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8 DEBRIS FLOW MANAGEMENT AND MITIGATION OPTIONS

by A Sloan, L Shackman, F Macgregor and M G Winter

8.1 MANAGING THE ASSET

The trunk road network comprises a long linear asset, much of which passes through hilly terrain, with varying potential for the development of debris flows and other disruptive events. The roads themselves carry different traffic flows and therefore the resulting consequences or losses from such events are variable.

This section explores the various management and mitigation practices that have been adopted in both the UK and overseas. It then goes on to recommend some of these as potential techniques for use on the Scottish trunk road network. It is suggested that a three-step management tool in Study 1, Part 2 be adopted, as follows:

• Detection: The identification of the occurrence of an event or the precursor conditions that could lead to an event.
• Notification: The dissemination of information relating to the hazard(s).
• Action: The proactive process by which intervention reduces the exposure of the road user to the hazard.

This Detection-Notification-Action (or DNA) process has been developed as a tangible approach to reducing the hazards to which the public are exposed when using the trunk road network. It is feasible to introduce this on parts of the network, particularly at locations where the hazards posed by debris flows are recognised as being real and present. It is considered that the DNA process should be an intrinsic element of a fully developed Asset Management System.

8.2 APPROACHES TO LANDSLIDE MANAGEMENT

In developing management processes for problems in Scotland it would be prudent to learn lessons from countries where landslide management has been practised for some time and where losses arising from landslides are significant. It is to be noted that in an international context the scale, frequency and consequential losses from landslides in Scotland have fortunately been relatively minor to date. The following sections briefly examine procedures developed for use in the United States of America ( USA) and Hong Kong although it is worth noting that substantial work has been carried out in Australia ( AGS, 2000) and elsewhere in the world.

8.2.1 United States of America

Landslides in the USA, for example, collectively constitute a serious hazard and result in significant losses. The US Geological Survey ( USGS) estimates an average of 25 to 50 deaths annually are attributable to landslides as well as financial costs of between $1 billion and $3 billion per annum and untold environmental and societal disruption. To this USGS has developed a management system (Spiker and Gori, 2003) based upon the following elements:

• Research.
• Hazard mapping, the benefits of which are particularly emphasised by Anon (2004).
• Real time monitoring.
• Loss assessment.
• Information collection, interpretation, dissemination and archiving.
• Guidelines and training.
• Public awareness and education.
• Implementation of loss reduction measures.

• Emergency preparedness, response and recovery.

Amongst other conclusions drawn from an assessment of the USGS proposals (Anon, 2004) it was also concluded that:

• The dissemination of collected information on landslide hazards was of critical importance in the implementation of an effective risk reduction programme.
• Finally it was concluded that the wish to implement a loss reduction programme would cost money and that a budget should be set for both the development and the implementation of such a process.

8.2.2 Hong Kong SAR

The benefits of introducing a slope management system in the developed world are perhaps most dramatically observed from the experience of Hong Kong. In 1972 and 1976 landslides occurred that killed 100 and 18 people respectively. In response to this the precursor of the Geotechnical Engineering Office ( GEO) was established with the basic mandate to improve public safety.

The GEO has developed slope management systems that are rigorously adhered to throughout Hong Kong. These involve defining maintenance requirements, examinations, risk analyses, real-time warning systems in relation to intense rainfall, increased community awareness through education programmes, and direct engineering works. This process has reduced dramatically the risk to public safety despite the rapid growth of Hong Kong (Chan, 2000).

Early stages of the Hong Kong programme concentrated on hazard definition through the developments of asset inventories, mapping and geological/geotechnical assessments. It was realised that such a technical based approach was insufficient alone to reduce the risk posed by landslides. Consequently developmental work was carried out into inspection and maintenance regimes, risk analysis, warning systems in relation to heavy rainfall, education of the community and increasing public awareness to the presence of slope hazards. This included the setting of regulatory instruments to control the construction of new slopes and the remediation of existing slopes ¹⁷.

The foregoing is simply a small sample of the wide range of systems implemented around the world for slope management. The examples do however serve as useful reference systems, albeit covering geographical areas more prone to landslides than Scotland.

8.2.3 United Kingdom

In the UK other managers of long linear infrastructure are Network Rail and the Highways Agency in England. Both these organisations have asset management systems specifically dealing with slopes. However, the Highways Agency in particular only really deals with slopes in the near-field relative to their road infrastructure. It does not, in general, have to contend with slopes that extend some hundreds of metres vertically and many more metres horizontally from their infrastructure as does the trunk road authority in Scotland.

Of particular relevance for comparison purposes is the system employed on Scotland's railways. This is of relevance because the railway in Scotland is faced with similar problems to those experienced by the country's trunk road system. The railways in Scotland run in the same topography and quite often along the same glens as the trunk road system. The rail network in Scotland has suffered from some significant landslides in the past. Partly as a consequence of these failures Network Rail has a system whereby all of the earthworks (embankments and cuttings) on the network are examined on a regular basis as a risk management procedure. The process is summarised in Figure 8.1.

It is important to note that the primary response of Network Rail to a perceived heightening of risk is often to close the railway and use buses to transport their customers by public road. This is clearly not an option for the trunk road authority.

Figure 8.1 - Summary of the Network Rail slope management procedures.

The elements of the process within the red outline on the above diagram are those that are specified in Network Rail's published standards. The management system revolves around the visual examination of slopes on a cyclic basis after an initial assessment. Those posing the greatest hazard are examined every year whilst those that are more benign are examined every 10 years. Those slopes of marginal hazard rating are scheduled for re-examination every five years. The processes by which the slopes have to be examined are specified in Network Rail company standards along with the level of competence required of the examiner. Physical monitoring or engineering works are carried out on slopes where the risk is considered to be such that visual inspection alone cannot be relied upon to manage the risk.

It is clear that management systems developed in relation to the potential for landslides recognise that it is not possible to prevent such events from occurring at every location. The aim is to manage the situation by understanding the hazards and potential losses arising from landslides and reducing losses to acceptable levels.

Key factors which define a system to be developed for dealing with debris flow hazards can be summarised as follows:

• The relatively small scale of the problem in Scotland.
• The emphasis on the trunk road network.
• Particular topographical and climatic conditions in Scotland.
• The particular emphasis on debris flows on Scotland's mountains.

Reporting Landslide Events on the UK Rail Network

The initial report of an incident on the railway, including a landslide, can come from a train driver, a member of the public or a railway worker. The nature of the incident and its location is reported to Network Rail 'Control', a 24 hour a day control centre for all aspects of the entire network. All rail workers are briefed as to the telephone number for 'Control' and it is displayed prominently on rail structures such as bridges.

Upon receipt of a notification of a landslide, 'Control' will notify the on-call engineer from the engineering department with responsibility for earthworks. This engineer makes a decision as to the best course of action. This, more often than not is to call Network Rail's Earthworks Examination engineers. This is a firm of independent consultants appointed to undertake the cyclic examinations of the network. This firm also provides 24 hour cover for call-outs to landslide events. The purpose of the call-out is primarily to undertake an examination and record the nature of the landslide.

Engineers on site assess the magnitude of the problem. Management decisions are made based upon the evidence collected from the site. The line may be already closed, or open but in a dangerous condition. In either case emphasis is placed on re-opening the line for full speed train operation in as expedient manner as possible, whether engineering works are required prior to re-opening or not.

8.3 ASSET MANAGEMENT FOR TRUNK ROAD SLOPES

8.3.1 The Concept of Loss

Asset Management requires knowledge of the degree of risk associated with the nature of the hazard, the likelihood of debris flows developing and knowledge of the consequences if the debris flow occurred. A set of criteria developed to understand and recognise hazards, although it is also recognised that it is highly unlikely that a stage of predicting debris flows or any other landslides with any degree of certainty will be reached.

Defining the consequences of such an event to allow a meaningful and complete assessment of risk is extremely complex. The identification and quantification of loss associated with landslides is as important an element of risk assessment as hazard identification. Take, for example, the debris flows that occurred at the same time and very close to those that closed the A83 in August 2004 but did not reach the road. While the hazard was essentially the same, within discernible limits, the losses were significantly different. This fact has been recognised in Section 6 and the term hazard ranking used as shorthand for an assessment that is qualitative or semi-quantitative.

Accepting that it will not be possible to reduce the risk to zero in each and every case introduces that realisation that there must be a level of loss that is acceptable and that to define this one must identify a boundary between acceptable loss and unacceptable loss. This concept is useful in determining risk as it gets away from the need to accurately define loss, instead the losses can be rated as either acceptable or unacceptable. For example it may transpire that on certain routes it is unacceptable to have a road closure at all or for no more than a short period of time. In other locations the level of acceptable loss may be drawn at personal injury ( i.e. road closure is acceptable for a period of days but injury to the road user is unacceptable).

Care should be taken not to set the aspirations of loss reduction to a level that cannot be justified in financial terms due to either the magnitude of the problem and/or inherent technical complexity. Stage 2 of this study will need to broadly define acceptable and unacceptable losses in the context of the exposure assessment discussed in Section 6.3.

8.3.2 Asset Management Strategies

The options available for the development of an asset management strategy fall into two groups defined herein as follows:

• Category 1: Reactive Approach.
• Category 2: Proactive Approach.

The Category 1 approach accepts that debris flows will occur and seeks to formalise the response to such events. This would involve little or no change to the current arrangements whereby the Operating Company is mobilised in the event of a landslide occurring on the trunk road network. Technical and practical assessments are made at the scene and actions such as diversion and road re-opening decided upon and implemented appropriately. With the absence of a debris flow hazard assessment model, this type of approach is one which requires to be adopted where such types of event can potentially occur.

A reactive approach may be appropriate where small non-life threatening slips are expected. In the terminology defined above this equates to situations where the level of consequential losses are small. The magnitude of the debris flows experienced in the summer of 2004 were such that the reactive approach is considered to be inappropriate in that the consequential losses could so easily have been significantly greater.

To this end it is considered that the better approach is that of the proactive Category 2 type. This can be summarised as the development and implementation of a step-by-step approach to managing the risk posed by debris flows. This would involve identifying risk and reducing this where the levels were determined as being unacceptably high, the aim being to minimise losses due to debris flows.

The Category 2 approach necessarily comprises several stages to be worked through in the development and implementation of management procedures to minimise potential losses. The complexity of each stage and the magnitude of the effort required will be dependent on the magnitude of the problem as a whole and the aspirations of the Scottish Executive.

The steps that require to be addressed are summarised as follows:

• Develop an inventory of slopes likely to pose a hazard to the road network.
• Carry out hazard assessment of each slope.
• Debris flow mapping.
• Assess the acceptable loss at each site.
• Assess the hazard posed at each site.
• Assess whether this hazard is acceptable or not.
• In cases where hazard is unacceptable decide on hazard reduction measures.
• In cases where hazard is acceptable decide how to measure change in hazard.
• Education and knowledge dissemination.

The options available to deliver each stage are discussed in more detail below. The purpose of the discussion is not to present technical detail rather to discuss the options from a management perspective and to reinforce the manner in which these elements of a system as largely described in previous sections fits into an asset management strategy.

The development of an inventory of slopes does not lend itself well to natural slopes, not least due to the likely very high number of low risk slopes that would be categorised. More appropriate is to ensure that areas likely to pose a hazard to the road network zones are delineated. The determination of such areas is described in Section 6 along with the approach to hazard assessment and the development of maps of debris flow hazards. The results of these will need to be taken forward into a loss, hazard ranking and loss acceptability assessment.

8.3.3 Assessment of Loss, Risk and its Acceptability

Without an assessment of loss it will not be possible to assess risk in a fully quantitative manner. However, as discussed in Section 6 it may be that the potential for loss can be summarised on a route by route or part route basis and a semi-quantitative/qualitative assessment may not only be acceptable but potentially desirable. In managing the asset, as discussed below, it will be necessary to compare the risk at individual slopes to determine which ones should be considered in more detail for risk mitigation measures. If losses along sections of road are deemed to be the same, then the comparison of risk distils to a comparison of geotechnical hazard.

The concepts of acceptable loss and unacceptable loss are discussed in forgoing text. An action for the next stage of the study is to ascertain on a route by route basis the degree of acceptable loss and thereafter acceptable risk. As a minimum this will be set at a level less than personal injury to the road user. It is to be noted that the commissioning of this study indicates that the degree of loss suffered in the summer of 2004 was unacceptable. In these events no one was injured but several road users had fortunate escapes and it may be considered that it is the realisation of what could have happened that is unacceptable.

The key test of the risk assessment process as an element of the overall management of the network is to assess whether the risk determined is acceptable or unacceptable. This single element is probably the most critical aspect of the asset management process. Up until this point the process has been steered by determining risk. In comparing slopes and deciding whether or not to recommend risk reduction measures a degree of objectivity and experience is required. It requires consideration of not only the level of risk but other factors as well, not least amongst these being financial constraints.

In the case where the risk is considered to be acceptable the option exists to do no further work. However there is always the possibility of unknown factors in the condition of a slope that could introduce an unsuspected hazard. In addition there is always the inherent, and in the case of debris flow assessment the very real, possibility of uncertainty in assessing the true extent of the hazard and, indeed, the risk. Consequently it is likely that some form of repeat examination will be required after the initial categorisation of hazard and risk. The next stage of the study will address the need for repeat examination and reassessment of slopes.

Further to this it may be the case that risk reduction measures cannot be put in place at all the slopes identified as posing significant risk. In this eventuality it may be that repeat examination is required in lieu of risk reduction measures.

8.3.4 Risk Reduction Measures

In the case of slopes that are categorised as being of sufficiently high risk that the situation cannot be managed by repeat examinations it will be necessary to take action to reduce the risk or, in this case, hazard ranking. Risk, being the product of hazard and consequence, can be managed by tackling either or both of these two elements. Examples of these are discussed in detail in Section 8.4, albeit in terms of the hazard, exposure and hazard ranking approach described in Section 6.

8.4 MITIGATION TECHNIQUES

The foregoing details the processes of managing slopes to understand the potential for debris flows. The process 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 unacceptable it will be necessary to undertake some form of mitigative action to either reduce the hazard or to reduce the exposure of the road user.

To reduce the hazard to the road user either the magnitude of the hazard and/or the potential exposure or losses that are likely to arise as a result of debris flow must be reduced. To reduce the exposure of road users, 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 they must be warned to exercise caution at appropriate times and places.

8.4.1 Exposure Reduction

The reduction of exposure lends itself to the use of a simple and memorable three-part management tool, as follows:

• Detection: The identification of either the occurrence of an event, by instrumentation ( e.g. tilt meters or acoustic sensors) or observation ( e.g. Closed-Circuit Television, CCTV, monitoring or visual patrols during high likelihood periods), or by the measurement and/or forecast of precursor conditions ( e.g. rainfall).
• Notification: The dissemination of information relating to the hazard(s) and exposure(s), by for example Variable Message Signs ( VMS) including NADICS signs, media announcements (radio, TV, traffic guidance systems and the web) and "landslide patrols" in marked vehicles.
• Action: The proactive process by which intervention reduces the exposure of the road user to the hazard, by for example road closure, convoying of traffic ¹⁸ or traffic diversion.

This DNA approach can be operated for either precursor conditions that potentially lead to landslide events in high hazard ranking areas, namely rainfall, or to actual landslide events that have taken place.

Precursor (Preparatory or Trigger) Conditions

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 and that trialled in California both pass information on the heightened likelihood of landslide development to the public as a result of rainfall monitoring. This is achieved in a not dissimilar fashion as that in which information on extreme weather events is passed to the public in the UK.

In the case of Hong Kong ¹⁹ a comprehensive network of automatic rain gauges covers much of the region to record and send data to a central control point for analysis in real time. This is combined with short-term forecast data to enable managers to monitor the rainfall situation as it develops and make informed decisions in an expedient fashion.

If such predictive capability was installed in Scotland then it would be possible to develop systems to reduce the exposure of the road user to the effects of debris flows. However, it must be understood that in Hong Kong more than 20 years of experience have been acquired. This means that a sound knowledge of the relationship 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 be required.

Notification: 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. The issue of a Landslip Warning also triggers an emergency system within various Government Departments that mobilizes staff and resources to deal with landslide incidents. A Landslip Warning is issued when it is predicted that numerous (more than about ten) landslides will occur. Nonetheless 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.

In Scotland it is clearly important that a variety of public announcements are used when there is a heightened likelihood of landslide development in an area. This might involve a variety of systems including websites ( e.g.NADICS), variable message sign systems and media (radio and TV) announcements notifying drivers that their potential exposure to the hazards posed by landslides is real and present. Announcements could also be linked into traffic guidance systems such as TrafficMaster ^TM.

Action: If such a system were devised and implemented for Scotland and warnings were received that heavy rain was falling in an area or was approaching an area recognised as being of high hazard ranking then a number of options are available for action. First the road length (or lengths) deemed to be threatened could be closed. This might be effected by installing barriers such as the snow barriers present on some of Scotland's roads. The decision to reopen the road would need to be taken after the intense rainfall had passed and an inspection of the route had taken place. The dis-benefit to this approach is that given the relatively rare occurrence of debris flows, at least those that interact with the trunk road network, and the high levels of rainfall that Scotland receives, a number of false alarms could be expected. The public at large could, potentially, become disillusioned at what could be seen as a very conservative approach.

Alternatively trained operatives could be deployed on high hazard ranking sections of road during periods of predicted or actual high rainfall. These operatives could escort people through the high hazard ranking sections of road.

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

Event Occurrence

Detection: The movement of slope material can be monitored and the resulting information used in a similar way to rainfall data. The data is measured in real time and used as a management tool. Monitoring instruments can be located such as to record movement from potential debris flow or positioned such that notification is received if debris reaches or gets close to a road.

In relation to the former, the seeding area for debris flows can be very large and high on the hillside. This introduces difficulty in pinpointing the optimum location for the installation of the monitoring system and doubt as to whether the debris will reach the road. Installing instrumentation to indicate whether debris has reached a road has precedence: there is a location on the Scottish rail network at Glen Douglas where an instrumented fence has been installed. The purpose of this is to recognise when a fall of ground impinges the line. Similarly the railway through the Pass of Brander above the A85 at Loch Awe has a system whereby any rolling rocks or debris flows trigger signals on the railway that shut the line and stop trains.

It is likely that any instrumentation would be electronic with remote reading of data sent back to a central control point.

Whether such a system is sufficient in isolation is questionable but it is 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.

A range of possible instrumentation types is presented briefly, as follows:

• Borehole or Shallow Inclinometers: Instrumentation installed in the ground that monitors ground movement. Of use when movement is know to be occurring and is to be monitored over a period of time.
• Tilt Meters: Instrumentation installed on a structure to determine rotation of the structure. The rotation is measured by electronic tilt switches. To be installed in road or hill side barriers to indicate movement of the ground or impact of a debris flow.
• 'Trip Wire': Instrumentation to be installed along the strike of the slope that records whether debris has moved on the hillside. In this case a cable is physically moved by debris either as material strikes it or the fixed ends or fixed points of the cable move relative to one another. Movement can be detected by a change in electrical resistance in the cable.
• 'Ball of String': Generally used to detect movement broadly along the dip of a slope. A fixed point is placed to stable ground and a freely rotating drum of wire is attached. The free end of the wire is attached to a point on potentially unstable ground. Movement is detected by the rotation of the drum as the movement causes additional wire to be paid out.
• Telemetry: Movement of the slope at discrete locations is recorded remotely by measuring distances form a set point.
• Acoustic Meters: Instrumentation that detects small amounts of noise/vibration caused by small movements preceding larger landslide events.
• Remote Sensing: Remote assessment of slope instability using techniques ranging from CCTV monitoring to satellite imagery. Monitoring might initially be by human/visual means while automatic movement detection systems are developed.

An alternative approach is to use operatives to detect debris flow events by introducing landslide patrols during periods of high rainfall. As previously noted it is essential that such operatives are trained in what to look for and that patrols should operate in pairs for safety reasons.

Notification: In the first instance, a landslide event having occurred, notification must be to the Operating Company and the infrastructure owner. The decision must then be made rapidly to close the road or to keep it open. The nature of debris flows is such that in most cases the road will be blocked and therefore closed to all intents and purposes. Secondly the public must be warned by media announcements.

Action: In terms of positive actions that may be taken after a debris flow event the range of actions is similar to that available for Precursor Conditions and described above. However, it is important to note that closing the road in the area immediately adjacent to the event is not an adequate response. Debris flow propensity is generally believed to affect long lengths of hillside and an evaluation of the vulnerable area must be performed in order to ensure that and appropriate length of road is closed.

In all cases re-opening of the road must 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 acceptable level. Current 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 at an acceptable level.

8.4.2 Hazard Reduction

The challenge with hazard reduction is in identifying locations that are of sufficiently high hazard and exposure to warrant spending significant sums of money on engineering works. The lengths of road that have already been identified in Section 7 are significant. The costs associated with installing remedial works over the entirety of such lengths are almost certainly both unaffordable and unjustifiable. Moreover the environmental impact of such engineering work should not be underestimated, having a lasting visual impact at the least and potentially more serious impacts. It is considered that such works should be limited to locations where their worth can be proven.

Notwithstanding the foregoing, simple measures can be taken 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.

Typically, the reduction in 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. As described in earlier sections of this report such slides tend to be dynamic and are quite often initiated some distance above the road. When the slides reach the road they are relatively fast moving high energy flows. The energy of these systems has a significant impact in the nature of the engineering works that can be used to reduce the hazard to the road and its user. Hence, there are three broad approaches to selection of hazard mitigation works:

• Accept that debris flows will occur and protect the road.
• Carry out engineering works to reduce to opportunity for a debris flow to occur.
• Realign the road.

In relation to the first option there are not many examples of such engineering works in Scotland or the rest of the UK, but in upland areas of mainland Europe such engineering is relatively common place. The energy of the debris flow is such that a rigid barrier constructed to protect the road would have to be designed for very high loads. The problem with a rigid barrier is that the debris flow has significant momentum and to bring the slide to a sudden stop, as is the case with a rigid barrier, requires the dissipation of a lot of energy, instantaneously imparting very high loads.

Road Protection

Debris Flow Shelters: Stone shelters or 'avalanche shelters' are engineered structures that form canopies over a section of road prone to rock fall or debris flows. These structures are usually formed from reinforced concrete. There is an example of such a structure on the A890 north-east of Stromeferry in the north-west highlands. This structure straddles both the road and railway at that location (Figure 8.2).

Figure 8.2 - Stone shelter on A890 northeast of Stromeferry.

In these structures energy is dissipated by placing a depth of granular material on the roof on which the debris flow lands.

Debris Flow Overshoots: In situations where the energy is anticipated to be very high, modifications can be made to the above 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 much energy. This shaping or profiling involves constructing a ski-jump type reinforced concrete structure. Material falling simply slides over the roof and continues down the hillside.

Barrier Fences: Fences can be constructed to act as effective barriers to halt debris flows. Such fences are designed to be flexible so that the 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. Figure 8.3 shows such a fence installed on the Inverness to Kyle of Lochalsh railway in Scotland. Such fences do 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-posts only ( e.g. Winter et al., In Preparation).

Flexible fixed position fence structures are common place 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.

Improving Channel Flow: In certain circumstances it may be possible to improve channel flow down to the road and beneath the road by for example widening culverts. Such works would improve the potential for debris flows to avoid the road.

Debris Flow Prevention

In relation to the second option, preventing the slide happening in the first place, applicable engineering solutions will vary depending very much upon individual circumstances. Debris flows can have a relatively large source area and in the case of recent examples in Scotland, be located very high up on the hillside above the road. In most circumstances the potential for carrying out conventional remedial works to restrain the material before it starts to move is 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 applicable and economic are likely to be limited.

The link between debris flows and intense rainfall has been established previously in this document. As a result water management can reduce the potential for debris flow initiation.

In the circumstances of the large debris flows that occurred in the summer of 2004 it is considered that on hill drainage improvement would have had little impact due to the scale of the events. In other locations positive action to improve drainage may well have a beneficial effect. This would include improving channel flow and forming drainage around the crest of certain slopes to take water away in a controlled manner.

Figure 8.3 - Flexible catch fence.

Road Realignment

Road realignment can be used as part of the Scottish Executive's structural management activities in order to improve the road in terms of both alignment and junction layout, in particular to reduce accidents and also to ensure compliance with current design standards. In cases where the hazard ranking from debris flows is high and other factors indicate that some degree of reconstruction is required, road realignment may be a viable option. Similar expedients have 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.

8.4.3 Partnership, Education and Knowledge Dissemination

It is recognised ( USGS and GEO: Malone, 1998) that widespread public awareness of landslide hazards enables individuals to make informed decisions as to where to live what property to buy where to locate businesses and so on. For local decision makers such knowledge allows for better town planning and the locating of critical facilities. In this study the decisions that people would have to make are limited to road usage. However, it is considered that a more inclusive approach will result in a wider understanding of losses, for example road closures. In parallel, the dissemination of such knowledge widens the base of decision making responsibility and as such public acceptance of loss is likely to increase.

To this end it is suggested that the next stage of the study addresses, the most appropriate ways to increase public awareness, evaluate the effectiveness of different types of message and messaging systems, and the most appropriate methods to disseminate information.

It should be recognised that different users will look for differing levels of information. This could range from roadside warnings in its simplest form to more detailed data presentations, possibly web site based where information detailing the management procedures is posted.

It is not only the public that should be engaged in debris flow education, but road maintainers and local authorities as well. In a similar way any alterations road layouts or new road developments should have the assessment of the potential impact on debris flow and land slips in general as an integral requirement of the Approval in Principle process.

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