{"id":681,"date":"2019-06-11T14:52:30","date_gmt":"2019-06-11T14:52:30","guid":{"rendered":"https:\/\/opentextbc.ca\/physicalgeology2ed\/chapter\/15-2-classification-of-mass-wasting\/"},"modified":"2021-12-08T20:32:00","modified_gmt":"2021-12-08T20:32:00","slug":"15-2-classification-of-mass-wasting","status":"publish","type":"chapter","link":"https:\/\/opentextbc.ca\/physicalgeology2ed\/chapter\/15-2-classification-of-mass-wasting\/","title":{"raw":"15.2 Classification of Mass Wasting","rendered":"15.2 Classification of Mass Wasting"},"content":{"raw":"It\u2019s important to classify slope failures so that we can understand what causes them and learn how to mitigate their effects. The three criteria used to describe slope failures are:\r\n<ul>\r\n \t<li>The type of material that failed (typically either bedrock or unconsolidated sediment)<\/li>\r\n \t<li>The mechanism of the failure (how the material moved)<\/li>\r\n \t<li>The rate at which it moved<\/li>\r\n<\/ul>\r\nThe type of motion is the most important characteristic of a slope failure, and there are three different types of motion:\r\n<ul>\r\n \t<li>If the material drops through the air, vertically or nearly vertically, it\u2019s known as a <strong>[pb_glossary id=\"1810\"]fall[\/pb_glossary]<\/strong>.<\/li>\r\n \t<li>If the material moves as a mass along a sloping surface (without internal motion within the mass), it\u2019s a <strong>[pb_glossary id=\"1812\"]slide[\/pb_glossary]<\/strong><strong>.<\/strong><\/li>\r\n \t<li>If the material has internal motion, like a fluid, it\u2019s a <strong>[pb_glossary id=\"1813\"]flow[\/pb_glossary]<\/strong>.<\/li>\r\n<\/ul>\r\nUnfortunately it\u2019s not normally that simple. Many slope failures involve two of these types of motion, some involve all three, and in many cases, it\u2019s not easy to tell how the material moved. The types of slope failure that we\u2019ll cover here are summarized in Table 15.1.\r\n<table class=\"aligncenter\" style=\"width: 100%;\" border=\"1\"><caption>Table 15.1 Classification of slope failures based on type of material and type of motion<\/caption>\r\n<thead>\r\n<tr>\r\n<td style=\"text-align: center;\" colspan=\"4\"><a href=\"#skiptable15.1\">[Skip Table]<\/a><\/td>\r\n<\/tr>\r\n<tr>\r\n<th scope=\"col\">Failure Type<\/th>\r\n<th scope=\"col\">Type of Material<\/th>\r\n<th scope=\"col\">Type of Motion<\/th>\r\n<th scope=\"col\">Rate of Motion<\/th>\r\n<\/tr>\r\n<\/thead>\r\n<tbody>\r\n<tr>\r\n<td>Rock fall<\/td>\r\n<td>Rock fragments<\/td>\r\n<td>Vertical or near-vertical fall (plus bouncing in many cases)<\/td>\r\n<td>Very fast (Greater than 10s of metres per second)<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Rock slide<\/td>\r\n<td>A large rock body<\/td>\r\n<td>Motion as a unit along a planar surface (translational sliding)<\/td>\r\n<td>Typically very slow (millimetres per year to centimetres per year), but some can be faster<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Rock avalanche<\/td>\r\n<td>A large rock body that slides and then breaks into small fragments<\/td>\r\n<td>Flow (at high speeds the mass of rock fragments is suspended on a cushion of air)<\/td>\r\n<td>Very fast (Greater than tens of metres per second)<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Creep or solifluction<\/td>\r\n<td>Soil or other overburden; in some cases, mixed with ice<\/td>\r\n<td>Flow (although sliding motion may also occur)<\/td>\r\n<td>Very slow (millimetres per year to centimetres per year)<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Slump<\/td>\r\n<td>Thick deposits (a metre to 10s of metres) of unconsolidated sediment<\/td>\r\n<td>Motion as a unit along a curved surface (rotational sliding)<\/td>\r\n<td>Slow (centimetres per year to metres per year)<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Mudflow<\/td>\r\n<td>Loose sediment with a significant component of silt and clay<\/td>\r\n<td>Flow (a mixture of sediment and water moves down a channel)<\/td>\r\n<td>Moderate to fast (centimetres per second to metres per second)<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Debris flow<\/td>\r\n<td>Sand, gravel, and larger fragments<\/td>\r\n<td>Flow (similar to a mudflow, but typically faster)<\/td>\r\n<td>Fast (metres per second)<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<h1 id=\"skiptable15.1\">Rock Fall<\/h1>\r\n[caption id=\"attachment_666\" align=\"alignright\" width=\"550\"]<a href=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/06\/contribution-of-freeze-thaw-to-rock-fall.png\"><img class=\"wp-image-666\" src=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/06\/contribution-of-freeze-thaw-to-rock-fall.png\" alt=\"\" width=\"550\" height=\"290\" \/><\/a> Figure 15.2.1 The contribution of freeze-thaw to rock fall.[\/caption]\r\n\r\nRock fragments can break off relatively easily from steep bedrock slopes, most commonly due to frost-wedging in areas where there are many freeze-thaw cycles per year. If you\u2019ve ever hiked along a steep mountain trail on a cool morning, you might have heard the occasional fall of rock fragments onto a <strong>[pb_glossary id=\"1411\"]talus slope[\/pb_glossary]<\/strong>. This happens because the water between cracks freezes and expands overnight, and then when that same water thaws in the morning sun, the fragments that had been pushed beyond their limit by the ice fall to the slope below (Figure 15.2.1).\r\n\r\nA typical talus slope, near Keremeos in southern B.C., is shown in Figure 15.2.2. In December 2014, a large block of rock split away from a cliff in this same area. It broke into smaller pieces that tumbled down the slope and crashed into the road, smashing the concrete barriers and gouging out large parts of the pavement. Luckily no one was hurt.\r\n\r\n[caption id=\"attachment_667\" align=\"aligncenter\" width=\"900\"]<a href=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/keremeos-2.png\"><img class=\"wp-image-667\" src=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/keremeos-2.png\" alt=\"\" width=\"900\" height=\"327\" \/><\/a> Figure 15.2.2 Left: A talus slope near Keremeos, B.C., formed by rock fall from the cliffs above. Right: The results of a rock fall onto a highway west of Keremeos in December 2014.[\/caption]\r\n<h1>Rock Slide<\/h1>\r\nA rock slide is the sliding motion of rock along a sloping surface. In most cases, the movement is parallel to a fracture, bedding, or metamorphic foliation plane, and it can range from very slow to moderately fast. The word <strong>[pb_glossary id=\"1814\"]sackung[\/pb_glossary]<\/strong> describes the very slow motion of a block of rock (millimetres per year to centimetres per year) on a slope. A good example is the Downie Slide north of Revelstoke, B.C., which is shown in Figure 15.2.3. In this case, a massive body of rock is very slowly sliding down a steep slope along a plane of weakness that is approximately parallel to the slope. The Downie Slide, which was first recognized in the 1950s, prior to the construction of the Revelstoke Dam in the late 1970s, was moving very slowly at the time (a few centimetres per year). Geological engineers were concerned that the presence of water in the reservoir (visible in Figure 15.2.3) could further weaken the plane of failure, leading to an acceleration of the motion. The result would have been a catastrophic failure into the reservoir that would have sent a wall of water over the dam and into the community of Revelstoke. During the construction of the dam they tunneled into the rock at the base of the slide and drilled hundreds of drainage holes upward into the plane of failure. This allowed water to drain out so that the pressure was reduced, which reduced the rate of movement of the sliding block. BC Hydro monitors this site continuously; the slide block is currently moving more slowly than it was prior to the construction of the dam.\r\n\r\n[caption id=\"attachment_541\" align=\"aligncenter\" width=\"900\"]<a href=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/McConnell-Thrust-at-Mt.-Yamnuska.png\"><img class=\"wp-image-541\" src=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/The-Downie-Slide-1024x490.png\" alt=\"\" width=\"900\" height=\"431\" \/><\/a> Figure 15.2.3 The Downie Slide, a sackung, on the shore of the Revelstoke Reservoir (above the Revelstoke Dam). The head scarp is visible at the top and a side-scarp along the left side.[\/caption]\r\n\r\n[caption id=\"attachment_669\" align=\"alignright\" width=\"600\"]<a href=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Site-of-the-2008-rock-slide-at-Porteau-Cove.jpg\"><img class=\"wp-image-669\" src=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Site-of-the-2008-rock-slide-at-Porteau-Cove.jpg\" alt=\"\" width=\"600\" height=\"434\" \/><\/a> Figure 15.2.4 Site of the 2008 rock slide at Porteau Cove. Notice the prominent fracture set parallel to the surface of the slope. The slope has been stabilized with rock bolts (visible near to the top of the photo) and holes have been drilled into the rock to improve drainage (one is visible in the lower right). Risk to passing vehicles from rock fall has been reduced by hanging mesh curtains (background).[\/caption]\r\n\r\nIn the summer of 2008, a large block of rock slid rapidly from a steep slope above Highway 99 near Porteau Cove (between Horseshoe Bay and Squamish). The block slammed into the highway and adjacent railway and broke into many pieces. The highway was closed for several days, and the slope was subsequently stabilized with rock bolts and drainage holes. As shown in Figure 15.2.4, the rock is fractured parallel to the slope, and this almost certainly contributed to the failure. However, it is not actually known what triggered this event as the weather was dry and warm during the preceding weeks, and there was no significant earthquake in the region.\r\n<h1>Rock Avalanche<\/h1>\r\n[caption id=\"attachment_670\" align=\"aligncenter\" width=\"1024\"]<a href=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/meager-rock-avalanche-2019.png\"><img class=\"wp-image-670 size-large\" src=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/meager-rock-avalanche-2019-1024x753.png\" alt=\"\" width=\"1024\" height=\"753\" \/><\/a> Figure 15.2.5 The August 2010 Mount Meager rock avalanche, showing where the slide originated (red arrow, 4 km upstream) and its path down a steep narrow valley.\u00a0 The yellow arrows show how far up the valley the avalanche extended.[\/caption]\r\n\r\nIf a rock slides and then starts moving quickly (metres per second), the rock is likely to break into many small pieces, and at that point it turns into a <strong>[pb_glossary id=\"1815\"]rock avalanche[\/pb_glossary]<\/strong>, in which the large and small fragments of rock move in a fluid manner supported by a cushion of air within and beneath the moving mass. The 1965 Hope Slide (Figure 15.0.1) was a rock avalanche, as was the famous 1903 Frank Slide in southwestern Alberta. The 2010 slide at\u00a0Mount Meager (west of Lillooet) was also a rock avalanche, and rivals the Hope Slide as the largest slope failure in Canada during historical times (Figure 15.2.5).\r\n<h1>Creep or Solifluction<\/h1>\r\n[caption id=\"attachment_671\" align=\"alignright\" width=\"400\"]<a href=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/depiction-of-the-contribution-of-freeze-thaw.png\"><img class=\"wp-image-671\" src=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/depiction-of-the-contribution-of-freeze-thaw.png\" alt=\"&quot;&quot;\" width=\"400\" height=\"251\" \/><\/a> Figure 15.2.6 A depiction of the contribution of freeze-thaw to creep. The blue arrows represent uplift caused by freezing in the wet soil underneath, while the red arrows represent depression by gravity during thawing. The uplift is perpendicular to the slope, while the drop is vertical.[\/caption]\r\n\r\nThe very slow\u2014millimetres per year to centimetres per year\u2014movement of soil or other unconsolidated material on a slope is known as creep. <strong>[pb_glossary id=\"1816\"]Creep[\/pb_glossary]<\/strong>, which normally only affects the upper several centimetres of loose material, is typically a type of very slow flow, but in some cases, sliding may take place. Creep can be facilitated by freezing and thawing because, as shown in Figure 15.2.6, particles are lifted perpendicular to the surface by the growth of ice crystals within the soil, and then let down vertically by gravity when the ice melts. The same effect can be produced by frequent wetting and drying of the soil. In cold environments, <strong>[pb_glossary id=\"1817\"]solifluction[\/pb_glossary]<\/strong> is a more intense form of freeze-thaw-triggered creep.\r\n\r\nCreep is most noticeable on moderate-to-steep slopes where trees, fence posts, or grave markers are consistently leaning in a downhill direction. In the case of trees, they try to correct their lean by growing upright, and this leads to a curved lower trunk known as a \u201cpistol butt.\u201d\u00a0 An example is shown on Figure 15.2.7.\r\n\r\n[caption id=\"attachment_672\" align=\"aligncenter\" width=\"800\"]<a href=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Pistol-Butt.jpg\"><img class=\"wp-image-672\" src=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Pistol-Butt.jpg\" alt=\"\" width=\"800\" height=\"432\" \/><\/a> Figure 15.2.7 Pistol-butt shaped trees on a slope that is experiencing creep[\/caption]\r\n<h1>Slump<\/h1>\r\n[caption id=\"attachment_546\" align=\"alignright\" width=\"600\"]<a href=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/joints-developed-in-a-rock-1.png\"><img class=\"wp-image-546\" src=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/motion-of-unconsolidated-sediments-in-an-area-of-slumping-1024x456.png\" alt=\"\" width=\"600\" height=\"267\" \/><\/a> Figure 15.2.8 A depiction of the motion of unconsolidated sediments in an area of slumping.[\/caption]\r\n\r\nSlump is a type of slide (movement as a mass) that takes place within thick unconsolidated deposits (typically thicker than 10 metres). Slumps involve movement along one or more curved failure surfaces, with downward motion near the top and outward motion toward the bottom (Figure 15.2.8). They are typically caused by an excess of water within these materials on a steep slope.\r\n\r\n[caption id=\"attachment_674\" align=\"alignright\" width=\"600\"]<a href=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/slump-along-the-banks-of-a-small-coulee-near-Lethbridge.jpg\"><img class=\"wp-image-674\" src=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/slump-along-the-banks-of-a-small-coulee-near-Lethbridge.jpg\" alt=\"\" width=\"600\" height=\"404\" \/><\/a> Figure 15.2.9 A slump along the banks of a small coulee near Lethbridge, Alberta. The main head-scarp is clearly visible at the top, and a second smaller one is visible about one-quarter of the way down. The toe of the slump is being eroded by the seasonal stream that created the coulee.[\/caption]\r\n\r\nAn example of a slump in the Lethbridge area of Alberta is shown in Figure 15.2.9. This feature has likely been active for many decades, and moves a little more whenever there are heavy spring rains and significant snowmelt runoff. The toe of the slump is failing because it has been eroded by the small stream at the bottom.\r\n<h1><\/h1>\r\n<h1>Mudflows and Debris Flows<\/h1>\r\n[caption id=\"attachment_675\" align=\"alignright\" width=\"400\"]<a href=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/lethbridge-2.png\"><img class=\"wp-image-675\" src=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/lethbridge-2.png\" alt=\"\" width=\"400\" height=\"495\" \/><\/a> Figure 15.2.10 A slump (left) and an associated mudflow (centre) at the same location as Figure 15.2.9, near Lethbridge, Alberta.[\/caption]\r\n\r\nAs you saw in Exercise 15.1, when a mass of sediment becomes completely saturated with water, the mass loses strength, to the extent that the grains are pushed apart, and it will flow, even on a gentle slope. This can happen during rapid spring snowmelt or heavy rains, and is also relatively common during volcanic eruptions because of the rapid melting of snow and ice. (A mudflow or debris flow on a volcano or during a volcanic eruption is a <em>lahar<\/em>.) If the material involved is primarily sand-sized or smaller, it is known as a mudflow, such as the one shown in Figure 15.2.10.\r\n\r\nIf the material involved is gravel sized or larger, it is known as a debris flow. Because it takes more gravitational energy to move larger particles, a debris flow typically forms in an area with steeper slopes and more water than does a mudflow. In many cases, a debris flow takes place within a steep stream channel, and is triggered by the collapse of bank material into the stream. This creates a temporary dam, and then a major flow of water and debris when the dam breaks. This is the situation that led to the fatal debris flow at Johnsons Landing, B.C., in 2012. A typical west-coast debris flow is shown in Figure 15.2.11. This event took place in November 2006 in response to very heavy rainfall. There was enough energy to move large boulders and to knock over large trees.<strong>\u00a0<\/strong>\r\n\r\n[caption id=\"attachment_676\" align=\"aligncenter\" width=\"800\"]<a href=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/buttle.png\"><img class=\"wp-image-676\" src=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/buttle.png\" alt=\"\" width=\"800\" height=\"466\" \/><\/a> Figure 15.2.11 The lower part of debris flow within a steep stream channel near Buttle Lake, B.C., in November 2006.[\/caption]\r\n\r\n<div class=\"textbox textbox--exercises\"><header class=\"textbox__header\">\r\n<p class=\"textbox__title\">Exercise 15.2 Classifying slope failures<\/p>\r\n\r\n<\/header>\r\n<div class=\"textbox__content\">\r\n\r\nThese four photos show some of the different types of slope failures described above. Try to identify each types and provide some criteria to support your choice.\r\n<table style=\"width: 100%;\" border=\"0\">\r\n<tbody>\r\n<tr>\r\n<td>\r\n\r\n[caption id=\"attachment_677\" align=\"aligncenter\" width=\"400\"]<a href=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Classifying-Slope-Failures1.jpg\"><img class=\"wp-image-677\" src=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Classifying-Slope-Failures1.jpg\" alt=\"A soft grassy hill. The centre part of the hill gave away and shifted downwards.\" width=\"400\" height=\"152\" \/><\/a> Figure 15.2.12a[\/caption]\r\n\r\n&nbsp;<\/td>\r\n<td>\r\n\r\n[caption id=\"attachment_678\" align=\"aligncenter\" width=\"400\"]<a href=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Classifying-Slope-Failures2.jpg\"><img class=\"wp-image-678\" src=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Classifying-Slope-Failures2.jpg\" alt=\"A pile of rocks, dirt, and sticks at the bottom of a slope.\" width=\"400\" height=\"322\" \/><\/a> Figure 15.2.12b[\/caption]<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>\r\n\r\n[caption id=\"attachment_679\" align=\"aligncenter\" width=\"400\"]<a href=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Classifying-Slope-Failures3.jpg\"><img class=\"wp-image-679\" src=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Classifying-Slope-Failures3.jpg\" alt=\"\" width=\"400\" height=\"330\" \/><\/a> Figure 15.2.12c[\/caption]<\/td>\r\n<td>\r\n\r\n[caption id=\"attachment_680\" align=\"aligncenter\" width=\"400\"]<a href=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Classifying-Slope-Failures4.jpg\"><img class=\"wp-image-680\" src=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Classifying-Slope-Failures4.jpg\" alt=\"\" width=\"400\" height=\"340\" \/><\/a> Figure 15.2.12d[\/caption]<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<table style=\"width: 100%;\" border=\"0\">\r\n<tbody>\r\n<tr>\r\n<td>a:<\/td>\r\n<td>b:<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>c:<\/td>\r\n<td>d:<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\nSee Appendix 3 for <a href=\"\/physicalgeology2ed\/back-matter\/appendix-3-answers-to-exercises\/#exercisea15.2\">Exercise 15.2 answers<\/a>.\r\n\r\n<\/div>\r\n<\/div>\r\n<h3>Media Attributions<\/h3>\r\n<ul>\r\n \t<li>Figure 15.2.1, 15.2.2: \u00a9 Steven Earle. CC BY.<\/li>\r\n \t<li>Figure 15.2.3: \u00a9 Google Earth. <a href=\"https:\/\/www.google.com\/permissions\/geoguidelines\/\">Used with permission<\/a> for non-commerical purposes.<\/li>\r\n \t<li>Figure 15.2.4: \u00a9 Steven Earle. CC BY.<\/li>\r\n \t<li>Figure 15.2.5: \"2010 Mt. Meager rock avalanche\" \u00a9 Isaac Earle. CC BY.<\/li>\r\n \t<li>Figure 15.2.6, 15.2.7, 15.2.8, 15.2.9, 15.2.10, 15.2.11, 15.2.12: \u00a9 Steven Earle. CC BY.<\/li>\r\n<\/ul>","rendered":"<p>It\u2019s important to classify slope failures so that we can understand what causes them and learn how to mitigate their effects. The three criteria used to describe slope failures are:<\/p>\n<ul>\n<li>The type of material that failed (typically either bedrock or unconsolidated sediment)<\/li>\n<li>The mechanism of the failure (how the material moved)<\/li>\n<li>The rate at which it moved<\/li>\n<\/ul>\n<p>The type of motion is the most important characteristic of a slope failure, and there are three different types of motion:<\/p>\n<ul>\n<li>If the material drops through the air, vertically or nearly vertically, it\u2019s known as a <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_681_1810\">fall<\/a><\/strong>.<\/li>\n<li>If the material moves as a mass along a sloping surface (without internal motion within the mass), it\u2019s a <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_681_1812\">slide<\/a><\/strong><strong>.<\/strong><\/li>\n<li>If the material has internal motion, like a fluid, it\u2019s a <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_681_1813\">flow<\/a><\/strong>.<\/li>\n<\/ul>\n<p>Unfortunately it\u2019s not normally that simple. Many slope failures involve two of these types of motion, some involve all three, and in many cases, it\u2019s not easy to tell how the material moved. The types of slope failure that we\u2019ll cover here are summarized in Table 15.1.<\/p>\n<table class=\"aligncenter\" style=\"width: 100%;\">\n<caption>Table 15.1 Classification of slope failures based on type of material and type of motion<\/caption>\n<thead>\n<tr>\n<td style=\"text-align: center;\" colspan=\"4\"><a href=\"#skiptable15.1\">[Skip Table]<\/a><\/td>\n<\/tr>\n<tr>\n<th scope=\"col\">Failure Type<\/th>\n<th scope=\"col\">Type of Material<\/th>\n<th scope=\"col\">Type of Motion<\/th>\n<th scope=\"col\">Rate of Motion<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Rock fall<\/td>\n<td>Rock fragments<\/td>\n<td>Vertical or near-vertical fall (plus bouncing in many cases)<\/td>\n<td>Very fast (Greater than 10s of metres per second)<\/td>\n<\/tr>\n<tr>\n<td>Rock slide<\/td>\n<td>A large rock body<\/td>\n<td>Motion as a unit along a planar surface (translational sliding)<\/td>\n<td>Typically very slow (millimetres per year to centimetres per year), but some can be faster<\/td>\n<\/tr>\n<tr>\n<td>Rock avalanche<\/td>\n<td>A large rock body that slides and then breaks into small fragments<\/td>\n<td>Flow (at high speeds the mass of rock fragments is suspended on a cushion of air)<\/td>\n<td>Very fast (Greater than tens of metres per second)<\/td>\n<\/tr>\n<tr>\n<td>Creep or solifluction<\/td>\n<td>Soil or other overburden; in some cases, mixed with ice<\/td>\n<td>Flow (although sliding motion may also occur)<\/td>\n<td>Very slow (millimetres per year to centimetres per year)<\/td>\n<\/tr>\n<tr>\n<td>Slump<\/td>\n<td>Thick deposits (a metre to 10s of metres) of unconsolidated sediment<\/td>\n<td>Motion as a unit along a curved surface (rotational sliding)<\/td>\n<td>Slow (centimetres per year to metres per year)<\/td>\n<\/tr>\n<tr>\n<td>Mudflow<\/td>\n<td>Loose sediment with a significant component of silt and clay<\/td>\n<td>Flow (a mixture of sediment and water moves down a channel)<\/td>\n<td>Moderate to fast (centimetres per second to metres per second)<\/td>\n<\/tr>\n<tr>\n<td>Debris flow<\/td>\n<td>Sand, gravel, and larger fragments<\/td>\n<td>Flow (similar to a mudflow, but typically faster)<\/td>\n<td>Fast (metres per second)<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<h1 id=\"skiptable15.1\">Rock Fall<\/h1>\n<figure id=\"attachment_666\" aria-describedby=\"caption-attachment-666\" style=\"width: 550px\" class=\"wp-caption alignright\"><a href=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/06\/contribution-of-freeze-thaw-to-rock-fall.png\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-666\" src=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/06\/contribution-of-freeze-thaw-to-rock-fall.png\" alt=\"\" width=\"550\" height=\"290\" srcset=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/06\/contribution-of-freeze-thaw-to-rock-fall.png 710w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/06\/contribution-of-freeze-thaw-to-rock-fall-300x158.png 300w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/06\/contribution-of-freeze-thaw-to-rock-fall-65x34.png 65w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/06\/contribution-of-freeze-thaw-to-rock-fall-225x119.png 225w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/06\/contribution-of-freeze-thaw-to-rock-fall-350x185.png 350w\" sizes=\"auto, (max-width: 550px) 100vw, 550px\" \/><\/a><figcaption id=\"caption-attachment-666\" class=\"wp-caption-text\">Figure 15.2.1 The contribution of freeze-thaw to rock fall.<\/figcaption><\/figure>\n<p>Rock fragments can break off relatively easily from steep bedrock slopes, most commonly due to frost-wedging in areas where there are many freeze-thaw cycles per year. If you\u2019ve ever hiked along a steep mountain trail on a cool morning, you might have heard the occasional fall of rock fragments onto a <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_681_1411\">talus slope<\/a><\/strong>. This happens because the water between cracks freezes and expands overnight, and then when that same water thaws in the morning sun, the fragments that had been pushed beyond their limit by the ice fall to the slope below (Figure 15.2.1).<\/p>\n<p>A typical talus slope, near Keremeos in southern B.C., is shown in Figure 15.2.2. In December 2014, a large block of rock split away from a cliff in this same area. It broke into smaller pieces that tumbled down the slope and crashed into the road, smashing the concrete barriers and gouging out large parts of the pavement. Luckily no one was hurt.<\/p>\n<figure id=\"attachment_667\" aria-describedby=\"caption-attachment-667\" style=\"width: 900px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/keremeos-2.png\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-667\" src=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/keremeos-2.png\" alt=\"\" width=\"900\" height=\"327\" srcset=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/keremeos-2.png 1300w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/keremeos-2-300x109.png 300w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/keremeos-2-768x279.png 768w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/keremeos-2-1024x372.png 1024w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/keremeos-2-65x24.png 65w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/keremeos-2-225x82.png 225w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/keremeos-2-350x127.png 350w\" sizes=\"auto, (max-width: 900px) 100vw, 900px\" \/><\/a><figcaption id=\"caption-attachment-667\" class=\"wp-caption-text\">Figure 15.2.2 Left: A talus slope near Keremeos, B.C., formed by rock fall from the cliffs above. Right: The results of a rock fall onto a highway west of Keremeos in December 2014.<\/figcaption><\/figure>\n<h1>Rock Slide<\/h1>\n<p>A rock slide is the sliding motion of rock along a sloping surface. In most cases, the movement is parallel to a fracture, bedding, or metamorphic foliation plane, and it can range from very slow to moderately fast. The word <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_681_1814\">sackung<\/a><\/strong> describes the very slow motion of a block of rock (millimetres per year to centimetres per year) on a slope. A good example is the Downie Slide north of Revelstoke, B.C., which is shown in Figure 15.2.3. In this case, a massive body of rock is very slowly sliding down a steep slope along a plane of weakness that is approximately parallel to the slope. The Downie Slide, which was first recognized in the 1950s, prior to the construction of the Revelstoke Dam in the late 1970s, was moving very slowly at the time (a few centimetres per year). Geological engineers were concerned that the presence of water in the reservoir (visible in Figure 15.2.3) could further weaken the plane of failure, leading to an acceleration of the motion. The result would have been a catastrophic failure into the reservoir that would have sent a wall of water over the dam and into the community of Revelstoke. During the construction of the dam they tunneled into the rock at the base of the slide and drilled hundreds of drainage holes upward into the plane of failure. This allowed water to drain out so that the pressure was reduced, which reduced the rate of movement of the sliding block. BC Hydro monitors this site continuously; the slide block is currently moving more slowly than it was prior to the construction of the dam.<\/p>\n<figure id=\"attachment_541\" aria-describedby=\"caption-attachment-541\" style=\"width: 900px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/McConnell-Thrust-at-Mt.-Yamnuska.png\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-541\" src=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/The-Downie-Slide-1024x490.png\" alt=\"\" width=\"900\" height=\"431\" \/><\/a><figcaption id=\"caption-attachment-541\" class=\"wp-caption-text\">Figure 15.2.3 The Downie Slide, a sackung, on the shore of the Revelstoke Reservoir (above the Revelstoke Dam). The head scarp is visible at the top and a side-scarp along the left side.<\/figcaption><\/figure>\n<figure id=\"attachment_669\" aria-describedby=\"caption-attachment-669\" style=\"width: 600px\" class=\"wp-caption alignright\"><a href=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Site-of-the-2008-rock-slide-at-Porteau-Cove.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-669\" src=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Site-of-the-2008-rock-slide-at-Porteau-Cove.jpg\" alt=\"\" width=\"600\" height=\"434\" srcset=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Site-of-the-2008-rock-slide-at-Porteau-Cove.jpg 806w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Site-of-the-2008-rock-slide-at-Porteau-Cove-300x217.jpg 300w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Site-of-the-2008-rock-slide-at-Porteau-Cove-768x556.jpg 768w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Site-of-the-2008-rock-slide-at-Porteau-Cove-65x47.jpg 65w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Site-of-the-2008-rock-slide-at-Porteau-Cove-225x163.jpg 225w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Site-of-the-2008-rock-slide-at-Porteau-Cove-350x253.jpg 350w\" sizes=\"auto, (max-width: 600px) 100vw, 600px\" \/><\/a><figcaption id=\"caption-attachment-669\" class=\"wp-caption-text\">Figure 15.2.4 Site of the 2008 rock slide at Porteau Cove. Notice the prominent fracture set parallel to the surface of the slope. The slope has been stabilized with rock bolts (visible near to the top of the photo) and holes have been drilled into the rock to improve drainage (one is visible in the lower right). Risk to passing vehicles from rock fall has been reduced by hanging mesh curtains (background).<\/figcaption><\/figure>\n<p>In the summer of 2008, a large block of rock slid rapidly from a steep slope above Highway 99 near Porteau Cove (between Horseshoe Bay and Squamish). The block slammed into the highway and adjacent railway and broke into many pieces. The highway was closed for several days, and the slope was subsequently stabilized with rock bolts and drainage holes. As shown in Figure 15.2.4, the rock is fractured parallel to the slope, and this almost certainly contributed to the failure. However, it is not actually known what triggered this event as the weather was dry and warm during the preceding weeks, and there was no significant earthquake in the region.<\/p>\n<h1>Rock Avalanche<\/h1>\n<figure id=\"attachment_670\" aria-describedby=\"caption-attachment-670\" style=\"width: 1024px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/meager-rock-avalanche-2019.png\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-670 size-large\" src=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/meager-rock-avalanche-2019-1024x753.png\" alt=\"\" width=\"1024\" height=\"753\" srcset=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/meager-rock-avalanche-2019-1024x753.png 1024w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/meager-rock-avalanche-2019-300x221.png 300w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/meager-rock-avalanche-2019-768x565.png 768w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/meager-rock-avalanche-2019-65x48.png 65w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/meager-rock-avalanche-2019-225x165.png 225w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/meager-rock-avalanche-2019-350x257.png 350w\" sizes=\"auto, (max-width: 1024px) 100vw, 1024px\" \/><\/a><figcaption id=\"caption-attachment-670\" class=\"wp-caption-text\">Figure 15.2.5 The August 2010 Mount Meager rock avalanche, showing where the slide originated (red arrow, 4 km upstream) and its path down a steep narrow valley.\u00a0 The yellow arrows show how far up the valley the avalanche extended.<\/figcaption><\/figure>\n<p>If a rock slides and then starts moving quickly (metres per second), the rock is likely to break into many small pieces, and at that point it turns into a <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_681_1815\">rock avalanche<\/a><\/strong>, in which the large and small fragments of rock move in a fluid manner supported by a cushion of air within and beneath the moving mass. The 1965 Hope Slide (Figure 15.0.1) was a rock avalanche, as was the famous 1903 Frank Slide in southwestern Alberta. The 2010 slide at\u00a0Mount Meager (west of Lillooet) was also a rock avalanche, and rivals the Hope Slide as the largest slope failure in Canada during historical times (Figure 15.2.5).<\/p>\n<h1>Creep or Solifluction<\/h1>\n<figure id=\"attachment_671\" aria-describedby=\"caption-attachment-671\" style=\"width: 400px\" class=\"wp-caption alignright\"><a href=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/depiction-of-the-contribution-of-freeze-thaw.png\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-671\" src=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/depiction-of-the-contribution-of-freeze-thaw.png\" alt=\"&quot;&quot;\" width=\"400\" height=\"251\" srcset=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/depiction-of-the-contribution-of-freeze-thaw.png 693w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/depiction-of-the-contribution-of-freeze-thaw-300x188.png 300w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/depiction-of-the-contribution-of-freeze-thaw-65x41.png 65w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/depiction-of-the-contribution-of-freeze-thaw-225x141.png 225w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/depiction-of-the-contribution-of-freeze-thaw-350x220.png 350w\" sizes=\"auto, (max-width: 400px) 100vw, 400px\" \/><\/a><figcaption id=\"caption-attachment-671\" class=\"wp-caption-text\">Figure 15.2.6 A depiction of the contribution of freeze-thaw to creep. The blue arrows represent uplift caused by freezing in the wet soil underneath, while the red arrows represent depression by gravity during thawing. The uplift is perpendicular to the slope, while the drop is vertical.<\/figcaption><\/figure>\n<p>The very slow\u2014millimetres per year to centimetres per year\u2014movement of soil or other unconsolidated material on a slope is known as creep. <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_681_1816\">Creep<\/a><\/strong>, which normally only affects the upper several centimetres of loose material, is typically a type of very slow flow, but in some cases, sliding may take place. Creep can be facilitated by freezing and thawing because, as shown in Figure 15.2.6, particles are lifted perpendicular to the surface by the growth of ice crystals within the soil, and then let down vertically by gravity when the ice melts. The same effect can be produced by frequent wetting and drying of the soil. In cold environments, <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_681_1817\">solifluction<\/a><\/strong> is a more intense form of freeze-thaw-triggered creep.<\/p>\n<p>Creep is most noticeable on moderate-to-steep slopes where trees, fence posts, or grave markers are consistently leaning in a downhill direction. In the case of trees, they try to correct their lean by growing upright, and this leads to a curved lower trunk known as a \u201cpistol butt.\u201d\u00a0 An example is shown on Figure 15.2.7.<\/p>\n<figure id=\"attachment_672\" aria-describedby=\"caption-attachment-672\" style=\"width: 800px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Pistol-Butt.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-672\" src=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Pistol-Butt.jpg\" alt=\"\" width=\"800\" height=\"432\" srcset=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Pistol-Butt.jpg 1989w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Pistol-Butt-300x162.jpg 300w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Pistol-Butt-768x415.jpg 768w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Pistol-Butt-1024x553.jpg 1024w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Pistol-Butt-65x35.jpg 65w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Pistol-Butt-225x121.jpg 225w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Pistol-Butt-350x189.jpg 350w\" sizes=\"auto, (max-width: 800px) 100vw, 800px\" \/><\/a><figcaption id=\"caption-attachment-672\" class=\"wp-caption-text\">Figure 15.2.7 Pistol-butt shaped trees on a slope that is experiencing creep<\/figcaption><\/figure>\n<h1>Slump<\/h1>\n<figure id=\"attachment_546\" aria-describedby=\"caption-attachment-546\" style=\"width: 600px\" class=\"wp-caption alignright\"><a href=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/joints-developed-in-a-rock-1.png\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-546\" src=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/motion-of-unconsolidated-sediments-in-an-area-of-slumping-1024x456.png\" alt=\"\" width=\"600\" height=\"267\" \/><\/a><figcaption id=\"caption-attachment-546\" class=\"wp-caption-text\">Figure 15.2.8 A depiction of the motion of unconsolidated sediments in an area of slumping.<\/figcaption><\/figure>\n<p>Slump is a type of slide (movement as a mass) that takes place within thick unconsolidated deposits (typically thicker than 10 metres). Slumps involve movement along one or more curved failure surfaces, with downward motion near the top and outward motion toward the bottom (Figure 15.2.8). They are typically caused by an excess of water within these materials on a steep slope.<\/p>\n<figure id=\"attachment_674\" aria-describedby=\"caption-attachment-674\" style=\"width: 600px\" class=\"wp-caption alignright\"><a href=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/slump-along-the-banks-of-a-small-coulee-near-Lethbridge.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-674\" src=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/slump-along-the-banks-of-a-small-coulee-near-Lethbridge.jpg\" alt=\"\" width=\"600\" height=\"404\" srcset=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/slump-along-the-banks-of-a-small-coulee-near-Lethbridge.jpg 840w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/slump-along-the-banks-of-a-small-coulee-near-Lethbridge-300x202.jpg 300w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/slump-along-the-banks-of-a-small-coulee-near-Lethbridge-768x517.jpg 768w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/slump-along-the-banks-of-a-small-coulee-near-Lethbridge-65x44.jpg 65w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/slump-along-the-banks-of-a-small-coulee-near-Lethbridge-225x152.jpg 225w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/slump-along-the-banks-of-a-small-coulee-near-Lethbridge-350x236.jpg 350w\" sizes=\"auto, (max-width: 600px) 100vw, 600px\" \/><\/a><figcaption id=\"caption-attachment-674\" class=\"wp-caption-text\">Figure 15.2.9 A slump along the banks of a small coulee near Lethbridge, Alberta. The main head-scarp is clearly visible at the top, and a second smaller one is visible about one-quarter of the way down. The toe of the slump is being eroded by the seasonal stream that created the coulee.<\/figcaption><\/figure>\n<p>An example of a slump in the Lethbridge area of Alberta is shown in Figure 15.2.9. This feature has likely been active for many decades, and moves a little more whenever there are heavy spring rains and significant snowmelt runoff. The toe of the slump is failing because it has been eroded by the small stream at the bottom.<\/p>\n<h1><\/h1>\n<h1>Mudflows and Debris Flows<\/h1>\n<figure id=\"attachment_675\" aria-describedby=\"caption-attachment-675\" style=\"width: 400px\" class=\"wp-caption alignright\"><a href=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/lethbridge-2.png\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-675\" src=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/lethbridge-2.png\" alt=\"\" width=\"400\" height=\"495\" srcset=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/lethbridge-2.png 545w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/lethbridge-2-243x300.png 243w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/lethbridge-2-65x80.png 65w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/lethbridge-2-225x278.png 225w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/lethbridge-2-350x433.png 350w\" sizes=\"auto, (max-width: 400px) 100vw, 400px\" \/><\/a><figcaption id=\"caption-attachment-675\" class=\"wp-caption-text\">Figure 15.2.10 A slump (left) and an associated mudflow (centre) at the same location as Figure 15.2.9, near Lethbridge, Alberta.<\/figcaption><\/figure>\n<p>As you saw in Exercise 15.1, when a mass of sediment becomes completely saturated with water, the mass loses strength, to the extent that the grains are pushed apart, and it will flow, even on a gentle slope. This can happen during rapid spring snowmelt or heavy rains, and is also relatively common during volcanic eruptions because of the rapid melting of snow and ice. (A mudflow or debris flow on a volcano or during a volcanic eruption is a <em>lahar<\/em>.) If the material involved is primarily sand-sized or smaller, it is known as a mudflow, such as the one shown in Figure 15.2.10.<\/p>\n<p>If the material involved is gravel sized or larger, it is known as a debris flow. Because it takes more gravitational energy to move larger particles, a debris flow typically forms in an area with steeper slopes and more water than does a mudflow. In many cases, a debris flow takes place within a steep stream channel, and is triggered by the collapse of bank material into the stream. This creates a temporary dam, and then a major flow of water and debris when the dam breaks. This is the situation that led to the fatal debris flow at Johnsons Landing, B.C., in 2012. A typical west-coast debris flow is shown in Figure 15.2.11. This event took place in November 2006 in response to very heavy rainfall. There was enough energy to move large boulders and to knock over large trees.<strong>\u00a0<\/strong><\/p>\n<figure id=\"attachment_676\" aria-describedby=\"caption-attachment-676\" style=\"width: 800px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/buttle.png\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-676\" src=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/buttle.png\" alt=\"\" width=\"800\" height=\"466\" srcset=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/buttle.png 920w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/buttle-300x175.png 300w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/buttle-768x447.png 768w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/buttle-65x38.png 65w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/buttle-225x131.png 225w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/buttle-350x204.png 350w\" sizes=\"auto, (max-width: 800px) 100vw, 800px\" \/><\/a><figcaption id=\"caption-attachment-676\" class=\"wp-caption-text\">Figure 15.2.11 The lower part of debris flow within a steep stream channel near Buttle Lake, B.C., in November 2006.<\/figcaption><\/figure>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<p class=\"textbox__title\">Exercise 15.2 Classifying slope failures<\/p>\n<\/header>\n<div class=\"textbox__content\">\n<p>These four photos show some of the different types of slope failures described above. Try to identify each types and provide some criteria to support your choice.<\/p>\n<table style=\"width: 100%;\">\n<tbody>\n<tr>\n<td>\n<figure id=\"attachment_677\" aria-describedby=\"caption-attachment-677\" style=\"width: 400px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Classifying-Slope-Failures1.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-677\" src=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Classifying-Slope-Failures1.jpg\" alt=\"A soft grassy hill. The centre part of the hill gave away and shifted downwards.\" width=\"400\" height=\"152\" \/><\/a><figcaption id=\"caption-attachment-677\" class=\"wp-caption-text\">Figure 15.2.12a<\/figcaption><\/figure>\n<p>&nbsp;<\/td>\n<td>\n<figure id=\"attachment_678\" aria-describedby=\"caption-attachment-678\" style=\"width: 400px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Classifying-Slope-Failures2.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-678\" src=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Classifying-Slope-Failures2.jpg\" alt=\"A pile of rocks, dirt, and sticks at the bottom of a slope.\" width=\"400\" height=\"322\" \/><\/a><figcaption id=\"caption-attachment-678\" class=\"wp-caption-text\">Figure 15.2.12b<\/figcaption><\/figure>\n<\/td>\n<\/tr>\n<tr>\n<td>\n<figure id=\"attachment_679\" aria-describedby=\"caption-attachment-679\" style=\"width: 400px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Classifying-Slope-Failures3.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-679\" src=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Classifying-Slope-Failures3.jpg\" alt=\"\" width=\"400\" height=\"330\" srcset=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Classifying-Slope-Failures3.jpg 482w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Classifying-Slope-Failures3-300x248.jpg 300w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Classifying-Slope-Failures3-65x54.jpg 65w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Classifying-Slope-Failures3-225x186.jpg 225w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Classifying-Slope-Failures3-350x289.jpg 350w\" sizes=\"auto, (max-width: 400px) 100vw, 400px\" \/><\/a><figcaption id=\"caption-attachment-679\" class=\"wp-caption-text\">Figure 15.2.12c<\/figcaption><\/figure>\n<\/td>\n<td>\n<figure id=\"attachment_680\" aria-describedby=\"caption-attachment-680\" style=\"width: 400px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Classifying-Slope-Failures4.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-680\" src=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Classifying-Slope-Failures4.jpg\" alt=\"\" width=\"400\" height=\"340\" srcset=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Classifying-Slope-Failures4.jpg 466w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Classifying-Slope-Failures4-300x255.jpg 300w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Classifying-Slope-Failures4-65x55.jpg 65w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Classifying-Slope-Failures4-225x191.jpg 225w, https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-content\/uploads\/sites\/298\/2019\/08\/Classifying-Slope-Failures4-350x297.jpg 350w\" sizes=\"auto, (max-width: 400px) 100vw, 400px\" \/><\/a><figcaption id=\"caption-attachment-680\" class=\"wp-caption-text\">Figure 15.2.12d<\/figcaption><\/figure>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<table style=\"width: 100%;\">\n<tbody>\n<tr>\n<td>a:<\/td>\n<td>b:<\/td>\n<\/tr>\n<tr>\n<td>c:<\/td>\n<td>d:<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>See Appendix 3 for <a href=\"\/physicalgeology2ed\/back-matter\/appendix-3-answers-to-exercises\/#exercisea15.2\">Exercise 15.2 answers<\/a>.<\/p>\n<\/div>\n<\/div>\n<h3>Media Attributions<\/h3>\n<ul>\n<li>Figure 15.2.1, 15.2.2: \u00a9 Steven Earle. CC BY.<\/li>\n<li>Figure 15.2.3: \u00a9 Google Earth. <a href=\"https:\/\/www.google.com\/permissions\/geoguidelines\/\">Used with permission<\/a> for non-commerical purposes.<\/li>\n<li>Figure 15.2.4: \u00a9 Steven Earle. CC BY.<\/li>\n<li>Figure 15.2.5: &#8220;2010 Mt. Meager rock avalanche&#8221; \u00a9 Isaac Earle. CC BY.<\/li>\n<li>Figure 15.2.6, 15.2.7, 15.2.8, 15.2.9, 15.2.10, 15.2.11, 15.2.12: \u00a9 Steven Earle. CC BY.<\/li>\n<\/ul>\n<div class=\"glossary\"><span class=\"screen-reader-text\" id=\"definition\">definition<\/span><template id=\"term_681_1810\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_681_1810\"><div tabindex=\"-1\"><\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_681_1812\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_681_1812\"><div tabindex=\"-1\"><p>the downward movement of rock or sediment on a slope as an intact mass<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_681_1813\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_681_1813\"><div tabindex=\"-1\"><p>the fluid-like motion of material during mass-wasting<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_681_1411\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_681_1411\"><div tabindex=\"-1\"><p>a sloped deposit of angular rock fragments at the base of a rocky escarpment<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_681_1814\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_681_1814\"><div tabindex=\"-1\"><p>an escarpment or trough at the top of a slow-moving rock slide (sackungen)<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_681_1815\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_681_1815\"><div tabindex=\"-1\"><p>a rapid turbulent flow of broken bedrock fragments down a steep slope<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_681_1816\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_681_1816\"><div tabindex=\"-1\"><p>the very slow (a millimetre to centimetre per year) flow of unconsolidated material on a gentle slope<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_681_1817\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_681_1817\"><div tabindex=\"-1\"><p>the flow of water saturated sediment or soil over a stronger and less permeable substrate<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><\/div>","protected":false},"author":90,"menu_order":2,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":"cc-by"},"chapter-type":[],"contributor":[],"license":[52],"class_list":["post-681","chapter","type-chapter","status-publish","hentry","license-cc-by"],"part":655,"_links":{"self":[{"href":"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-json\/pressbooks\/v2\/chapters\/681","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-json\/wp\/v2\/users\/90"}],"version-history":[{"count":5,"href":"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-json\/pressbooks\/v2\/chapters\/681\/revisions"}],"predecessor-version":[{"id":2350,"href":"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-json\/pressbooks\/v2\/chapters\/681\/revisions\/2350"}],"part":[{"href":"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-json\/pressbooks\/v2\/parts\/655"}],"metadata":[{"href":"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-json\/pressbooks\/v2\/chapters\/681\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-json\/wp\/v2\/media?parent=681"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-json\/pressbooks\/v2\/chapter-type?post=681"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-json\/wp\/v2\/contributor?post=681"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/opentextbc.ca\/physicalgeology2ed\/wp-json\/wp\/v2\/license?post=681"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}