Scottish Road Network Landslides Study - University of Glasgow

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18 BACKGROUND Many of the features of the slope at Invermoriston, such as its convex shape (i.e. steepening downslope) are characteristic of glacial valleys which are in turn typical of much of the landscape of Scotland. The event was preceded by rainfall of both long duration and high intensity. As a result of the debris flow the road was closed, damage was sustained to vehicles and a local hotel only narrowly escaped substantial physical damage. Debris flow events have also been observed at other times of the year. They have affected both the A890 and the railway at Stromeferry in January 1999, October 2000 and October 2001 (Figure 2.5). The January 1999 and October 2000 events were characterised by the mobilisation of material from a pre-existent landslide which slipped into a gully thus providing the source material for the debris flow event. The October 2001 event was propagated from a gully that had been infilled with silt, gravel and cobble fractions. In each case disruption to the road and railway was experienced. Figure 2.5 – Debris flow at A890 Stromeferry in October 2001. (Courtesy of and © copyright Alex Ingram.) The effects of forestry have frequently been identified as, at least, partial causes or propagators of debris flows in areas such as the Pacific NW of the USA (Brunengo, 2002). Logging or deforestation can have a dramatic effect on the drainage patterns of a slope, reducing root moisture uptake, slope reinforcement due to the root systems, and the physical restraints on downslope water flow for example. Such effects were especially noted as factors in the triggering of a translational landslide (not a debris flow) at Loch Shira adjacent to the A83 trunk road near Inverary in December 1994. Returning to the more recent debris flows of August 2004, these occurred at three main locations as discussed in the following paragraphs. The A83 was blocked at two locations in Glen Kinglas and at a point approximately 1km north of Cairndow and the road here was closed for two days. Numerous smaller debris flows were also observed on the hill slopes either side of the glen.
19 BACKGROUND The A9 was blocked by three main debris flows, two of which corresponded with areas of instability adjacent to the old A9 which runs parallel to, and upslope from, the present trunk road. In such circumstances both forest roads and minor roads can act to retard and concentrate the downslope flow of water and thus aid its penetration into the slope below. Such a mechanism has been a factor in a number of previous events such as the washout that blocked the A83 Rest and be Thankful in the vicinity of Roadman’s Cottage, in 1999. However, in the A9 Slochd failure of July 2002 it was the presence of the trunk road that contributed to the failure of the old road (used as a cycle path) and consequently to its own failure by undercutting. The presence of forest tracks was also identified as a factor in the debris flow at Invermoriston. In the A85 incident the road was blocked by two landslides. The southerly slip occurred first and as advice was being offered to motorists by Operating Company staff a second landslide occurred to the north of the first. The two landslides effectively trapped 20 vehicles, and 57 occupants were airlifted to safety by RAF and Royal Navy helicopters (see cover photograph). In its latter phases the northerly debris flow surmounted a spur of rock and an unoccupied Operating Company vehicle that had been parked in the lee of the spur was swept over the edge of the road and some distance downslope before it came to rest against a tree. Since August 2004 further landslides have affected the Scottish road network on the A82 near Letterfinlay alongside Loch Lochy (January 2005). In addition rock falls have affected the A832 near Kinlochewe (December 2004) and the A82 1.5 miles north of the Corran Ferry junction (January 2005). 2.3 CLIMATIC ISSUES The climate of Scotland in terms of its rainfall may be very broadly divided into east and west (see Figures 2.6 and 2.7). Data presented by the Meteorological Office (Anon, 1989) indicates that in the east rainfall generally peaks in August while in the west the maximum rainfall levels are reached during the wider period September to January (Figure 2.6). Although rainfall levels in the west are relatively low in August they increase from a low point in May. Both scenarios indicate that the soil may be undergoing a transition from a dry to a wetter state at or around August, indicating an increased potential for debris flow and other forms of landslide activity. The central area, as represented by Pitlochry in Figure 2.6, has a mix between the rainfall characteristics of the ‘east’ and the ‘west’. The rainfall peak is both lower and shorter (December and January) than in the west, but there are also small sub-peaks in August and October. A broadly similar pattern is found for Perth. Monthly average rainfall for 1951 to 1980 (mm) 140 120 100 80 60 40 20 0 Edinburgh (Royal Botanic Gardens), 26m AMSL, Average Annual Rainfall 626mm. Pitlochry, 144m AMSL, Average Annual Rainfall 824mm. Tiree, 9m AMSL, Average Annual Rainfall 1106mm. J F M A M J J A S O N D J F M A M J J A S O N D J F M A M J J A S O N D Figure 2.6 – Annual average rainfall data for points in Scotland.
Page 1 and 2: Scottish Road Network Landslides St
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Page 5 and 6: MINISTERIAL FOREWORD The landslide
Page 7 and 8: 3 WORKING GROUP Dr Roger Moore: Rog
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Page 13 and 14: 1 INTRODUCTION TO LANDSLIDE HAZARDS
Page 15 and 16: 11 INTRODUCTION Section 6 describes
Page 17 and 18: 13 BACKGROUND However, Varnes (1978
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Page 49 and 50: 4 DEBRIS FLOW TYPES AND MECHANISMS
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69 CONTRIBUTORY FACTORS There is ho
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5.1.2 Geological, Geotechnical and
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73 CONTRIBUTORY FACTORS Other hydro
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75 CONTRIBUTORY FACTORS to the fail
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77 CONTRIBUTORY FACTORS 5.3 FACTORS
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79 CONTRIBUTORY FACTORS upon the lo
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6 PROPOSED METHODOLOGY FOR DEBRIS F
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85 PROPOSED METHODOLOGY presence of
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87 PROPOSED METHODOLOGY The exposur
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A84 South of Strathyre (8km) HIGH H
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HIGH HAZARD AREAS AND EARLY OPPORTU
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8 DEBRIS FLOW MANAGEMENT AND MITIGA
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8.2.3 United Kingdom MANAGEMENT AND
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MANAGEMENT AND MITIGATION OPTIONS D
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MANAGEMENT AND MITIGATION OPTIONS a
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MANAGEMENT AND MITIGATION OPTIONS N
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MANAGEMENT AND MITIGATION OPTIONS r
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MANAGEMENT AND MITIGATION OPTIONS F
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9 SUMMARY AND RECOMMENDATIONS FOR D
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SUMMARY AND RECOMMENDATIONS The ini
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SUMMARY AND RECOMMENDATIONS extensi
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115 REFERENCES Chan, R. K. S. 2000.
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117 REFERENCES Moore, R., Lee, E. M
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APPENDIX - PROJECT WORKSHOP AGENDA
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Scottish Road Network Landslides Study - University of Glasgow

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