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4 DEBRIS FLOW TYPES AND MECHANISMS

by I M Nettleton, S Martin, S Hencher and R Moore

4.1 FLOWS

4.1.1 Classification of Flows

The flow type ( see Section 2.1) of landslide movement is classified according to whether the materials involved are rock or engineering soils (Varnes, 1978).

In the context of the recent flow type landslides on the road network in Scotland it is the engineering soil flows that are pertinent. The descriptions of these materials and the corresponding classifications are shown in Table 4.1.

Table 4.1 - Engineering soils and associated flow types (after Varnes, 1978; Cruden and Varnes, 1996; Hutchinson, 1988).

Figure 4.1 shows the predominantly fine materials deposited by an Earth Flow at one of the events on the A9 Trunk Road north of Dunkeld (August 2004). Coarser material was in evidence at the other two main locations.

Figure 4.1 - Hillslope flow which has formed its own channel by erosion (A9 north ofDunkeld, August 2004). (Courtesy of Alan MacKenzie, BEAR.)

Figure 4.2 shows the predominantly coarse debris deposited by Debris Flows on the A887 Trunk Road at Invermoriston (August 1997). This is by far the most common type of flow encountered on the road network in Scotland.

Figure 4.2 - Debris flow material on the A887 Trunk Road at Invermoriston, August 1997. The debris consists predominantly of coarse material (gravel, cobbles and boulders), with some finer material (clay, silt and sand) and tree trunk debris. (Photograph courtesy of Northpix.)

Pierson and Costa (1987) classified flow type landslides on the basis of the flow velocity and sediment concentration and their rheological classification of sediment water flows is shown in Figure 2.2. This rheological approach to classification may be particularly appropriate for the development of remedial measures such as channels, overshoots, culverts and so forth. Based on Pierson and Costas' (1987) classification it is considered that most recent Scottish flow landslides would fall within the Debris Flow category - which description is adopted as an all-encompassing term for this work. Peat flows are discussed further in Section 4.3.4.

4.2 DEBRIS FLOWS

4.2.1 Debris Flow Materials

Debris flows usually comprise a mixture of fine (clay, silt and sand) and coarse (gravel, cobbles and boulders) materials with a variable quantity of water. The resulting mixtures often behave like viscous "slurries" as they flow down slope. They are often of high density, 60% to 80% by weight solids (Varnes, 1978; Hutchinson, 1988), and may be described as being analogous to "wet concrete" (Hutchinson, 1988).

Debris flows are potentially very destructive as they cause significant erosion of the substrates over which they flow, thereby increasing their sediment charge and further increasing their erosive capabilities. The density and rapid movement of debris flow materials yield a mass with significant energy (Table 4.2). This has the ability to pick-up and transport even large and/or well secured objects, thereby giving rise to the potential for significant damage. Examples of such "accidental detritus" (Johnson and Rodine, 1984) picked-up by debris flows include tree trunks, branches, large boulders, parts of structures and vehicles (see Figures 4.1 and 4.2).

Table 4.2 - Landslide rates of movement (WP/WLI, 1995).

The debris flows experienced in Scotland occur on hillsides with relatively thin (typically <3m), predominantly unconsolidated and relatively coarse-grained superficial deposits (Ballantyne, 1986). The Scottish superficial deposits that are most prone to debris flows are shown in Table 4.3.

Table 4.3 - Superficial deposits prone to debris flows. Descriptions based on McMillan and Powell (1999).

Superficial deposits dominated by a fine-grained soil matrix, and exhibiting apparent cohesion, are much less prone to debris flows. Due to their apparent cohesion and lower permeability these materials tend to be less prone to erosion than coarser grained frictional materials. The lower permeability will reduce infiltration into these soils (Ballantyne, 1986). Examples of these "cohesive", fine-grained matrix superficial deposits include some of the glacial tills which are generally overconsolidated ( e.g. lodgement till). Figure 4.3 shows a hillslope debris flow where the failure has not cut down into a glacial till with a fine-grained "cohesive" matrix.

Figure 4.3 - Hillslope debris flow on the North side of Maol Chean Dearg, 2004. Note the failure surface lies on top of a band of superficial deposits which contain a higher proportion of more silty/clayey material, that possesses an apparent "cohesion".

4.2.2 Debris Flow Forms

Two forms of Debris Flows are distinguishable, based on the topographic and geological characteristics of their locations.

Hillslope (Open-Slope) Debris Flows

These form their own path down valley slopes as tracks or sheets (Cruden and Varnes, 1996), before depositing material on lower areas with lower slope gradients or where flow rates are reduced: e.g. obstructions, changes in topography (Figures 4.4, see also Section 4.3.2 and Figure 4.15). The deposition area may contain channels and levees.

Channelised Debris Flows

These follow existing channel type features: e.g. valleys, gullies, depressions, hollows and so on (Figures 4.4, 4.5 and 4.6). The flows are often of high density, 80% solids by weight (Cruden and Varnes, 1996), and have a consistency equivalent to that of wet concrete (Hutchinson, 1988). Hence, they can transport boulders that are some metres in diameter, for example a 9 tonne boulder was reported at the debris flow on the A85 at Glen Ogle ( see Section 5).

Figure 4.4 - Hillslope (a) and channelised (b) debris flow.

Figure 4.5 - Hillslope/channelised debris flow on the A890 Stromeferry Bypass, October 2001. The figure in orange is at the base of the source area. Note the drainage pipe, to the side of the gully, installed to take water from an interceptor trench above the debris flow scarp and convex slope break at the pine tree.

Figure 4.6 - Stilling basins filled with coarse debris flow material the base of Frenchman's Burn on the A890 Stromeferry Bypass. The basins have a combined capacity of 100m ³. The upper basin dam was formed using "armour" stone blocks from the stream while the lower basin dam was formed using gabion baskets.

Coarser material may form natural levees or accumulate as debris dams (Figure 4.7) at obstacles ( e.g. trees and large boulders and so on.) or changes in channel gradient, thus leaving finer material in suspension to continue down the channel. Suspended material in channel flows will typically be deposited in lower gradient sections of channels, where channels widen and upon emergence from the channel.

In practice many debris flows may start as the hillslope form, but during the course of flowing down slope they may enter channel type features, form their own channel flow tracks in superficial deposits or may cut through superficial deposits and then be channelled down preexisting channel features in rockhead: e.g. an infilled stream or gully (Figure 4.8).

Figure 4.7 - Boulder and Tree Trunk Debris Dam containing an estimated 50m ³ to 75m ³ of debris, which was subsequently broken up to prevent catastrophic failure and resulting erosional effects. Frenchman's Burn on the A890 Stromeferry Bypass.

Figure 4.8 - Location of debris flow scour where channel cut down through superficial deposits over a buried cliff. In excess of 100m ³ of material was picked up at this one location. Note the large boulders and tree trunks in the foreground. Debris Flow on the A887 road at Invermoriston, August 1997.

4.3 PRINCIPLES OF RAPID LANDSLIDE DEVELOPMENT

In understanding the mechanisms of debris flows it is helpful first to consider the mechanisms of landslides more generally, not least as these frequently form all or part of the trigger event. Fundamentally, all landslides are the result of gravitational forces causing the ground to fail. Once the failure starts, the debris will travel downhill, sometimes in a highly mobile state due to mixing with water. There is potential for failure in any sloping ground but, all things being equal, the steeper the ground the more prone it is to land sliding.

The susceptibility of a particular hillside to failure is expressed as a " Factor of Safety" as illustrated in Figure 4.9. For any potential failure surface, there is a balance between the weight of the potential landslide ( driving force or shear force) and the inherent strength of the soil or rock within the hillside ( shear resistance). Provided the available shear resistance is greater than the shear force then the Factor of Safety will be greater than 1.0 and the slope will remain stable. If the Factor of Safety reduces to less than 1.0 through some change in conditions, the model predicts failure.

Figure 4.9 - Landslide development.

The shear force is mostly a component of the weight of the rock/soil making up the potential landslide. If water gets into the slope however this may have several impacts on the shear force. Water pressure can actively encourage movement of the landslide downhill. Saturation can increase the weight of the sliding mass. Other destabilising factors can be vibrations from nearby traffic, blasting and earthquakes. Damaging earthquakes are rare in Scotland.

Clearly the highest shear forces will be in steeper ground but generally that ground will also be inherently strong (otherwise it would not stand so steeply) and therefore may have a similar Factor of Safety as shallower ground. That said, if a failure occurs in steep ground, then the effects may be particularly severe because the debris may gain momentum quickly and travel a long way.

The shear resistance is provided by the natural strength of the soil or rock. This can be very prone to the effect of water. The resistance along the potential sliding plane depends, among other factors, upon the weight of the potential sliding mass.

4.3.1 Causes of Debris Flow

Hillside debris flows typically start as a sliding detachment of material (upland debris slide, peat slide, rock slide etc.), usually initiated during heavy rainfall, which subsequently breaks down into a disaggregated mass in which shear surfaces are short-lived and usually not preserved. The failure mass usually combines with surface water flow, which typically results in high mobility and run-out.

Channelised debris flows may develop as a result of the mobilisation and entrainment of sediments by extreme flows confined within stream valleys, which may include the collapse of natural landslide dams that may have partly or completely blocked channels and stream valleys for some period prior to the event. For this reason, it is particularly important to investigate entire catchments in respect of channelised debris flow hazard and risk assessment.

From the above, it may be concluded there are two principal causes of debris flows:

• The initiation of a source upland landslide that develops into a hillside debris flow.
• The mobilisation and entrainment of sediments by extreme flows within stream valleys.

With regard to the causes of landslides these are well documented by others ( e.g. Jones and Lee, 1994; Moore et al., 1995). Ultimately, landslides occur when the force of gravity exceeds the strength of soils and rocks forming slopes. In such circumstances, slope failure occurs to restore the balance between the destabilising forces (stresses) and the resisting forces (shear strength) along the surface of rupture or shear surface. Therefore, a landslide may be regarded as a dynamic process that changes a slope from an unstable to a more stable state.

The causes of landslides are generally separated into two types:

• Preparatory factors which work to make the slope increasingly susceptible to failure without actually initiating it, and
• Triggering factors which initiate movement.

As for all landslides, debris flows are caused by a combination of preparatory and triggering factors. The interrelationship of these factors controls the likelihood and timing of events at different sites (Figure 4.10).

When considering the actual causes of upland landslides this relative simplicity gives way to complexity, as there is a great diversity of causal factors. In broad terms, however, they may be divided into internal causes that lead to a reduction in shear strength and external causes which lead to an increase in shear stress (Table 4.4). In summary, the main causes of upland landslides in Scotland are likely to include:

• Reduction of soil and rock strength over time due to weathering and slope ripening,
• Historical land use changes, including deforestation, road construction, disturbance of natural drainage, etc,
• Increased rainfall and storm intensity due to climate change, and
• High transient pore water pressures arising from intense rainstorms.

Figure 4.10 - Long term development of upland slopes and their susceptibility to rapid landslides.

Table 4.4 - Causes of landslides.

Preparatory Factors

Certain conditions are needed for the initiation of upland landslides including some or all of the following:

• Steep hillsides promoting gravity induced slope failure,
• Weak jointed rocks exposed in rock slopes and cliffs,
• Weak soils, colluvium or peat overlying weathered rock,
• Low vegetation exposing soils to weathering processes,
• Poor drainage, surface water flow and soil piping, and
• Extreme climatic conditions.

In upland environments, winter weather conditions involve freezing and thawing processes which act to weaken the soil and rock structure. Dry summer conditions may cause desiccation of soils (particularly peat), opening large cracks and providing routes for the ingress of surface water. These weathering processes result in weakened soil structures and loss of material strength.

The products of weathering often form a mantle of weak soils overlying harder rocks which provide an interface or potential shear surface along which slope failure may propagate. Where rocks are exposed at surface, deep penetration of weathering along rock joints and discontinuities can significantly weaken the integrity of the rock mass and provide detachment surfaces for rock falls and slides. Jacking by tree root forces may open existing fractures allowing deeper penetration of water and frost penetration.

The long term weathering of soils and rocks make upland slopes increasingly susceptible to failure. Such processes are often described as the 'preconditioning' or 'ripening' of slopes. They are often overlooked as a major cause of landslides given the long timescales over which they operate but they are a fundamental control in the location and timing of upland landslide events.

Historical land use changes and construction activities are also important factors in the preconditioning of slopes for upland landslides. The effects of deforestation are well documented Sidle et al. (1985) and construction activities involving cut and fill and drainage works can lead to slope failures sometime after the works are completed.

Such processes of deterioration are illustrated schematically in Figure 4.11 (upper right hand). The gradual deterioration is represented by a curve in which the Factor of Safety reduces over a period of time which may comprise tens or hundreds of years. The vertical lines represent the temporary reduction in Factor of Safety caused by relatively short-term, transient events. In the course of time, the slope will deteriorate to the point where it is vulnerable to a transient event - causing a reduction in the Factor of Safety to a value below 1.0. Whether that event results in catastrophic failure or relatively minor movement and distress depends on the slope, circumstances and severity of event (including how long it lasts).

In general it can be assumed that for any given hillside there will be a whole range of locally susceptible areas with different current Factors of Safety (as per Figure 4.11, lower diagram). One might consider the hillside to comprise an inventory of different slopes of different susceptibilities. The susceptibility at each location will be a function of the strength (or weakness) of the soil at that location but also many other factors such as local slope angle (and therefore shear stress), catchment leading to that location (which will influence water pressures and erosion potential), local topography leading to concentrations of surface flow, erosion and undermining and vegetation cover (deep rooted trees will help hold the soil together).

Figure 4.11 - Mechanisms of long term hillslope deterioration.

A minor rainstorm (say a one in 1 year storm) will probably not result in any discrete landslides although it will contribute to the general deterioration of the hillside which might be measurable given sophisticated instruments.

A more severe event (say a one in 10 year storm) may cause a few failures in sections of hillside where the ripened factor of safety is approaching 1.0 (say 1.0 to 1.1).

A much more severe event (say a one in 100 year storm) may cause all slopes to fail within a much wider range (say 1.0 to 1.3). Not only will the intensity of such a storm initiate discrete failures but the length of time that heavy rain continues during such a storm will make the debris more mobile so that it can flow a long way and impact on more structures than would otherwise be the case - the event may be disastrous.

Road cuts may be particularly susceptible to triggering events as illustrated in Figure 4.12. Fundamentally, if the cutting had not been made, the natural slope would have gradually deteriorated in geological time (100s or 1,000s of years probably). However the process of cutting the slope leads to a rapid reduction in the Factor of Safety because of increased shear stress (over-steepening) and probable changes in the groundwater conditions. Such deleterious effects can be mitigated against by the construction of engineering works such as retaining walls or in some other way strengthening the soil /rock.

Figure 4.12 - Mechanism model for cutting a new slope.

Examples of preparatory factors observed in recent Scottish debris flows are shown in Table 4.5.

Table 4.5 - Examples of debris flow preparatory factors observed in Scotland.

Triggering Factors

Triggering events result in the initiation and mobilisation of upland landslides (Table 4.4). In upland environments, the most significant triggering factor is likely to be the development of transient high pore water pressures along pre-existing or potential rupture surfaces. High pore water pressures are typically generated as a result of extreme antecedent (long-duration) rainfall conditions and intense rainstorms, both of which can result in high groundwater levels and perched groundwater conditions. If the soil becomes fully saturated surface water flow may occur which can result in erosion and triggering of hillside debris flows. Examples of such features are common in upland Scotland. It is noted that extreme rainstorms of different intensities, frequency and storm-paths can result in a very different pattern of landslide initiation and debris flow response.

The permeability of soils and the speed by which surface water can be transmitted to potential rupture surfaces is a key factor in the initiation of upland landslides. The interface between permeable soils and relatively impermeable substrate can lead to the development of cleft water pressures along soil and rock discontinuities and artesian pore water pressures along potential rupture surfaces. Certain geological situations are particularly prone to the effects of water infiltration, for example where permeable soil overlies less permeable bedrock. In such circumstances rapid increases in pore water pressures can trigger slope failure and mobilisation of landslides.

Debris Flow Propagating Factors

Many debris flows are of a size that would not lead to any significant events, and examples of such can be seen on many hillsides in Scotland. However, whilst flowing down slope or channel some these debris flows may encounter particular features that can exacerbate them. This may lead to even quite modest debris flows escalating into large ones with potentially significant destructive effects, which are out of proportion to the initial event.

Based upon Scottish experience, it is usually combinations of these propagating factors that lead to the large debris flows that have a significant impact upon the road network. These propagating factors are not well documented in the literature and are therefore some examples are presented in Table 4.6 and Figure 4.13.

Figure 4.13 - Relict rockhead cliff surviving from, in this case, glacial times (a) and convex slope break (b).

4.3.2 Mechanisms of Debris Flow

Source Area

Slides in Soils and Peat: Steep upland slopes which are mantled by a cover of unconsolidated soils or peat are particularly susceptible to debris slides and hillside debris flows. Debris slides and peat slides involve shear failure of the unconsolidated material or peat at the interface with the underlying weathered rock, which typically varies between 1m and 5m below ground surface. Rapid increases in pore water pressure along the interface result in significant reductions in effective shear strength, leading to rupture or shear failure along the soil-rock interface.

Table 4.6 - Debris flow propagating factors.

Propagation of shear failure may occur in an upslope or downslope direction depending on the point of maximum strain. Where the slope is undercut or where the interface daylights on the slope, it is likely that shear failure will propagate upslope due to unloading. Where the slope is in tension, usually marked by curvi-linear tension cracks defining the potential headscarp, shear failure will propagate downslope due to loading. Bulging of the landslide toe is a characteristic of the latter mechanism which marks the location where the shear surface ruptures to the ground surface. In both cases, when 50% or more of the shear surface has developed, rapid failure is likely to develop given a favourable pore water pressure regime.

Figure 4.14 - Location of the October 2001 Debris Flow on the A890 Stromeferry Bypass. The main preparatory factors are highlighted.

Falls and Slides in Rocks: Upland mountain slopes or rock exposures where slope angles are close to, or parallel, to the dip of the rock are particularly susceptible to rock falls or rock slides. They are often characterised by pronounced headscarps and flanks which are relatively free of debris, and a pronounced scree slope or debris fan at the base of the slope.

Detachment surfaces usually correspond to faults, joints and other structural discontinuities where rock strengths are considerably less than those of the parent rock due to the effects of long term weathering and transient pore water pressures. Rapid increases in pore water pressures along rock discontinuities are a major cause of rock falls and rock slides. Ice-wedging along joints may also be important.

Debris Flow

Once the fall or slide is in motion and depending on the coherency of the displaced mass, the failure breaks up on impact and as the slide avalanches downslope. The failure may develop into a debris flow when the debris comes into contact with surface water and stream flow, dramatically decreasing the viscosity of the debris-water mix ¹⁴. As a general rule, where the constituent particles of the slide debris cease to be in contact and become supported by fluids, a change in mechanism from debris slide to debris flow takes place. This process is illustrated in the slope failure that occurred along the A83 in Scotland (Figure 4.15). This transition may be very rapid once the slide debris makes contact with surface water or stream flow (Figure 4.16).

Figure 4.15 - Upland debris slide and flow development along at Cairndow on the A83 in 2004.

Debris flows consist of a mixture of fine and coarse material, with a variable quantity of water, which forms a muddy slurry that flows downslope, often in gravity induced surges. Debris flows generally mobilise as a result of soil saturation, surface water flow, and high pore water pressures developed within unconsolidated surface soils. Debris moves as a combination of viscous flow and mass movement under gravity.

4.3.3 Propagation and Run-out Factors Affecting Debris Flow

Rapid upland landslides and debris flows can develop into large run-out flows. Whether or not upland landslides develop into hillside debris flows and channelised debris flows depends to a large extent on a number of conditions or 'run-out' factors, these being:

• The supply and mixing of surface water with the landslide mass in motion.
• The erosive capacity of flooded upland streams (channelised debris flows).
• An available source of sediment for entrainment in channelised debris flows.
• Slope steepness and length of slope for gravity induced slides and falls.
• The connectivity between hillslopes and upland stream channels.

Figure 4.16 - Mechanism models for open hillside failure caused by water.

Water plays a major role not only in the initiation of failure but also in the way that the debris then flows or slides and the distance that it travels. Figure 4.17 illustrates many of the important factors culminating in the exposure of society to safety and economic consequence (item 7 on Figure 4.17).

The connectivity between upland landslides and stream channels is a very significant factor in the propagation and run-out potential of debris flows. For relatively high frequency, shallow, open hillside landslides, debris typically remains on the hillslopes or is deposited on the lower valley slopes rather than being directly mobilised as channelised debris flow. However, for low frequency high magnitude events, debris stored upon hillslopes and within valley floors provides a source of generally unconsolidated sediment that can be entrained and mobilised by channelised debris flows. It follows that the accumulation of unconsolidated debris from numerous hillside landslides over time can provide a large volume of sediment capable of being mobilised in a single episodic channelised debris flow event.

Figure 4.17 - Debris flow characteristics.

Where open hillside landslides or debris flows deposit directly into stream channels at peak stage, mobilised sediments will be entrained and transported as a viscous flow (Figure 4.18). Where landslides deposit into stream channels at other times, landslide dams may form causing temporary lakes. As the volume of the lake increases, the erosive stresses imposed on the unconsolidated material forming the dam eventually exceed its holding capacity, leading to the collapse and break up of the dam. The collapse of landslide dams can be sudden, releasing surges of water and debris downstream, in the form of debris flows, with destructive effects.

Debris flows have high erosive energy and are capable of entraining material as they propagate downslope or downstream (Figure 4.19). The entrainment of slope and valley deposits often contributes a significant proportion of the volume of debris flows. This is especially significant in channelised debris flows where colluvial and alluvial deposits occur as ribbon-like stores along stream channel banks and beds (often as angular and sub-rounded boulders within a sandy matrix) or broader accumulations in valley floors (valley floor stores). Moore et al. (2002) indicate that, in steep upland catchments, debris flow run-out volumes can be as much as eight times greater than the total volume of the source landslides. Five main stages of debris flow propagation were recognised within the catchment: initiation and detachment of material from hillslopes; transport and delivery of this material into the channel system; storage of material within the channel system (and also, in the short-term, on hillslopes before delivery to the channels); entrainment and run-out from the catchment; and deposition on the debris fan. The linkages between these stages are critical.

Figure 4.18 - Entrainment processes increasing the size and nature of run-out characteristics, Channerwick, Shetland Islands.

Figure 4.19 - Connectivity of Hillslopes and upland streams and their implications on the run-out characteristics of debris flows, Channerwick, Shetland Islands.

4.3.4 Channerwick Peat Slide and Debris Flow Example

An example is provided of the dramatic peat slide failures triggered by an intense rainstorm in September 2003 at Channerwick, Shetland Islands. The rainstorm was part of a slow moving front which pushed south-eastwards across Scotland overnight and anecdotal evidence indicates an average intensity of 33mm/hr. The intensity of the storm resulted in widespread flooding of the hillsides (Figure 4.20) and burns and the initiation of rapid peat slides. The latter developed into hillside debris flows with long run-outs, causing widespread damage to roads and other infrastructure. Landslides occurred on slopes with angles between approximately 7 and 25 ^o.

Figure 4.20 - Upland intense rainfall characteristics at Hoswick Burn Shetland Islands south mainland, 2003.

A key factor was the timing of the storm which followed a dry summer when groundwater levels would have been low reducing the load of the peat blanket. Cracks within the peat will also have formed providing conduits of surface water flow to the peat-weathered rock interface. During the intense rainstorm, surface water filled the tension cracks and soil pipe networks that connect the upper slope flushes and bogs to the lower slope peat blanket. The relative impermeability of the weathered rock interface beneath the basal amorphous peat will have caused a sudden increase in pore water pressure reducing the effective shear strength of the overburden. Given the relatively low normal load of the peat overburden it is likely that the pore water pressures could have resulted in 'lifting' (buoyancy effect) of the peat blanket above the interface. Elsewhere, 'bogbursts' are widely reported where artesian groundwater conditions develop within the soil pipe network and where rupture surfaces break out at the ground surface.

Once movement is initiated, the partially saturated peat is 'rafted' downslope, initially upon the shear surface and subsequently down steep sodden grassed slopes. The rafted blocks of peat move rapidly as a debris slide with the peat blocks breaking down into smaller units as the slide progresses (Figure 4.21).

Figure 4.21 - Changes in peat transport mechanisms, Channerwick, Shetland Islands.

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