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Unformatted text preview: Topic 12 Hillslopes 1. Introduc?on Slopes (interfluves) basic element of landscape Hillslope erosion may : involve weathered material or rock involve water to varying degrees be quasicon?nuous or episodic Colluvium: Material deposited by hillslope processes 2. Water Erosion on Hillslopes Rainsplash Terminal velocity: 6 mm raindrop ~ 9 m/s Kine%c Energy: KE = m v2 KE is imparted by raindrop to soil surface Geography 211 (Fall 2009) 11/9/09 1 Driving forces: KE of raindrop (mass and velocity) Slope gradient Resis?ng forces: Soil and vegeta?on proper?es Vegeta?on vs. bare surface Grain size (size vs. cohesion) Compac?on Slopewash SheeYlow Rills Driving forces: discharge (downhill changes) slope gradient Resistance factors: vegeta?on grain size compac?on Rainsplash and overland flow: Rainsplash effec?ve if no overland flow Rainsplash decreases infiltra?on Increase surface runoff Increases flow turbulence Geography 211 (Fall 2009) 11/9/09 2 = g d S : shear stress (N/m2) g: gravity (m/s2) : density of water (kg/m3) d: depth (m) S: gradient d downslope Therefore downslope xc: cri?cal loca?on/depth (driving stresses = surface resistance) Above xc : belt of no erosion Below xc: erosion xc 3. Mass Movements (Introduc?on) Types of Movement Slide/Slump (e.g., landslide) Flow (e.g., debris flow) Heave (e.g., creep) debris flow earthflow Geography 211 (Fall 2009) 11/9/09 3 4. Slow Mass Movement Soil Creep Heavetype movement: heave soil moisture/freezesthaw Deforma9on: slow, plas?c deforma?on Par%cle creep Con%nuous creep 5. Fast Episodic Mass Movements rapid, episodic downslope movement of rock/sediment under influence of gravity Failure plane: sediment over bedrock/?ll bedding planes (rock) fractures (rock) Planar Curved Geography 211 (Fall 2009) 11/9/09 4 Basic Mechanical Principles Driving force (shear stress): = m g sin a varies from: 0 on horizontal slope maximum on ver?cal slope Resis?ng force (shear strength): S.S. = c + (n u) tan angle of internal fric?on = f (size/shape/arrangement of par?cles) n = m g cos a n varies from: maximum on horizontal surface 0 on ver?cal slope Safety factor = Resis?ng forces Driving forces Stable >1.3 Unstable <1 Condi?onally stable 11.3 Geography 211 (Fall 2009) 11/9/09 5 A. Low moisture content Rockfalls Rockfall: Glacier Na%onal Park, U.S.A Rockslides B. Moisture content important Debris Slides (someBmes called landslide) rela?vely cohesive mass of sediment low to moderate water content transla?onal/rota?onal Rota%onal debris slide Debris slide scar Transla%onal debris slide Debris Avalanches (Sturzstroms) high velocity long travel distances rock/sediment ice, water may fluidize air layer? Huascaran debris avalanche: Before and after Geography 211 (Fall 2009) 11/9/09 6 C. Higher Moisture Content Mudflow mainly clay channelized high water content Debris Flow fine/coarse debris channelized high water content Debris flow Debris flow Mudflow: Mt. St. Helens Earth flows are fairly slow, occurring over a few hours or so
Earth flows differ from mudflows and debris flows in that they: tend to be slower are not confined to channels have a lower water content 6. Talus Slopes and Angle of Repose Talus: deposit of large rock fragments talus cone talus apron Talus Talus cones Geography 211 (Fall 2009) 11/9/09 7 Talus cones Geography 211 (Fall 2009) 11/9/09 8 ...
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- Fall '08