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6 PROPOSED METHODOLOGY FOR DEBRIS FLOW ASSESSMENT

by M G Winter, F Macgregor and L Shackman

6.1 HAZARD ASSESSMENT

The purpose of the proposed hazard assessment is to determine stretches of the trunk road network most likely to be affected by debris flow activity. This will involve the sequential discarding of unlikely areas, at least in the early stages.

Two initial sifts are likely to be undertaken. The first will differentiate between peat and non-peat drift deposits. The second will be based on slope angle. A relatively high slope angle (around 26 ^o to 50 ^o) will be applied to debris flow formation in non-peat deposits with a slope angle of around 8 ^o applied to the run-out zone (between, for example, the area in which debris flows might form and a trunk road). Soils that are known to exhibit cohesion may have a hazard reduction factor applied as they may be less susceptible to debris flow activity. A relatively low slope angle (possibly as low as 5 ^o) will be applied to areas that are covered by peat. This approach means that most effort in assessing hazard is expended in those areas of greatest actual hazard rather than an initial 'blanket' approach which expends considerable effort in all areas.

The National Landslide Hazard Assessment ( NLHA), based upon NEXTMap and other data, for landslide hazard zonation ( see Section 3.3) can be rapidly adapted to suit the purposes of a bespoke initial assessment. In addition to the factors described above, some account of engineering soil type is also incorporated. The NLHA data set incorporates information from both published and unpublished drift and bedrock geology maps, the unpublished information not being otherwise readily available to any other form of assessment. Proxy data in respect of friction angle ( Ø') have been developed from drift and bedrock geology descriptions and are held within the data set.

It is understood that the Meteorological Office have data on regular storm tracks for intense rainstorms across England and Wales. While it is not entirely clear whether such information is currently available for Scotland, it would be a very useful means of refining the initial evaluation of hazard if available. A proxy for antecedent rainfall data could also be delivered by means of the 30-year average rainfall figures for Scotland. However, some care is required as such data may not reflect the current rainfall patterns and also ignore the issue of low water content-high suction slopes that can be vulnerable to intense rainfall events ( see also Section 2.3).

It is also perfectly feasible to attach a higher assignment of hazard for conditions relating to stream channel and catchment areas, thus emphasising the perceived hazard of debris flow development associated with stream beds.

A key issue is the swathe width to be examined and clearly this must correspond to the catchment adjacent to the road and the location of the watershed may well be the most appropriate measure to define the relevant swathe. However, the effort required to undertake the assessment is, to a large extent, independent of the geographical area included. This opens up the possibility of undertaking the assessment for the entire land mass of Scotland, rather than simply for land adjacent to trunk roads, thus providing valuable data for Local Authority roads. This approach also has the advantage that work is not necessary to identify the limits of all of the catchments adjacent to the trunk road network, albeit that these are available electronically from the Flood Estimation Handbook (Anon, 1999) which includes details such as return period, capacity, flow and other data.

In addition to the basic hazard assessment other key outputs have the potential to be sourced from the GIS, as follows:

• Inventory: An inventory of areas of high hazard in Scotland and, more specifically, areas that have a potential to affect the trunk road network may be developed. This could be refined to determine the lengths of the trunk road network subject to hazard from debris flow activity. Inventories could also be developed for local authority roads as a pan-Scotland approach is proposed. Some care will be needed to ensure that the inventory is not too restrictive as criticism may well be levelled at the system if debris flows subsequently occur outside the areas identified. Notwithstanding that, not all areas identified in the inventory will be selected for further investigation and/or management measures or mitigation. There remains a possibility that such areas will be subject to debris flow activity in the future, albeit that the possibility will be substantially less than those areas that are selected for further action.
• Mapping: The mapping of potential debris flow areas is a potentially valuable tool to portray the hazard assessment results. Not only could the initial, GIS-based hazard assessment be illustrated in this way, but results of the subsequent site-specific, more detailed hazard assessments could also be incorporated. In addition, both the exposure levels and the hazard ranking (see Section 6.2) could be illustrated in map form. The foregoing is, of course, subject to the data being available in a suitable format.

Once areas of high potential hazard are identified then more substantive, site specific efforts may be expended in using a system developed on the basis of the assessment factors described in Section 6.3. As a first step it is proposed that a 'ground-truthing' exercise be undertaken by making a desktop comparison of the results from the initial GIS-based assessment with those areas of high hazard assessed during the Project Workshop and detailed in Section 7.

In terms of the site specific work it is crucial that a range of sites representing as fully as possible the full range of conditions likely to be encountered relative to debris flows in Scotland is evaluated. Further, while the evaluation of each site should be undertaken by one member of the Working Group an independent check should be undertaken by another member of the Working Group. In each case the results must be compared, evaluated and audited by the Project Team.

It is important to emphasise that any form of hazard assessment will determine the most likely areas to suffer debris flow activity. It remains possible that areas identified as having lower likelihood may experience such events if circumstances come together to provide the necessary triggers. As has been pointed out on many occasions the precise nature of the ground is uncertain and residual hazards must be expected. However, the judicious use of a system such as that proposed should ensure that apparently anomalous events are rare and that the hazards are managed to the best effect within budgetary constraints.

6.2 HAZARD RANKING

The assessment of hazard in isolation simply details the areas most likely to be affected - the likelihood of occurrence; it does not consider the consequences of such events. In order to enable the appropriate prioritisation of management budgets for potential debris flow activity on the trunk road network the exposure due to the interaction of debris flows on the network must also be evaluated.

Traditionally the product of the hazard and exposure (or consequence) is defined as the risk. However, there are a number of ways in which exposure might be considered. In an ideal world the exposure resulting from debris flow activity would be determined in all its contexts. Such contexts include the exposure to life and limb, social/employment factors (including the effect upon tourism), environmental factors and economic factors. To include all such factors would be a major undertaking and is almost certainly beyond current capabilities in terms of fully understanding the interaction between the factors and ensuring that there is no double-counting (or even treble-counting) of the exposure factors. A thorough view of risk assessment in the context of landslides is presented by Lee and Jones (2004).

Accordingly the product of hazard and exposure is referred to in the limited, but direct, sense in which it is evaluated as a hazard ranking. The hazard ranking may be seen as a qualitative/semi-quantitative risk assessment as opposed to the fully quantitative conventional risk assessment approach.

The complexity of the interactions of exposure factors means that many are underpinned by a few relatively simple measures such as traffic flow, road geometry (especially sightlines), and the length and, indeed, the existence of a diversion route. These factors are capable of capturing a simplified assessment of exposure and thus being imposed on the basic assessments of hazard to provide the hazard ranking described above. The process does not, however, represent a full risk assessment and nor is such a process either necessary, or desirable, in this case.

Clearly, debris flow activity on the busy A9 to the north of Perth (traffic flow around 13,500 vehicles per day - all vehicles two-way, 24 hour AADT¹⁵) would have a far greater effect due to the higher traffic flows (and higher number of people dependent upon such traffic movements) than on the much more lightly trafficked A835 between Ullapool and Braemore Junction (traffic flow around 2,900 vehicles per day), for example. If two such lengths of road are found to have the same level of debris flow hazard ( i.e. the same likelihood of a debris flow interacting with the road) then some means of distinguishing between the two and adopting a prioritisation approach to management and mitigation is required.

Using the simplified exposure evaluation technique described above, it is thus almost certain that of the two examples cited the A9 would be assigned a higher priority than the A835. This is entirely appropriate as the interests affected (businesses, commuters, tourists, etc) by such events would be much greater than on the A835. In addition, the traffic flows on the A9 are much higher and the chances of personal injury are therefore proportionately higher, albeit that this aspect is offset to some extent by the presence of generally better sightlines and geometry on the A9.

Accordingly it may be seen that once the level of hazard has been determined then a further assessment of the exposure must be applied to a given situation to yield what we describe henceforth as a hazard ranking. The purpose of this hazard ranking is primarily to distinguish between areas with similar hazard levels to allow budgetary decisions to be made on an informed basis. Secondly, as indicated above, it is clear that areas with lower hazard levels may yield higher hazard rankings than areas with higher hazard levels which may yield lower hazard rankings. The foregoing, purely hypothetical, comparison of the A9 with the A835 may well be a typical example of such a situation. The effects of an event on the A9 being so much greater than one on the A835 that the actual level of hazard alone does not determine the need for action or otherwise.

The factors generated at the Project Workshop were very detailed and comprehensive ( see Section 3.2). However, it is clear that many factors have the same, or similar, root. For example it could be argued that depositional regime is the root factor for others such as density, relative density, air voids, void ratio, permeability and even saturation.

In this context it is clear that some effort is required in simplifying the factors determined from the Project Workshop. Indeed, this section does not address how they will be defined, but merely identifies the most important combined factors. Combining the factors and the method for doing so is a matter to be addressed at an early stage of Study 1, Part 2.

The factors given below are considered to be a strong reflection of those that must be incorporated into a hazard assessment and ranking system. Clearly some are likely to be used at a very early stage ( e.g. a GIS-based assessment) while others will be incorporated into a site-specific assessment methodology. However it is also recognised that some refinement of these factors will be undertaken as the construction of a working hazard assessment and ranking system is constructed.

6.3.1 Hazard Factors

In Section 5 key contributory factors to debris flow hazard are discussed in the context of those that affect likelihood of occurrence and those that affect the consequences of the event.

The first stage assessment, as described in Section 6.1, considers two categories of debris flow hazard assessment.

• The first will effectively seek areas of peat on slope angles of 5 ^o or greater. While the presence of streams in the peat will be evaluated these will almost inevitably be present and further work on a site specific basis will be required.
• The second will deal with all other types of surface deposit. The slope angle, engineering soil type and presence or otherwise of a stream will all be taken account of. Note that the presence of a trunk road above, not just below, a hazard zone may present a threat to that road.

Other factors will be incorporated as the availability of data permits.

Once the first stage assessments have been undertaken then more detailed examinations of areas of high hazard will be required. It is likely that all areas of high hazard in peat will require site specific assessments. The key issues for further desk-based assessment of areas of high hazard in non-peat deposits are as follows:

• The presence of a trunk road within the area of high hazard or the presence of a suitable run-out zone (slope angle 8 ^o or greater) between the area of high hazard and the trunk road.
• The presence of other topographical features that may enhance the likelihood of debris flow occurrence. These include terraces, ditches (natural or otherwise) or breaks in the slope which may have either a positive or negative impact on debris flow formation and transportation and rock outcrops and other natural or artificial barriers that may retard the formation or passage of a debris flow.
• The existence of a history of landslide activity at the location. Such information is available from the National Landslide Database and BGS digital maps as well as from experience and observation.
• Factors relating to bedrock will require some further investigation. Much of the available research on Scottish debris flows has indicated a limited history of debris flows in areas of schist bedrock materials for example. However, much on-the-ground experience contradicts this, especially in localities where the direction of bedrock dip has been found to approximately coincide with the slope aspect.
• Catchment data such as runoff coefficients and the catchment size and shape (Anon, 1999).
• The presence of spring lines.
• Deforestation and afforestation as factors potentially increasing and decreasing the likelihood of debris flow activity at a given location. In the case of deforestation the direction of old planting furrows should be taken into account as these may direct water into the area of high hazard. Afforestation is a particularly important factor to consider in the context of arresting or retarding debris flow runout.
• The presence of features such as public or forest roads between the area of high hazard and the trunk road. These may slow the progress of water and thus increase the deleterious effects of water ingress immediately below the feature and/or the presence of culverts passing under such roads may delay the downslope passage of debris and thus increase the debris load of future events.

Storm track data will be incorporated if available, and 30-year average rainfall data will be used as a proxy for antecedent rainfall if the advice of the Meteorological Office concurs with its use in this context.

Slope height, slope aspect, earthquakes and the underlying geological formation are all considered to be factors that have limited influence on the potential for debris flow development. However, where slope angle and the dip/direction of bedrock are known to be coincident then this might be a factor that adds to the perceived level of hazard. In addition the presence of a layer of drift and/or weathered bedrock deposits is considered vital for the development of debris flows: this must be neither so thin as to provide inadequate material to develop a debris flow nor so thick as to damp the dynamic flow.

Detailed geotechnical factors, other than as described above and including the location of the water table, are unlikely to be available at other than a detailed site appraisal stage in which specific mitigation measures are being evaluated.

6.3.2 Exposure Factors

There are three main factors that would ideally be incorporated into the assessment of exposure for the system. These are as follows:

• Traffic flows which not only give an estimate of the likely number of vehicles that will be delayed due to an event, but also give an, admittedly indirect, evaluation of factors such as the potential for personal injury and indeed the potential damage to the local economy.
• Factors related to road geometry, such as sightlines and carriageway width, determine the forward visibility available to drivers at a given location. This, in turn, describes the potential visibility of a hazard and therefore the potential for the driver to see it in time to stop or take other appropriate avoiding action. Clearly sightlines will become less relevant at night when the distance that a driver can see will be determined by the efficacy of the vehicle lighting.
• Diversion length improves the estimate of the potential damage for the local economy, albeit still in an indirect sense. This will be improved if the suitability of the diversion for the disrupted traffic levels, see item (a) above, and for HGVs can be assessed. Clearly if there is no diversion then the hazard ranking will need to reflect this fact.

Having discussed these factors it is also clear that the other landslide hazard assessment and ranking system in use on Scotland's trunk road network needs to be taken into account. The Rock Slope Hazard Index system ( ROSHI), developed by McMillan and Matheson (1997) for the Scottish Executive's use on trunk roads, considers only rock slopes. However, it gives a hazard ranking for the purposes of budgetary prioritisation of management and mitigation measures. Clearly having the two systems running in parallel and on an entirely different basis would severely restrict the ability of the Executive to make rational decisions on expenditure and to compare rock fall hazards with debris flow hazards. As such it is important that the end results from the two systems can be compared.

It should, however, be recognised that the approach to the ROSHI is specific to rock slope instability and it is not the most desirable approach for the development of a system for debris flows. For example, the assessment of debris flow lends itself to a GIS-based initial assessment. This means that areas that satisfy specific, multiple criteria are identified. In contrast the ROSHI takes a sequential approach to building up a hazard ranking number (see below). As such it is proposed that the present debris flow system adopts the same number of hazard ranking categories and that efforts are made to ensure that these are as comparable as possible with the ROSHI. Not least among these efforts should be that the assessment of exposure used in the ROSHI categories is adopted for the debris flow system, with changes only as required to reflect the different nature of the hazard.

The exposure factors used in the ROSHI are as follows:

a) Sightline.
b) Road type (single track, single carriageway, wide single carriageway, dual carriageway,
two-lane motorway, three-lane motorway) - carriageway width and NESA/ COBA speed-
flow relationships.
c) Traffic flow (24-hour, 2-way, AADT).
d) The existence of services and/or other structures above the road.
e) The existence of a downwards slope, river or loch immediately to the opposite side of the road from a potential failure.

Factors (a) to (c) are closely related to those described in Section 6.3.2.

Essentially, the possible range of values of each factor is split into sub-ranges and the intermediate range assigned a parameter value of unity. Factor values in higher and lower ranges are assigned higher and lower parameter values, respectively.

Sightline parameters (item (a) above) are based upon stopping distances from the Highway Code. Thus for single track and single carriageway roads a parameter value of unity is set to the sightline range of 40m to 60m, indicating that vehicles travelling within the speed limit are as likely to hit a block on the road as not ¹⁶. For sightlines less than 40m a greater proportion of vehicles are likely to hit the block and the parameter is increased accordingly. Similarly for sightlines above 60m a greater proportion of vehicles will stop before hitting the block and the parameter value decreases. The equivalent sightline range for dual carriageways and motorways, corresponding to a parameter value of unity, is 60m to 100m.

Carriageway width (item (b) above) gives an indication of the potential space available for avoidance manoeuvres in the event of a rockfall both at the time and if the rockfall has come to rest on the road. The ROSHI parameter values range from 0.7 to 1.2 with unity set for the 6m to 8m width range. However, as a debris flow event is likely to cover the full road width in a very short period of time the opportunity for avoidance manoeuvres is severely limited and carriageway width is thus less pertinent to debris flows. A typical debris flow is likely to close the road for a longer period of time than a typical rockfall and, as previously observed, diversion issues come to the fore in place of avoidance.

Traffic flow (item (c) above) is used along with other data in the ROSHI to derive traffic parameter values. Speed flow relations ships combined with AADT 2-way flows are used to obtain an indication of speed from, which parameter values for each of the six different road types in item (c) above are derived. Although it is also claimed that design speed is incorporated into the assessment of traffic parameter values, it is not clear from McMillan (1995) how this achieved.

Factors (d) and (e) have relatively little influence on the overall outcome and are used to make small percentage adjustments to the overall hazard ranking score.

Factor (d) does however raise the question of whether the Scottish Executive should expend additional effort in including statutory undertakers' services in the hazard ranking assessment. This could potentially divert the assessment from its primary aims of protecting life and limb of road users and the economic activity to which Scotland's road network is so vital. As the presence of statutory undertakers' services is not immediately apparent and they will potentially be protected by any mitigation works undertaken it is suggested that such services not be included in the evaluation of exposure.

The way in which ROSHI combines factors related to exposure and hazard is complex. Data and their effects are amalgamated by data type. Thus factors such as sightlines and carriageway widths are combined into the assessment along with slope angle, slope height and other factors relating to slope geometry in a section entitled, not surprisingly, geometry. This approach makes it very difficult to assess exactly how each factor contributes to the assessment of exposure. Notwithstanding this it is clear from an assessment of McMillan (1995) that while this approach may be unusual it does not introduce errors, only difficulties in comparing the outputs with other systems. In the proposed debris flow system it is recommended that the assessment of hazard remain separate from that of exposure in order to ensure that the development of each is clear.

The proposed evaluation of exposure for debris flows thus becomes

i) Sightline.
ii) Carriageway width (small percentage change to the hazard ranking).
iii)Traffic flow (24-hour, 2-way, AADT) and road type/speed-flow relationship as an indicator of relative traffic speed.
iv)Diversion existence, length and suitability.
v) The existence of vulnerable structures in the path of any potential debris flow (small percentage change to the hazard ranking).
vi)The existence of a downwards slope, river or loch immediately to the opposite side of the road from a potential failure (small percentage change to the hazard ranking).

The associated categories of hazard ranking and their scoring in the ROSHI are follows:

• No [immediate] action required (score 0 to 1).
• Review in five years (score >1 to 10).
• Detailed inspection (within two years) (score >10 to 100).
• Urgent detailed inspection (score >100).

It is recognised that the above descriptive categories are not entirely suited to debris flows. However the important issue will be to ensure that the two systems are as comparable as possible while recognising that the hazard evaluated and therefore the approach to their evaluation are different. Thus measurement of exposure must be as similar as possible for the two systems, as described above, and there must be the same number of, broadly comparable, hazard ranking categories.

Once a hazard ranking is established for a range of sites then priorities can be set within the context of planned maintenance and capital works. Areas and sites viewed as the most vulnerable can then be subjected to well-targeted and justified management and mitigation actions as discussed in Section 8.

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