by M G Winter, F Macgregor and L Shackman

The process described in the previous section culminates in a decision on whether the hazard ranking, in the context of the safe operation of the road network at any location, is acceptable or not. At those locations where the hazard ranking is deemed unacceptable, some form of mitigative action is required. To reduce the hazard ranking (or risk) to the road user to acceptable levels, either the magnitude of the hazard and/or the potential exposure or losses that are likely to arise as a result of any debris flow, must be reduced.

The reduction of the exposure of road users forms the main focus of the work here. In this case the debris flow event is taken as a given and either the number of people exposed to the hazard must be reduced, for example by closure of the road, or warning must be given to exercise caution at appropriate times and places.

To reduce the hazard itself, physical intervention is required but in many cases the options will be of higher cost and more intrusive. It is anticipated that relatively few locations will justify expenditure to this degree.


The reduction of exposure lends itself to the use of a simple and easily-remembered, three-part management tool (Winter et al., 2005a) as Detection-Notification-Action (DNA), as follows:

  • Detection: The identification of either the occurrence of an event (e.g. by instrumentation/monitoring or observation) or by the measurement and/or forecast of precursor conditions (e.g. rainfall).
  • Notification: The notification of either the likely or actual occurrence of an incident to the authorities: including the Police, Traffic Scotland, Transport Scotland and the relevant Operating Company.
  • Action: The proactive process by which intervention reduces the exposure of the road user to the hazard, by for example road closure or traffic diversion. This also includes the dissemination of hazard(s) and exposure information by for example signs, media announcements and ‘landslide patrols’ in marked vehicles.

In the current situation, the DNA approach to mitigation must be reactive to debris flow events. There may be a case for reacting to extremely heavy rainfall events. However, a caveat to this is the need to consider carefully at what levels the triggers should be set, insofar as the relation between rainfall and landslides/debris flows in Scotland is by no means fully understood.

In the longer-term, the detection of precursor triggering conditions (i.e. rainfall) may enable both the Notification and Action phases to be taken in anticipation of the occurrence of major events. However, to do this an extensively enhanced rainfall detection network will be required across Scotland. Even once this is in place it is fully expected that it will require some considerable time and effort to ensure that sufficient data has been obtained and analysed so as to be able to introduce a reliable warning system. Even then atypical events, which are not the subject of warnings, and false alarms are to be expected. A programme of public and media education and awareness-raising is likely to be desirable to minimise any potential adverse reaction to such scenarios. Such an approach is discussed in more detail in Section 9 and by Winter et al. (2007c).

8.1.1 Event Occurrence

Detection: The movement of slope material can be monitored in real time and used as a management tool. Monitoring instruments such as tilt meters and acoustic sensors can be installed and located so as to record movement from potential debris flow or positioned such that notification is received if debris reaches or gets close to a road (e.g. trip wires).

If movement monitoring is being considered, it must be appreciated that the seeding area for debris flows can be very large and can be located high on the hillside. This introduces considerable difficulty in pinpointing the optimum location for the installation of the monitoring system and raises uncertainty as to whether the debris will reach the road. As an example, an instrumented fence is used on the Scottish rail network at Glen Douglas to recognise when material impinges upon the line. Similarly, a system to detect rock falls and debris flows is used at the Pass of Brander above the A85 at Loch Awe to raise an alarm to shut the line and stop trains.

Whether such a system would be sufficient in isolation, and in the context of a road, is questionable. It is, however, considered that in conjunction with rainfall monitoring and possibly the deployment of operatives, the likelihood of road users being affected by debris flow events could be reduced significantly. It is likely that any instrumentation would be electronic with remote reading of data sent back to a central control point. The range of possible electronic instrumentation types (data sent to a central control point) is presented in Winter et al. (2005a) but includes the following: borehole or shallow inclinometers, tilt meters, ‘trip wire’, ‘ball of string’, acoustic meters, and remote sensing. The selection of appropriate instrumentation is a highly site-specific activity and thus requires very detailed evaluation of a number of factors, not least physical access and availability of telecommunications.

An alternative approach is to use visual observation to detect debris flow events either by closed-circuit television or, more practically given the constraints imposed by darkness and poor visibility, for long stretches of hillside, by introducing ‘landslide patrols’ during periods of high rainfall. It is essential that such landslide patrol operatives are trained in what to look for and that patrol vehicles should operate in pairs for safety reasons. Given the wide range of locations at which debris flow activity may be experienced the use of patrols might prove to be a more practical alternative, the costs of instrumenting and monitoring extensive lengths of slope being potentially prohibitive. Furthermore, the issue of inadvertent activation of systems such as trip wires by, for example, livestock and hill-walkers would need to be addressed in the context of the road network, access to which is less constrained than is the case for the rail network. In addition, the value of observations made by the general public should not be underestimated, especially given the proliferation and ubiquity of mobile telecommunications.

Notification: In the immediate aftermath of the occurrence of a debris flow event, notification must reach the Police, the Operating Company and the infrastructure owner. The decision must then be made rapidly as to what action is to be taken (see below). The nature of debris flows is such that in most cases the road will be blocked and therefore closed to all intents and purposes. It is important that such closures are formalised at safe locations distant from further potential events.

It is important to note that if landslide patrols, comprising trained personnel, are used as part of the Detection process then the functions of that role must also be extended to ensure that the proper authorities are notified promptly. It should also be noted that the effectiveness of such patrols for detection will be extremely limited in other than full daylight. It may well be that such patrols have more value in rendering assistance to the public in the aftermath of an event than in actually spotting an impending flow.

Action: Following a debris flow a number of positive options for action are available. First the road length (or lengths) affected could be closed and the appropriate pre-planned diversion routes put in place. However, it is important to note that closing the road only in the area immediately adjacent to the event is not an adequate response. Debris flow propensity generally affects long lengths of hillside and an evaluation of the vulnerable area must be performed in order to ensure that an appropriate length of road is closed.

Closure might, for example, be achieved by installing barriers similar to the snow gates present on some of Scotland’s roads (Figure 8.1). Such an approach is applied to the Sea-to-Sky Highway in British Columbia, a route well-known for its propensity to disruption due to debris flow, and gates are in place on this route for the specific purpose of closing the road in the event of debris flow (Figure 8.2).

Figure 8.1 – Snow gates on the A9. These are used to close the road over Drumochter Pass when it is impassable due to snowfall. These and similar installations could also be used to close the road in the event of debris flow.

Figure 8.1

A road closure may only be ordered by the Police. However, in practice, such decisions are usually made in consultation with and/or on the recommendation of other appropriate bodies. In this case such bodies are likely to include Traffic Scotland, who collate and distribute information about the factors likely to affect traffic flows, and the relevant Operating Company.

Warning the public of hazards is an important feature of any Action programme. In Scotland there is a variety of potential means of making public announcements when either debris flows have occurred or there is heightened likelihood of their occurrence in an area. This might involve real-time systems such as traffic information websites, variable message signs (VMS) (Figure 8.3) and media (radio, TV and web) announcements notifying drivers that their potential exposure to the hazards posed by debris flows is real and present. Announcements could also be linked into traffic guidance systems. The overt use of landslide patrols, as describe above, can assist in this process by heightening the awareness of road users to potential hazards.

Figure 8.2 – Gates used to effect road closure following landslide events on the Vancouver to Whistler, Sea-to-Sky Highway, British Columbia, Canada.

Figure 8.2

Figure 8.3 – A variable message sign located on the A9. The network of such signs in Scotland is being significantly increased and will form a crucial part of the strategy for warning road users of hazards including landslides.

Figure 8.3

Static signs may also be used to convey both general information on the nature and locations of potential hazards and also to convey specific instructions (Winter et al., 2007a) (see Section 8.1.3).

It is important to ensure that measures such as the provision of signs, especially VMS, and landslide gates are at suitably strategic locations on the network. Such locations should allow drivers to make, and implement, a decision as to whether they proceed with their planned route, use an alternative or cancel their journey. For example, the VMS sign illustrated in Figure 8.3 is located on the approach to a major interchange and the snow gates illustrated in Figure 8.1 are located at the beginning of a stretch of dual carriageway where provision is made for traffic to be able to turn around.

The purpose of any road closure is to ensure that road-users are strongly discouraged from entering high risk areas. VMS signs may be used to alert the public to road closures that may be on their route and also to provide information at times of heightened hazard. In general such signs work on a basis of:

  • Problem.
  • Location.
  • Effect.
  • Guidance.

VMS signs are based upon either a 3 by 18 or a 4 by 15 character grid and thus suitable wording for signs might include:




The text in parentheses is intended for use on 4 by 15 character grid signs and would be omitted on 3 by 18 signs.

Correctly-trained operatives also could be deployed on high hazard ranking sections of road during periods of predicted or actual high rainfall. One approach might be that these operatives, in their vehicles, could escort people through the high hazard ranking sections of road in convoy. However, it must be understood that while this moves platoons of traffic past a potential hazard in a relatively short time, if a convoy were to be hit, the outcome could be more serious than might otherwise be the case.

In all cases, re-opening of the road, or its return to normal operation, can only occur after a thorough inspection of the road and the adjacent slopes has been undertaken to ensure that the likelihood of further debris flow events is at an acceptably low level. Current (and recommended) practice is to undertake ground-based inspections only when the adverse weather has abated and only to reopen the road once such inspections indicate that the residual hazard and exposure are again at an acceptable level.

In terms of public information, there is a strong argument for pre-empting the potential incidence of debris flow events, particularly in areas where these are relatively frequent (e.g. A83) and having pre-prepared Press Releases and Ministerial Briefings available in advance. Such information could be located within part of a ‘Core Briefing’.

8.1.2 Precursor (Preparatory or Trigger) Conditions

This subject is discussed in more detail in Section 9 in terms of both the underpinning science and the detailed operational issues that will need to be addressed. However, it is appropriate to detail the management strategy in terms of DNA at this juncture.

Detection: Debris flows are initiated, in the main, by heavy rainfall in combination with other conditions. Forecast and real-time rainfall data for an area with adverse topographic or other conditions is extremely useful information. If high rainfall is forecast or recorded in such areas then the potential for debris flows will be higher. In certain parts of the world weather forecasting and thereafter rainfall monitoring in real time are two of the controlling factors in landslide management. For example, the very successful system run in Hong Kong, as well as systems developed in other parts of the world (see Section 9), monitors rainfall and passes information on the resulting heightened likelihood of landslide activity to the public.

In the case of Hong Kong, a comprehensive network of automatic rain gauges covers much of the region to record and send data to a central control point for real time analysis. This is combined with short-term forecast data to enable managers to monitor the development of rainfall events and make informed decisions in an expedient fashion.

Whilst a predictive capability is under development in Scotland (Winter et al., 2007c; see also Section 9) with the intention of reducing the exposure of road users to the effects of debris flows, it must be understood that in Hong Kong, for example, around 30 years of experience has been acquired. This means that a sound knowledge of the relation between rainfall and landslides is in place relating to the local climate and geology. It is clear that some considerable time would be required to build a similar knowledge base for Scotland, possibly a minimum of five years. A significant investment in instrumentation, data analysis and maintenance would however also be required. Notwithstanding this, ongoing developments of the rainfall radar network mean that it can reasonably be expected that the use of these techniques will become an increasingly rich source of data in the future.

Notification: In Hong Kong, the conditions for issuing a ‘Landslip Warning’ are that a prediction has been made that numerous (more than about 15) landslides will occur. At this point the relevant Government bodies must make a rapid and effective decision to issue the warning. In Scotland, a decision as to which geographical area the warning should be applied will need to be made.

Action: Once again in Hong Kong, if the conditions for a landslip warning are met then the public are alerted to reduce their exposure to possible danger from landslides. Pre-defined procedures are also executed to ensure that resources are mobilised to deal with incidents. In addition, the issue of a landslip warning triggers an emergency system within various Government Departments that mobilizes staff and resources to deal with landslide incidents. Although a landslip warning is issued when it is predicted that numerous landslides will occur, it is accepted that isolated landslides may occur from time to time when a landslip warning is not in force and that landslip warnings will occasionally be issued and not be followed by landslides. Landslip warnings are issued by means of website notices, media announcements and notices prominently displayed in public buildings and areas.

Such a system implemented in Scotland would mean that, once warnings are received that heavy rain is forecast or falling in an area recognised as being of high or very high hazard ranking, a number of options are available for action. These are broadly similar to those described under event occurrence. However, in the case of road closures it is necessary to be aware of a significant disbenefit of this approach if applied in anticipation of any event occurrence. In contrast to the situation in Hong Kong (where a landslip warning is issued only when multiple events are forecast), the relatively rare occurrence of Scottish debris flows – at least those that interact with the trunk road network – and the high levels of rainfall that Scotland receives, means that a number of false alarms could be expected. The public at large could, potentially, become desensitized to what could be seen as an overly conservative approach.

An alternative approach could be to simply notify the public of the heightened likelihood of debris flow development in an area, as described above, and to take no further action until an event occurred.

8.1.3 Static Signing to Reduce Exposure

Static signing can provide a valuable addition to the Action-based signing described in Section 8.1.1. Static signing has the advantage of being permanent, indicating zones of hazard and also of raising awareness of such hazards.

A brief review of the manner in which landslides (and some other) hazards are portrayed on static signs in other parts of the world is presented in Appendix E. In essence, most countries use a symbol similar to standard rock fall symbol used in the United Kingdom (Figure 8.4). This may be accompanied by a sub-plate indicating the distance over which the hazard exists. In the UK, at present, this sign specifically relates to rock fall (TSRGD, 2002).

Figure 8.4 – UK rockfall sign (TSRGD, 2002: Diagram 559) indicating risk of falling or fallen rocks.

Figure 8.4

However, the aim herein is to determine a form of sign suitable for signing other types of landslides, primarily debris flow. In the absence of other models and suitable graphics for other types of landslide it is recommended that the standard rock fall sign be adopted for signing other types of landslide by the addition of a sub-plate stating ‘Landslides’. The sub-plate should also indicate the distance (in miles or yards: TSRGD, 2002, Schedule 16, Regulation 17(1), Item 6) over which the hazard extends (e.g. Figure 8.5). Alternatively, signs could be placed at both ends of the hazard length with ‘Landslides’ on the sub-plate at the start of the section and either ‘End’ on the sub-plate at the finish or no sub-plate at all with the symbols scored through. It should be noted that the use of such non-standard symbols, in specific instances, would require approval from Scottish Ministers. Additionally, changes to the Regulations (TSRGD, 2002) would require the agreement of the UK Government as represented by the Department for Transport.

Figure 8.5 – Indicative for of proposed signing for debris flow hazard.

Figure 8.5

It is important to ensure that such signs are not over-used. It is recommended that they be restricted to sites of significant landslide hazard ranking (those with a hazard ranking of 100 or greater) as identified in Section 7. At some sites, those with availability of electrical power or suitability for solar power, flashing lights may be provided to give a degree of temporal alert in line with the VMS signs described above. This approach has the potential to overcome the potential for such signs becoming ignored due to familiarity. In time, these, along with the VMS network, could be linked into a landslide warning system as described in Section 9.

Other types of sign warn specifically of actions to be taken to minimise the exposure of hazards related to stream-based debris flows (Figure 8.6).

8.1.4 Education to Reduce Exposure

In British Columbia, more general hazard information signs (Figure 8.7) are located in lay-bys and indicate areas in, and routes on, which hazards might be encountered. Signs which give information on the nature and background to the hazards are also provided. Both of these types of sign, in addition to providing information on the type and location of the hazard, provide advice on what not to do in the hazard areas.

Figure 8.6 – Sign indicating exposure reduction measures for stream-based debris flow hazards (Vancouver to Whistler, Sea-to-Sky Highway in British Columbia in Canada).

Figure 8.6

Figure 8.7 – Landslide hazard information sign (Vancouver to Whistler, Sea-to-Sky Highway in British Columbia in Canada).

Figure 8.7

Static educational materials could play a valuable role in helping the public to understand the nature of landslide hazards in Scotland and also of placing such hazards in a balanced context. Such information should describe the geological and geomorphological setting of the area and include landslides as one of the inevitable consequences of the setting. Including reference to landslides in the broad natural history of the area would also help in the dissemination of such worthwhile information.

An excellent example of the type of sign in question is located beside the A938 at Dulnain Bridge. It primarily illustrates the manner in which the distinctive Roche Moutonée features were formed during glacial times, but also provides information on the local flora and fauna as well as a small fire hazard warning (Figure 8.8).

Figure 8.8 – Educational sign adjacent to the A938 at Dulnain Bridge near Grantown-on-Spey.

Figure 8.8

Suitable locations for similar educational materials might include National Park Gateways, service areas (e.g. Bankfoot, House of Bruar and Ralia on the A9) and could be integrated into other information sources at these locations with the agreement of the operators. The involvement of the Local Authorities, Scottish Natural Heritage and, where appropriate, the National Park Authorities and the Forestry Commission would be beneficial.

An information leaflet (see Appendix F) has been drafted for posting on the Transport Scotland website and possibly for wider circulation at a later date. The leaflet is intended to help the process of informing and educating the public as to the nature of landslides and what to do and what not to do in the event of being caught up in such an event. It particularly focuses upon the need to adopt a precautionary approach in terms of individual exposure to landslide risk.


The challenge with hazard reduction is in identifying locations that are of sufficiently high hazard ranking to warrant spending significant sums of money on engineering works. The costs associated with installing remedial works over long lengths of road are difficult to justify in economic terms and may well be unaffordable. Moreover the environmental impact of such engineering work should not be underestimated, having a lasting visual impact at the least and potentially other more serious impacts. It is considered that such works should be limited to locations where their worth can be proven.

Notwithstanding this, simple measures such as ensuring that that channels and gullies are kept open can be effective in terms of hazard reduction. This requires that the maintenance regime is fully effective both in routine terms and also in response to periods of high rainfall, flood and slope movement.

It is also important that maintenance and construction projects currently in design take the opportunity to limit any hazards by incorporating, where suitable, measures to achieve higher capacity or better forms of drainage, or debris traps. In particular, critical review of the alignment of culverts and other conduits close to the road should be carried out as part of any planned maintenance or construction activities.

Typically, achieving a reduction in the hazard will entail physical engineering works to change the nature of a slope or road to reduce the potential for either initiation and/or the potential for a debris flow to reach the road once initiated. Debris flows are dynamic in nature and quite often originate some distance above the road; when they reach the road they are relatively fast-moving, high-energy flows. The energy of these systems is a significant factor in determining the nature of the engineering works that can be used to effectively reduce the hazard to the road and its user. Hence, there are three broad approaches to the selection of hazard reduction works:

  • Road Protection: Accept that debris flows will occur and take measures to protect the road. Potential solutions include debris basins, lined debris channels, debris flow shelters, overshoots and barriers (including ditches, walls and fences).
  • Debris Flow Prevention: Carry out engineering works to reduce the opportunity for a debris flow to occur.
  • Road Realignment: Realign the road.

These options are also reviewed in detail by Winter et al. (2005a; 2006b; 2007a). In the context of this project and the Scottish environment, it is anticipated that few of any such actions will be appropriate to deal with the widely-dispersed hazards extant on the Scottish trunk road network. Their use should be limited to locations where their worth can be demonstrated within the broader context of construction and maintenance budgets and priorities. However, it is worth outlining the potential measures that might be used in appropriate situations.

8.2.1 Road Protection

In relation to the road protection approach there are not many examples of this kind of engineering work in Scotland or the rest of the UK, but in some upland areas of mainland Europe such engineering is relatively commonplace. The energy of the debris flow is such that any rigid barrier constructed to protect the road would have to be designed for very high loads. In essence a debris flow has significant momentum and to bring it to a sudden stop, as is the case with a rigid barrier, would require the dissipation of a lot of energy, instantaneously imparting very high loads. Examples of solutions which have proved feasible are described in the following paragraphs.

An Austrian Standard for the ‘Design of [debris flow] structural mitigation measures’ is currently in draft and is based upon the requirements of Eurocode 1 (Huebl & Proske, 2008). In time this document, as well as those by VanDine (1996) and Couture & VanDine (2004), may prove to be a useful source should such structures be required.

Debris Basins: Each debris basin comprises a large decant structure and a downstream barrier designed and constructed as an earthfill dam capable of retaining water to full height in the event of the drainage outlet(s) becoming completely blocked (Figures 8.9 and 8.10). The basin illustrated in Figure 8.9 is estimated to be approximately 200m across with a well-defined stream bed running towards the outlet structure (Figure 8.10). The outlet structure is not insignificant in size (Figure 8.11), however compared to the basin as a whole (Figure 8.9) it appears relatively small. One or more debris basins may be used in a given catchment.

Figure 8.9 – Debris basin showing the downstream barrier and drainage outlet, Mackay Creek, North Vancouver, British Columbia, Canada.

Figure 8.9

Figure 8.10 – Debris basin showing the downstream barrier and drainage outlet, Mackay Creek, North Vancouver, British Columbia, Canada.

Figure 8.10

In larger examples a concrete spillway is often incorporated into the downstream face of the barrier to protect the earthfill from erosion in the event of overtopping. Irregular surface features may be used to slow the passage of the debris (Figures 8.12 and 8.13). The channel below the structure may be lined with either concrete or with concrete/boulders to control both flood flows and debris flows in the event of over-topping (Couture & VanDine 2004) The former approach will facilitate rapid movement of water and debris downslope while the latter will slow the flows and provide a degree of temporary retention as described below.

Figure 8.11 – Downstream drainage outlet of debris basin, Mackay Creek, North Vancouver, British Columbia, Canada. The black disc visible on the base of the structure is 72mm in diameter.

Figure 8.11

Such structures on the scale used in British Columbia may not be viable in Scotland for a number of reasons, including the smaller scale landscape along with aesthetic and other environmental considerations. However, where cyclical hazards are identified as having a short return period, smaller scale structures may be appropriate (e.g. Figure 8.14).

Lined Debris Channels: Where storage space upstream of the road is limited an alternative approach may be taken by allowing material to move safely beneath the road and on to a safe repository area, usually a large body of water such as the sea or a loch, or similar. Couture & VanDine (2004) illustrate the use of a steel-fibre reinforced shotcrete lining in smooth well-aligned stream channels in order to move material smoothly and swiftly below the road (Figure 8.15). It is also recognised that relatively low cost, simple improvements to channel flow down to and beneath the road may have a beneficial effect; this may be achieved by widening culverts, for example. An alternative approach is illustrated in Figures 8.16 and 8.17, where boulders have been embedded into the concrete channel lining in order so as to provide a degree of retardation to the flow of water and debris.

Debris Flow Shelters: Rock shelters or ‘avalanche shelters’ are engineered structures that form canopies over a section of road subject to high hazard levels from rock fall or debris flows. These structures are usually formed from reinforced concrete. There is a Scottish example of such a structure on the A890 north-east of Stromeferry (Figure 8.18) in the north-west highlands (see Winter et al., 2005a). This structure straddles both the road and railway at that location. Energy is dissipated by placing a depth of granular material on the roof on which the debris flow lands.

Figure 8.12 – Concrete spillway on the downstream face of a debris basin barrier, Charles Creek, Sea-to-Sky Highway, British Columbia, Canada. The drainage outlet may be seen in the centre of the spillway.

Figure 8.12

Figure 8.13 – Detail of a concrete spillway on the downstream face of a debris basin barrier, Harvey Creek, Sea-to-Sky Highway, British Columbia, Canada. The drainage outlet may be seen in the centre of the spillway.

Figure 8.13

Figure 8.14 – Debris stilling basins at Frenchman’s Burn, A890 Stromeferry, Highland. (Courtesy of Ian Nettleton, Coffey Geotechnics.)

Figure 8.14

Figure 8.15 – Stream/debris channel, Alberta Creek, Sea-to-Sky Highway, British Columbia, Canada.

Figure 8.15

Figure 8.16 – Stream/debris channel, Harvey Creek, Sea-to-Sky Highway, British Columbia, Canada.

Figure 8.16

Figure 8.17 – Detail of stream/debris channel, Harvey Creek, Sea-to-Sky Highway, British Columbia, Canada.

Figure 8.17

Figure 8.18 – Stone shelter on A890 north-east of Stromeferry.

Figure 8.18

Debris Flow Overshoots: In situations where the energy is anticipated to be very high, modifications can be made to debris flow shelters to allow the debris flows to pass over the top of the structure. This is done by shaping the top of roof of the shelter such that the falling material passes over the structure without dissipating its energy. This shaping or profiling involves constructing a ‘ski-jump’-type reinforced concrete structure. Flow material simply slides over the roof and continues down the hillside.

Barriers and Fences: Fences can be constructed to act as effective barriers to halt debris flows. Such fences are designed to be flexible so that the kinetic energy of the debris flow is dissipated over a short period of time, thus reducing the forces that the structure has to cater for. These systems have been shown to work well. Such a fence has been installed on the Inverness to Kyle of Lochalsh railway in Scotland (see Winter et al., 2005a). Such fences do however require maintenance after the impact of a debris flow. A related approach has been taken to the arrest of rockfalls using highly flexible fences with fixed end-points only (see Figures 8.19 and 8.20).

Flexible fixed-position fence structures are commonplace in upland areas of mainland Europe and, while the UK does not have engineering design standards for such structures, experience is available and formalised procedures do exist, particularly in Switzerland.

Less flexible barriers may also be used to trap or divert debris flow and may be formed using stiffer components. Such structures may include gabion baskets as illustrated in Figure 2.28. However, more common are check dams and baffles which are used to slow and partially arrest flow within a defined channel. Barriers may also be constructed across hillsides in order to protect larger areas where open hillside flows are a risk and/or channelised flows may breach the stream course.

VanDine (1996) cites the use of check dams and baffles and also gives some design guidance for such structures. VanDine also relates the use of low cost earth mounds that act as impediments to debris flow. However, one of the main issues with the use of such structures, low cost or otherwise, is that they are effective in slowing and arresting flow primarily in the debris fan area. The situation in Scotland is that most, if not all, of the roads potentially affected by debris flow are located in either the high energy transport zone or the upper reaches of the debris fan. Roads located on debris fans frequently run close to a loch side and therefore the opportunity for the use of these types of measure tend to be limited.

Figure 8.19 – Flexible rockfall catch fence, Abbey Craig, Stirling.

Figure 8.19

Figure 8.20 – Flexible fence with a fallen rock that has been successfully arrested, Abbey Craig, Stirling.

Figure 8.20

Rigid barriers were built as debris flow defence structures at Sarno to the east of Naples in Italy following the events of May 1998 in which 159 people were killed (see also Appendix G.3.2) (Versace, 2007). At Sarno itself a series of debris basins has been constructed. That illustrated in Figure 8.21 has a capacity of around 176,000m3. Rigid barriers in the form of combination reinforced concrete barrier-and-trench structures extend across the foot of the hills for up to a kilometre either side of the basin; check dams have also been constructed in the main stream channels (Versace et al., 2007). The works have been the subject of extensive landscaping and sports fields and other facilities such as cycle tracks (e.g. Figure 8.22) have been incorporated. The cost of the works, including the ancillary works, has been estimated at between €20M and €30M (Versace et al., In Press).

Figure 8.21 – Debris basin with a capacity of approximately 176,000m3 at Sarno in Italy.

Figure 8.21

Figure 8.22 – Debris basin with a peripheral cycle track at Sarno in Italy.

Figure 8.22

In addition to the problems of locating barriers suitably, there remains the issue that (whatever type of barrier is used) provision must be made for maintenance – particularly to allow the regular removal of retained material.

8.2.2 Debris Flow Prevention

The engineering solutions applicable to the prevention of debris flow will depend greatly upon the individual circumstances. Debris flows can have a relatively large source area and be initiated very high up on the hillside above the road. In most circumstances the opportunities for carrying out conventional remedial works that would restrain the material before it starts to move are considered to be very limited. There may be particular conditions where a combination of techniques such as gravity retaining structures, anchoring or soil nailing may be applicable. However, in general terms the cases where these are both practicable and economically viable are likely to be limited.

The link between debris flows and intense rainfall has been established previously in this document. As a result, effective runoff management can reduce the potential for debris flow initiation. In the circumstances of the debris flows that occurred in the summer of 2004, it is considered that on-hill drainage improvement would have had little impact because of the scale of the events. In other locations and situations positive action to improve drainage might well have a beneficial effect. Such measures could include improving channel flow and forming drainage around the crest of certain slopes to take water away in a controlled manner.

8.2.3 Road Realignment

Road realignment is undertaken as part of Transport Scotland’s route improvement activities in order to improve the road in terms of both alignment and junction layout, in particular to reduce accidents and to ensure compliance with current design standards. In cases where the debris flow hazard ranking is high and other factors indicate that some degree of reconstruction is required, road realignment may be a viable option. This type of expedient has historically been used on the Scottish rail network, for instance at Stromeferry, Penmanshiel and Dolphinston, where hazards have been sufficiently significant to justify the high cost of such realignments.

Figure 8.23 shows the A86 trunk road at Loch Laggan. At this site there were clear signs of distress to the carriageway caused by an extensive series of what are, individually, relatively minor landslides. However, taken collectively they posed a serious threat to the stability of the road and combined with the steep hillside, the narrow existing carriageway with a poor alignment, and the need to undertake a full depth reconstruction of the pavement the decision was made to realign the road (Figure 8.24).

Figure 8.23 – Deformation of the A86 carriageway alongside Loch Laggan due to a series of minor landslides.

Figure 8.23

Figure 8.24 – Aerial photograph showing the realignment of the A86 carriageway alongside Loch Laggan. The new alignment may be seen to the north-west of the old alignment (north is towards the top of the image).

Figure 8.24

8.2.4 Drainage

Clearly it is important that issues surrounding drainage and culverts are considered. These fall into two main categories.

First, routine inspection and clearing of drainage channels and culverts must be seen as a priority on the trunk road network and its surroundings. Ensuring that ditches and culverts on the trunk road network are kept clear forms part of the responsibilities of the Operating Companies. The issue of more distant stream channels and the potential for these to temporarily dam and subsequently promote debris flow is however more difficult. Nonetheless, a degree of cooperation with land owners immediately adjacent to high hazard ranking areas of the trunk road network, in order that mutually beneficial improvements to the drainage regime may be undertaken, could prove productive and is an approach that should be pursued.

Second, major systemic improvements to the drainage at road level, including enlarged/enhanced culverts and other drainage features to accommodate debris should be considered. Increasing the capacity of drainage systems also fits well with the changes to UK National Standards (DMRB 4.2.3 HD33/06) implemented in response to forecast climate change. These are also described in the example case study (Section 8.3.2). In addition to capacity it is important that alignment and shape are optimised – there should be a predisposition towards straightness and cross-sections should be based upon efficient circular and square shapes. The importance of carefully considering the drainage provision at the slope-road interface is highlighted in Section 8.3.2. There would be benefit in a using a more formalised approach for drainage auditing, inspection and maintenance on sites at risk from debris flow.

8.2.5 Land Management

The presence of forestry is known to be a positive feature in the minimisation of debris flow in terms of both occurrence and magnitude. However, more importantly, commercial deforestation can significantly increase the propensity for debris flow. Indeed, 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 and slope reinforcement due to the root systems, increasing infiltration in some areas while removing physical constraints on downslope water flow in others. Furthermore, it is considered that the effects of deforestation can leave land in a more susceptible condition than it might have been if the tree-planting had not been undertaken in the first place.

The practice of clear-felling, whilst not so widespread as it once was in Scotland, can have particularly severe effects as the whole hillside is denuded of vegetation. Where the practice of clear-felling has been abandoned in favour of leaving areas of trees standing then this has largely been from the point of view of improving the aesthetics of the remaining hillside. While this is in itself a laudable objective, there is a clear need to adjust and adapt such practices in order that hillside stability is not decreased in addition to addressing aspects (as notably used in British Columbia in Canada) that could be of great benefit to the practices adopted in Scotland. This should be seen as a priority follow-up to this project in order to begin the process of ensuring that the current situation is not made worse by potentially ill-considered deforestation operations over the coming decade. This process will require dialogue with the Forestry Commission.


The overall objective behind this study work is to improve the safety of the road-using public allied to improving journey-time reliability. It is also recognised that by better managing the effects of landslides the effects of severance on remote communities are reduced, and this contributes to the objectives of accessibility and social inclusion.

To deliver this overall objective the following management actions (which are in effect the ‘A’ of the DNA, or Detection–Notification–Action, process described earlier), as described more fully in Sections 8.1 and 8.2, are considered essential:

  • Integration of landslide-specific requirements into the VMS network.
  • The erection of static signs to indicate the beginning, extent and end of sites of significant landslide hazard ranking (sites with a hazard ranking of 100 or greater) as identified in Section 7.
  • The implementation of a systematic landslide patrols approach.
  • Consideration of the need for landslide gates at locations where a physical closure may be deemed necessary. An obvious hazard area where such an approach would be appropriate is the A83 in the Rest and be Thankful area.
  • In consultation with other stakeholder organisations, the provision of information signs in lay-bys, rest areas and at entry points to National Parks for example. Suitable sites for such provision might also include the rest areas on the A9 at Ralia and House of Bruar and the lay-by at Duck Bay on the A82.
  • The content of the draft leaflet on ‘Scottish Roads and Landslides’ (Appendix F) should form part of the material for the information signs described above. The leaflet should be made available in electronic form (on the Transport Scotland and Traffic Scotland websites) and in possibly in hardcopy at the sites described above at a later date.
  • The need for more systematic reviewing of the drainage provision in areas at risk from debris flows should be considered by Transport Scotland.
  • A strategy for dealing with land management issues in the light of debris flow potential should be considered by Transport Scotland in consultation with other stakeholders such as the Forestry Commission.
  • The proactive detection of debris flows by means of rainfall monitoring is set-out in detail in Section 9 and forms a vital part of the management actions described here.

8.3.1 Closure and Decision Points

Traffic Scotland is responsible for the management of traffic on the Scottish trunk road network. Decisions on potential closure points should be undertaken in consultation with them and the local police force. Work in terms of Emergency Standard Diversion Routes (ESDRs) and Transport Scotland’s Asset Management Plan (AMP) is ongoing and will determine both closure points and diversion routes to be implemented in response to all types of incident across the network, including landslides.

However, in terms of any potential landslide incident, or series of incidents, there would be a number of potential types of closure and decision point. These would be:

1. Primary destination decision point: Early signing of closures in order to enable traffic to remain on direct primary routes to destination – most suited to long distance traffic. This type of decision is being developed, along with diversionary routes for the entire trunk road network, by the ESDR study being undertaken by Transport Scotland.

2. Junction-based decision point: The point of closure beyond which only traffic with a need for access to points between this closure and that described at item (3) below should pass. This essentially diverts through traffic whilst allowing local traffic maximum use of the available network. Diversions are signed from this point. This type of signing is also, to a large extent, being undertaken via the ESDR. Locations for static signs are identified by the start and end National Grid References of the High and Very High Hazard Ranking sites detailed in Table 7.1.

3. Point of physical closure: This is the point at which traffic is prohibited from passing because the road is either unsafe and/or not passable. This is very much a site-/case-specific issue and the point of closure will depend upon the location of incident(s) and the likelihood of further nearby incidents, amongst other factors. Physical closures are more a matter for the Operating Company.

Clearly, in some situations, two or more of the decision/closure points described in items (1) to (3) above may coincide.

For example, assume a hypothetical blockage of the road due to a landslide on the A87 in Glen Shiel (between Cluanie Inn and Glen Shiel). This example illustrates the three types of diversion/closure (Figure 8.25).

Figure 8.25 – Map showing the A87 between the A82 and Kyle of Lochalsh (the base map is 1:250,000 but is here not to scale). Prioritised route sections highlighted as part of the GIS-assessment interpretation process (see Section 5) are also shown. (© Crown Copyright. All rights reserved Scottish Government 100020540, 2008.)

Figure 8.25

In terms of primary destination decisions, westbound traffic (e.g. for Kyle of Lochalsh) from the A82 south of Invergarry would be signed to follow the A82 to Inverness and then to follow the A9, A835, A832 and A890 to Kyle of Lochalsh. Conversely traffic from Inverness intending to follow the A87 westbound would be signed to follow the same diversion route but from Inverness.

In terms of junction-based decisions, the road would be closed to westbound traffic, ‘except for local access’, at the A87/A887 junction at Bun Loyne and physical closure would be effected just to the west of Cluanie Inn.

For eastbound traffic, the primary destination decision would be made at the junction with the A87 and the A890 to the east of Kyle of Lochalsh with traffic directed to follow the A890, A832, A835 and A9 for Inverness. The junction-based decision would be effected at the same point with a closure placed ‘except for local access’ on the A87. Physical closure of the A87 would be effected just to the east of Shiel Bridge.

8.3.2 Case Study

Longer-term actions to deal with landslide issues come in a variety of forms, ranging from relatively cost-effective improvement to drainage through high cost defence structures to complete realignment of a section of road.

A good example illustrating this range of actions is provided by the A83 between Ardgarten and the Rest and be Thankful. Recent incidents (October 2007, see Section 2.3, and April 2008) at this location highlighted a potential need for action and a number of possibilities are highlighted below.

Two of the key issues relating to the former incident were the small size of the culvert at this location and the open ditch drain alongside the road at the toe of the slope. The open ditch drain carries water from further up the hill towards the Rest and be Thankful and discharges through this culvert (and others further down the road). The sizing of the upper culvert, whilst not being the most critical factor in terms of the above event, has the potential to cause problems, in terms of blockage and subsequent over-topping, similar to those that occurred at Glen Ogle in August 2004 (see Section 2.2). Also, the drainage ditch at the toe of the slope blocked during the October 2007 event at the A83 and water from it was, as a result, diverted across the road. This caused a separate erosive event downhill of the road and consequent loss of stability of the road structure itself.

In the first instance it seems clear that some short to medium-term action in terms of the drainage provision along the stretch of the A83 between Ardgarten and the Rest and be Thankful is required. Certainly a reconfiguring of the drainage ditch at the toe of the slope, such that it is covered and therefore much less likely to block, is required. In tandem with this work, an assessment of the capacity of the current culverts along this length should be made with a view to increasing capacity and improving shape (cross-sectional and longitudinal) where appropriate. The feasibility of providing debris traps also should be considered between the toe of the slope and the road itself or, indeed, on higher ground if necessary due to space constraints. While it is accepted that it may be difficult to configure such traps on the steep hillside, serious consideration should be given to including them where it is possible to do so. Their location and size should be considered in the light of the potential volumes of debris. It must be emphasised that all such actions should be undertaken along the complete section from Ardgarten to the Rest and be Thankful and not solely in the immediate locality of any incidents which occur or have occurred.

Larger scale construction measures, such as debris shelters have been suggested as a possible solution to the debris flow problem in this area. However, these can only be implemented in the longer term; they are typically not only massive and expensive structures; they are also visually intrusive. If such large scale engineering works are to be contemplated it may be more acceptable to engineer the level of the road in order to allow debris to pass below it –but this would effectively entail a total reconstruction of the road on, or close to, the existing alignment. The disruption to traffic during such lengthy construction operations would need to be fully taken into account should such an option be considered.

A more effective but potentially more costly long-term action may be to realign the road on the opposite side of the valley, possibly at a lower altitude than the current route. This is an action that should not be considered lightly. While recent debris flows have not been observed historically on that side of the valley the disruption to the landscape caused by the construction and maintenance of a road could well lead to a change from the current situation. A decision to take up such an option should only be contemplated after thorough review of all of the information available, including the GIS-based assessment (see Section 4), and a thorough desk-based and walkover investigation of the site to assess the geomorphological, geological and geotechnical issues and potential hazards and risks. In addition, the effects of ongoing deforestation works (as at June 2008) on the opposite side of the valley should be taken into account.

The foregoing considers a variety of options for remediating the hazards and risks for a length of the A83 of around 6.3km. Whilst few would argue other than that this is the most badly affected section of this route in term of debris flows, it is by no means the only length of this route to be so affected, as was demonstrated by the events of August 2004 (see Section 2.2). The main recommendation for long-term action at the A83 is that a thorough Route Action Plan (RAP) be undertaken. This should take into account the landslide potential in the area in addition to the customary considerations such as the strategic nature of the route, traffic levels (including the likely future demand) and level of service required.