9 SUMMARY AND RECOMMENDATIONS FOR DEBRIS FLOWS IN SCOTLAND
by M G Winter, F Macgregor and L Shackman
9.1 SUMMARY
In August 2004 a series of landslides in the form of debris flows occurred in Scotland. Some of these affected the A83, A9 and A85, which form part of the trunk road network. These incidents were well reported in the media.
While debris flows occur with some frequency in Scotland, they only rarely affect the trunk road network or for that matter the main local road network. However, when they do impact on the road network the degree of damage they do, in terms of the infrastructure and the loss of utility to road users, can have a major detrimental effect on both economic and social aspects of the use of the asset. Additionally, there is a high potential for such events to cause serious injury and even loss of life although, fortuitously, such consequences have been limited to date.
The events of August 2004 followed a sustained period of heavy rainfall and, in addition, intense localised storms contributed to the triggering of at least some of the resulting debris flows. Rainfall of up to 300% of the monthly average fell in certain parts of Scotland during August 2004.
Within the recent past, debris flow activity in Scotland has occurred largely in the periods July to August and November to January, but there is no certainty that such a pattern will be continued in the future. However, eastern parts of Scotland do receive their highest levels of rainfall in August. Additionally, climate change models indicate that rainfall levels will increase in the winter but decrease during the summer months but that intense storm events will increase in number. These factors, therefore, may change both the frequency and the annual pattern of debris flow events.
The impacts of such events, when they do happen, are particularly serious during the summer months due to the major contribution that tourism makes to Scotland's economy. Nevertheless, the impacts of debris flow events during the winter months should not be underestimated.
Following the events of August 2004, this study was commissioned to take stock of the present situation on the trunk road network and to determine a sustainable approach to the management to such occurrences in the future. The process chosen involves the assessment and ranking of hazards with a system of management and mitigation also being proposed. This system is based upon the principles of Detection, Notification and Action ( DNA) applied both to the response to landslide events and to precursor rainfall conditions.
9.2 RECOMMENDATIONS
9.2.1 Early Opportunities
A number of areas of perceived high hazard were identified at the Project Workshop. The lengths of the road and the slope lengths they involve are substantial. Accordingly, it is considered unrealistic to undertake suitably prioritised further evaluations at this stage. The proposal is for the outputs of the GIS-based assessment to be used to corroborate the identification of the localities identified at the Project Workshop and, in addition, as a validation tool for the site specific assessment methodology.
In the meantime it is important that maintenance and construction projects currently in design take the opportunity to limit any hazards or exposure by incorporating, where suitable, measures such as higher capacity or better forms of drainage, or debris traps. Peer group consultation in the form of the involvement of the Overseeing Organisation and its Independent Geotechnical Checker, the corresponding specialists within the Operating Companies, design organisations or other appropriate organisations is an essential part of this process.
In the realm of minimising the potential impacts of debris flows on the network, some retargeting of maintenance actions could be productive. The checking of gullies, ditches and catchpits, with a wider view than that of merely keeping the roadway itself clear of water, could be undertaken as part of regular inspections. Where ineffectiveness of the drainage system, or underperformance under updated drainage criteria, is suspected, this should be considered in conjunction with the inspection regime for the roadside side slopes and remedial action addressed via an appropriate structured asset management plan. Additionally, critical review of the alignment of culverts and other conduits close to the road ought to be carried out as part of inspection and reporting procedures.
Certain monitoring measures are already under consideration - for example, the installation of a rain gauge in the A83 Rest and be Thankful area, where debris flows are generally small but relatively frequent, potentially yielding more comparable data in a short time frame. The use of any such data gained, in conjunction with longer-duration data available from the Meteorological Office, needs to be managed appropriately to serve a worthwhile and consistent function. At a later stage, informed selection of locations for discrete placement of additional rain-gauging facilities could be productive, and should be considered in the light of experience of managing the information from current sources.
An important action which could be introduced on an early basis is bringing NADICS into the management loop with regard to route advice when weather conditions conspire to create situations where sections of the network might be considered 'at-risk'.
9.2.1 Study 1, Part 2
The initial stage of Study 1, Part 2 will be to develop the methodology for the assessment of hazard and exposure to provide a hazard ranking, together with the selection of an appropriate management approach. The second stage will be to test the methodology before applying it more widely to the trunk road network.
Figure 9.1 presents a flowchart of the work to be undertaken.
The initial stage of this work is itself divided into four elements and can be summarised as follows:
- Development of a debris flow hazard and exposure assessment system to provide a hazard ranking of 'at-risk' areas of the road network.
- Undertaking a computer-based GIS assessment as a first stage in the hazard assessment process.
- Undertaking site specific hazard and exposure assessments of areas identified by the GIS as being of higher hazard.
- The identification and development of appropriate management processes for each category of hazard ranking.
Figure 9.1 - Management and mitigation options within Study 1, Part 2.
The GIS-based assessment will be used as a first stage in the hazard assessment process. This will enable site specific assessments to be targeted in order to obtain better value from such relatively resource-intensive activities. It will also allow the elimination of large areas of the network having minimal hazard.
It is also particularly important to note that the site-specific assessment will not be a 'driveby' survey; it will require a highly specialised detailed site examination which will need to be carried out using an overall consistent approach. Prior to undertaking any site surveys it is important that the system is established for consistently describing and identifying hazards and the associated exposure. Some of the factors that will need to be incorporated in such a system, such as slope angle and the broad nature of the geology, will be incorporated into the GIS assessment. Other, more detailed, factors such as the effects of forestation will need to be incorporated into the site-based survey. Once a hazard assessment has been completed it may be combined with an assessment of the exposure of the road user to that hazard to give a hazard ranking. This will allow, in-turn, an appropriate management option to be selected from the range of options to be developed.
There are a number of potential options which could be applied to the management of debris flows. These are addressed in the following paragraphs.
The 'Do-Nothing' approach is intended to be applied to sites of low hazard ranking for which substantial expenditure is inappropriate. For such sites, whilst it is not possible to eliminate the chance of a landslide event affecting such areas it is seen as unlikely, largely unforeseeable and/or the exposure is less serious than at other locations where resources may be better expended.
The 'Do-Minimum' option, with the potential to mitigate the impacts of debris flows to some extent involves simply ensuring that forward plans are in place to ensure that diversion routes are available and may be exploited in an expedient and well organised manner. Diversion route maps and contingency plans are currently held for many areas of the trunk road network.
Whilst it is not possible to eliminate the chance of a debris flow event affecting such areas any occurrence is seen as unlikely and largely unforeseeable and any residual exposure cannot readily be quantified and is unlikely to justify the commitment of additional resources which may be better expended at other locations.
'Do-Something 1' is the first management option where site specific action is contemplated. Such action is essentially exposure reduction by managing the access to and/or actions of the road-using public on the network at times either when events occur or precursor rainfall has indicated a high likelihood of landslides occurring.
In the case of short-term to medium-term reaction to such occurrences, then the Detection-Notification-Action ( DNA) approach can be implemented by pre-planned actions such as issuing an advisory warning or closing the road. There may be a case for reacting to extremely heavy rainfall events in a similar fashion, especially with warnings. A caveat to this is the need to consider carefully at what levels the triggers should be set, in so far as the relationship between rainfall and landslides in Scotland is by no means fully understood.
Considering the longer-term approach, precursor triggering conditions ( i.e. rainfall) may enable many of the actions described above to be taken prior to the occurrence of major events. Either an extensively enhanced network of rain gauges installed across Scotland or access to data derived from radar and of sufficient resolution would be required. Such work initially be concentrated on known storm tracks, if these are available from the Meteorological Office, and vulnerable slopes. Clearly, if this approach is taken then a close consultation with both the Geotechnical Engineering Office in Hong Kong, which has extensive experience of operating such a system albeit in different climatological and geological conditions, and the UK Meteorological Office would be highly desirable.
It is fully expected that it will take some considerable time and effort to ensure that sufficient data has been obtained and analysed so as to be able to introduce a warning system. Even then it must be expected that atypical events, which are not the subject of warnings, may occur. Also a number of false alarms may inevitably be expected. A programme of public and media education and awareness-raising is also likely to be desirable to minimise any potential adverse reaction to such scenarios.
'Do-Something 2' involves more major works in order to achieve hazard reduction (as opposed to exposure reduction in the 'Do-Something 1' case). The approaches involved entail physical measures such as the protection of the road, reduction of the opportunity for a debris flow to occur or realignment of the road away from the area of high hazard. Such options need to be considered in the context of the policy governing the Scottish Executive's overall trunk road maintenance and construction programme. In general, these are likely to be of high cost necessitating their restriction to the very few areas of highest hazard ranking.
Clearly, and as illustrated in Figure 9.1, M onitoring and Feedback is fundamental to the success of the system and key to deriving best value from the arrangements proposed. The system developed is an active one and lessons learned from future landslide events, whether they occur in areas of high or very high hazard ranking or not, will produce valuable data which needs to be taken into account in adjusting the parameters that form the cornerstone of the assessment methodology.
There exists a need to ensure that actions identified by the existing Rock Slope Hazard Index system (as developed in the early 1990s) are carried out on a priority budget basis. These will include both maintenance works and re-inspection activities. While the rock slope system and the proposed landslide system have very different structures, great efforts have been made to ensure that the critical exposure evaluation and the output categories are capable of being mutually compatible.