The Scottish road network debris flow project: Contribution of the Geology and Landscape (Northern Britain) Programme

This report describes the methods used by GLNB staff who contributed to the assessment of potential debris-flow hazard to Scottish trunk roads, as part of a study commissioned by Transport Scotland. The first part of the report covers the first phase of the work, which involved verifying and amending the causal factor values for rock/sediment type shown on the amended GeoSure/DigMap 50 (LEX_D classification) for all of the units that crop out within the ‘potential debris flow catchments’ in Scotland, based on local knowledge. This resulted in amendments to 97 lithology values, and assigning lithology scores for 22 units that had had no values originally assigned to them It also involved reviewing all permeability scores and making alterations to some 190 permeability values (out of a total dataset of 3,706). The second part of the report describes the methods adopted for rapidly providing new attributed polygons for Superfical Deposits in areas of interest identified by Transport Scotland, where existing DigMap50 data was absent or known to be inadequate. Some new Quaternary data was produced from existing large scale analogue data sets that could be quickly digitized and new digital 1:25 000 scale datasets which were concurrently being produced by cartographic services. For the remainder of the areas of interest, a preliminary simplified interpretation of the distribution of the main Quaternary units (eg peat, fluvial deposits, raised beaches till, moraines, and bedrock near surface) was made by hand, using experienced field geologists to interpreted tailored NextMap data, overprinted on large-scale topographic base maps. This attributed line-work was then captured, in Arc9, by cartographic services staff and integrated with the adjacent DigMap50 tiles. This produced a seamless coverage of digital polygons for the Superficial Deposits within the potential debris flow catchments. The methods adopted produced the highest quality, most cost-effective and internally consistent interpretations which could be made, within the time-frame and costing constraints of the project.