Lecture Notes Set 9 Mass Wasting Pt 2 PDF

Mass Wasting/ Landslides Part 2 1 Mass Wasting: Examples MASS WASTING (aka. SLOPE FAILURE): failure and downslope movement of rock or unconsolidated materials due to gravity. UNCONSOLIDATED MATERIALS: Material derived from the disintegration and erosion of rocks on the land's su...

Mass Wasting/ Landslides Part 2 1 Mass Wasting: Examples MASS WASTING (aka. SLOPE FAILURE): failure and downslope movement of rock or unconsolidated materials due to gravity. UNCONSOLIDATED MATERIALS: Material derived from the disintegration and erosion of rocks on the land's surface, including clay, silt, sand and gravel. Now let us consider the wide range of MASS WASTING examples that our encountered for: Massive boulder rolls through vineyard! ✓ ROCKS ✓ UNCONSOLIDATED MATERIALS 2 FALL vs SLIDE vs FLOW: What’s the Difference? FALL: A material dropping through air, vertically or almost vertically. SLIDE: A material moving as a mass along a SLOPING surface (NOTE: there is NO internal motion within the mass itself). FLOW: Downslope movement of a material that also has internal motion, like a fluid (e.g., water, lava). Regardless of name, most MASS WASTING examples include two or more of the above (e.g., stones can SLIDE before FALLING off cliff). 3 Mass Wasting: Falls FALLS are an EXTREMELY RAPID or VERY RAPID form of MASS WASTING seen with rocks (ROCKFALL) and unconsolidated materials (SOIL FALL). ROCKFALLS: ✓ rapid downslope movement of dislodged rocks that originate above a steep enough slope ✓ rocks may experience falling, rolling, bouncing, and/or sliding ✓ falling rocks include broken chunks from BEDROCK OUTCROPS or BOULDERS coming down mountainsides/cliffs. ROCKFALL 4 Mass Wasting: Falls DEFINITIONS to ensure complete understanding: BEDROCK: ✓ hard, solid rock (i.e., CONSOLIDATED ROCK) under UNCONSOLIDATED surface materials such as SOIL, SAND or GRAVEL. ✓ can extend hundreds of meters below Earth’s surface The town of Bedrock ☺! ✓ exposed (i.e., above Earth’s surface) BEDROCK seen on mountain tops, along coastlines, in quarries and on plateaus (plateau = region of relatively level/flat ground with a higher ROCKFALL elevation than surrounding areas) OUTCROPS: visible exposures of BEDROCK occurring due to EROSION or TECTONIC PLATE activity. HYDROSTATIC PRESSURE: Pressure exerted by any fluid (e.g., water) in a confined space. 5 Mass Wasting: Falls FALLS are an EXTREMELY RAPID or VERY RAPID form of ROCKFALL MASS WASTING seen with rocks (ROCKFALL) and unconsolidated materials (SOIL FALL). Aside from factors discussed in previous lecture (i.e., SHEAR FORCE, SHEAR STRENGTH, ROCK STRENGTH), ROCKFALLS and SOIL FALLS can also be driven (in part) by: ✓ HYDROSTATIC PRESSURE and the presence of water in SOIL FALL BEDROCK cracks ✓ Freeze (i.e., expansion) and thaw (i.e., contraction) cycles ✓ heavy snowfall and subsequent melt ✓ plant roots in BEDROCK cracks. ✓ erosion by rivers/streams and/or wave action below (aka. UNDERCUTTING) ✓ quarrying (i.e., mining) activities. 6 Mass Wasting: Slides SLIDES can range from EXTREMELY SLOW to EXTREMELY RAPID. This form of MASS WASTING is seen with rocks and unconsolidated materials. There are several types of SLIDES to consider. ROCK SLIDE: A rock slide buries equipment. ✓ sliding motion of rocks along a sloped surface ✓ can range from VERY SLOW to MODERATEY FAST. ROCK AVALANCHE: ✓ sliding motion of rocks along a sloped surface ✓ larger rocks moving downhill quickly (at speeds on the order of meters/second) fragment (i.e., BREAK DOWN) to form smaller rocks/pieces that keep moving down hill/slope A rock avalanche. 7 Mass Wasting: Slides SLIDES can range from EXTREMELY SLOW to EXTREMELY RAPID. This form of MASS WASTING is seen with rocks and unconsolidated materials. There are several types of SLIDES to consider. SLUMP: SLUMP (ROCK LAYER SLIDE) on a CURVED SLOPE. ✓ sliding encountered when have thick (> 10 meters) layer/slice of UNCONSOLIDATED MATERIAL on a CURVED SLOPE. ✓ (may also observe same for rock layer on CURVED SLOPE) ✓ can occur EXTREMELY SLOWLY to MODERATELY FAST. ✓ mass of UNCONSOLIDATED MATERIAL gradually moves DOWNWARD and spreads OUTWARD. SLUMP (UNCONSOLIDATED MATERIAL SLIDE) on ✓ often caused by presence of excess water between a CURVED SLOPE. UNCONSOLIDATED layer and CURVED SLOPE 8 Mass Wasting: Slides SLIDES can range from EXTREMELY SLOW to EXTREMELY RAPID. This form of MASS WASTING is seen with rocks and unconsolidated materials. Moving downward and spreading outward. There are several types of SLIDES to consider. SLUMP: ✓ sliding encountered when have thick (> 10 meters) layer/slice of UNCONSOLIDATED MATERIAL on a SLUMP (UNCONSOLIDATED MATERIAL SLIDE). CURVED SLOPE. ✓ (may also observe same for rock layer on CURVED SLOPE) ✓ can occur EXTREMELY SLOWLY to MODERATELY FAST. ✓ mass of UNCONSOLIDATED MATERIAL gradually moves SLUMP (UNCONSOLIDATED MATERIAL SLIDE) on DOWNWARD and spreads OUTWARD. a curved SLOPE. ✓ often caused by presence of excess water between UNCONSOLIDATED layer and CURVED SLOPE 9 Mass Wasting: Slides SLIDES can range from EXTREMELY SLOW to EXTREMELY RAPID. This form of MASS WASTING is seen with rocks and unconsolidated materials. There are several types of SLIDES to consider. SLUMP: NOTE: SLUMP (UNCONSOLIDATED MATERIAL ✓ sliding encountered when have thick (> 10 meters) SLIDE) occurs above a CURVED SLOPE. layer of UNCONSOLIDATED MATERIAL on a CURVED SLOPE. ✓ can occur EXTREMELY SLOWLY to MODERATELY FAST. ✓ mass of UNCONSOLIDATED MATERIAL gradually moves DOWNWARD and spreads OUTWARD. ✓ often caused by presence of excess water between NOTE: SLUMP (UNCONSOLIDATED MATERIAL SLIDE) occurs above a CURVED SLOPE. UNCONSOLIDATED layer and CURVED SLOPE 10 Mass Wasting: Flows FLOWS can range from EXTREMELY SLOW to EXTREMELY RAPID. This form of MASS WASTING is generally associated with unconsolidated materials. There are several types of FLOWS to consider. CREEP: Evidence of ✓ very slow (mm/year to cm/year) movement of CREEP. soil/UNCONSOLIDATED materials down a SLOPE. ✓ prevalent on SLOPED surfaces ✓ an example of very slow FLOW but difficult to rule out some SLIDING in many cases. ✓ results in cracked roads Trees exposed to CREEP end up with curved lower trunk. Curved trunk is ✓ results in trees, fenceposts, grave markers, etc… that known as “PISTOL BUTT” lean DOWNHILL. 11 Mass Wasting: Flows FLOWS can range from EXTREMELY SLOW to EXTREMELY RAPID. This form of MASS WASTING is generally associated with unconsolidated materials. There are several types of FLOWS to consider. CREEP down a SLOPED surface can be enhanced by: Evidence of CREEP; downhill ✓ frozen subsoil preventing water drainage and wet material above movement of soil. slowly moves downhill ✓ freeze/thaw ✓ frequent wetting/drying of soil SOLIFLUCTION: ✓ CREEP triggered by freeze-thaw in colder climates. Evidence of CREEP; ✓ gradual movement of wet soil/wet UNCONSOLIDATED materials downhill movement of down a slope soil. 12 Mass Wasting: Flows FLOWS can range from EXTREMELY SLOW to EXTREMELY RAPID. This form of MASS WASTING is generally associated with unconsolidated materials. There are several types of FLOWS to consider. MUDFLOWS and DEBRIS FLOWS: ✓ often RAPID to EXTREMELY RAPID movement of MUDFLOW in Taiwan soil/sediment/UNCONSOLIDATED materials down a SLOPE. SEDIMENT/UNCONSOLIDATED MATERIALS SATURATED with water (e.g., due to heavy rain, ice/snow thaw) lose strength and ability to stay/stick together. SEDIMENT/UNCONSOLIDATED MATERIAL grains are pushed apart to extent that SATURATED MIXTURE can actually FLOW. MUDFLOW and DEBRIS FLOW in California, USA 13 Mass Wasting: Flows FLOWS can range from EXTREMELY SLOW to EXTREMELY RAPID. This form of MASS WASTING is generally associated with unconsolidated materials. There are several types of FLOWS to consider. MUDFLOWS and DEBRIS FLOWS: MUDFLOW Since SEDIMENT/UNCONSOLIDATED MATERIALS are SATURATED with water, then FLOW can occur even down GENTLE SLOPES. MUDFLOW: FLOW of (water) saturated UNCONSOLIDATED material whose grains are SAND-SIZED or smaller. DEBRIS FLOW: FLOW of (water) saturated UNCONSOLIDATED material whose grains are GRAVEL-SIZED or larger. DEBRIS FLOW 14 NOTE: Soil Particle Sizes SOIL particles vary in size. Scientists classify SOIL particles into GRAVEL/STONES, SAND, SILT and CLAY. CLAY particles are smallest with DIAMETERS < 0.002 mm. SILT particles have DIAMETERS ranging from 0.002 mm to 0.05 mm. SAND particles have DIAMETERS ranging from 0.05 mm to 2.0 mm. GRAVEL/STONES have DIAMETERS > 2.0 mm. Comparison of SAND, SILT and CLAY particles. 15 NOTE: Soil Particle Sizes SOIL particles vary in size. PORE SPACE is the space that exists between soil particles PORE SIZE: size of spaces that exist between SOIL PARTICLES. PORE SIZE IN: GRAVEL > SAND > SILT > CLAY. When all PORE SPACES (i.e., the entire PORE VOLUME) filled with water, SOIL is SATURATED. SATURATION of soil particles. 16 Mass Wasting: Flows There are several types of FLOWS to consider. MUDFLOWS and DEBRIS FLOWS: As DEBRIS FLOWS involve movement of LARGER particles than MUDFLOWS, they typically require STEEPER SLOPES than MUDFLOWS. DEBRIS FLOW, Switzerland. Example of a common process resulting in a DEBRIS FLOW: 1. UNCONSOLIDATED MATERIAL from a stream or riverbank collapses into the water. 2. UNCONSOLIDATED MATERIAL initially blocks water flow, then becomes SATURATED, ultimately mixing and FLOWING with water. 3. Major and sudden downstream FLOW of previously blocked water Riverbank collapse in Australia. + SATURATED UNCONSOLIDATED MATERIAL occurs, potentially causing significant damage (downstream). 17 Mass Wasting: Flows Example of a common process resulting in a DEBRIS FLOW: 1. UNCONSOLIDATED MATERIAL from a stream or riverbank collapses into the water. 2. UNCONSOLIDATED MATERIAL initially blocks water flow, then becomes SATURATED, ultimately mixing and FLOWING with water. 3. Major and sudden downstream FLOW of previously blocked water + SATURATED UNCONSOLIDATED MATERIAL occurs, potentially causing significant damage (downstream). Remnants of DEBRIS FLOW in Remnants of DEBRIS FLOW BC, Canada 18 Mass Wasting: Flows NOTE: MUDFLOWS and DEBRIS FLOWS can be caused by VOLCANOES! VOLCANIC ACTIVITY can suddenly melt ice/snow. Melted ice/snow can SATURATE UNCONSOLIDATED MATERIALS, causing said materials to FLOW downstream/down SLOPE with water. A LAHAR mixing with river water and FLOWING in Guatamala (1989) LAHAR: MUDFLOW and DEBRIS FLOW occurring on a VOLCANO or associated with VOLCANIC ACTIVITY. LAHAR is a flowing mixture of water, debris, volcanic and unconsolidated materials. ONCE MORE: LAHAR: MUDFLOW and DEBRIS FLOW occurring on a VOLCANO or associated with VOLCANIC ACTIVITY. 19 Mass Wasting: Topple TOPPLES are another form of MASS WASTING worth mentioning. TOPPLE: forward ROTATION and movement of a mass/slice of ROCK or UNCONSOLIDATED MATERIAL (earth, debris) out of a slope TOPPLE: forward ROTATION and can occur EXTREMELY SLOWLY to RAPIDLY movement of a mass/slice of ROCK or UNCONSOLIDATED MATERIAL triggered by factors including ice/snow thaw, WEATHERING, UNDERCUTTING, and FLUVIAL EROSION. results in formation of debris pile (i.e., debris cone or TALUS CONE) at base of slope. TALUS CONE: A debris or rock pile at base of slope/mountain/hill. DEBRIS: loose, natural material consisting especially of broken pieces TALUS CONES at the of rock. base of multiple slopes 20 Mass Wasting: Topple TOPPLES are another form of MASS WASTING worth mentioning. TOPPLE: forward ROTATION and movement of a mass/slice of ROCK or UNCONSOLIDATED MATERIAL (earth, debris) out of a slope ROCK TOPPLE can occur EXTREMELY SLOWLY to RAPIDLY triggered by factors including ice/snow thaw, WEATHERING, UNDERCUTTING, and FLUVIAL EROSION. results in formation of debris pile (i.e., debris cone or TALUS CONE) at base of slope. DEBRIS/UNCONSOLIDATED TALUS CONE: A debris or rock pile at base of slope/mountain/hill. MATERIAL TOPPLE 21 Mass Wasting: Topple DEFINITIONS to ensure complete understanding: EROSION: geological process in which materials are worn away and potentially transported naturally by wind or water. UNDERCUTTING: EROSION of material at the foot of CLIFF or steep SLOPE. UNDERCUTTING caused by waves. FLUVIAL EROSION: EROSION caused by flowing water (on the adjacent ROCK/UNCONSOLIDATED MATERIALS. WEATHERING: breaking down or dissolving of ROCKS and MINERALS on Earth’s surface. WEATHERING driven by factors such as water, ice/snow, acids, salts, plants, animals and temperature fluctuations. Once ROCKS broken down into bits of rock and minerals, EROSION can transport these bits away. An UNDERCUT slope. 22 Mass Wasting: Topple DEFINITIONS to ensure complete understanding: How are EROSION and WEATHERING different? Both involve breakdown of materials. However, EROSION generally implies worn-down materials are also transported away. In WEATHERING – materials are also worn down but not transported away. That’s disappointing! 23 Mass Wasting: So How Do we Define Landslides? Ultimately, let us assume a LANDSLIDE = MASS WASTING (aka. SLOPE FAILURE) We will assume they are same. Thus, a definition of LANDSLIDE based on what we have seen: LANDSLIDE: failure and downslope movement of rock or Flood followed by LANDSLIDE in Saguenay-Lac-St-Jean, unconsolidated materials due to gravity. Quebec (2023) movement of ROCK or UNCONSOLIDATED MATERIAL (debris or earth) MASS down a SLOPE due to gravity. 24 See you next lecture!!! Landslide in St. Jude, Quebec kills 4 in 2010 25