run-off via conditions in planning approvals. A range of SuDS options are available, but guidance states that infiltration into the ground, where feasible, is the preferred method for managing surface water run-off on site without increasing flood risk downstream. Regulations on SuDS vary locally, and should be verified with your local planning authority.

The GeoSmart SuDS Infiltration Suitability Map should be consulted as part of the site development and design process or when considering land purchase.

The SuDS map supports the following requirements:

• Validating planning applications at Pre-Planning Stage
• Planning Conditions
• Building and flood mitigation design
• Due diligence
• Code for Sustainable Homes
• SuDS approval boards (SABs)
• Lending, mortgages, and pension funds

The map provides an initial assessment of the feasibility of infiltration SuDS at a site and recommendations on site investigations to support Infiltration system design, where relevant. It is intended to support property professionals at an early stage in the development planning process in understanding the SuDS options available at a site and the risks and costs associated with them.

A sustainable drainage system (SuDS) is designed to replicate, as closely as possible, the natural drainage from the Site (before development) to ensure that the flood risk downstream of the Site does not increase as a result of the land being developed. SuDS can also significantly improve the quality of water leaving the Site and can enhance the amenity and biodiversity that a site has to offer.

There are a range of SuDS options available to provide effective surface water management that intercept and store excess run-off. When considering these options the preferred destination of the run off should be assessed using the order of preference outlined in the Building Regulations Part H document (HM Government, 2010) and DEFRA’s Draft National Standards for SuDS (2011):

1. Discharge to the ground
2. Discharge to a surface water body
3. Discharge to a surface water sewer
4. Discharge to a local highway drain
5. Discharge to a combined sewer

For general information on SuDS see www.susdrain.org.

• House builders and developers
• Property professionals such as architects, planning consultants and land agents
• Businesses
• Private individuals
• Insurance brokers and lenders
• Local Authorities and regulators

The GeoSmart SuDS Infiltration Suitability Map has full coverage over Great Britain with a resolution of 50m.

The GeoSmart SuDS Infiltration Suitability Map map consists of an Infiltration Potential Map which represents three categories, Low, Moderate, High, which indicate the potential for successfully implementing an infiltration system at a given location. The mapped infiltration potential is based on data related to the geology of underlying superficial deposits and bedrock, their permeability, the thickness of the superficial deposits, where they exist, and high groundwater levels.

• Low Potential: There is a low potential for infiltration SuDS in parts of the Site. It is likely that the underlying geology at the Site, or in areas of the Site, is relatively impermeable which would limit the effectiveness of a proposed infiltration SuDS scheme
• Moderate Potential: Infiltration SuDS are potentially suitable subject to further field investigation. One or more of the following three factors may have led to the moderate Potential classification for a given cell: a) the permeability of the receiving formation can vary over a large range, and field investigation is necessary to evaluate whether the infiltration capacity at that location will be sufficient for the successful deployment of an Infiltration SuDS system. b) Superficial Deposits at that location may be thin and a significant difference in permeability is expected between bedrock and Superficial deposits. For these locations, the thickness of superficial deposits needs to be verified and the design of the Infiltration system must take into consideration a potentially significant change in permeability or infiltration capacity over the depth of the system. c) The risk of a shallow winter-time water table and groundwater flooding in cases of extreme elevated groundwater levels is significant. For these locations, the height of the seasonal water table needs to be verified based on field data to ensure that it remains below the base of the Infiltration system. Furthermore, a detailed, site-specific assessment of groundwater flood risk, and the functioning of the soakaway under such conditions is recommended
• High Potential: Infiltration SuDS are likely to be suitable due to the high permeability of underlying formation. Where infiltration is into permeable superficial deposits overlying potentially impermeable bedrock, the thickness of superficial deposits seems sufficient for standard soakaway design. Groundwater flood risk is low or negligible. Still we would advise field verification of permeability and seasonal water table

For sites less than 1 ha, a close-up of the Infiltration Potential map for the site can be obtained via the SuDSmart Standard, Plus and Pro Reports, along with site analyses and recommendations related to site investigations to inform infiltration system design and additional supporting information. For larger sites, the GeoSmart SuDS Infiltration Suitability Map can be obtained for any area boundary. The map is also available for purchase at county or national scale.

What will the GeoSmart Sustainable Drainage Map tell me about my property?
The map will tell you whether infiltration is likely to be a feasible solution for managing rainfall runoff at your site. Where relevant, it will also provide information that supports the initial conceptual design of an infiltration system and recommendations on field investigations required to support the detailed design of such a system. Whether infiltration techniques are feasible or not will determine the suitability of other SuDS schemes going forward. The map can therefore provide information early on in the property development process related to the range of SuDS options available to a site and expected costs of implementing SuDS at a site.

The GeoSmart SuDS Infiltration Suitability Map incorporate various national scale datasets including 1:50,000 scale Geological Maps produced by the British Geological Survey and Permeability Data and Superficial Deposit Thickness. GeoSmart’s SuDS Infiltration Suitability Map also uses information from GeoSmart’s Groundwater Flood Risk Map, which is based on national scale datasets of topography and groundwater levels, in addition to bespoke in-house hydrogeological and risk models.

The map is updated on a 6-month cycle to take advantage of newly released data as soon as it comes out

The map has been through a rigorous internal QA process. At every update cycle, it is reviewed against a national database of site-specific SuDS assessments to verify its validity and identify any required model improvements to be incorporated into the next updated release.