2 LANDSLIDE EVENTS 2.2 EVENTS OF AUGUST 2004 2.3 OTHER EVENTS 2.4 SEASONALITY
2 LANDSLIDE EVENTS
by M G Winter, A P Heald, J A Parsons, D Spence, F Macgregor and L Shackman
2.1 LANDSLIDES AND DEBRIS FLOW
In recent times, extreme rainfall in Scotland has led to events that have been described in the media under the generic term ‘landslide’. The major events of August 2004 intersected with the A83, A9 and A85 trunk roads (see Section 2.2).
While these 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 also been observed in recent years at Invermoriston, intersecting the A887, and at Stromeferry, intersecting the A890 local road (e.g. Nettleton et al., 2005a). Other events have been observed, at various times, at A83 Rest and be Thankful, A9 Slochd, A95 Craigellachie and A84 Strathyre, for example.
The word ‘landslide’ covers a range of types of gravitational mass movement. 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. This is an important distinction and not simply an academic nicety. Failure to make such a distinction can very easily lead to inappropriate approaches being proposed and inappropriate data being collected.
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 (Pierson & Costa, 1987 and as discussed further by Winter et al., 2005d). Stürzstrom, debris avalanches and grain flows are generally air-lubricated slides and are beyond the scope of the work of this study. Similarly normal and hyperconcentrated streamflow are typical of flooding, showing some similarity to the August 2004 events in Boscastle in south-west England, and are not considered further herein.
The remaining categorisations of debris flow and earth flow are the flow types with which we are concerned here and for simplicity are for now referred to simply as debris flows. These sediment-water flow 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 any road network is, irrespective of the consequences, a matter of coincidence. Debris flows affecting the trunk road network do not have as their cause its construction 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.
At this point 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).
It is, however, clear that the August 2004 events in Scotland had the potential to cause injury and even death. Fortunately, such potential was not on the same scale as the reality that is experienced elsewhere in the world on a regular basis, such as following the Kashmir (Pakistan) earthquake in October 2005 and the catastrophic landslide in The Philippines in February 2006.
2.2 EVENTS OF AUGUST 2004
2.2.1 Introduction
The rainfall experienced in Scotland in August 2004 was substantially in excess of the norm. Some areas of Scotland received more than 300% of the 30-year average August rainfall, while in the Perth & Kinross area figures of the order of between 250% and 300% were typical. Although the percentage rainfall during August reduced to the west, parts of Stirling and Argyll & Bute still received between 200% and 250% of the monthly average3. Subsequent analysis of radar data indicated that at Callander, some 20km distant from the events at the A85 (see Section 2.2.4), 85mm of rain fell during a four hour period on 18 August. Some 48mm fell in just 20 minutes and the storm reached a peak intensity of 147mm/hour. The 30-year average rainfall for August in Scotland varies between 67mm on the east coast and 150mm in the west of Scotland (Anon, 1989).
The rainfall was both intense and long lasting and a large number of landslides, in the form of debris flows, were experienced in the hills of Scotland. A small number of these intersected the trunk, or strategic, road network, notably 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). These locations are illustrated in Figure 2.2.
Figure 2.2 – Map showing the trunk road network, including motorways, in Scotland. The locations of the three main debris flow event groups that affected the trunk road network (1, 2, 3) in Scotland in August 2004 are shown as are areas in which the local Highland road network was affected by debris flows in August 2004, December 2004 and October 2006.
While there were no major injuries to those affected, 57 people had to be airlifted to safety when they became trapped between the two main debris flows at Glen Ogle. However, the real impacts of the events were economic and social, in particular the effects of the severance of access to relatively remote communities. The A85, carrying up to 5,600 vehicles per day (all vehicles two-way, 24 hour AADF – Annual Average Daily Flow), was closed for four days. The A83, which carries around 5,000 vehicles per day, was closed for slightly over a day and the A9, carrying 13,500 vehicles per day, was closed for two days prior to reopening, initially with single-lane working under convoy. The disruption experienced by local and tourist traffic, as well as to goods vehicles, was substantial.
The traffic flow figures are for the most highly-trafficked month of the year for each of the roads, either July or August. Minimum flows occur in either January or February and are roughly half those of the maxima. The figures reflect the importance of tourism and related seasonal industries to Scotland’s economy.
This section provides an overview of the events of August 2004, based upon that of Winter et al. (2006a).
2.2.2 A83 Glen Kinglas/Cairndow – 9 August
The A83 in Argyll & Bute was blocked at two locations in Glen Kinglas, 0.5km and 2.5km from the junction with the A815, and at a point approximately 1km north of Cairndow. In addition to causing the road to be closed for slightly over a day, the debris flow at Cairndow (Figure 2.3) also had a substantial effect on a residential property immediately upslope from the road (Figure 2.4). Numerous smaller debris flows were also observed on the hill slopes either side of Glen Kinglas.
Figure 2.3 – Debris fan containing boulders (estimated up to 9 tonnes) at A83 Cairndow.
The A83 is a single-carriageway route and in Glen Kinglas lies at approximately 100m AOD, at the toe of the steep undulating slopes of Binnein an Fhidhleir and Stob Coire Creagach, which extend some 700m to 800m above the road. The slopes are generally vegetated with grass, bracken and occasional heather cover with a few forested areas on the lower slopes adjacent to the road. Rock outcrops locally on the slopes, but mainly on the higher ground. The slope is frequently incised with watercourses. These are culverted below the road and into Kinglas Water, located a short distance to the south of the A83.
In early August 2004 the hillsides were in a saturated condition following a relatively wet spell during the preceding weeks. This was followed by a relatively short period of exceptionally heavy rainfall.
Typically the flows commenced in the steep upper reaches of the slopes at around 500m AOD (Figure 2.5). At the head of each a shallow scarp, less than 1.5m, was observed. The waterlogged material is assumed to have flowed into existing water courses providing a more erosive sediment charge, resulting in erosion up to between 10m and 15m either side of the channels. Deposition occurred at the toe of the slope where the gradient slackens.
Figure 2.4 – Debris flow above the A83 to the west of Cairndow showing the effects on a roadside cottage and the trunk road immediately downslope from the cottage.
Figure 2.5 – Upland debris slide and flow development at Cairndow on the A83. (Courtesy and © copyright of Halcrow.)
Several hundred tonnes of material are estimated to have blocked the road at the two locations in Glen Kinglas with possibly two to three times this amount at the Cairndow slide. The debris blocking the road comprised very silty sand and gravel with frequent cobbles and boulders, the largest of which was estimated to weigh nine tonnes (Figure 2.3). Smaller boulders remained within the watercourses, although none was considered to be a further threat to the road.
2.2.3 A9 North of Dunkeld – 11 August
The heavy rain that triggered the A83 events continued for three more days over much of Scotland and precipitated further debris flows, three of which affected the A9 just to the north of the Jubilee Bridge near Dunkeld in Perth & Kinross.
At this location the A9 is single-carriageway and passes at the foot of a steep slope on its eastern side. The River Tay is a short distance to the west. The old A9, now a minor local road (C502), traverses the hillside above the trunk road. The upper part of the slope between these two roads is wooded, whereas the lower part is vegetated by broom with few trees. The lower part of the slope was steepened at the time of construction of the present trunk road, to a gradient close to 1 in 2 (vertical to horizontal) whereas the upper part is slightly less steep, steepening again at the top to form the bench on which the old A9 was constructed. Above the old A9 the wooded hillside continues to rise for approximately 250m in elevation.
The upper part of the slope is notable for the presence of a superficial layer of yellow fine sand that is both slightly denser and lighter in colour than the underlying uniformly graded fine sand. This denser material is absent from the lower part of the slope and it seems likely that material of this nature was removed when the lower part of the slope was steepened to accommodate the A9 trunk road.
It is clear that, as a result of the exceptional rainfall, a large amount of surface water runoff descended the slope above the old A9, both along the course of existing streams and on the open hillside between. When it reached the old road, the drainage system was unable to contain or disperse such large volumes of water. The surface runoff travelled along the old road, spilling over the edge onto the slope below in a number of places, as illustrated in Figure 2.6 and effectively concentrating the flows at these locations.
In at least five major locations and a number of minor locations, this overspill of water caused failure of the outer edge of the old A9 road (Figure 2.7). In two such locations (the central and northern flows, see Figure 2.6) the flow of water, charged with debris from these failures brought with it trees from the upper part of the slope. A deposition zone, approximately 20m wide at the central flow, corresponded with this stage of the event, which resulted in the flooding and trees on the A9 that were first observed and reported by trunk road users.
Following the first stage, the power of the flood increased and it entered an erosive phase in which gullies, some 3m to 4m deep and up to 6m wide, were scoured. These gullies stretched from the top of the cut slope (or above) to the A9 trunk road verge. Once the vegetation was stripped and the underlying fine sand was mobilised, it flowed freely down the slope and onto the trunk road. The erosion gullies did not extend into the upper part of the slope due to the slightly lesser gradient and the presence of the more erosion resistant layer of the lighter yellow fine sand (Figure 2.8).
Figure 2.6 – Influence of old road on debris flow at A9 Dunkeld. The central flow is shown and the northern flow can also be seen on the left of the picture. Photograph dated 11 August 2004. (Courtesy of Alan Mackenzie, BEAR.)
Figure 2.7 – Instability at the downslope edge of the old A9 above the central debris flow. Photograph dated 12 August 2004. (Note that the brown pipe running horizontally across the backscarp is a telecommunications duct.)
The southern flow (see Figure 2.9), differed only in detail from the central and northern flows. A low point and a change in crossfall of the old A9 caused a large amount of the water flowing along the road to spill onto the upper part of the slope. The ground immediately below the old A9 did not fail, because of either the presence of more mature trees in this area and/or an unknown detail change in the road construction. The water then appears to have flowed down the slope to the top of the cut slope, where a deep gully was eroded. This was similar in form to, but rather wider and deeper than, the central and northern gullies, and deposited a large amount of sand on the trunk road (Figure 2.9). The top of this southern gully is marked by exposed rocks, both within the gully and at surface immediately above. It thus seems likely that the extension of this gully was limited by the presence of bedrock or boulders.
Figure 2.8 – The top of the central erosion gully at the top of the cut slope. The overhang of the denser yellow fine sand is clearly visible and it appeared that this material was better able to shed the water and debris. Photograph dated 12 August 2004.
Figure 2.9 – The southerly debris flow at the A9 north of Dunkeld. The flow has formed its own channel by erosion. Photograph dated 12 August 2004. (Courtesy of Alan Mackenzie, BEAR.)
Both forest roads and minor roads can act either to retard or to 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 below (now used as a cycle path) and consequently to the failure of the A9 itself by undercutting. The presence of forest tracks was also identified as a contributory factor in the debris flow which occurred at the A887 Invermoriston in August 1997 (Winter et al., 2005a; 2005b; Nettleton et al., 2005a).
An brief account of the repair work undertaken to the drainage systems at and immediately below the old A9 (C502) is given by Fossett et al. (2006)
2.2.4 A85 Glen Ogle – 18 August
Following the A83 and A9 incidents, the rainfall in the area decreased for several days but on 18 August a short but exceptionally intense rainstorm occurred in west Stirlingshire and triggered two debris flows that blocked the A85 in Glen Ogle north of Lochearnhead. The southerly slip occurred first and, as advice was being offered to motorists by Trunk Road Operating Company staff, a second landslide occurred to the north of the first. Some 20 vehicles were trapped between the two debris flows, and 57 occupants were airlifted to safety by RAF and Royal Navy helicopters (Figure 2.10).
Figure 2.10 – Fifty-seven occupants of the 20 vehicles that were trapped between the two debris flows in Glen Ogle were airlifted to safety. (© Perthshire Picture Agency: www.ppapix.co.uk.)
The A85 trunk road through Glen Ogle is a relatively narrow single carriageway and climbs north-westward from Lochearnhead, at an elevation of approximately 100m AOD, to a pass at the head of the glen at around 290m AOD before descending into Glen Dochart to the north. From Glen Ogle Farm (approximately 1.2km from Lochearnhead) northwards, the road climbs up the eastern flank of the valley to the top of the pass; it is along this section that the most significant flows occurred.
The hillside above the road rises some 400m at approximately 1 in 2. It is covered with bracken and heather with isolated boulders and areas of crags. Below the road the gradient of the slope decreases rapidly to the Glen Ogle Burn. The two slips followed steep streams that descend this hillside and are culverted beneath the road. The southerly stream descends through an area covered by heather and bracken while the northerly stream descends a partially rocky area of the hillside.
As a result of the exceptional rainfall, and possibly because of the additive of the high level of antecedent rainfall, the soils in the upper catchments to the streams became saturated, triggering slides into the headwaters of both streams. The culverts rapidly became blocked and debris spilled across the road (Figure 2.10) and down the slope beyond (Figure 2.11). Most of the debris came to rest on the slope below the road but a small proportion reached the Glen Ogle Burn. This burn was also in spate at the time and rapidly removed the debris that reached it.
Figure 2.11 – View of the northern A85 Glen Ogle debris flow two days after the event, showing the sharp bend in the channel just above road level.
2.2.4.1 Northern Flow
Both terrestrial and helicopter-based examinations of the northern flow (Figure 2.11) undertaken two days after the events indicated two independent sources. To the north is an arcuate scar from which a shallow translational slip broke away. The turf and upper soil travelled over the surface of the vegetation below and entered the upper part of the stream gully. However, scarring indicates that instability occurred independently at the very top of the gully. It is not known which instability occurred first although both slides appear to have generated only a relatively small amount of debris. The debris was, however, channelled into and down the steeply inclined bed of the stream and scoured the gully, removing turf and soil. It is likely that the volume of water and debris increased further down the gully and that the consequent damage was increased in areas closer to the road.
In the middle and lower parts of the flow, large and small boulders and trees were mobilised in addition to soil and turf. In that locality the schistose bedrock is generally encountered at shallow depths and scouring appears not to have been deep but rather to have spread laterally. However, it appears that bedrock was loosened in places in the area of a small waterfall a short distance above the road. The dominant component of the debris comprised fine particles but many cobbles and boulders were also in evidence. Several boulders of up to 10 tonnes were deposited on the road and one boulder, estimated at 90 tonnes, was deposited some 10m above the road. From eyewitness accounts it would appear that the debris reached the road in pulses. These were most likely associated with the temporary damming of the stream by debris or by new areas of instability in the stream banks. Similar observations were made regarding the August 1997 debris flow which affected the A887 at Invermoriston (Winter et al., 2005b; Nettleton et al., 2005a).
The west-flowing stream channel then takes a sharp, right-angled turn to the south approximately 10m before it reaches the road due to a bluff of rock. This outcrop steers the stream channel into a course that runs sub-parallel to the road before the stream makes another sharp, right-angled turn to the west to pass under the road by means of a high arched culvert and descends the lower slopes to the Glen Ogle Burn.
On the afternoon of 18 August the initial part of the debris flow followed the course of the stream. However, at some point the culvert became blocked with boulders up to 2m in size and fallen trees, causing the water and debris to flow over the road, largely destroying the parapet of the culvert. As the energy of the debris flow increased, it reached a point where some or all of it failed to negotiate the first corner and it swept over the rock bluff and crossed the road some 40m to 50m to the north of the culvert. An unoccupied Trunk Road Operating Company vehicle that had been parked in the lee of the spur was swept over the edge of the road and for some distance downslope before it came to rest against a tree (Figure 2.12). A wide debris fan was left on the slope between the road and Glen Ogle Burn (Figures 2.12 and 2.13).
2.2.4.2 Southern Flow
The failure in the southerly stream was less extensive than that in the northern, the erosion scar being both narrower and less deep. This may have been due to the flow having less momentum than the northerly flow as this stream appears to be less continuously steep. Much of the material was coarser than that from the northern slip, being predominantly cobble-sized (see Figure 2.10). Otherwise, the general mechanism appears to have been similar, although in this case there is no major bend in the stream. The culvert is smaller and rapidly became blocked and debris spilled across the road causing damage to the outer face of the culvert and the outer edge of the road.
Figure 2.12 – A85 Glen Ogle showing the Trunk Road Operating Company vehicle that was swept away by the northern debris flow.
Figure 2.13 – The debris fan formed by the northern debris flow in Glen Ogle viewed from the A85 trunk road, looking towards Glen Ogle Burn and down the valley towards Lochearnhead.
2.3 OTHER EVENTS
There has been a number of landslide – including debris flow – events in Scotland since August 2004. Relatively minor events affected the road network, albeit not always the trunk road network, at the A832 near Kinlochewe in December 2004, on the A82 approximately 1.5 miles north of the Corran Ferry junction in January 2005 (details of this event are rather sketchy but it was most likely a small rockfall), on the A82 at Letterfinlay on 7 January 2005 and on the A814 in January 2006. Other events affecting the local road network in Highland in August, October and December 2004, September 2005 and October 2006 as described in Section 2.3.1. The events of October 2006 also affected the trunk road network as did later events in December of that year – primarily on the A82 at Letterfinlay and on the A9 north of Inverness at Berriedale, Helmsdale and Portgower.
However, perhaps the most serious single event to affect the trunk road network since August 2004 is that which occurred at approximately 0330 hours on Sunday 28 October 2007 on the eastern approach to the Rest and be Thankful. The event intersected the trunk road at approximate National Grid Reference (NGR) NN 23600 07000.
Figure 2.14 illustrates the event and the surrounding hillside; the photograph is taken from the opposite side of Glen Croe and evidence of numerous past events can be clearly seen. Figure 2.15 illustrates the event in more detail and it is clear that the system of mass movement comprises two discrete but related events.
Figure 2.14 – View of the hillside above and below the approach from the east to the Rest and be Thankful (from NGR NN 23160 06559 on the opposite side of Glen Croe). Not only can the event dated 28 October 2007 be clearly seen but evidence of numerous past events can be seen on the surrounding hillside.
A detailed site walkover revealed that the flow above the road commenced with a relatively small slide (or slides) into an existing drainage channel. This then triggered the movement of a large amount of marginally stable material in and around the stream channel which was deposited at road level. The Operating Company (Scotland TranServ) estimated that around 400 tonnes of material were deposited at road level. This material blocked the open drain which runs carries water along the road to a series of culverts beneath. While the material from above the road had limited impact upon the slopes below the road, water diverted from the drain was channelled across and over the edge of the road causing some significant undercutting of the slope below and associated deposition further down the hill as can be seen in Figure 2.15.
While not necessarily germane to the events reported here, it was observed that the culvert at this location was of a small size (around 400mm) and most likely of only marginal adequacy for water flows let alone for effectively carrying debris. In addition it is clear that the culvert does not follow a straight path, a feature that would reduce its capacity and increase the potential for blocking. Additionally, this means that water has been flowing from the culvert at an angle to the hillside of considerably less than ninety degrees. This is the most likely cause of the erosion observed in Figure 2.15 below the main road and to the left of the recent scar. The issues relating to this particular stretch of road are discussed further in Section 8.
Figure 2.15 – View of the debris flows above and below the A83 on the approach to the Rest and be Thankful (from NGR NN 23160 06559 on the opposite side of Glen Croe). The head scar is at approximately 370m AOD, the A83 at 240m AOD and the old road at 180m AOD.
2.3.1 Landslides in the Highlands
2.3.1.1 Background
In the Scottish Highlands, the combination of hard metamorphic and igneous rocks, glacially steepened valley slopes and high rainfall is ideal for generating debris flows and slides.
The bulk of the Highlands falls into the physiographic regions of ‘Western Plateaux and Foothills’, ‘Dissected Central Mountains’ and ‘Eastern Mountains Plateaux’ (Sissons, 1976), all of these regions being characterised by steep valley sides (>30o) and a mantle of varying thickness of granular morainic material and weathered rock. Several major geological structures also traverse the region, for example, the Great Glen Fault, and the existence of these features tends to increase the availability of shattered rock material.
In terms of levels of precipitation that may provoke a landslide event, the average annual rainfall in some Highland areas can exceed 4,000mm, occurring either in short bursts (summer convective storms) or in persistent medium to heavy falls (mainly autumn/winter).
Landslides affecting the road network in some way occur somewhere in the Highlands almost annually. Many affect only minor routes with little disruption to traffic. They are usually small-scale events and are cleared within a short space of time. However, some significant landslides have, as reported earlier in this section, have affected main trunk routes giving rise to severe disruption to traffic, as, generally speaking, there are few available diversion routes.
2.3.1.2 Landslide Types and Locations
Landslides of various types occur throughout the Highlands. These may include rotational slips, mainly in the soils associated with the Mesozoic rocks of Skye and east Sutherland (see Figure 2.16), mass movement of boulder fields (see Figure 2.17) and – by far the most common variety – debris flows/slides. Events of this latter variety have occurred on a number of occasions within the period 2004 to 2006.
Figure 2-16 – Part of a rotational slip at Flodigarry, Skye.
Figure 2.17 – Road at Duntulm, Skye being moved by mass movement of boulder field, accelerated by erosion at toe
Many of the landslides affecting roads occur in the western, more mountainous areas of the Highlands, the most noted locality being the A890 at the Stromeferry Bypass which has been known to be affected by landslides on several occasions in the course of a year. The less mountainous eastern areas are however not immune to landslide events, with several incidents having occurred within recent years. This is probably the Highland Council’s most problematic road with regard to debris flow activity. It continues to be the subject of ongoing study by consultants and a rigorous programme of inspection and maintenance on the part of the Council. The problems and issues relating to this locality have been discussed elsewhere (Nettleton et al., 2005a; 2005b).
Although the majority of the roads affected by slides and debris flows in the Highlands are B, C and unclassified routes, many of these are nevertheless locally very significant, in some cases being the only access routes to remote communities.
Recent landslides affecting the local road network in the Highlands have most notably occurred during August and December of 2004 and in October 2006. The slide in December 2004 at Glenelg was the largest in extent and volume of material. The August 2004 events were concentrated in Easter Ross and the Black Isle but are not discussed further herein. The approximate location of these events are illustrated in Figure 2.18.
Shiel Bridge to Glenelg Road, December 2004: On 6 December 2004, following a sustained period of heavy rainfall, a debris flow occurred on the C46 road between Shiel Bridge and Glenelg at Cnoc Fhionn in the Lochalsh area. Approximately 1,500m3 of material was brought down and the route was disrupted for two days (Figures 2.19 and 2.20).
Whilst many of the smaller landslides, and particularly debris flows, may have been exacerbated by factors such as poor roadside drainage, blocked field drains and other anthropogenic factors, the larger scale events, such as the Shiel Bridge to Glenelg slide, seem to be largely natural in origination.
Figure 2.18 – Map showing the trunk road network, including motorways, in Scotland. The locations of the debris flows on the local Highland road network are shown. The numbers 1a, 1b, 2 and 3 refer to the locations described in Figure 2.2.
Figure 2.19 – Head of slip near source at Glenelg.
Although not a principal transport route, the road is however of very high importance locally, being the only overland link between the communities of Arnisdale, Glen Beag, Glenelg and the outside world.
Figure 2.20 – Debris causing road closure at Glenelg.
The local geology has led to the formation of a step-like hillside topography (Figure 2.21) and Figure 2.22 shows distinct linear features which have acted as accumulation zones for water. This has led to a potential for slippage over the entire hillside.
Figure 2.21 – Glenelg Landslip, likely failure mechanism.
Figure 2.22 – Slips on hillside above and below road at Glenelg.
Easter Ross and Sutherland, October 2006: A number of local roads and properties in localities between Dingwall and Helmsdale were affected by flooding and landslides on 25 and 26 October 2006.
Torboll (north of Dornoch), October 2006: A combination of toe erosion and a cascade of water down a steep slope on the road led to complete collapse of the natural slope below the unclassified road between The Mound and Bonar Bridge at Torboll (to the north of Dornoch). Part of the already-narrow carriageway was undermined (Figure 2.23) and subsequently progressively collapsed, with the safety barrier being left suspended (Figure 2.24).
Figure 2.23 – Landslide at Torboll, 26 October 2006.
Quebec Bridge – (south of Tain), October 2006: Drainage system problems, which have caused difficulties previously in this locality, led in this instance to a massive amount of water being channelled down the roadway, causing erosion and slippage and major damage to the bridge structure (Figure 2.25).
Figure 2.24 – Torboll, showing ongoing erosion, 14 February 2007.
Figure 2.25 – Landslide at Quebec Bridge, October 2006.
B9176 Struie road at New Bridge (north of Alness), October 2006: Two debris flow slides on the uphill side of the road at New Bridge on the B9176 Struie road in Easter Ross deposited large amounts of debris on the carriageway on 26 October 2006 (Figure 2.26). The water and debris continued to flow over the road causing significant erosion and deposition of material on the downhill side also (Figure 2.27). Blocked agricultural drains above the slope on the uphill side were thought to have been a main contributing factor. Major repair and reconstruction work was necessary, especially on the downhill side, but in addition small debris-retaining gabion structures were constructed on the uphill side to attempt to contain any reoccurrence (Figure 2.28).
Figure 2.26 – Struie slide at south end.
Figure 2.27 – Erosion and deposition below road, Struie.
Figure 2.28 – Gabion debris trap, Struie.
A862 Ardullie (north of Dingwall), October 2006: A minor slip occurred above the road (Figure 2.29) at Ardullie, just west of the Cromarty Bridge, depositing material on the road and causing erosion of the downhill (seaward) slope (Figure 2.30). A void appeared adjacent to the seaward side retaining wall indicating that the downhill embankment slope had been subject to movement.
Figure 2.29 – Ardullie slip above road.
Figure 2.30 – Erosion on downhill side, Ardullie.
2.4 SEASONALITY
Observation indicates that, within the recent past, debris flow activity in Scotland has occurred largely in the periods July to August and November to January, with the latter season occasionally stretching to October and February. There is, of course, no certainty that such a pattern will be continued in the future, even though eastern parts of Scotland do receive their highest levels of rainfall in August. Additionally, climate change models indicate that rainfall levels will increase in the winter but decrease during the summer months and that intense storm events will increase in number. These factors, therefore, may change both the frequency and the annual pattern of debris flow events.
In recent years debris flow events do 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 a large number of debris flows are initiated on the Scottish hills. HoweveroweverH, 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 magnifying 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.
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. 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 (Nettleton et al., 2005a).
Logging or deforestation can have a dramatic effect on the drainage patterns of a slope, reducing root moisture uptake and removing the physical restraints on downslope water flow (for example), as well as disrupting root systems that help to reinforce to slope. 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 January 1994.