OR/16/010 What information does the dataset provide: Difference between revisions
m 1 revision imported |
|||
(No difference)
|
Latest revision as of 16:03, 23 June 2016
Dearden, R. 2016. User guide for the infiltration SuDS map: summary. British Geological Survey Internal Report, OR/16/010. |
Overview
This section describes the four summary maps and the data from which they are derived.
- Infiltration constraints summary layer focuses on the infiltration constraints summary layer.
- Drainage summary layer focuses on the drainage summary layer.
- Ground stability summary focuses on the ground stability summary layer.
- Groundwater protection summary layer focuses on the groundwater protection summary layer.
Each polygon has four key attributes; score, short description, detailed description, and advice. The score is used in the creation of the summary maps; a lower score reflects greater suitability and a higher score reflects lesser suitability.
Infiltration constraints summary layer
The infiltration constraints summary layer highlights areas where there is potential for a hazard if water is infiltrated to the ground. In such areas, infiltration SuDS should only be installed if the potential for, or the consequences of, the constraint are considered not to be significant. Table 2 shows the attribute used in this summary layer.
Score | Short description | Detailed description |
4* | Very significant constraints are indicated | There is a very significant potential for one or more geohazards associated with infiltration |
*_Polygons_with_the_highest_score_(4)_are shown independently in the ‘Infiltration Constraints Summary’ layer to provide an overview of where hazards may occur if water is infiltrated to the ground.
If an area is identified as having an infiltration constraint, there is significant potential for one of the following hazards to be present:
- Soluble rock hazards
Some types of ground contain layers of material that can dissolve in underground water. This can cause cavities to develop. Cavities created by dissolution of soluble rocks can collapse, resulting in subsidence of the land above. More commonly, changes in ground or surface water flow can flush away unconsolidated sediment, potentially leading to the collapse of overlying materials leading to subsidence at the surface. Infiltration may exacerbate this problem causing acute collapse around infiltration SuDS. - Landslide hazards
In deposits that are highly susceptible to landslide, the infiltration of water to the ground may decrease the strength of the deposit, resulting in slope instability. This may occur if water is infiltrated to the ground on or above the susceptible area. In such deposits, the installation of infiltration SuDS should proceed only following a full appraisal of the ground conditions. - Shallow mining hazards (non-coal)
In areas where current or past underground mining has resulted in cavities at shallow depths, the infiltration of water may destabilise material above or within a cavity potentially resulting in ground collapse in highly susceptible areas. In such areas, infiltration of water to the ground may initiate or exacerbate the instability of material leading to collapse around the infiltration system. This dataset only considers mining for commodities other than coal. For information regarding underground and opencast coal mining, please contact the Coal Authority, Mining Reports, 200 Lichfield Lane, Mansfield, Nottinghamshire, NG18 4RG; telephone 0845 762 6848 or at www.coal.gov.uk. - Made ground
Made ground, including ground that has been infilled or landscaped has an unknown composition and structure. Infiltration through such material may result in ground instability or in the remobilisation of contaminants from within that ground. - Persistently shallow groundwater
In areas where the water table is shallow either persistently or seasonally, the installation of infiltration SuDS requires further consideration. In such areas, potential rises in shallow groundwater as a result of infiltration, may result in the inundation of subsurface storage chambers, or possibly the emergence of groundwater at the ground surface (termed groundwater flooding).
This map does not state which hazard is present, this information is provided in the Infiltration SuDS Map: Detailed. For more information about the hazards above and the limitations of the dataset, see Dearden (2016)[1].
Drainage summary layer
The drainage summary layer provides an overview of the extent to which the ground will drain. Table 3 shows the four attributes used in this summary layer.
Score | Short description | Detailed description |
1 | Highly compatible for infiltration SuDS | The subsurface is likely to be suitable for free-draining infiltration SuDS. |
2 | Probably compatible for infiltration SuDS | The subsurface is probably suitable for infiltration SuDs although the design may be influenced by the ground conditions. |
3 | Opportunities for bespoke infiltration SuDS | The subsurface is potentially suitable for infiltration SuDS although the design will be influenced by the ground conditions. |
4 | Very significant constraints are indicated | There is a very significant potential for one or more geohazards associated with infiltration. |
The drainage summary layer is derived from the following datasets:
- Infiltration constraints summary layer
- Superficial deposit permeability
- Superficial deposit thickness
- Bedrock permeability
- Depth to water level
- Geological indicators of flooding
If a site scores a value of 1, the subsurface is likely to be highly permeable, with a deep water table and not underlain by floodplain deposits that may respond rapidly to changes in river levels. In this environment, the installation of infiltration SuDS is likely to be straightforward. Sites that score a value of 2 may be characterised by a spatially variable permeability or a water table that may be within 1 m of the base of the infiltration system, or both. These areas are probably compatible for infiltration SuDS, but the system design may be influenced by the local ground conditions. Sites that score a value of 3 may be poorly draining, or have a shallow water table, or are located on floodplain deposits, or have some combination of these characters. In these areas, the subsurface may potentially be suitable for infiltration SuDS, but the design will be strongly dependent on the local ground conditions. Finally, sites that score a value of 4 have a constraint that needs investigation to determine whether the potential for, or the consequences of the constraint are likely to be significant.
The dataset does not state the properties of the subsurface, this information is provided in the Infiltration SuDS Map: Detailed.
Ground stability summary
The ground stability summary layer provides an overview of the potential for ground instability as a result of infiltration. The summary layer is derived from BGS’ GeoSure datasets. Table 4 shows the four attributes used in this summary layer.
Score | Short definition | Detailed definition |
1 | Geohazard unlikely | Increased infiltration is very unlikely to result in ground instability. |
2 | Potential for geohazard | Ground instability problems may be present or anticipated. Increased infiltration is unlikely to result in ground instability. |
3 | Significant potential for geohazard | Ground instability problems are probably present. Increased infiltration may result in ground instability. |
4 | Very significant constraints are indicated | There is a very significant potential for one or more geohazards associated with infiltration. |
Geohazards considered include:
- Soluble rocks
As discussed in Infiltration constraints summary layer, infiltration may result in ground collapse where the geological deposits are susceptible to dissolution. - Landslide hazards
As discussed in Infiltration constraints summary layer, the infiltration of water into deposits that are susceptible to landslide, may decrease the ‘strength’ of the deposit, resulting in slope instability. - Compressible ground
When the ground is compressed by a building or other load, water in the pore space can be squeezed out, causing the ground to compress. If water is added to the ground through an infiltration system, the compressibility may alter, possibly initiating settlement. - Swelling clays
Clays susceptible to shrink and swell, can change volume significantly according to how much water they contain. If water is added to the ground through an infiltration system, susceptible clays may swell, possibly introducing differential uplift. - Running sands
Running sand conditions occur when loosely-packed sand, saturated with water, flows into an excavation or other type of void. This can lead to subsidence of the surrounding ground. Running sand is potentially hazardous during the installation of infiltration SuDS if a void is excavated. - Shallow mining hazards (non coal)
As discussed in Infiltration constraints summary layer, infiltration may destabilise material bridging above or within a mined cavity, potentially resulting in ground collapse. - Collapsible ground
Collapsible deposits can collapse when they have been loaded and then become saturated by water. If the ground below a building collapses it may cause the building to sink, tilt, crack or distort. Infiltration will result in an increase in water content, which may affect the strength of the ground.
The stability of an area with a score of 1 is not anticipated to be impacted as a result of infiltration. Areas with a score of 2 or 3 may need investigation prior to infiltration; however the hazards present should not prevent infiltration SuDS from being used. Finally, for areas with a score of 4, the potential for and consequences of the identified hazard should be fully appraised.
The dataset does not state which hazard is present, this information is provided in the Infiltration SuDS Map: Detailed .
Groundwater protection summary layer
The groundwater protection summary layer provides an overview of subsurface factors that may impact the planning and design of infiltration SuDS in respect of protecting groundwater quality. Table 5 shows the four attributes used in this summary layer.
Score | Short description | Detailed description |
1 | Low susceptibility | The groundwater is not expected to be especially vulnerable to contamination. Infiltration water should be free of contaminants. |
2 | Moderate susceptibility | The groundwater may be vulnerable to contamination. Infiltrating water should be free of contaminants. |
3 | Considerable susceptibility | The groundwater is likely to be vulnerable to contaminants. Infiltrating water should be free of contaminants. |
4 | Very significant constraints are indicated | Made ground is present at the surface. Infiltration may increase the possibility of remobilising pollutants. |
The summary layer is derived from the following datasets:
- Source protection zone dataset
The source protection zone dataset sourced from the Environment Agency, and Natural Resources Wales define zones around public water supply abstraction points where additional protection is required to safeguard drinking water quality. In these areas, activities that may affect the quality of the drinking water abstraction may be restricted. This data is included here to show where restrictions on the installation of infiltration SuDS may apply. - Predominant flow mechanism
Some geological deposits will attenuate surface water pollutants more than others. The predominant flow mechanism (intergranular flow, fracture flow or a mixture of both) in the unsaturated zone is one proxy for determining the extent to which attenuation is likely. - Made ground
As discussed in Infiltration constraints summary layer, infiltration through made ground may impact groundwater quality as such deposits can contain contaminants that may be remobilised from the unsaturated zone into the groundwater.
In consideration of the above three factors, lower scores indicate areas where groundwater is less susceptible to surface water contaminants and higher scores represent those areas that may be more susceptible. This groundwater quality assessment is limited to an assessment of the datasets listed above and does not constitute a full risk assessment, which will need to be undertaken. Importantly, it does not consider the presence of contaminated land.
The dataset does not state which factor may be important, this information is provided in the Infiltration SuDS Map: Detailed .
Data summary
The datasets from which the Infiltration SuDS Map: Summary is derived, are shown in Table 6.
Data layer | Layer ID | Original dataset | Dataset owner | Scale |
Infiltration constraints summary | ||||
Soluble rock constraints | SuDS_infiltrationconstraints_solublerocks_2016 | GeoSure v7 | BGS | 1:50 000 |
Landslide constraints | SuDS_infiltrationconstraints_landslides_2016 | GeoSure v7 | BGS | 1:50 000 |
Shallow groundwater constraints | SuDS_infiltrationconstraints_shallowGW_2016 | Groundwater Flooding Susceptibility v6.1 | BGS | 1:50 000 |
Made ground constraints | SuDS_infiltrationconstraints_madeground_2016 | DiGMapGB-50 v7 | BGS | 1:50 000 |
Shallow mining constraints | SuDS_infiltrationconstraints_shallowmining(non-coal)_2016 | Mining hazard (non-coal) GB v7 | BGS | 1:50 000 |
Drainage summary | ||||
Depth to water table | SuDS_drain_depthtowater_2016 | gwlevelgb (2016) | BGS | --- |
Superficial deposit thickness | SuDS_drain_superficialthickness_2016 | Basic superficial thickness model, version 2.1, GI_SDTM v2.1 | BGS | 1:50 000 |
Superficial deposit permeability | SuDS_drain_superficialpermeability_2016 | Permeability indices v7 | BGS | 1:50 000 |
Bedrock permeability | SuDS_drain_bedrockpermeability_2016 | Permeability indices v7 | BGS | 1:50 000 |
Floodplains | SuDS_drain_geologicalindicatorsofflooding_2016 | Geological indicators of flooding v6 | BGS | 1:50 000 |
Ground stability summary | ||||
Soluble rocks | SuDS_stability_solublerocks_2016 | GeoSure v7 | BGS | 1:50 000 |
Landslides | SuDS_stability_landslides_2016 | GeoSure v7 | BGS | 1:50 000 |
Compressible ground | SuDS_stability_compressibles_2016 | GeoSure v7 | BGS | 1:50 000 |
Swelling clay | SuDS_stability_swellingclay_2016 | GeoSure v7 | BGS | 1:50 000 |
Running sand | SuDS_stability_runningsand_2016 | GeoSure v7 | BGS | 1:50 000 |
Shallow mining | SuDS_stability_shallowmining_2016 | Mining hazard (non- coal) GB V7 | BGS | 1:50 000 |
Collapsible ground | SuDS_stability_collapsibles_2016 | GeoSure v7 | BGS | 1:50 000 |
Groundwater protection summary | ||||
Source protection zones | SuDS_waterquality_sourceprotectionzone_2016 | Downloaded 2015 | EA | -- |
Predominant flow mechanism | SuDS_waterquality_predominantflowmechanism_2016 | Permeability indices v7 | BGS | 1:50 000 |
Made ground | SuDS_waterquality_madeground_2016 | DiGMapGB-50 v7 | BGS | 1:50 000 |
Reference
- ↑ DEARDEN, R. 2016. User guide for the Infiltration SuDS Map: Detailed. British Geological Survey Open Report, OR/11/061.