Tofino BC Geography

Landslides in Nootka and Clayoquot Sound

MASS WASTING : AN OVERVIEW OF THE PROCESSES AND AN INDEPTH LOOK AT NOOTKA SOUND, BRITISH COLUMBIA

John Platenius

ABSTRACT

Mass wasting is a common occurrence in all soils saturated with water, and is defined as the transport of regolith down slope with the sole aid of gravity.  Many different categories and subcategories can be assigned to mass wasting, including: slumps, falls and slides, slurry flows, debris flows, creep.  Nootka Sound, located on the central western coast of Vancouver Island, British Columbia Canada, is most susceptible to three types of mass wasting: debris flows, debris slides, and creep.  Bedrock geology shows very little significance in determining the location of mass wasting in Nootka Sound.  Despite this conspicuous incongruity with bedrock, Nootka Sound does show a predictable pattern where dense areas of mass wasting can be expected: sites greater than two kilometers from the coast, with southeast-facing, concave slopes that have an angle between 35º and 40º.  Industrial logging and road building will significantly increase the frequency of mass wasting in the study area.

INTRODUCTION

Like all classifications of nature, mass wasting overlaps and interacts with other agents of weathering and erosion.  Despite the continuum in which these processes naturally occur, mass wasting has a discrete definition: the transport of regolith down slope with the sole aid of gravity.  Mass wasting occurs without any help from transporting mediums such as water, ice, and wind.  However, water is a very important player in mass wasting events.  When regolith becomes saturated with water, friction is reduced between rock particles and the bedrock that they lie on top of.  With a reduction in friction, and an increase in mass in the form of water, the regolith is more susceptible to down slope movement—mass wasting (Skinner and Porter, 1989).

The purposes of this paper are threefold: 1) to describe mass wasting processes, 2) to categorize the types of mass wasting to be expected in Nootka Sound, and 3) to surmise the factors that are associated with mass wasting in Nootka Sound. 

WHAT IS MASS WASTING?        

There are many different categories assigned to mass wasting.  All of these categories are placed on a continuum based on their particle composition and the velocity at which they travel.  Skinner and Porter (1989) divide terrestrial mass wasting into two coarse categories: slope failures and sediment flows, both of which contain a variety of subcategories.

Slope failures

Slope failure is the downslope movement of debris through slumping, falling, or sliding.  Slope failure happens in sudden catastrophic events, usually in conjunction with or shortly after a change in landscape composition (i.e.: fire, road construction), or after a drastic increase in rainfall.  Slope failure can be divided into three subcategories of mass wasting: slumping, falls and slides, and sliderock and taluses.

Slumping

Slumps occur along concave features of a hill slope, and usually arise in a series of concentric slump blocks (Fig. 1).  When the regolith becomes saturated with water, friction along the slip line between the bedrock and the regolith decreases, and slumping will occur.  Areas with a steep gradient that experience heavy and frequent rainfall and/or earthquakes are particularly susceptible to slumping (Skinner and Porter, 1989).  The addition of roads to areas already susceptible to slumping increases the probability of such an event (Jones et al, 2000).   Roads oversteepen the slope, and add exposed sediment and debris on the downslope side of the road (Fig. 2).

Falls and Slides

Free falling rocks or debris that have become detached from a cliff or steep slope are called rockfalls or debris falls.  Rockfalls are common in mountainous areas that have steep slopes containing a high concentration of exposed bedrock.  A rockfall can be a single rock dislodging from a steep slope and free falling, or a large mass of rock that breaks apart just after it dislodges and collects as a conspicuous pile of shattered rocks at the base of a cliff.  Debris falls involve the same process as rockfalls, but they are composed of a mixture of regolith, rock, vegetation, and soil—debris.  Rockslides and debris slides are the sudden downslope movement of rock or debris along a steeply inclined surface, often a bedding surface.  Falls and slides are common in glaciated mountains, or any other areas that contain a high concentration of steep slopes (Skinner and Porter, 1989; Fig. 1).

Taluses and Sliderock

A mass of rock that collects on a slope below a cliff is called a talus, and the sediments that make up taluses are called sliderock.  The particle size of sliderock varies from grains of sand to large boulders, the finer grains typically settling to the bottom of the pile.  Taluses are formed from rockfalls and rockslides, where the rock comes to rest at the angle of repose—the steepest angle that the rock can still remain stable.  The angle of repose is typically between 33º and 37º.  Some of the larger rocks deposited in rockfalls and rockslides will come to rest beyond the toe of the talus because they have more momentum and will travel further than the smaller sediments (Skinner and Porter, 1989; Fig 1).

Sediment Flows

Sediment flows consist of the deformation of a mixture of materials, including regolith, rock debris, water and air.  The force of gravity causes the deformation of sediments, and is continuous and irreversible.  The way that these materials flow depends on three factors: 1) the relative proportion of water, solids, and air, 2) how the grain sizes of the solid fraction are distributed, and 3) the chemical and physical properties of the sediments.  Sediment flows can be placed into two different categories: slurry flows and granular flows.  These categories can be subdivided further, depending on the velocity at which they travel (Skinner and Porter, 1989).

Slurry Flows

 Slurry flows are the mass movement of sediments downslope that are fully saturated with water.  The water is trapped among the sediments and is transported down the slope in a soupy mixture of flowing mass.  This mixture is dense enough that large boulders can become trapped and moved along in the flow.  Depending on the velocity of the flow, slurry flows have three subdivisions: solifluction, debris flows, and mud flows.

Solifluction is the slow, viscous downslope movement of sediments saturated with water.  The movement of solifluction is so slow that it cannot be detected without careful measurement over a long interval of time.  This slow movement forms characteristic landscape features such as overlapping lobes and sheets of debris, and can often fold and deform surface vegetation. 

Debris flows are conspicuous downslope movements that are composed of unconsolidated regolith, where most of the sediments are coarser than sand.  Debris flows can vary in their velocity from 1 m/yr, to as fast as several km/h, and often form distinct tongues of sediment, similar to the morphology of glaciers.  Similar to slumping, debris flows are common after a period of heavy rainfall, when the ground becomes oversaturated with water and has a lower friction at a surface contact.  Not surprisingly, debris flows are often triggered by a slump that occurs just above an area where a debris flow follows.

When a debris flow has a water content high enough to give it a fluid consistency, it is called a mudflow.  The consistency of mudflows can range from that of a thick, wet concrete texture, to as runny as a muddy stream.  Because mudflows are highly mobile, they tend to travel along the floors of valleys.  Mudflows are common in areas with little vegetation and episodic rain events, like the American southwest, where they often begin in canyon streams and continue after exiting the canyon as thin mud sheets across the open country.  Areas of high volcanism are also subject to mudflow activity.  After a volcanic event, layers of ash build up and become highly susceptible to moving as a mudflow (Skinner and Porter, 1989).

Granular Flows

A granular flow is the downslope movement of a mass of sediments where the mass is supported by grain-to-grain contact.  The sediments involved in granular flows may be quite dry, having air filling the pores, or the sediments could be fairly saturated with water.  Although it seems counterintuitive, granular flows can take place with saturated sediments if the water can escape easily due to the grain size and distribution of the sediments.  Depending on their velocity, granular flows can be further subdivided into three categories: creep, earthflows, and debris avalanches.

Creep is the very slow, downslope movement of regolith.  Loose, incoherent particles moving predominately by creep are collectively called colluvium.  Colluvium tends to be constructed of chaotically sorted angular particles, as opposed to most other types of mass wasting that are composed of rounded, well sorted particles.  This configuration of colluvium allows geologists and sedimentologists to discern the difference between colluvium and particles that were deposited by sediment flows.

Earthflows are very similar to creep, but they move at a faster rate—usually between 1 m/day and 360 m/h—and they consist of unconsolidated regolith, like debris flows.  Earthflows occur in areas with high amounts of rainfall, on moderately steep slopes.

Debris avalanches are granular flows that move at a very high velocity (> 36 km/h).  These events are rare and very destructive.  Debris avalanches occur after large rockfalls or rockslides, where the velocity and slope are great enough to carry a large mass of broken rocks downslope at a very fast rate (Skinner and Porter, 1989). 

PREDICTING MASS WASTING IN NOOTKA SOUND

Study Area

Nootka Sound is located on the west coast of Vancouver Island (Fig 3.).  The boundaries of this study area have been determined by the Nootka Sound map-area (92 E), as described in Muller et al (1981).  The study area is characterized as three distinct physiographic sub-units: Fiord-land, Vancouver Island Range, and Estevan Coastal Plain (Fig 3.; Howes, 1981).  The majority of this study area is Fiord-land.

The climate of Nootka Sound is dominated by high amounts of rainfall, 80% of which falls between October and April (EC, 1998).  According to Howes (1981), high intensity rainstorms are frequent in the Fiord-land physiographic sub-unit, and annual precipitation can increase by more than 2.6 meters east of the fiord inlets, near the convergence of the Vancouver Island Range.  The yearly rainfall average at Estevan Point (see Fig 3.) is 3.1 meters (EC, 1998), so we could expect annual rainfall levels to approach 5 meters near the convergence zone—a wet climate indeed.  Nootka Sound’s vegetation predominately consists of central pacific coastal forests, which have been logged extensively (Ricketts et al., 1999; Devall, 1993).  There are many logging roads throughout Nootka Sound, especially in the northeast corner of the study area (Muller et al., 1981).

Late Paleozoic and early Mesozoic granitic intrusions, metamorphosed intrusions, and basalts dominate the geology of Nootka Sound.  Cenozoic sedimentary layers are predominate on the Estevan Coastal Plain, and are distinctly flat compared to the fiord-land region (Howes, 1981; Muller et al., 1981).  There are several block faults in the study area, most of which are north, northwest trending.  All of northwestern Vancouver Island was subjected to more than one period of glaciation, the maximum extent of which occurred approximately 15,000 years ago (Howes, 1981).  Northern Vancouver Island lies in the most active seismic region in Canada.  There were 1,768 earthquakes recorded in northern Vancouver Island between 1899 and 1976, 267 of which were strong enough to be felt by humans (Fig. 4; Howes, 1981).

Mass Wasting in Nootka Sound

Howes (1981) has prepared a paper that includes the distribution of slope failures on a gross scale in northern Vancouver Island, in which he divides the northern half of the island into three zones: northeastern, southeastern, and western.  The Nootka Sound study area falls almost entirely in the Howes’ western zone, the boundary of which falls just east of the Vancouver Island Range boundary (see Fig 3.).

Howes characterizes the western zone as having a high abundance of debris flows and debris slides.  (Note that Howes refers to debris slides as debris avalanches, and due to the conspicuous absence of recorded slumps, it is assumed that slumps were included in his debris slide category.)  Ninety-five percent of all the debris flows and slides observed in the western zone occurred on colluvial sediments.  The abundance of debris flows and slides increases in the western zone as the slope of the hill increases.  The high frequency of debris flows and slides in the western zone is undoubtedly due to the high frequency and intensity of rainfall (especially during the winter months) and the glacially oversteepened slopes.  The abundance of debris flows and slides in colluvium can be logically attributed to the high occurrence of colluvial sediments reported in the western zone (Howes, 1981).

Although most of Nootka Sound’s terrain is composed of colluvium, it should be noted that there are a small number of bedrock exposures in the study area, especially in the Vancouver Island Range sub-unit (Howes, 1981).  These exposures are susceptible to rockfalls and rockslides, particularly during earthquake episodes (Skinner and Porter, 1989). 

The terrain of the Estevan Coastal Plains is characterized as marine sediments and gravel, most of which take the form of beaches and terraces (Muller et al., 1981).  Because of the lack of relief, very little mass wasting is to be expected in the Estevan Coastal Plains sub-unit.

Soil creep is predicted to be the predominant low velocity mass wasting event in Nootka Sound.  I make this prediction solely based on the high abundance of colluvial sediments that dominate the study area.

DISCUSSION

Factors Affecting Mass Wasting in Nootka Sound

Given the abundance of debris flows and slides in the study area and their potential hazards, I will focus this discussion on the factors that could affect these two mass wasting events. There are many factors that affect debris flow and slide activity.  Jakob (2000) identifies four factors that affect landslide activity in Clayquot Sound: distance-to-coast, slope shape, slope gradient, and bedrock geology.  (Jakob’s study area is mostly situated just south of Nootka Sound, but some does overlap my study area.)  Using Jakob’s study, it can be inferred that areas in Nootka Sound that are likely to experience debris flows and slides will be more than two kilometers from the coast, on concave slopes with a gradient between 35º and 40º.  Slope gradients between 25º and 35º are much more susceptible to failure after they have been logged.  Debris flows and slides are predicted to occur more frequently on southeast-facing slopes due to the predominate southeast wind direction, particularly during stormy winter months (EC, 1998; Jakob, 2000).

Both Jakob and Howes (1981) found very little evidence suggesting that bedrock geology in northern Vancouver Island influenced debris flows or slides.  It should be mentioned, however, that bedrock has been found to play a significant role in debris sliding on watersheds on the northeast side of Vancouver Island, where 70% of the slides occurred on Karmutsen volcanics (Guthrie and VanderFlier-Keller, 1998).  The terrain of the study site on the northeastern side of the island is significantly different, consisting mostly of glacial till, not the colluvium that is common in Nootka Sound.  Given that 58% of the slides in Guthrie and VanderFlier-Keller’s study occurred in morainal deposits, the relationship between bedrock geology and debris slides in Nootka Sound remains questionable.  Nevertheless, the Karmutsen Formation is exposed in Nootka Sound, particularly in the northern and eastern sections of the study site (Muller et al., 1981).  Given Guthrie and VanderFlier-Keller’s results, further study may be needed to assess the relationship of bedrock geology to debris sliding in Nootka Sound.

Debris flows and slides are expected to occur more frequently in logged areas in Nootka Sound (Jacob, 2000; Montgomery et al., 2000).  The frequency of landslides (mostly debris flows and slides) in Clayquot Sound was found to be nine times higher in logged areas than in “undisturbed forest” (Jakob, 2000).  The higher frequency of failures in logged terrain is most likely due the removal of rooted vegetation and an increase in road density.  Landslides will be more frequent in logged terrain particularly after the stumps and roots have decayed, and before significant shrub growth has occurred (Nagel, pers. comm).  Road density increases the susceptibility of slopes to slumping through oversteepening (Fig. 2), and further accelerates the chances of debris falls and slides (Skinner and Porter, 1989; Jones et al., 2000).

Although debris slides and flows are a natural part of Nootka Sound’s landscape processes, there has undoubtedly been a significant increase in their frequency due to the intensity of logging on steep slopes.  The debris that mass wasting delivers to anadromous fish populations is an important part of their breeding cycle.  However, the delivery of too much debris has been shown to severely harm several different species of salmon (Nagel, pers. comm.).  It is imperative to the long-term survival of salmon, and perhaps other river and shallow marine species, that forestry operations in Nootka Sound consider the impact that logging will have on the frequency of debris flows and slides in the areas that are timbered.
FIGURE CAPTIONS

Figure 1.

This figure shows five types of slope failures: slump, rockfall, rockslide, debris fall, and debris slide (After Skinner and Porter, 1989).

Figure 2.

This figure shows the oversteepening of slopes that accompanies road building.  The debris that is accumulated on the down slope side of the road becomes a potential hazard and trigger for debris slides (After Nagel, pers. comm.).

Figure 3.

This figure displays the location of Nootka Sound in relation to Vancouver Island, British Columbia, Canada, and the three zones that Howes (1981) uses to divide northern Vancouver Island in order to assess the distribution of slope failure on a gross scale (After Howes, 1981; Jacob, 2000).

Figure 4.

This figure displays the proximity of Canada’s major earthquake zones to the study site (After Howes, 1981).

 

Figure 2.

Figure 3.

Figure 4.

WORKS CITED

Devall, B., ed., 1993, Clearcut: The tragedy of industrial forestry: San Francisco, Sierra Club Books, 291 pp..

Environment Canada (EC), 1998, Canadian climate normals 1961-1990: Estevan Point, British Columbia: Accessed online April 11, 2001 at http://www.cmc.ec.gc.ca/climate/normals/BCE006.HTM.

Guthrie, R.H., and VanderFlier-Keller, E., 1998, The contribution of geology, to debris slides on Vancouver Island, B.C.  In Moore, D. and Hungr, O., Proceedings: Eighth international congress International Association for Engineering Geology and the Environment, Volume 3: Rotterdam, A.A. Balkema, p. 1993-1999.

Howes, D.E., 1981, Terrain inventory and geologic hazards: Northern Vancouver Island: Vicoria, B.C. Ministry of Environment, 105 pp..

Jacob, M., 2000, The impacts of logging on landslide activity at Clayquot Sound, British Columbia: Catena, v. 38, p. 279-300.

Jones, J.A., Swanson, F.J., Wemple, B.C., and Snyder, K.U., 2000, Effects of roads on hydrology, geomorphology, and disturbance patches in stream networks: Conservation Biology, v. 14, n. 1, p. 76-85.

Montgomery, D.R., Schmidt, K.M., Greenberg, H.M., and Dietrich, W.E., 2000, Forest clearing and regional landsliding: Geology, v. 28, n. 4, p. 311-314.

Muller, J.E., Cameron, B.E.B., and Northcote, K.E., 1981, Geology and mineral deposits of Nootka Sound map-area (92 E) Vancouver Island, British Columbia: Ottawa, Geologic Survey of Canada, 53 pp..

Nagel, G., Personal communication. April, 12, 2000, Prescott College, Prescott, Arizona.

Ricketts, T.H., et al., eds., 1999, Terrestrial ecoregions of North America: A conservation assessment: Washington, D.C., Island Press, 485 pp..

Skinner, B.J., Porter, S.C., 1989, The dynamic earth: New York, John Wiley and Sons, 541 pp..