Ministry of Environment Terrain

Landslides, Hazards, Consequences and Risks

This Chapter introduces landslides in general, landslides in British Columbia, and the concepts of landslide hazards, consequences and risks as they pertain to terrain stability mapping.

2.1 Landslides

A landslide event is defined as "the movement of a mass of rock, debris or earth down a slope" (Cruden 1991). The word 'landslide' also refers to the geomorphic feature that results from the event. Other terms used to refer to landslide events include 'mass movements', 'slope failures', 'slope instability' and 'terrain instability'. In spite of the simple definition, landslide events are complex geological/geomorphological processes and are therefore difficult to classify. The classification system most commonly used in North America, and used in this report, is modified from Varnes (1978) to reflect the common usage in British Columbia (Table 2.1). The classification is based upon material type and type of movement, and is similar to the updated classification of slope movements suggested by Cruden and Varnes(1996).

2.1.1 Material Type

The material involved in a landslide is classified into two groups, 'bedrock' and 'soil'. Soil, which is generally unconsolidated surficial material, is further subdivided into 'debris' and 'earth' depending upon its texture.

Bedrock refers to earth materials that have lithified by some rock-forming process. Its strength depends not only on the rock type but also on the degree of weathering and the density and orientation of the discontinuities, which are generally the planes of weakness in the rock mass. For instance, if a strong, hard granite contains many fractures, the rock mass may be no stronger than a coarse grained soil.

Debris is composed of predominantly coarse grained soil (bouldery through to gravel and sand-sized materials), or as mentioned above, can also include highly fractured bedrock. The strength of coarse grained soil is generally derived from friction between the grains. Woody debris such as trees or logs, or other organic material, is sometimes incorporated with the inorganic debris.

Earth refers to predominantly fine grained soil (primarily of silt and clay sized materials). The strength of fine grained soil is generally derived from cohesion, the chemical and electrical bonding between the small particles.

2.1.2 Type of Movement

Falls take place rapidly by free-fall, bouncing, or rolling, and may develop into either slides or flows.

Topples consist of the rapid rotation of a unit of rock or soil about some pivot point. Toppling may not lead to either falls, slides or flows.

Slides involve the movement along one or more distinct surfaces. Slides are subdivided into 'rotational slides' and 'translational slides', depending upon the shape of the failure plane.

Rotational slides, also referred to as slumps, involve movement along a curved failure plane. Often the failure plane did not exist before movement occurred. Rotational slides usually involve relatively few distinct rock or soil units.

Translational slides involve the movement of many rock or soil units along a plane. If few distinct units are involved, the movement is referred to as a 'translational block slide'. Often the failure plane existed before movement occurred.

Most rotational and translational slides occur rapidly, however, some earth slumps and slumps in weak rocks can occur slowly, over many days or even years.

Lateral spreads are dominated by lateral extension of the ground, accompanied by shear or tensile forces, and a general subsidence of the ground surface. They generally occur relatively slowly.

Flows describe movement that resembles a viscous fluid. Some flows occur slowly, others occur rapidly. Velocity within the flowing mass is usually decreases with depth and laterally. In most cases, water is an integral component. Creep is a type of flow that occurs very slowly.

Complex landslides involve the combination of two or more types of movement. Commonly one type of movement starts the material moving, such as a debris slide, and once underway the material takes on the character of another type of movement, such as a debris flow. The name of the complex movement is a combination of the types of movement, in order of occurrence, such as a debris slide-debris flow. The rate of movement depends on the types of movements and material types involved.

2.2 Landslides in British Columbia

At present there is no comprehensive inventory of landslides in British Columbia, however, most types of landslides occur in the province. A number of the publications that review landslides in the province include Eisbacher (1979); Evans and Gardiner (1989); Evans (1991); Evans (1992); VanDine (1992); BC Ministry of Energy, Mines and Petroleum Resources (1993), and BC Ministry of Transportation and Highways (1996). The following are examples of common landslide types in British Columbia.

Rock falls and rock topples are associated with steep, near vertical or overhanging natural bedrock bluffs and steep bedrock excavations. Where they occur frequently, the bedrock is usually moderately to highly fractured with intersecting fractures. Rock falls and rock topples vary in size from a single piece of rock to many thousands of cubic metres. They often occur rapidly without warning. Rock falls and rock topples occur in all areas of British Columbia. Small rock falls are common along many of the province's transportation routes.

Debris and earth falls and topples are associated with steep, near vertical or overhanging natural soil bluffs or excavations. They vary in size from a single boulder or block of soil to many hundreds of cubic metres. They often occur rapidly and without warning. Debris and earth falls and topples occur throughout the province, wherever the appropriate vertical relief exists. They are common in the dryer southern interior and are a concern along a number of the interior transportation routes.

Rock slumps most frequently involve large tracts of land, up to several kilometres across. They are usually located along river banks or along steep valley sides and are generally associated with weak, fine textured bedrock types. Glacier unloading is postulated as one triggering mechanism of rock slumps. They can occur moderately fast, all at once, or slowly and progressively. Large slow-moving rock slumps are commonly found bordering river valleys, in particular in northeastern British Columbia, for example along the Peace River and Laird River valleys.

Rock block slides and rock slides are generally associated with stronger rock types that fail along pre-existing planes of weakness. They usually occur rapidly in strong rocks and more slowly in weak rocks. Once the initial failure has occurred they can continue to move slowly and/or intermittently. They can vary in size from very small, involving one or several blocks of rock, to extremely large.

Rock block slides and rock slides occur in all mountain ranges within the province. An example of a large rock block slide is Downie Slide north of Revelstoke along the Columbia River valley. Hope Slide east of Hope, which occurred in January 1964, is an example of a large rock slide. Some rock slides can have extremely long runout zones and become debris flows. The resulting complex landslide is often referred to as a 'rock slide-avalanche', or simply a 'rock avalanche'. An example of a rock slide with a long runout is the 1959 Pandemonium Creek rock avalanche in the Southern Coast Ranges which travelled nearly 8 km along a stream valley inclined at only 6° to 9°.

Earth slumps are usually located along river banks, road cuts or steep valley sides. They can involve the displacement of one or more rotational blocks of weak, predominantly fine grained soil. They can occur extremely slowly to rapidly, all at once, or slowly but progressively. They can stabilize then remobilize, and often retrogress with time. Earth slumps vary in size from small, involving several cubic metres, to large, involving many hundreds of thousands of cubic metres.

Earth slumps are common in glaciolacustrine sediments along the interior valleys of the province, in particular the Thompson, Columbia and Okanagan valleys. Many spectacular examples of large rapid earth slumps were triggered by flood irrigation of silt benches at the turn of the century. With today's restrained irrigation techniques, landslides of this type occur less frequently. Earth slumps also occurr underwater, as submarine earth slumps, within delta fronts. Examples are those which occurred along Howe Sound and Douglas Channel, near Kitimat.

Debris slides are common in areas with steep slopes and high rainfall. They often occur during periods of intense rainfall. They tend to be shallow failures and usually occur along planes of weakness between looser, overlying soil such as colluvium or weathered till, and denser, underlying material, such as unweathered till or bedrock. Debris slides are also common along road fills. They vary greatly in size from very small, involving an area a few square metres, to large, involving up to many hectares. Once started, they usually travel rapidly and can develop into debris flows. Debris slides are ubiquitous in all parts of the Coast Mountains, Vancouver Island and the Queen Charlotte Islands. They also frequently occur in the wetter parts of the interior of the province, such as the Columbia Mountains.

Debris flows can occur on open slopes or in pre-existing channels. Open slope debris flows are also referred to as 'debris avalanches'. Channellized debris flows have in the past been referred to as 'debris torrents'. Both open slope and channelized debris flows involve the rapid movement of liquefied, predominantly coarse grained soil and sometimes large organic debris, on steep terrain. They may be initiated by debris slides or rock slides. The volume of the debris often increases downslope as a result of slope erosion and/or channel scouring. Debris flows occur in surges and often come to rest many hundreds to thousands of metres from the initiation zone. Debris flows are common in all mountain regions of British Columbia. They can vary from small, involving several tens of cubic metres, to large, involving many thousands of cubic metres.

Earth flows are large, slow or rapid moving landslides of predominantly fine grained soil and/or weathered volcanic bedrock. They usually involve relatively large tracts of land. Earth flows are common in the Interior Plateau. The 1993 Mink Creek slide near Terrace is an example of a rapid earth flow in glaciomarine sediments.

Soil creep is a shallow, slow-moving form of an earth flow involving thin layers of near-surface soil. Where permafrost is involved, the movement is referred to as 'solifluction'. Soil creep is found throughout the province, while solifluction is found in northern British Columbia and in the higher alpine regions of the province.

2.3 Landslide Hazards, Consequences and Risks

The following summarizes some of the terms relating to terrain stability or landslide hazard and risk assessment. It is adapted from Morgan et al (1992), Fell (1994) and Sobkowicz et al (1995). Common abbreviations are included in parenthesis.

2.3.1 Landslide Hazards

The word 'hazard' is derived from the Arabic word for 'a die' (singular of dice) and is often related to 'chance or probability', as in the phrase 'to hazard a guess'. This definition is reflected in the United Nations definition of natural hazard: "the probability of occurrence of a potentially damaging natural phenomenon" (Varnes 1984). In reference to landslides, Fell (1994) defines 'hazard' as "the magnitude of the event times the probability of its occurrence".

In British Columbia, however, 'hazard' is also often used to describe the damaging phenomenon, as in 'natural hazard', 'geological hazard', 'landslide hazard', or a specific type of landslide hazard, such as, a 'debris flow hazard'.

Hazard (H), as used in this report, is a condition or event that puts something or someone, in a position of loss or injury, or in a position of potential loss or injury. A landslide hazard results from a potential or actual landslide occurrence.

Probability of occurrence (P) is the chance or probability that a landslide hazard will occur. It can be expressed in relative (qualitative) terms or probabilistic (quantitative) terms. Examples of relative terms are 'very high', 'high', 'moderate' and 'low', or 'very frequent', 'frequent', 'infrequent' and 'seldom'.

Probability of occurrence can be expressed as an 'annual probability of occurrence' (P_a), or a 'long term probability of occurrence' (P_x), where '_x' is a given number of years. The following statistical equation converts P_a to P_x:

(P_x) = 1-(1-(P_a))^x

For example, the probability of occurrence of a landslide hazard in a 50 year period given an annual probability of occurrence of 1 in 475 is:

(P50) = 1-(1-(1/475))⁵⁰
= 0.10
= 10%

For natural hazards that occur frequently in the same location such as floods, a statistical probability of occurrence can be determined by rigorous analysis. Landslide hazards, however, usually occur infrequently in a given location, therefore an estimated probability of occurrence is often determined by judgement combined with empirical evidence. Such estimates may be arrived at by consensus among a number of specialists and are referred to as Bayesian-like prior probability estimates, or simply 'Baysian-like estimates'. True Baysian estimates can only be tested if events have a relatively high frequency of reoccurrence, for example snow avalanches (McClung and Tweedie). Baysian statistics are described in texts such as (Freund 1973 and Lapin 1983).

The results of Bayesian-like estimates are often presented as ranges. The ranges presented in Table 2.2 are useful benchmarks in that they decrease in a regular stepped manner and they relate to some physical factors as well as to existing hazard acceptance.

Magnitude (M) is the volume of displaced material involved in a landslide hazard. The magnitude can be expressed qualitatively by words such as 'small', 'medium' or 'large', or quantitatively as an actual volume or range of volumes. It should be emphasized that some landslide events, such as debris flows, may take place as a number of separate smaller events or surges, and the magnitude of the surge versus the total magnitude of the event must be differentiated. From air photos it is often difficult to estimate the actual or potential thickness of a landslide Therefore the area affected by the landslide hazard, with some assumption of thickness, is sometimes used as a rough estimate of magnitude.

As in earthquake engineering, the magnitude of a landslide hazard can be related with the probability of occurrence of that hazard. An example of a single 'magnitude-probability of occurrence' relation is:

a colluvial fan that is subject to debris flow with an estimated magnitude of 10,000 m3, with an estimated annual probability of occurrence of 1:200.

Intensity (I) is a collection of physical parameters that describe the destruction or destructive potential of a landslide hazard, such the downslope velocity, the thickness of the landslide debris and/or the impact forces. Intensity can also be expressed qualitatively, by words such as 'slow', 'moderate', and 'fast', or 'low', 'moderate' and 'high', or quantitatively. Intensity varies with location along and across the path of the landslide and therefore it should ideally be described using a spatial distribution function.

As in earthquake engineering, the intensity of a landslide hazard at a given location can be related to probability of occurrence of that hazard. An example of a single 'intensity-probability of occurrence' relation is:

a specific site on a colluvial fan that is subject to debris flows with estimated velocities > 5 m/sec and an estimated debris deposition thickness > 2 m, with an estimated annual probability of occurrence of 1:400.

For a range of magnitudes or intensities, and the corresponding range of probabilities of occurrence, the relation can be graphed (Figures 2.1). The two curves represent the ranges in confidence or uncertainty in assigning the parameters. The area beneath the magnitude (or intensity)-probability of occurrence curve is the product of the magnitude (or intensity) and the probability of occurrence, and represents the 'total hazard' as defined by Fell (1994).

Figure 2.1 Example of magnitude or
intensity - annual probability of occurrence relation.

2.3.2 Landslide Consequences

Landslides hazards can result in a wide variety of downslope consequences, including environmental, social and/or economic. To have a consequence, there must be something or someone vulnerable to loss or injury, as described below.

Elements at risk (E) include any land, resources, environmental values, buildings, economic activities and/or people in the area that may be affected by the landslide hazard. The elements at risk can be quantified by placing a dollar value, or some other form of value, on them. Specialists are often required to identify and/or evaluate certain elements at risk. For instance a fisheries biologist should determine whether or not a stream is a fish stream and detemine the value of that resource.

Vulnerability (V) is the degree of damage caused by a landslide hazard to the elements at risk. It is usually expressed in relative terms, using words such as 'no damage', 'some damage', 'major damage', 'and total loss', or by a numerical scale between 0 (no damage) and 1 (total loss). An assessment of vulnerability often requires specialist input, such as engineers for structures and resource managers for natural resources.

Vulnerability can also be subdivided, for example into spatial vulnerability (V_s -- will a particular area be affected by the event?), temporal vulnerability (V_t -- will the area be occupied by a person at the time of the event?), and life vulnerability (V_xl -- will there be loss of life due to the event). When expressed by a numerical scale, the subdivided vulnerabilities can be multiplied together to obtain the total vulnerability (V = V_s x V_t x V_l) (Morgan et al 1992).

Consequence (C) is the resulting loss or injury, or the potential loss or injury. It is the product of the elements at risk and the vulnerability (E x V), and can be quantified if the element at risk is expressed as a value and the vulnerability is expressed numerically.

When a consequence is expressed qualitatively, it is sometimes referred to as a 'consequence rating'. The phrase, 'there is a high probability that landslide debris will reach the creek, cause siltation and damage fish habitat', is an example of a consequence rating.

2.3.3 Landslide Risks

Landslide risk considers both the landslide hazards and the consequences. Simply stated, risk is the product of the probability that a landslide hazard will occur and the consequence of that occurrence:

R = P x C

Specific risk (R_s) is the product of the annual probability of occurrence and the vulnerability (R_s = P_a x V) for a specific element at risk. Depending on the quality of the data and the methods used to express annual probability of occurrence and vulnerability, specific risk can be expressed qualitatively or quantitatively.

Total risk (R) is the sum of the specific risks, or the sum of the product of the annual probability of occurrence, the elements at risk and the vulnerability (R = P_a x E x V). As for specific risk, depending on the methods used to express annual probability of occurrence, elements at risk and vulnerability, total risk can be expressed qualitatively or quantitatively.

Risk cost (R_c) is the annual cost, or annualized cost, of the expected losses from the landslide hazard.

In many cases involving landslide hazards, public safety is an overriding consideration. The following 'risk to life' concepts are modified from Morgan et al (1992).

Probability of death of an individual (PDI), also known as 'risk to life', is the probability that a specific person will be killed as a result of a specific landslide hazard. It is a variation of the risk procedures described above. PDI is the product of the annual probability of the hazard, the person being spatially in the path of the event when it occurs, the person being temporally in the path of the event when it occurs and the person being killed as a result. Mathematically, PDI is expressed as:

If a given area is potentially subject to more than one type of independent landslide hazard, the individual PDIs are additive.

Probability of death of a group (PDG) is the probability that a specific hazard will result in a minimum number of casualties. Because the numbers of people vary in space and time, PDG is much more complex to determine.

Severity (S) is sometimes used in association with PDI and PDG, and is the product of P_s x P_t x P_l, described above. It is somewhat analogous to consequence (C).

As for the other parameters, PDI, PDG and severity can be expressed qualitatively by words such as 'low', 'moderate' or 'high', or quantitatively as an actual number.

For a range of consequences or severities, and the corresponding range of probabilities of occurrence, the relation can be graphed (Figures 2.2). The two curves on each figure represent the ranges in confidence or uncertainty in assigning the parameters. The area beneath the consequence (or severity)-probability of occurrence curve is the product of the consequence (or severity) and the probability of occurrence, and represents the 'total risk' as defined by Morgan et al (1992) and Fell (1994).

Figure 2.2 Consequence or severity - annual
probability of occurrence relation.

The final stage in landslide risk assessment is to determine the acceptability of the estimated risk. In the case of environmental or economic risks, acceptability is often carried out by means of a cost-benefit analysis, comparing estimated annual risk costs with annual capital and maintenance costs of any or all remedial measures. In the case of risks to life, comparisons of estimated PDI or PDG are made against accepted societal standards. It is the responsibility of the terrain stability mapper to provide technical input. It is not the mapper's responsibility to determine the acceptability of the risk.

2.3.4 Applications and Limitations

As discussed above, many of the landslide hazard, consequence and risk parameters can be expressed in relative (qualitative) or numerical (quantitative) terms. If reliable data is available quantitative terms are preferred, as they provide the most precise, objective mapping. Because landslide hazards occur relatively infrequently in the same location, unlike other natural hazards such as floods, probabilities of occurrence and other quantitative assessments cannot be based on standard statistical methods, and are usually based on Baysian-like estimates--essentially 'subjective estimates' that cannot be tested, but can be critically reviewed. Even when quantiative terms are used, numerical ranges are useful to convey the degree of uncertainty perceived by the mapper. For example, a statement such as 'moderate sized debris flows, with a magnitude range of 10,000 to 20,000 m³, can occur frequently, with an estimated annual probability of occurrence of 1:100 to 1:500', conveys the degree of uncertainty. Quantitative results can easily be applied to cost-benefit analyses to aid decision making.

Unless users of such quantitative assessments understand the limitations of the methods, however, they may be misguided by the apparent precision provided by the numbers.

With less reliable data, qualitative estimates can be made and qualitative terms can be used to express the landslide hazard and risk parameters. If using a qualitative scale, it is recommended that the same principles of landslide hazard and risk assessment should be kept in mind. A drawback to using qualitative terms is that terms such as 'low', 'medium' and 'high' mean different things to different people, and hence map users may interpret different meanings than intended by the mapper.

Table 2.1 Abbreviated Classification of Landslides

Table 2.2. Example of Relative Terms and Ranges of Annual Probability of Occurrence