Scottish Road Network Landslides Study - University of Glasgow

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20 BACKGROUND The soil water conditions necessary for debris flows may be generated by long periods of rainfall or by shorter intense storms. It is however widely accepted that Scottish debris flow events are usually preceded by both extended periods of heavy rainfall (otherwise known as antecedent rainfall) and intense storms. Figure 2.7 – Example of Meteorological Office 30-year monthly average rainfall data for October (image courtesy of the Meteorological Office). Climate change models for Scotland in the 2080s 5 indicate that in the summer precipitation will decrease but increase in the winter. However the models are generally considered to be incapable of predicting localised summer storms. These storms are believed to be at least partially responsible for triggering the events of August 2004, and climate data may not give a full picture of the relationship between precipitation and landslides. Furthermore, it is 5 Source: http://www.ukcip.org.uk and Personal Communication from D J Price (2005).
21 BACKGROUND important to note that climate models generally predict averages and that the error limits can be substantial. Predicted changes in the number of ‘intense’ wet days generally indicate a net increase of less than one day per annum by the 2080s, with slightly fewer intense wet days in the summer and more in the winter. However, by the 2080s extreme storm event rainfall depths are predicted to increase by between 10% and 30%, with intense winter rainfall increasing slightly more than this and that in spring/autumn by slightly less. Summer extreme rainfall depths are predicted to increase by between 0% and 10%. Peak fluvial flows are anticipated to increase progressively during the 21 st century. Eastern Scotland is expected to experience larger increases than north-west Scotland for example. The occurrence of snow and the associated contribution of snowmelt to both fluvial flow and groundwater are, on the other hand, predicted to decrease. Reductions in snowfall are predicted to be greater for the eastern and southern parts of Scotland and least for the central upland areas. Changes in the factors discussed above coupled with increased potential evapotranspiration, particularly in the summer, and a longer growing season, leading to increased root uptake, are expected to have substantial effects on soil moisture. The models predict a 10% to 30% decrease in soil moisture for summer/autumn and an increase of 3% to 5% in the winter. The winter figures reflect the fact that soils can only contain so much water and most Scottish soils are already close to saturation in the winter. Reduced soil moisture during the summer and autumn months may mean that the short term stability of some slopes formed from granular materials is enhanced by suction pressures. Soils under high levels of suction are vulnerable to rapid inundation, and a consequent reduction in the stabilising suction pressures, under precisely the conditions that tend to be created by such as short duration, localised summer storms. In addition, non-granular soils may form low permeability crusts during extended dry periods as a result of desiccation. Providing that these do not experience excessive cracking due to shrinkage, then they may increase runoff to areas of vulnerable granular deposits. Such actions could lead to the rapid development of instabilities in soil deposits, potentially creating conditions for the formation of debris flows. The complicating factors are the potential inability of current climate models to resolve storm events and the precise nature of the localised failure mechanisms that will lead to the initiation of an individual debris flow. It is highly unlikely that the measurement of soil suction could provide a practical and reliable means of debris flow forecast. The UKCIP (UK Climate Impacts Programme) report considers three periods: the 2020s, the 2050s and the 2080s. In general terms small changes are noted in the predictions for the 2020s. These changes increase slightly for the 2050s and slightly further still for the predictions for the 2080s, reflecting the temporal trends in temperature and precipitation. Whilst climate models generally predict averages and the associated error limits can be substantial, it is also important to note that inter-annual variability is predicted to increase for many climate factors. This means that average changes, as discussed above, may mask more important variability effects.
Page 1 and 2: Scottish Road Network Landslides St
Page 3 and 4: Electronic copies of this report ma
Page 5 and 6: MINISTERIAL FOREWORD The landslide
Page 7 and 8: 3 WORKING GROUP Dr Roger Moore: Rog
Page 9 and 10: 5 EXECUTIVE OVERVIEW Outlining poss
Page 11 and 12: 7 EXECUTIVE OVERVIEW The initial st
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
Page 19 and 20: 15 BACKGROUND squares that showed s
Page 21 and 22: 17 BACKGROUND events that do not im
Page 23: 19 BACKGROUND The A9 was blocked by
Page 27 and 28: 23 BACKGROUND earthworks stability
Page 29 and 30: 3 DEBRIS FLOW INFORMATION SOURCES b
Page 31 and 32: 27 INFORMATION SOURCES whereas stre
Page 33 and 34: 29 INFORMATION SOURCES Detailed haz
Page 35 and 36: 7. Meteorological: a) Antecedent ra
Page 37 and 38: 33 INFORMATION SOURCES 1:50,000 sca
Page 39 and 40: 35 INFORMATION SOURCES socio-econom
Page 41 and 42: 37 INFORMATION SOURCES Figure 3.6 -
Page 43 and 44: 39 INFORMATION SOURCES team using a
Page 45 and 46: 41 INFORMATION SOURCES events would
Page 47 and 48: NEXTMap Britain Digital Terrain Mod
Page 49 and 50: 4 DEBRIS FLOW TYPES AND MECHANISMS
Page 51 and 52: 47 TYPES AND MECHANISMS damage. Exa
Page 53 and 54: 49 TYPES AND MECHANISMS Figure 4.4
Page 57 and 58: 53 TYPES AND MECHANISMS The shear r
Page 59 and 60: Preparatory Factors 55 TYPES AND ME
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Page 69 and 70: 65 TYPES AND MECHANISMS detachment
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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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83 PROPOSED METHODOLOGY ground is u
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85 PROPOSED METHODOLOGY presence of
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87 PROPOSED METHODOLOGY The exposur
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89 PROPOSED METHODOLOGY Once a haza
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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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