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2 BACKGROUND TO SCOTTISH LANDSLIDES AND DEBRIS FLOWS

by M G Winter, L Shackman, F Macgregor and I M Nettleton

2.1 LANDSLIDES

Recent extreme rainfall in Scotland has led to events that have been described in the media using the generic term 'landslide'. These events have intersected with the A83 (between Glen Kinglas and to the north of Cairndow), A9 (to the north of Dunkeld) and A85 (Glen Ogle) trunk roads.

While the recent happenings have been of both high magnitude (in terms of the amount of material moved) and severe (in terms of their impact on the trunk road network and the exposure of its users) it is important to understand that they are by no means unique. Similar events have been observed in recent years by Nettleton et al. (In Press) at Invermoriston, intersecting the A887, and at Stromeferry, intersecting the A890 local road. Other events have been observed at A83 Rest and be Thankful, A9 Slochd, A95 Craigellachie and A84 Strathyre, for example.

Many systems have been proposed for the classification of landslides, however, the most commonly adopted systems are those of Varnes (1978) and Hutchinson (1988).

The International Geotechnical Societies' UNESCO Working Party on World Landslide Inventory ( WP/ WLI) was formed for the International decade for Natural Disaster Reduction (1990 to 2000). The WP/ WLI (1990) report "A Suggested Method for Reporting a Landslide" uses Varnes' (1978) classification and reports that it is the most widely used. The World Road Association ( PIARC) report "Landslides: Techniques for Evaluating Hazard" (Escario et al., 1997) also presents a classification based on Varnes'.

Figure 2.1 presents the five kinematically distinct types of landslide identified by Varnes (1978), as follows (after Escario et al., 1997):

a) Falls: A fall starts with the detachment of soil or rock from a steep slope along a surface
on which little or no shear displacement takes place. The material then descends largely by
falling, bouncing or rolling.
b) Topples: A topple is the forward rotation, out of the slope, of a mass of soil and rock about a point or axis below the centre of gravity of the displaced mass.
c) Slides: A slide is the downslope movement of a soil or rock mass occurring dominantly on the surface of rupture or relatively thin zones of intense shear strain.
d) Flows: A flow is a spatially continuous movement in which shear surfaces are short lived,
closely spaced and usually not preserved after the event. The distribution of velocities in
the displacing mass resembles that in a viscous fluid.
e) Spreads: A spread is an extension of a cohesive soil or rock mass combined with a general subsidence of the fractured mass of cohesive material into softer underlying material. The rupture surface is not a surface of intense shear. Spreads may result from liquefaction or flow (and extrusion) of the softer material.

However, Varnes (1978) also presented a sixth mode of movement, Complex Failures. These are failures in which one of the five types of movement is followed by another type (or even types). For such cases the name of the initial type of movement should be followed by an "en dash" and then the next type of movement: e.g. rock fall-debris flow ( WP/ WLI, 1990).

The EPOCH (1993) project (The Temporal Occurrence and Forecasting of Landslides in the European Community) produced a European classification based on Varnes (1978). For the purpose of this work Varnes' (1978) classification has been adopted with amendments from Cruden and Varnes (1996). This approach is consistent with the UNESCO Working Party on World Landslide Inventory ( WP/ WLI, 1990; 1991; 1993).

Figure 2.1 - Types of landslide: (a) falls, (b) topples, (c) slides, (d) flows, and (e) spreads (after Escario et al., 1997).

The recently observed landslide events have been typical of flow-type landslides. The influence of substantial flows of water, the stripping of superficial deposits, and the speed with which debris has both flowed and been deposited have all been apparent. In many cases the initial trigger appears to have been the displacement of relatively small amounts of material, often into a stream channel. This has added a substantial debris charge to already high and potentially damaging water flows. The combination of water with high sediment loadings then has substantial erosive power. In other cases highly saturated materials have slumped rapidly downslope in a manner not dissimilar to that illustrated in Figure 2.1(d).

Such events are typically described as 'debris flows' and are distinguished from most other types of landslides involving shear by the dynamic as opposed to broadly static nature of the failure mechanisms ³. This is an important distinction and not simply an academic nicety. Failure to make such a distinction could very easily lead to inappropriate data being collected and inappropriate approaches being proposed.

Flows are largely dynamic in their trigger mechanisms and are generally characterised by rapid erosion and movement with high proportions of either water or air acting as a lubricant for the solid material that generally comprises the bulk of their mass. In their classification of such flows, Pierson and Costa (1987) have illustrated the types of sediment-water flows using a two-dimensional matrix of mean velocity and sediment concentration. This has been adapted and simplified and is illustrated in Figure 2.2. Only pure water (0%) and dry sediment (100%) are marked on the sediment concentration axis as exact values depend on the particle size distribution and the physical-chemical composition. In addition, easily visualised mean velocities of mixed units are used, serving to emphasise the conceptual nature of this figure.

Stürzstrom, debris avalanches and grain flows are generally air lubricated slides and are beyond the scope of the work of this report, except in as much as this work relates to the existing Rock Slope Hazard System ( see Section 6.3). The large area under the curve at the bottom left hand of the figure has no mechanism to suspend sediment and can thus be neglected, as this essentially relates to flooding rather than landslides. Similarly normal and hyperconcentrated streamflow are typical of flooding, bearing a closer relationship to the August 2004 events in Boscastle in south-west England, and are not considered further herein.

The remaining categories of debris flow and earth flow, as defined by Pierson and Costa, are the flow types with which we are concerned here and for simplicity are for now referred to simply as debris flows. These flow types, together with peat flows, are discussed further in Section 4. The sediment-water flows are defined as plastic with movement occurring over a wide range of potential velocities. These features are broadly characteristic of the debris flow types experienced in Scotland in recent years.

Debris flows occur, in the main, because of the character of natural slopes, the deposits of which they are comprised, and the amount and duration of rainfall (and consequent infiltration) to which they are subject. The fact that they impact on a road network is, irrespective of the consequences, coincidental in the phenomenological sense. Debris flows affecting the trunk road network are not caused by its construction and/or management, except in unusual circumstances. However, some aspects of the built environment, including a road network, may contribute to the outcomes of such events.

It is important to note that debris flows are neither a recent phenomenon nor an uncommon occurrence. The first church in the Falkland Islands, for example, was wrecked in 1886 when a "river of liquid peat … roared down from the hills" (Winchester, 1985). Closer to home, a cloud burst in 1744 resulted in the flow and associated erosion of the gulley below the summit of Arthur's Seat known today as the Gutted Haddie (McAdam, 1993). Innes (1983a) made a survey of Scotland based upon aerial maps and marked those 10km by 10km grid squares that showed some sign of debris flow activity (Figure 2.3), clearly indicating that such activity is far from unusual.

Figure 2.2 - Simplified rheological classification of sediment-water flows (after Pierson and Costa, 1987). Flow types are given in green text.

It is clear that the August 2004 events in Scotland had the potential to cause injury and even death. However, such potential was not on the same scale as the reality that is experienced elsewhere in the world on a regular basis. For example, in September 2004, torrential rain triggered massive floods and landslides in SW China, killing in excess of 170 people and injuring many dozens more ⁴.

Figure 2.3 - The extent of recorded debris flow activity in Scotland (from Jones and Lee,1994; after Innes, 1983a). Note that the figure does not record any activity in the area around the Rest and be Thankful, for example. It seems unlikely that there was no such activity prior to 1983 when the figure was first published and the data set should thusnot be seen as exhaustive.

In recent years debris flow events appear to have had an increasing effect on the Scottish trunk and local road network, together with the Scottish rail network. At face value this suggests that such events have become more common. Such a conclusion would however be somewhat speculative as comprehensive, detailed records are not generally available for events that do not impact upon man's activities. What does appear clear from simple observation is that many debris flows are initiated on the Scottish hills. However, only a relatively small number turn into major events that impact upon road networks or other forms of infrastructure. This implies that in order to manage the impacts of debris flows it is necessary to understand the preparatory factors (that make a slope vulnerable to debris flows), the trigger factors (that lead to initiation of flows) and any propagation and/or magnification factors. This theme is developed further in Section 4.

A number of debris flows have historically occurred in the month of August. One example is an event that intersected the A887 at Invermoriston in 1997 (Figure 2.4). This event was studied in detail and found to have been triggered at a point almost 300m vertically and around 2,000m horizontally from the road, close to the source of the stream which subsequently contained most of the event. A number of contributory factors were established (Nettleton et al., In Press), including the following:

• The lack of water storage volume within the catchment, both above and below ground.
• The ploughing of agricultural land increases and accelerates runoff into streams.
• The presence of downslope bedding planes.
• Low permeability bedrock.
• The presence of forestry bridges which temporarily arrested the flow allowing material to accumulate and subsequently remobilise with greater erosive power.
• The presence of a buried cliff providing a large amount of debris at a point close to the road.
• A steep slope close to the road.

Figure 2.4 - Debris flow at Invermoriston (A887) in August 1997. (Courtesy ofNorthpix.)

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.

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).

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.

Figure 2.6 - Annual average rainfall data for points in Scotland.

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 forOctober (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 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.

2.4 CURRENT INSPECTION AND MAINTENANCE ARRANGEMENTS

The current term contracts for the management and maintenance of the Scottish trunk road network require that embankments and cuttings are inspected (Section 2.7 of Schedule 7 Part 1 to the Contract: Embankments and Cuttings). Guidance on inspections and on failure modes and their identification together with procedures for remedial works are given in HA48/93 Maintenance of Road Earthworks and Drainage ( DMRB 4.1.3). HA48/93 has recently been superseded by HD41/03 Maintenance of Highway Geotechnical Assets ( DMRB 4.1.3), which has replaced HA48/93 for use on the trunk road network in England. HD41/03 is heavily slanted towards the Highways Agency's organisational procedures and system of geotechnical checking, but the principles are suitable to be applied where appropriate to the trunk road network in Scotland. Having been the conforming standard at the time of letting the current term maintenance contract, HA48/93 remains active for use on the trunk road network in Scotland.

The Operating Companies are required to carry out detailed inspections of all embankments and cuttings to check for any indication of instability. Evidence of instability is reported using Form A and remedial works proposed on Form B of Appendix A of HA48/93.

Although the actual requirement is for the Operating Companies to inspect embankments and cuttings, Form A includes for the reporting of instability in both those two categories and additionally in natural slopes. Notwithstanding this, the HA48/93 itself is focussed upon cuttings and embankments with only a brief note (paragraph 3.3) on the potential for the reactivation of slab slides (a variant upon that illustrated in Figure 2.1(c)) by the excavation of a cut slope or by loading with an embankment.

Inspections are required to be carried out (Section 1.6 of the Schedule 7 Part 1 to the Contract: Detailed Inspection Requirements) at intervals of one year and no later than 14 days after the anniversary of the previous inspection. Further, in the North West Unit area, for example, additional earthwork monitoring requirements are specified. These are as follows:

• A83 Rest and be thankful (west of Arrochar): three monthly inspection and levelling survey of the road surface.
• A83 Loch Shira (east of Inverary): six monthly inspections.
• A83 Artilligan Sea Wall (south of Ardrishaig): two weekly inspections of sea wall and rock protection.
• A84 Doctor's Corner (Loch Lubnaig): visual inspection of previous slip area.

The possible need for additional inspection requirements in the light of a recent report on rock slope stability along the A83 is also highlighted.

As part of their routine maintenance operations the Operating Companies are required to remove debris from behind netting, repair and replace netting, remove debris in rock traps and from behind rock fences. Other maintenance activities are to be the subject of an Order following the submission of Forms A and B in HA48/93.

For the Scottish trunk road network schemes, the geotechnical process is subject to Geotechnical Certification as operated by the Scottish Executive and its Independent Geotechnical Checker. A high degree of comfort is therefore assured that all aspects of earthworks stability have been properly addressed in design and construction. At the end of the maintenance period responsibility for inspection passes to the Scottish Executive and the relevant Operating Company.

2.5 POTENTIAL THIRD PARTY ISSUES

The landslides which occurred during August 2004, see Section 2, all occurred either directly or indirectly as a result of rainfall and consequential debris flows on land outwith the trunk road boundary. Many of the other landslides which have occurred in Scotland have also been instigated on land outwith the road boundary. This raises questions as to how such land can be accessed and controlled in order that future events can either be prevented, minimised or managed. In addition, the responsibilities of the third parties who own or control such land need to be clarified in relation to such occurrences.

With regard to trunk roads a number of powers are available to the Scottish Ministers as roads authority to assist in such matters under the Roads (Scotland) Act 1984 (House of Commons, 1984). Such powers are also available for use by local roads authorities for roads under their control.

The various sections of the Act which are of relevance are as follows:

• Section 30 -this section provides for works to be carried out by the roads authority in order to protect the road against hazards of nature, including landslide.
• Section 104(1)(a) authorises the roads authority to acquire land, either on a compulsory or voluntary basis, for the protection of a public road.
• Section 109 provides, by reference to Schedule 5, distance limits for acquiring land compulsorily, but in terms of Section 109(3) those distance limits do not apply for purposes connected with the protection of a public road.
• The roads authority therefore has the power to acquire land to construct a barrier or carry out other works to protect a road from landslide even although that barrier or work might be remote from the road itself.
• Section 31 makes provision for drainage of a public road including preventing surface water from falling on to the road.
• Section 32 authorises the roads authority to make contributions towards drainage works or flood prevention operations which may be desirable for the protection of a public road.
• Section 93 imposes an obligation on the roads authority to take steps to obviate any danger on land beside or near to a road.
• Section 95 deals with the deposit of mud or other materials from vehicles on to roads.
• Section 99 requires the owner and occupier of any land to prevent any flow of water or other matters from that land on to the road.
• Section 102 deals with the ploughing of unenclosed land adjoining a public road

It is worth considering what the potential liability of third parties such as owners and occupiers of land adjoining a road is in relation to landslides and what might be the impact on land values and the economy in general.

It is clear that the primary responsibility for the protection of a road lies with the roads authority. However, liability may attach to third parties in certain circumstances, possibly, for example, where an adjoining landowner has been negligent and damage to the road as a result of that negligence is foreseeable. It would be necessary to carefully consider the individual circumstances of any incident resulting in damage to a road to ascertain whether any liability does attach to a third party. Where we are dealing with landslides caused solely by torrential rain, it may be very difficult to show liability for damage resulting to a road attaches to any third party.

Certain duties and liabilities relating to the protection of roads are currently imposed on third parties. The Scottish Ministers could as a matter of policy impose further duties and liabilities. However, such an imposition may have the effect of diluting the primary responsibility for the protection of a road which currently lies with the roads authority and transferring it to owners and occupiers of land adjoining roads. Such a policy may impact on land values and the economy more generally, and this aspect would have to be taken into consideration when formulating the policy.

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