3 Desk Study
In order to identify peat landslide hazard potential, all development sites should be subject to a front end desk-study, comprising two main components:
(i) Review of existing site information; and
(ii) Site reconnaissance to verify assertions made in the review.
This provides a cost-effective front-end means of identifying site factors conducive to peat instability and should act as a framework for specifying targeted site investigation in the event that significant indicators of instability are identified. The scope of a site investigation for clarification of peat landslide hazard may be considerably reduced if sufficient attention is given to this front-end activity. Refer to Figure 1.1.
3.2 Review of existing site information
The extent of the development site should be clearly identified at the outset of the study, and illustrated with adjacent land down-slope and up-slope of the site on maps and plans prepared in the desk study document. Once the site extent has been identified, appropriate efforts should be made to collect any and all relevant information relating to the site. The time spent in data collection and review should reflect the nature and scale of the investigation and the volume of information available for the site.
3.2.1 Sources of site information
Currently available sources of information to be considered may include, among others:
(a) Previous site information including technical reports, feasibility reports, previous ground investigation information and Envirocheck reports;
(b) Geological information, specifically the regional field guide relevant to the site in question;
(c) British Geological Survey publications on superficial deposits;
(d) Soil Survey of Scotland (Macaulay Institute) Soil Memoirs;
(e) Academic literature and publications about the site;
(f) Newspaper archives; and
(g) Local knowledge from landowners, farmers and local residents about the site.
a) to d) provide fundamental site information of relevance to not only peat landslide hazard assessments, but of value in later stages of geotechnical design if and when consent has been granted. These should be considered essential sources for the front end desk study. e) to g) provide additional information and are often particularly informative where a history of instability at a development site has already been recognised by landowners, researchers or has been recorded by the local press.
It should be noted that maps indicating peat cover should not be taken as definitive statements on its presence or absence. The depth and extent of peat deposits may vary sharply over short distances as a function of local underlying geology, past and ongoing geomorphological activity and management history. It is for this reason that desk-study must be informed by site reconnaissance survey, to ensure that existing information is robust and reliable.
3.2.2 Sources of mapping data
Several sources of mapping data may be of use in conducting peat landslide hazard assessments, some of these are summarised in Table 3.1 below.
Table 3.1 Recommended sources of mapping
Description and Purpose
Ordnance Survey Land-Line data or 1:10000 to 1:25000 digital raster tiles
To be used for base mapping in a Geographical Information System ( e.g.ESRI ArcView, MapInfo) and therefore preferably provided in a digital format
Landmark historical mapping data
To be overlain on existing Ordnance Survey mapping to identify long term changes in ground conditions at a site, and therefore preferably supplied in digital format
British Geological Survey geological maps and/or 1:10000 to 1:25000 digital raster tiles
To be overlain on existing Ordnance Survey mapping to identify extent and character of solid and drift geology, and in particular the location of impermeable rocks or till, preferably supplied in digital format..
Digital BGS data is required for Geographical information systems although paper 1:10,000 mapping may be more appropriate for a detailed desk study.
Macaulay Institute soils maps at 1:250000 scale
To be digitised from hard copy and overlain on existing Ordnance Survey mapping to indicate 'soil' characteristics, including: presence of peat and runoff potential.
3.2.3 Sourcing and interpretation of historical and contemporary aerial photographs
Ortho-rectified digital aerial photography for much of the UK is now available from GetMapping TM, sorties having been undertaken in 1999, 2000 and 2005. Unfortunately , coverage in Scotland is not complete, but where available provides good quality images of recent ground conditions and should be used as follows:
- Identify the presence of existing failure scars and debris runout;
- Identify pre-conditioning factors for failure (where visible at the resolution of the photography);
- Identify evidence of other pre-development ground conditions of relevance to ground works but not exclusively associated with landslides, including vegetation cover, drainage regime and dominant drainage pathways; and
- Identify evidence for land management practices with the potential to influence ground conditions ( e.g. burning, artificial drainage, peat cutting).
At the time of writing, on-line interactive programs such as Google Earth TM and local.live.com TM provide useful and free sources of aerial photography from which gross changes in peatland morphology can be identified. In some parts of Scotland, the quality of coverage on Google Earth TM is sufficiently resolute to identify historical peat landslide scars without the need to purchase aerial photography exclusively for this purpose.
3.2.4 Use of digital topographic datasets
Digital elevation models ( DEMs) compiled from radar-derived aerial surveys can provide detailed information on site topography including elevation, slope angle and slope aspect. These data should be used as follows:
- To characterise the overall site relief ( e.g. steep with pronounced convex slopes; gentle and undulating) and identify topographic controls on drainage ( e.g. hillslope summits and footslopes, major catchments, sub-catchments and gullies);
- To classify the site into slope classes ( e.g. 0-5°, 5-10°) on the basis that certain slope ranges may be more or less susceptible to specific failure mechanisms (section 2.1); and
- To identify north and south-facing slopes on the basis that slopes with differing aspects may have differing hydrological characteristics in relation to sun exposure ( e.g. rates of snow melt).
Digital topographic datasets in the UK are increasingly available from a variety of suppliers, or can be flown by commission if required. Datasets are normally geo-referenced and can be layered in a GIS with the mapping datasets described previously.
3.2.5 Remote sensing data and satellite imagery
Data collected by remote sensing includes aerial photographs, digital topographic datasets ( e.g. from Next MAP) and multispectral datasets illustrating ground conditions ( e.g. moisture content). Until recently, earth observation applications to peat landslide investigations have relied upon the interpretation of aerial photographs. Satellite imagery has lacked the spatial resolution to provide detailed images at the scale of an individual landslide. However, a new generation of satellite and airborne technology offers opportunities to investigate and map individual peat landslides, or terrain susceptible to peat landslides to a scale of 1:2000, or greater. The main satellite and aerial imagery sources which may be applicable to landslide investigations are summarised below and covered in more detail in Appendix A:
- Optical satellite imagery (Landsat thematic mapper): for identification of flow tracks, ground fissures and subtleties in peat morphology;
- Microwave (Synthetic Aperture Radar Interferometry, In SAR): for vegetation type, moisture content and collation of digital elevation models;
- Multispectral video: for mapping of groundwater systems; and
- Hyperspectral scanners: mapping of geological units in areas of poor exposure using soil moisture content as a proxy, estimation of soil thicknesses prone to landsliding.
Where pre-existing datasets are available, these can be of value in understanding site conditions. However, commissioning of such datasets for a single scheme would normally be considered cost prohibitive.
3.3 Site reconnaissance survey
Site reconnaissance survey can provide a rapid means of identifying the required scope for further ground investigation. If overall site characteristics do not appear to correspond to those associated with peat landslides, and if there has been no previous history or evidence of failures, the extent to which peat landslide hazard governs future ground investigation may be significantly reduced.
3.3.1 Purpose of reconnaissance survey
Preliminary site reconnaissance should comprise inspection of the site ground conditions to produce an initial interpretation of the site in the context of the surrounding environment. Ideally, this should be undertaken subsequent to the review of aerial photographs of the site (see previous chapter), with the reconnaissance site-walkover survey acting to ground truth key features identified on aerial photographs and/or maps. Features identified from aerial photographs and verified during walkover and additional features noted during survey should be recorded to produce a summary plan in map form for subsequent and more detailed investigation.
During site walkover, a preliminary understanding of peat characteristics and the nature of the peat-substrate interface should be gained by probing and by retrieving cored samples using hand coring techniques. Peat depths should be probed using either a hand auger, gouge or 'Russian' type hand-driven corer (Aaby and Berglund, 1986). Softer-substrates ( e.g. soft clays) may also be retrieved and indicate the nature of the peat-substrate contact. Harder substrates (bedrock) will not yield materials, but this lack of retrieval will still provide information on the nature of the interface. All materials encountered, and depths of changes in strata should be logged and recorded. Logging can be undertaken on-site or samples removed from the site and logged remotely. Samples should be collected across the development site and the number and distribution of samples should reflect the following factors:
(a) Topography: peat depths are likely to be shallower on steeper slopes, and therefore sufficient samples/probes should be taken to reflect the range of slope angles identified over the development site;
(b) Vegetation: the physical characteristics of peat will vary according to their hydrological setting, usually reflected in surface vegetation, samples should be taken to reflect the range of major vegetation types ( e.g. heathers, mosses, grasses); and
(c) Climate and hydrology: the date of the survey and the general weather conditions should be recorded during the site visit. The hydrology of the site should be recorded and map where possible in including any evidence of surface and subsurface drainage pathways and the depth of water strikes encountered during peat probing.
(d) Land management: peat will also vary according to local land management practices, with peat that has been subject to burning, draining or cutting exhibiting differing characteristics to adjacent undisturbed peat.
(e) Proposed infrastructure: if known, the planned location of infrastructure (roads, turbines, etc) should guide selection of locations for peat sampling, although it should be noted that the infrastructure layout may be subject to change pending the results of the peat landslide hazard investigation.
Dynamic probing can also be used to provide information on peat depth and variability in strength with depth through the stratigraphy.
The responsibility for determining the number and location of sample sites lies with the competent person(s) identified in section 1.7. Assuming that sample sites are representative, these preliminary results will provide a useful basis for more detailed specification of ground investigation, should it be required.
3.3.2 Extent of survey
The debris from peat landslides may extend from very short distances on shallow slopes, to several hundreds of metres over steepening convex valley sides. If debris runs out into gullies, streams or rivers, it may travel for kilometres as part of a peaty debris flood. Consequently, the point of impact may be some distance from the location of the peat landslide scar. The extent of the survey area will therefore be unique for each development, according to catchment size, slope configuration, the presence of stream channels, the position of adjacent infrastructure and the development location. The competent person(s) should justify the extent of reconnaissance survey on this basis.
3.3.3 Review of project status
Upon completion of the front-end desk study and site reconnaissance survey, sufficient information should be available to summarise the status of key controls and indicators of peat instability at a site. Figure 3.1 provides simple criteria for identifying the need for further investigation with respect to peat landslide hazard at a development site. Assessment criteria are listed at five decision levels, based upon simple summary assessments of the presence of peat, the presence of prior instability or of features indicative of instability, the topography of the site, and the possibility of construction induced instability. If little or no peat is present, or if engineering works are planned away from peat areas that display only minimal indicators of potential instability, an early exit from the peat landslide hazard assessment is available.