A series of rainfall-induced debris flow events in August 2004 affected the Scottish road network, and at Glen Ogle 57 people were airlifted to safety. Although there were no major injuries, the social and economic impacts were significant, particularly the severance of access to and from relatively remote communities. A study was commissioned with the overall purpose of ensuring that the hazards posed by debris flows were systematically assessed and ranked, thus allowing actions at specific sites to be prioritised effectively within available budgets. The methodology used to undertake a pan-Scotland, GIS-based assessment of debris flow susceptibility is described, as is the approach taken to interpret the resulting imagery in order to establish those sections of road alignment subject to hazards. The hazard scores assigned using this approach were subsequently modified in the light of the results of site-specific inspections. The ranking of hazards based upon the potential exposure of road users to debris flow hazards and the potential socio-economic impacts is also described, and a map illustrates the locations of the highest hazard-ranking sites. The success of the system is briefly discussed in the context of subsequent events, and the approach to management and mitigation is outlined.
IntroductionThe widely reported Scottish debris flow events of August 2004 were caused by rainfall substantially in excess of the norm, with some areas experiencing over three times the 30-year monthly average, and storm intensities of up to around 150mm/h (Winter et al., , 2006(Winter et al., , 2009a.Long-lasting and intense rainfall led to a large number of landslides, in the form of debris flows, in the hills of Scotland. Critically, some of these affected important parts of the major road network (Figure 1) linking not only towns and cities but also smaller, remote communities. Notable events occurred at the A83 between Glen Kinglas and to the north of Cairndow (9 August), the A9 to the north of Dunkeld (11 August), and the A85 at Glen Ogle (18 August) (Figure 2).Although there were no major injuries, 57 people were taken to safety by helicopter after being trapped between the two main debris flows on the A85 in Glen Ogle. However, the real impacts were social and economic, particularly the severance of access to and from relatively remote communities. The A83, carrying up to 5000 vehicles per day (all vehicles two-way, 24 h annual average daily traffic, AADT), was closed for slightly in excess of a day; the A9 (carrying 13 500 vehicles per day) was closed for 2 days prior to reopening, initially with single-lane working under convoy; and the A85 (carrying 5600 vehicles per day) was closed for 4 days. The traffic flow figures are for the most highly trafficked month of the year (July or August). Minimum flows occur in either January or February, and are roughly half those of the maxima, reflecting the importance of tourism and related seasonal industries to Scotland's economy. Substantial disruption was thus experienced by...
