Identifying site suitability for sustainable drainage systems (SuDS)
SuDS map and summary
Climate change is compelling local authorities to think smarter about sustainable property development. Reconciling the pressure for housing with better environmental management is bringing forward new guidance on improved site drainage and flood mitigation.
Our unique SuDS Infiltration Suitability Map reveals the locations best suited to sustainable drainage to meet new compliance standards for planning submissions.
Government policy for England is to introduce SuDS to control site rainfall runoff via conditions in planning approvals. Infiltration into the ground is the preferred method for managing surface water runoff on site without increasing flood risk downstream.
Property professionals must assess the feasibility of infiltration SuDS at an early stage in support of the site development process. This will allow outline site assessment of the cost implications and initial master planning of the development prior to the requirement for detailed design.
Lead Local Flood Authorities also need clear guidance on the site suitability for infiltration SuDS when they are assessing proposed developments.
To discuss the SuDS screening map and how it can improve your planning submission for your client, contact us today.
GeoSmart SD50 provides an assessment of the capacity of the ground to receive infiltration depending on the nature, thickness and permeability of the underlying material and the depth to the high groundwater table.
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Your frequently asked questions answered:
Whilst groundwater flooding is generally less hazardous to human health than surface flooding, it is more hazardous to property for a given flood depth, producing 2 to 4 times the damage to building fabric and greater disruption to economic activity due to the longer duration of flood events. Also, the impact may be less about surface water depths or velocities and more about the extended saturation of the shallow subsurface with the following consequences:
An initial screening review will be sufficient for most areas that are at negligible risk of groundwater flooding. Areas that may be at significant risk of groundwater flooding generally require a more detailed site-specific assessment.
The Groundwater Flood Risk Map allows users identify whether groundwater may be a source of flooding at their site. It is available at multiple scales and risk resolutions.
GW5: The Groundwater Flood Risk map is available at 5m resolution for Great Britain. It classifies groundwater flood risk in every cell on a 5m grid covering Great Britain into one of 4 risk categories: Negligible, Low, Moderate, and High.
GW200-S: The Groundwater Flood Risk map is available on a 200m grid covering Great Britain as a Screening Map. It classifies groundwater flood risk for each cell on that grid into one of two categories: ‘Potentially At Risk’ and ‘Negligible Risk’, based on the maximum risk occurrence within that cell as represented on the higher resolutions products, GW5.
The map can tell you whether groundwater flooding is likely to be a concern at a specific site, and will provide recommendations based on the level of groundwater flood risk modelled for that site.
However, it does not provide an alternative to a proper site-specific assessment, and a detailed risk assessment should be used for any site where the impact of groundwater flooding would have significant adverse consequences.
The map classification shows on a national mapping scale the areas within which property may be at risk, but this should not be mistaken to mean that groundwater floods will occur across the whole of the groundwater flood risk zones.
Mapping limitations and a number of local factors may reduce groundwater flood risk to land and property even where it lies within mapped groundwater flood risk zones.
Bespoke in-house hydrogeological and risk models were used to process this data and produce the Groundwater Flood Risk Map. A national database of recorded groundwater flooding events collated by GeoSmart from various sources is used to calibrate and validate the Groundwater Flood Risk Map.
Severity relates to our expectations of the amount of property damage or other harm that groundwater flooding at that location might cause.
However, there are a number of reasons why the national map may be indicating risk where there is no actual risk at a local scale (mapping limitations), and also a number of local factors that may protect land and property even where it lies within confirmed flood risk zones (property protection factors).
Some of the main reasons why the map may be indicating risk in areas where flooding may not actually occur:
Property Protection Factors
Some of the main reasons why property within areas at risk of flooding may avoid being flooded:
In summary, it is recommended that a more detailed risk assessment by a suitably qualified specialist should be undertaken for any sites that occur in mapped zones of groundwater flood risk where the impact of groundwater flooding would have significant adverse consequences.
Decisions such as whether to proceed with a property transaction should not be made on the basis of the map classification alone. Even in the event that groundwater flood risk is confirmed through site-specific assessment, potential risk mitigation measures can usually be identified which could enable the property transaction or development to proceed.
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, and has been reviewed by external experts. At every update cycle, it is reviewed against a national database of groundwater flooding events to verify its validity.
Other sources of flooding should be considered in this case, which are not represented on the Groundwater Flood Risk Map. These include fluvial and tidal flooding, surface water flooding, and flooding produced by springs.
This suggests an important hydrological connection between the river and the groundwater in this area. Due to the groundwater contribution, flooding in this area may occur more frequently and for longer durations than would be expected from fluvial flooding alone.
Bedrock Flooding: After extreme rainfall, groundwater levels, in places, rise to intercept the ground surface. Rises in groundwater level may also be superimposed upon above average groundwater levels in the aquifer prior to the rainfall. Water tables typically reach the surface first in (dry) river valleys, where the emerging water can drain away.
With increasing groundwater levels, the carrying capacity of the drainage channel is exceeded and a surface flood commences. This is also known as ‘clearwater’ flooding because of the relative clarity of groundwater compared to fluvial flood water.
Permeable Superficial Deposits Flooding: Where groundwater in permeable superficial deposits (PSD) is in good hydraulic contact with a river or the coast, flooding can occur during periods of high river stage or tide. This mechanism is particularly exacerbated when the PSD overlie low permeability strata.
If a depression is set back from the river, with a connection via PSD then this might be prone to groundwater flooding even if it is protected from overland fluvial flooding. In conditions of less extreme floods, groundwater flooding often occurs in flood plains due to high in-channel river levels, before the river overtops its bank, or after it has retreated back into its channel.
It is therefore often difficult to distinguish from river flooding. Effectively, the subsurface flow path results in more extensive, frequent, and prolonged inundation of flood plains that occur on permeable superficial deposits connected to rivers.
Other mechanisms of groundwater flooding that are not represented on the Groundwater Flood Risk Map are: