« Previous | Contents | Next »
Listen
5 KEY CONTRIBUTORY FACTORS TO DEBRIS
FLOWS
by A Heald and J Parsons
5.1 HAZARD FACTORS AFFECTING DEBRIS FLOW
OCCURRENCE
A wide range of factors may have a part to play in the
triggering of particular debris flows in the Scottish
context, and indeed worldwide. Some of these may be
considered fundamental and must be in place, for example
steep slopes (except in particular geological circumstances
such as the presence of peat). Others may be considered
contributory, for example animal tracks, but may
nevertheless tip the balance of stability. In making a
rapid and practical estimate of the relative hazard of
debris flows on a national scale, it is necessary to weigh
the relevant factors and, as a first pass, include only the
highly influential factors in a simple model. Detailed
studies at particular sites may then, as a second or later
stage, include verification that more subtle factors are,
or are not, in place. It may be that these stages lend
themselves to a
GIS-based first pass approach, followed
by more detailed examination of aerial or satellite
photographs and by ground truthing.
A number of landslide studies worldwide have used, as a
primary indicator, the presence of pre-existing landslides,
determined either from historical records or from
geomorphological features. This has been used to determine
both landslide hazard and other variables, such as
magnitude and run-out distances. It is not clear that this
approach is appropriate to this case, as some debris flows
take place where there may be no precedent, or in the case
of channelised flows, evidence may have been lost by the
more regular processes of erosion. Furthermore, in
assessing the wider subject of risk, experience shows that
it is often the unprecedented event that causes the
greatest damage.
Various studies in Hong Kong (Evans and King, 1998) and
in Nepal (Hearn and Petley, pers. comm., 2002) indicate
that slope angle and geological unit alone provide good
correlation with landslide occurrence and these factors
formed the basis of simple predictive models. These studies
considered a wider range of landslide types than simply
debris flows and it may be that the underlying bedrock
geology has a more or less direct influence in relation to
the current study. The following sections discuss each of
the factors that may be considered to influence the
occurrence of debris flows in particular and in a Scottish
context.
5.1.1 Topographical Factors
Slope Angle
There seems little doubt that slope angle is a
fundamentally important factor influencing the occurrence
of debris flows. It should be borne in mind that the slope
angle required to trigger a flow may not be the same as the
slope angle required to maintain the mobility of the flow
in its run-out zone. However, there is some overlap since
there may be an angle above which scour will add further
material to the flow in its run-out zone.
It appears to be accepted that debris flows may be
triggered at angles above about 30_ and the recent flow
affecting the A83 at Cairndow (Figure 5.1) appears to be an
example of this. Similarly, the section of A83 leading up
to the Rest and be thankful, some 7km south east of
Cairndow, has a history of being blocked as a result of
instabilities in the slopes above. It is interesting to
note that the slope angles along this section are also
generally 30¡ to 40¡.
There is however some evidence of flows originating at
angles as low as 26_, for example, the A9 Dunkeld flows and
possibly also the A85 Glen Ogle flows. It appears that fine
granular superficial materials are likely to flow at lower
angles than coarser lithologies or cohesive soils. Much
lower angles are recorded in the special case of peat
flows. It may be that the A9 Dunkeld flows rely partially
on scour for their origin and may therefore be triggered at
a lower angle but it may also be a function of material
type since the A9 Dunkeld (Figure 4.1), and chiefly the A85
Glen Ogle, flows consisted of finer material than is
generally reported appear in the literature.
Figure 5.1 - Main characteristics of debris
flow at A83 Cairndow
Further afield, the 1998 Pachagrande debris flow that
destroyed the Macchupicchu power station in Peru appears to
have originated at a gradient of about 1v:2h (26.5
o) whilst studies in Hong Kong (Franks, 1999)
concluded that '… most landslide sources originate in areas
with slope angles greater than 30
o.
It may be possible to impose an upper limit on
susceptible slope angle on the basis that strata likely to
flow do not stand at angles greater than a certain value.
There does not seem to be very much information on this in
relation to Scottish conditions but it is considered likely
that a maximum angle would not be greater than 45
o to 50
o. An angle of 46¡ is suggested Section 3.1.2,
corresponding to the upper limit at which debris
accumulates.
It would be recommended that any first pass hazard
assessment should include slopes of 26
o to 50
o and that this factor is considered to be of
primary importance. Where the geological formation is peat,
then a lower minimum slope angle should be adopted.
Slope Height
It is not clear that any correlation exists between
slope height and susceptibility to debris flow. Of the
August 2004 events, the vertical height of the main A83
Cairndow and A85 Glen Ogle flows was similar at around 400m
to 500m from source to limit of run-out, but the A9 Dunkeld
flows were smaller by an order of magnitude. It may be
relevant that the Pachagrande flow, discussed above, and
the Huascaràn stürzstroms of 1962 and 1970 were almost an
order of magnitude greater. Given that the materials
involved behave substantially as granular soils and may be
modelled by a c=0 (purely frictional) analysis, then slope
stability theory supports the view that the probability of
failure is independent of slope height.
This aside, it is interesting to note that the source of
the majority of A83 Cairndow flows did appear to start at a
similar height on the hillside, as did many of the flows at
or around the A85 Glen Ogle, however it is considered that
this may be a function of some other factor (
e.g. drift thickness, bedrock, spring line or
change in slope angle) rather than a function of
height.
It is not considered that slope height should be
considered in the hazard model. It may be that, all other
factors being equal, a flow descending from 400m would be
more damaging than one descending from 40m and this could
be considered in assessing hazard exposure.
Slope Aspect
Slope aspect relative to the key elements of bedrock
structure is often considered an important factor in
landslide prediction. This seems most likely to be a
potential preparatory factor in the initiation of debris
flows when the slope aspect and the direction of dip of a
relatively smooth rockhead profile coincide. Similarly it
may be that a stepped rockhead profile, or one with inward
facing scarps relative to the slope aspect, are less likely
to be involved in the initiation of debris flows. Any
correlation between slope aspect and type or thickness of
drift cover is likely to be too complex and the effect too
subtle for incorporation in the first stage model. Effects
consequent upon steep northern slopes compared to gentler
southern slopes, for example, will be picked up by other
means.
It is striking that the major flows in each of the
August 2004 events all occurred on west facing slopes and
this can be extended to include the Stromeferry event. It
may be that the prevailing south-westerly weather patterns
drive a greater degree of rain into the slope causing a
greater degree of saturation. Other contemporaneous flows
at Cairndow faced south and a minority of the smaller 18
August flows in the Glen Ogle/Strathyre district faced
north, south and east. An east-facing flow affected the
B898 on the opposite side of the valley to the A9 Dunkeld
flows.
There is limited evidence that slope aspect alone is a
reliable predictor of debris flows and it s use in the
model would require careful consideration. This factor, in
combination with others, is explored further in Section
6.
Other Topographical Influences
The presence of active stream channels and gullies tends
to focus surface water runoff and hence make channelised
flow more likely. Terraces, ditches (natural or otherwise),
and breaks in slope may have a positive or negative
influence on the formation of debris flows depending upon
their form or location. Rock outcrops or other natural or
artificial barriers in the source, transportation or
deposition zones may retard the formation or impact of a
flow.
These are issues that may prove important at the stage
of detailed site appraisal and should be included in the
model at that stage.
5.1.2 Geological, Geotechnical and
Hydrogeological Factors
Geological Formation
Since the flows largely mobilise unconsolidated
deposits, the influence of bedrock geology may at first be
considered to be limited. Indeed, Vandine (1985) discounted
underlying bedrock as a predisposing factor for landslides
in British Columbia. It may be surprising then, that the
three areas affected in August 2004, and the earlier Rest
and be thankful instabilities, were all in areas underlain
by Dalradian schists; a rock type often associated with a
relatively low debris flow activity (Section 3.1.3).
Similarly, the Invermoriston flow is in an area of Moinian
schist and the Stromeferry flow is underlain by older
metamorphic rocks. Thus, while any direct correlation
between susceptibility to flow and bedrock type may not be
entirely clear at this stage, the tendency for schists and
similar metamorphic rocks to weather to produce fine soils
consisting of platey minerals, may be significant. Further,
the low permeability of these rock types is likely to limit
dissipation of pore water pressures by under drainage. At
least in one case, that of the A9 Dunkeld flows, the false
bedded silty fine sand that flowed does not appear to be
locally derived and thus may be attributed to coincidence.
The apparent correlation between debris flows and schist
should be considered in the light of the preponderance of
this and similar lithologies in the high relief areas of
Scotland. Further afield, Franks (1999) found that '…
volcanic rocks were generally more susceptible to
landslides than feldsparphyric rocks' in Hong Kong, but
thought that '… this may have been because the topographic
relief is greater where the bedrock is volcanic'.
The presence of a mantle of superficial deposits is of
fundamental importance to the susceptibility to debris
flows. It has been suggested that a critical thickness of
around 1m to 2m may be most favourable to triggering a flow
and this would appear to be supported by the source areas
of the debris flows at the A83 Cairndow, Rest and be
thankful, A85 Glen Ogle and from Hong Kong studies (Franks,
1999). The debris flow materials were predominantly finely
granular deposits, of glacial origin with the exception of
the A9 event, which was fluvial or fluvio-glacial. Given
that glaciation affected all of Scotland and that the
majority of, if not all, steep sided slopes are expected to
have a partial cover of glacial deposits, it is unlikely
that it will be possible to include this variable as a
factor in the model.
In summary, while the solid geological formation is not
in itself considered significant, the lithology of the
underlying bedrock is likely to be a secondary influence.
The presence and characteristics of a mantle of superficial
materials is of primary importance but, given that such a
mantle may be thin, this information is not readily
available in a
GIS model and may be difficult to
discern with certainty by any form of remote imagery. It
may be more practical to assume at first pass that
everywhere below the maximum slope angle has the requisite
mantle of superficial material and to filter out those
cases where this does not apply by walkover at the second
stage.
Landslide History
As discussed above, the pre-existence of landslides is
often considered to be a good predictor of future
instability. Although landslide history is an important
factor in predicting future instability, it is not clear
that it is as useful in predicting fast moving debris flows
as it is in forecasting more slow moving progressive
movements. However, evidence of past debris flows on a
slope is a good indicator that the conditions exist for
future flows and this may be considered an important factor
at the stage of a second, more detailed, pass. Further,
where a debris flow has occurred in the immediate past and,
for example, the vegetation has been removed to expose the
vulnerable soils beneath, there is no doubt that the area
is more susceptible to remobilisation if the trigger
conditions (
e.g. rainfall) should recur.
Geotechnical Factors
Soil properties including cohesion, grain size, shear
strength, moisture content, void ratio, relative density
and permeability are relevant to the occurrence of debris
flows. These are likely to be known only as a result of a
detailed ground investigation and should be picked up
during a second stage detailed site appraisal.
Earthquakes
Although flows worldwide have been triggered by seismic
activity (
e.g. Huascaràn 1962 and 1970), the occurrence
and strength of earthquakes in Scotland is so low that
their effect need not be considered here.
Hydrogeological Factors
Studies in Canada (Vandine, 1985) and California (Reneau
and Dietrich, 1987) indicate that surface drainage is an
important factor in controlling debris flow susceptibility,
demonstrated by the fact that most of the landslides
studied occurred within or adjacent to significant drainage
lines or hollows. This pattern would appear to hold true
for the A83 Cairndow, A83 Rest and be Thankful and the main
A85 Glen Ogle (Figure 5.2) events.
Figure 5.2 - Source area of A85 Glen Ogle
debris flow event.
The location of the ground water table is important in
the prediction of any slope instability but is difficult to
estimate except as a result of detailed ground
investigation. However, the presence of spring lines is an
important indicator. It may be possible to identify these
remotely from aerial or satellite photographs and published
geological information.
Other hydrogeological and hydrological features that are
relevant to the probability of occurrence of debris flows
include runoff coefficients and the size and shape of
catchments. Some of the factors may be obtained remotely
and from pre-existing data sets, but others would only be
obtainable from detailed site specific studies.
5.1.3 Meteorological Conditions
Rainfall
There can be little doubt that rainfall is one of the
single most important factors in triggering debris flows in
Scottish conditions. It is commonly accepted that the most
frequent climatic trigger for landslides worldwide is a
heavy rainfall event following a period of high antecedent
rainfall. Of the August 2004 events, it appears that the
A83 Cairndow and A85 Glen Ogle flows occurred after short
intense summer storms, albeit against a background of a wet
summer, whereas the A9 Dunkeld flows followed more
prolonged heavy rain.
The Meikle Tombane rain gauge approximately 7km from the
A9 Dunkeld flows measured 77.5mm of rain on 9 August 2004,
two days before the event. This quantity of rain on a
single day has a return period of approximately 50 years.
During the three days 9 to 11 August, 171.3mm of rain was
measured at Meikle Tombane and such a quantity of rain over
three days has a return period of just over 400 years.
The Lochearnhead rain gauge close to the A85 Glen Ogle
event measured 80.8mm of rain on 18 August and this has a
return period of 10 to 15 years. It is interesting to note
that the rainfall record indicates that 89mm of rain fell
here on 10 August 2004 and this has a return period of
about 20 years. This rain gauge records rainfall only on a
daily basis but anecdotal information suggests that the
rain was confined to a relatively short period for the day
of 18 August. If the rainfall measured on 18 August fell in
only six hours then the return period would be about 120
years, if in 4 hours then the return period would be 250 to
300 years.
It is also notable that the burns in Glen Ample and the
Keltie Water (draining Ben Vorlich and Stuc A'Chroin to the
south east of the debris flows that affected the trunk
roads) experienced much worse flood flow conditions than
the Glen Ogle burn. Three bridges in Glen Ample and five on
the Keltie Water were washed away. In terms of return
periods for these two catchments it is estimated, based on
observations in the glens, that the floods were greater
than 100 year events. In the Ogle Burn the flood debris
indicates a much smaller event, probably with less than a
10 year return period.
Information has been obtained from rain gauges about
20km away from the Cairndow event. Their return periods do
not suggest an extraordinary event but their distance away
from the site of interest may mean that they did not
properly sample the event rainfall where the flows
occurred.
Thus, it seems that rainfall events of both long and
short durations should be included in the model. However,
there are currently insufficient rainfall data to determine
how much rain has to fall over what time frame, before the
likelihood of debris flows becomes a concern. Further, it
is expected that these 'trigger levels' will vary from area
to area as soil composition and other topographical factors
come into play.
A major practical difficulty in incorporating rainfall
into any model predicting debris flows is predicting which
geographical areas of Scotland may be subject to
exceptionally heavy rain over the lifespan of the model. It
may well be considered that all areas of the highlands and
islands, and possibly the whole of Scotland, could be
equally subject to this factor. In that case rainfall
distribution is no longer a variable in any predictive
model, although, of course, rainfall level remains a
critically important variable. However with more
information from future instabilities, it may be possible
to set rainfall 'trigger' levels as a short term management
tool.
Other Meteorological Factors
Of the other meteorological influences, snow melt is
clearly a source of surface runoff and of saturation of
near surface sediments, thus increasing the likelihood of
instability. Conversely, frozen ground would be expected to
be an inhibitor of debris flows. Wind, in addition to the
possible effect discussed above under 'slope aspect' in
relation to driving rain into the slope, may also have the
secondary effect of uprooting trees with a consequent
detrimental effect on stability. These other meteorological
influences are considered either too subtle or too
unpredictable to form a useful basis for a debris flow
model.
It should be noted that these comments relate to the
long term prediction of the influence of meteorological
conditions on a particular slope over a period of many
years. The prediction that a particular slope has an
increased susceptibility due to a storm that is currently
occurring or imminently forecast, is quite a different
matter.
5.1.4 Factors Related to Vegetation and Land
Use
Vegetation Factors
Different types and densities of vegetation may be more
or less retardant to debris flows depending upon how they
affect soil infiltration rates and upon how their root
systems serve to hold the soil in place. Landslides in Hong
Kong during 1992/1993, occurred in terrain with low scrub
and grass rather than the dense tropical vegetation typical
of the region. Forestry in particular appears to reduce the
probability of debris flows and may be considered of
primary importance. In British Columbia, policy has
concentrated on controlling timber harvesting and
encouraging reforestation in the 'source zone'. Forests may
be picked up by
GIS and should be incorporated into the
susceptibility model at an early stage. Other types of
vegetation may be considered to be less influential and
also less readily identified remotely and should be
incorporated at a later stage.
Land Use Factors
Many land use factors may influence the likelihood of
debris flows. These include agricultural uses, the presence
of buildings or other man made features such as
hard-standing, infrastructure or drainage. The influence of
the old road in concentrating water flows was demonstrated
at the A9 Dunkeld failure (Figure 5.3) and forest tracks
could be expected to have a similar influence, as in the
case of the washout that blocked the A83 Rest and be
thankful in the vicinity of Roadman's Cottage, in 1999.
Conversely, in the A9 Slochd failure of July 2002, the
presence of the trunk road contributed in a similar way to
the failure of the old road (used as a cycle path) and to
its own failure by undercutting. In that case, a drainage
channel was another man-made feature that served to
concentrate runoff and hence contribute to the failure. In
the case of the Stromeferry flow, it was an old field
boundary/deer track that created a pathway for preferential
water flows.
Generally, these features are of local significance and
would be difficult to incorporate into a national model.
They should, however, be incorporated at the site specific
assessment stage.
Figure 5.3 - Influence of old road on debris
flow at A9 Dunkeld.
5.2 HAZARD FACTORS AFFECTING DEBRIS FLOW
RUN-OUT
5.2.1 Slope Angle, Height and Magnitude (Volume
of Material Delivered to Deposition Zone)
It is generally accepted that debris-supported flows (
i.e. those in which there is
particle-toparticle contact) including most or all of those
that have affected Scottish trunk roads in recent years,
will flow at slope angles at or above 11
o. The 1998 Pachagrande debris flow, referred to
above, is an example of a debris flow that conforms to this
limiting slope angle. Hungr
et al. (1987) defines a confined channel as one
with a width to depth ratio of less than five and reported
(Hungr
et al., 1984) that deposition will occur on slopes
of 10
o to 14
o for non-channelised flows and 8
o to 12
o for channelised flows. This agrees well with
studies in Hong Kong (Franks, 1999). Water-supported debris
flows (
i.e. where the particles are not generally in
contact) often flow at angles at or above 2
o. Observations of Scottish debris flows
indicate that they are arrested at angles steeper than 2
o.
As discussed in Section 5.1.1 above in relation to
probability, there seem to be no limiting factors related
to the height (or length) of run-out.
Along the Cairndow section of the A83, it was observed
that, of debris flows originating at a similar height,
'smaller' flows did not tend to reach the A83. Whilst there
may have been subtle differences in the factors affecting
the run out channel characteristics (
i.e. angle of slope), it may suggest that
there is a certain volume of material required to gain
sufficient momentum to reach the road. However, a more
detailed investigation would be required to confirm
this.
5.2.2 Channel Characteristics
In channelised flows, the cross-sectional shape of the
stream channel, its width and depth in particular, may be
expected to affect the length and volume of the run-out.
Similarly, the longitudinal shape of the channel may lead
to zones of deposition and zones of erosion along its
length and these may vary with the intensity of different
stages of the flood. Further, the smoothness of the channel
may promote a longer run-out and this may in turn be a
function of topography, geology (drift thickness, bedrock
type and structure), obstructions or constrictions (natural
or artificial), and history of debris flows.
The sinuosity of the channel may absorb the energy of
the flood and thus retard it. However, it may also result
in increased erosion on the outer sides of bends and in
this way debris may be added to the flow. Bends in the
channel may affect the direction of run-out and thus the
effects of the event. The classical example of this relates
to the 1970 Huascaràn stürzstrom in which the town of
Yungay was thought to be protected by a 150m high hill that
deflected the channel to the south. However, one branch of
the flow failed to turn the bend, surmounted the hill and
resulted in a reported 18,000 deaths in Yungay. The recent
Glen Ogle flow, though on a much smaller scale and
fortunately without casualties, followed a very similar
pattern. The early part of the flow followed a sharp
left-hand bend (Figure 5.4) in the stream channel, thus
damaging a culvert and a section of the road. A later pulse
did not turn the bend but had sufficient momentum to
continue straight ahead over a rock outcrop, sweeping away
a vehicle that might have been thought to be protected by
the outcrop. In this way, the width of the run-out was
increased and a greater length of the road was affected.
Conversely, in the case of the A83 at Cairndow a ridge at
the toe of a drainage channel successfully prevented one
debris flow from reaching the road.
Accordingly, the hydrological factors affecting run-out
can be seen to be complex and are thus best reviewed on a
site specific basis.
5.2.3 Vegetation and Land Use Factors
The surface conditions in the run-out zone may permit or
impede the run-out of the flow. Afforestation may be
particularly important in retarding flows as seen at
Cairndow, but other conditions, such as hard surfacing or
pasture land may be much more permissive to flows. Uprooted
trees can contribute to the power of the debris flow. This
was seen at the A9 Dunkeld, where trees formed part of the
debris that reached the road and trapped vehicles and at
Glen Ogle, where trees were swept into the culverts and
formed part of the blockage. Uprooted trees have caused
significant damage in larger scale events in the Himalaya,
for example in the Hinku valley of Nepal and at Punakha in
Bhutan where a temporary dam of trees deflected the flow
with a resultant loss of life.
On a Scottish scale afforestation is, in most cases,
likely to retard run-out and this may be considered an
important factor in assessing the effects of a debris
flow.
5.3 FACTORS AFFFECTING EXPOSURE TO DEBRIS FLOW
HAZARDS
The key factor in relation to the exposure that results
from a debris flow is whether or not the flow reaches a
vulnerable element. As this study is focused on trunk roads
and trunk road users, this key factor becomes simplified to
whether or not the flow is, or is not, expected to reach a
trunk road or associated infrastructure. Clearly, if there
is no possibility that the flow will reach a trunk road (or
associated infrastructure) then both the hazard and the
hazard ranking (
see Section 6) become, for the
purposes of this study, zero.
In cases where a trunk road is present within the
modelled run-out zone of a flow, it would be possible to
prioritise actions based on the scale of the exposure as
discussed below.
Figure 5.4 - View of the larger of the A85 Glen
Ogle debris flows, showing the sharp bend in the
channel just above road level.
5.3.1 Factors Related to Road Usage
Clearly, the potential exposure in relation to death or
injury to members of the public are greater where traffic
flows are greater. Debris flows tend to be fast-moving
compared to most other forms of landside and frequently
wash down very large boulders, as seen in the Cairndow
(Figure 5.5) and Glen Ogle events. Any washing down of
large boulders, or indeed other large items of debris, has
the potential to cause serious injury or fatality.
As trunk roads comprise, by definition, the country's
first level strategic road network, factors to be taken
into account by this study will include traffic flows,
sightlines and the availability and length of diversion
routes. Traffic flow relates to the likelihood of a debris
flow event affecting road users, whilst sightlines will
determine the potential for the road user to take avoiding
action. The availability and length of a diversion route
may be seen as an analogue for the economic impact of such
an event. This may be complicated by the possibility of
alternative routes becoming blocked by other
contemporaneous debris flows resulting from the same
weather conditions or other factors. In the cases of both
the recent Dunkeld and Glen Ogle events, minor roads in
each area were also blocked by separate but related
events.
Figure 5.5 - Debris fan containing boulders
(estimated up to 9 tonnes) at A83 Cairndow.
In summary, it may be considered that traffic flows and
are a key factor that may be utilised for prioritisation in
a national plan. The other factors discussed here are more
subtle and may be considered on a site specific basis.
5.3.2 Factors Related to Emergency
Response
The seriousness of an event may be exacerbated or
minimised by the ease of emergency response. For example,
at the A9 Dunkeld event the police were able to attend the
scene within a few minutes and to assist motorists from
their vehicles. This may not be the case in a more remote
location. At the A85 Glen Ogle event,
BEAR personnel were rapidly on the scene
and provided assistance but, with 20 vehicles and 57
motorists isolated between two debris flows, the decision
was wisely taken to effect evacuation by
RAF and Royal Navy helicopters. The
events of August 2004 suggest that when debris flows occur,
multiple events should be regarded as highly likely and
thus there is a reasonable chance of the public becoming
trapped or the main emergency becoming inaccessible to
emergency vehicles. Clearly, the use of helicopters can
reduce the effect of both remoteness and of multiple debris
flows.
Police and military personnel and trunk road maintenance
staff are trained in emergency procedures and during recent
events provided an excellent service. However, assessing
the likelihood and location of any further debris flows is
not part of their capability. Depending upon the location
of the emergency, there is always likely to be an interval
of several hours before a geotechnical specialist with
experience of landslides can attend the scene to assess the
current and near-future hazards.
In such events, the alarm is often raised rapidly by
motorists using mobile telephones. There are however areas
in Scotland where there is no mobile telephone coverage.
These may be areas that are susceptible to debris flow
activity and the seriousness of any event occurring in such
locations could therefore be exacerbated by this
factor.
It may be considered appropriate to include these
factors relating to access and the ability to rapidly raise
the alarm in the determination of hazard ranking of
particular routes. However, such areas are likely to be
remote, have lower traffic flows and therefore affect fewer
people. Such actions in the hazard ranking may therefore
undermine the need to target resources where there is the
greatest need, typically identified by the greatest traffic
levels.
5.3.3 Factors Related to the Local Value of the
Asset.
Factors considered here reflect the value of individual
assets on the network and the likely cost of repair, for
example damage to structures is likely to be more expensive
to repair than damage to the carriageway surface or to an
earthwork.
It is also important to consider the environmental
implications of a debris flow. Whilst the primary concerns
of the work here are in ensuring that the exposure of the
road using public to potentially dangerous and adverse
economic debris flow events is minimised, clearly some
account of the environmental impact of debris flow is
required.
Factors relating to environmental issues and designated
areas would need to be assessed on a site specific
basis.
5.3.4 Publicity and Political Factors
There is a potential for adverse publicity to be
associated with any event that causes a trunk road to be
closed although this may be diminished if, as in recent
events, casualties have been avoided and the response is
timely and efficient. The difficult question would be
whether roads should be closed on the basis of a forecast
event in any particular location and how the
non-realisation of such an event would be perceived by the
public and the media.
It is considered that the assessment of this factor is
beyond the remit of this study.
5.3.5 Secondary Effects
Debris flows may not only have a direct effect on a
trunk road but there may also be 'knockon' effects. For
example, the debris may dam a river causing impounding of
water and inundation upstream. This was the mechanism for
destruction of the Macchupicchu power station following the
1998 Pachagrande debris flow. Subsequent bursting of such a
temporary dam may cause further destruction downstream.
Either of these situations could be damaging to a trunk
road or to trunk road users. The potential for secondary
effects would need to be assessed on a site specific
basis.
5.4 SUMMARY OF KEY CONTRIBUTORY
FACTORS
A wide range of factors may contribute to the likelihood
of, and exposure to the effects of, a debris flow in a
particular location. Some of these are widely applicable
and others are subtle but may make a critical difference at
a particular location. Furthermore, some are readily
assessed using
GIS and/or remote sensing whereas others
are only discernible to the expert after close inspection.
It is likely that it will be necessary to base a first
stage hazard assessment and hazard ranking upon the more
widely applicable and readily obtainable factors and then
to carry out secondary and subsequent filters using more
site specific and difficult to assess factors.
Rainfall or other source of water is a critical factor
but it has been assumed that all parts of the trunk road
network may be subject to excessive rainfall and so this
has been excluded as a differentiating factor.
For a risk or hazard to exist at all, the conditions
must allow a debris flow to occur and must allow the
run-out of such a flow to reach a trunk road, a trunk road
user or other infrastructure or feature that can impact
upon that road or road user. As a first pass, there are
three critical factors that could be obtained rapidly and
remotely from a
GIS to assess whether these conditions
are in place:
- A source area where the slope angle is greater than
26
o and less than 50
o.
- A run-out zone where the slope angle is greater
than 8
o.
- A trunk road is present within either of the above
zones.
It should be noted that peat can flow at much lower
angles than these and it would be appropriate also to
perform an alternative first pass in which a search is
carried out for all trunk roads passing through areas of
peat.
Perhaps the next most important factors are those that
would allow prioritisation of particular routes or parts of
routes, particularly traffic flows, the strategic
importance of the route and the length and viability of
diversions.
There are a number of influential factors that should be
considered at the second stage and possibly the most
important of these is afforestation. Other significant
topographical features may be considered at this stage
along with the lithology of solid and drift geological
deposits and the landslide history. Perhaps the next most
critical factors relating to the seriousness of the event
may be the factors affecting the emergency response and
possibly the publicity and political factors.
Other factors such as vegetation and land use, animal
and anthropogenic factors, slope aspect, detailed
topography, geotechnical, hydrological and hydrogeological
factors, local structures, environmental implications and
secondary effects would need to be considered on a site
specific basis but it may be necessary to bear in mind the
possibility of 'knock-on' effects at all stages following
the first pass.
« Previous | Contents | Next »