OR/17/031 Technical information

Bee, E J, Pennington, C V L, Dashwood, C and Lee, K. 2017. A User Guide for the GeoSure Extra: Debris Flow Susceptibility Model for Great Britain (version 6.0) . British Geological Survey Open Report, OR/17/031.

Definitions

• Hazard: A potentially damaging event or phenomenon.
• Risk: The impact of the hazard on people, property or capital.

For example, a debris flow could be perceived as a hazard, but the likelihood of it causing structural damage would be the risk.

A high hazard does not necessarily translate to a high risk. For example, if a particular location has a relatively high ground stability hazard, but the infrastructure routes that are built there have taken this into account, and are designed to withstand the hazard, they will not have a comparable level of risk. This is because the likelihood of the hazard causing any loss has been reduced due to the design of the property.

The Debris Flow Susceptibility Model does not identify the cost of a hazard being realised or the exposure of assets or people, and therefore does not consider risk. The Debris Flow Susceptibility Model only examines the conditions that leave an area predisposed to a debris flow occurring.

BGS imperative

BGS GeoSure is a set of six national ground-stability layers developed in vector GIS format at 1:50 000 scale. The six layers include Compressible Ground, Collapsible Ground, Landslides, Running Sands, Soluble Rocks and Shrink–Swell. GeoSure aids decision makers, responds to the legislation and provides income to BGS via data licensing and GeoReport sales. The Debris Flow Susceptibility Model V6.0* is an additional dimension to the GeoSure Landslides surface layer and is designed for users specifically interested in debris flow potential.

[*NB: Version 6 refers to the version of BGS DiGMap used to create the bedrock and superficial permeability datasets used in its creation. This is to be consistent with the way that version numbers have been allocated to other BGS data products derived from DiGMap].

Scale

The Debris Flow Susceptibility Model Great Britain (Version 6.0) dataset is produced for use at 1:50 000 scale providing 50 m ground resolution.

Attribute table field descriptors

Table 1 Attribute table field descriptions

Field name	Field description
VALUE	An automatically generated number (1–5) to represent each discrete category in the dataset.
COUNT	The number of cells within the associated [Value] field.
LEGEND	Classification of hazard on a scale of A–E
SHORT_DESC	Description of the hazard
VERSION	Dataset name and version number (DFSM_GB_V6.0)

Creation of the dataset

The Debris Flow Susceptibility Model GIS layer is rated on an A–E classification, representing increasing hazard (Table 2).

Table 2 Debris Flow Susceptibility Model legend

Legend	SHORT_DESC	Longer description/Interpretation (not included in spatial layer)
A	Debris flows are not though to occur.	Debris flows are not thought to occur. This is due to a lack of available slope materials, high drainage rates or low slope angle.
B	Debris flows are not likely to occur.	Debris flows are not likely to occur. This is either due to a limited availability slope materials, sufficient drainage rates or low slope angles.
C	Debris flows may be present or anticipated.	Debris flows may be present or anticipated. The combinations of increasing slope angle, poor drainage condition and the presence of available material may increase the potential for failures to occur.
D	Debris flows are probably present or have occurred in the past.	Debris flows are probably present or have occurred in the past. The combinations of steep slopes, poor drainage conditions and an increased presence of available material suggest that debris flows are likely to be present at these sites.
E	Debris flows are highly likely to be present.	Debris flows are highly likely to be present. The heightened combinations of steep slopes, poor drainage conditions and the presence of available material suggest that debris flows are highly likely to be present at these sites.

To produce the Debris Flow Susceptibility Model layer, an assessment of hazard is made by:

• identifying the factors that are involved in creating the hazard
• assessing which are thought to be present at each location
• assessing how significant they are thought to be at each location

The factors are then combined to estimate the level of hazard. The level of potential hazard does not mean that a damaging debris flow event is going to happen but is an indication of how many causative factors may be present and how severe they are thought to be. The Debris Flow Susceptibility Model is created from a stack of grids that represent the following factors:

• the type of material (regolith) available to flow
• the gradient of the slope
• how free-draining the slope is
• influence of stream channels
• Quaternary history

Coverage

The Debris Flow Susceptibility Model covers Great Britain (Figure 8). This does not include the Isle of Man, the Channel Islands or Northern Ireland.

Figure 8 Extent of coverage of the Debris Flow Susceptibility Model. Contains Ordnance Survey data © Crown Copyright and database rights 2017.

Data format

The Debris Flow Susceptibility Model dataset has been created as an ESRI GRID raster file. The data can also be made available in ASCII or other formats on request, subject to the limitations and availability of translational software.

Limitations

• The Debris Flow Susceptibility Model has been developed at 1:50 000 scale and must not be used at larger scales.
• All spatial searches against the data should be done with a minimum 50 m buffer.
• The Debris Flow Susceptibility Model is concerned with potential ground stability related to NATURAL geological conditions only. This does not, therefore, include man made slopes such as embankments or cuttings.
• The Debris Flow Susceptibility Model is based on, and limited to, an interpretation of the records in the possession of the British Geological Survey at the time the dataset was created.
• An indication of natural ground instability does not necessarily mean that a slope will be affected by ground movement or subsidence. Such an assessment can only be made by inspection of the area by a qualified professional.
• The level of potential hazard does not mean that a damaging debris flow event is going to happen but is an indication of how many causative factors may be present and how severe they are thought to be.
• The words provided in the Table 5 are designed to provide a general indication of the meaning of the various Debris Flow Susceptibility Model hazards levels. If the data are to be used for advising specific sectors of end users in detail, e.g. home-buying, property insurance, site development and construction, then the BGS can provide additional end user guides and attribution details for the data. To find more about this, please contact our Business Solutions department through the Central Enquiries Desk using the contact details at the start of this document.
• A slope model was derived directly from the NEXTMapTM digital terrain model of Britain. The NEXTMapTM dataset was generated from an airborne survey at 5 m resolution. It is recognised that the NEXTMapTM elevation model does not always accurately represent the ground surface and produces erroneous elevation and slope data in some locations. This occurs because of the oblique way in which NEXTMapTM data are collected. Examples of this include the coast, verges of dense stands of trees and large structures such as warehouses or extensive stretches of seawall. There are artefacts of this problem found in this version of the debris flow model and values are therefore likely to be over-exaggerated in these areas.
• There are several areas within mainland GB which have values of ‘No Data’. These generally occur where there are inland bodies of water such as lakes. Where the underlying geological mapping within the Soil-Parent Material V6 data shows these areas to be ‘NA’, it was not possible to assign a lithology score and the cell is therefore recorded as ‘no data’. In some areas, which are coincident with water bodies, the geological map has attributed the underlying surface geology with a lithology. In such cases, a score was possible and as such, the cell was not assigned with a score rather than a ‘no data’ value.
• This model is limited to areas where debris flows are initiated, as well as parts of the debris flow track. It does not indicate where the material involved in the failure will flow.
• The model does not include any influence of land cover on the stability of a slope. The stabilising influence of certain types of vegetation is well documented in the literature, however the data available at the national-scale does not reflect vegetation-type updated on a regular basis. Therefore, any changes to vegetation (e.g. a felled forest would have a destabilising effect on the slope) would not be captured until the next release of data enabling incorrect assumptions to be made about debris flow potential.
• The Debris Flow Susceptibility Model is limited to the latest version of the Soil Parent Material Dataset. Version 6.0 will therefore be subject to future updates within the BGS data products delivery schedule.