Hillslope Processes GPH 111 Hillslope Processes Game Plan: What are HILLSLOPE PROCESSES and how do they fit within gemorphology “Innie” versus “Outie”
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Hillslope Processes GPH 111 Hillslope Processes Game Plan: What are HILLSLOPE PROCESSES and how do they fit within gemorphology “Innie” versus “Outie” Hillslopes Major TYPES of hillslope processes What determines the “slope of hills”? - Angle of Internal Friction (Repose) - Cohesion What do HILLSLOPES do? What are Hillslope Processes? The collective processes that lead to the transport of weathered debris through gravity from mountain slopes to fluvial systems Hillslopes within Geomorphology Sierra Estrella Mountains and the Gila River What is the Weathering Front? The boundary between weathered bedrock on the surface and fresh bedrock below Hillslope Processes in Geomorphology “Innie” Hillslopes “Outie” Hillslopes Most of the rainwater absorbed as groundwater Most of the rainwater flows along the surface Hillslope Processes Debris Flows Creep Sheetwash Rockfall Mud Flows Landslides Rainsplash Critters and Plants Rock Avalanches Hillslope Processes 1. 2. 3. Water “Innie” ________ – Very slow continuous movement of soil down slope ________ – Rapid movement of a cohesive mass of soil or bedrock ________ – Largest landslides that occur on the planet, 4. 5. 6. Water “Outie” ________ – Flowage of coarse rock debris intermixed with water ________ – Flowage of fine sediment intermixed with water ________ – Rapid individual freefall of rock debris off a steep face volume greater than 5 million cubic meters Water “Innie” and “Outie” 7. ________ – Removal of loose surface materials by overland flow 8. ________ – Force of raindrops on bare soil causing erosion 9. ________ – Disturbance from plant growth or animal activity Hillslope Processes Water “Innie” Creep – Very slow continuous movement of soil down slope Landslides – Rapid movement of a cohesive mass of soil or bedrock Rock Avalanches – Largest landslides that occur on the planet, volume greater than 5 million cubic meters Water “Outie” Debris Flows – Flowage of coarse rock debris intermixed with water Mud Flows – Flowage of fine sediment intermixed with water Rockfall – Rapid individual freefall of rock debris off a steep face Water “Innie” and “Outie” Sheetwash – Removal of loose surface materials by overland flow Rainsplash – Force of raindrops on bare soil causing erosion Critters and Plants – Disturbance from plant growth or animal activity Hillslope Processes Water “Innie” Creep Landslides Rock Avalanches Water “Innie” - Creep Slow downhill movement of soil under the influence of gravity and soil dynamics (shrink– swell or freeze–thaw action) Probably the dominant hillslope process! More Creep Water “Innie” - Landslides The rapid transport of sediment or bedrock down slope. Tend to fail as a slump-rotational or a translational movement. “Slump-Rotational” “Translational” Like taking a scoop of ice-cream out of hillslope More like a board sliding past another board Landslide Damage California El Salvador following a magnitude 7.7 earthquake, 2001 Landslide Damage The La Conchita Landslide,2005 (10 fatalities, 13 homes destroyed) The La Conchita Landslide,1995 (destroyed 9 homes) The biggest landslide events are termed “Rock Avalanches” Volumes larger than 5 million cubic meters “Rock Avalanches” “Rock Avalanches” “Rock Avalanches” Hillslope Processes Water “Outie” Rock Fall Debris Flow Mud Flow Water “Outie” - Rockfall Tend to be minor events, but collectively can generate substantial landscape change in some locations. Water “Outie” - Rockfall An important process on the Colorado Plateau. Debris Flows “Outie” Transport of coarse rock debris in a sediment rich current. Debris Flows and Us Debris Flows and Us Venezuela, 1999 after 8 inches of rain followed in two weeks by 36 inches of rain. Debris Flows “Outie” Phoenix Mts. Death Valley N.P. Cool Sunset from Death Valley Mudflows “Outie” Mostly fine grains intermixed with water involved in flowage down slope Mudflows “Outie” http://www.youtube.com/watch?v=ZVYGJYnJTi0 http://www.youtube.com/watch?v=51C7vEAVbxk&feature=PlayList&p= 5E960EA57644667E&playnext=1&playnext_from=PL&index=2 Hillslope Processes Water “innie” and “Outie” Overland Flow Rainsplash Critters and Plants Water “Innie” and “Outie” – Overland Flow Erosion of thin layers of sediment by broad continuous sheets of running water Water “Innie” and “Outie” Rainsplash Rainsplash: The transportation of sediment through the sudden impact of a water droplets on unconsolidated sediment. Water “Innie” and “Outie” - Critters and Plants “Sediment flux by gopher bioturbation was the dominant hillslope process.” Gabet and Dunne, 2003 Water “Innie” and “Outie” - Critters and Plants Root Throw: As trees fall over they transport sediment nestled in the roots. What Determines the Slope of Hills Angle of Internal Friction (similar to the “angle of repose”) Cohesion (similar to stickiness and includes affects from vegetation, water, and clay) Angle of Internal Friction Angle of Internal Friction: The angle of rest for hillslope particles just prior to failure. Usually steeper than the “angle of repose”. Angle of Internal Friction Importance of grain size 32.8° Large grain sizes 23.9° Smaller grain sizes What about particle angularity? Activity Instructions: Cohesion Materials: tray or tub, cup of water, 20 toothpicks, and two cups of dry sand Step 1. With the dry sand, try to get the material as steep as possible, and then measure the slope with the inclinometer. Step 2. With the use of twenty toothpicks, try again to get the sand as steep as possible, and then measure the slope with the inclinometer. Step 3. Dampen the sand with a small amount of water, and then again try to get the sand as steep as possible while measuring your results Step 4. Soak the sand in water, and then again try to get the sand as steep as possible while measuring your results Step 5. Consolidate your results and then hypothesize in your table groups the reason for your results. Cohesion - Moisture Wet – very unstable Dry – stable Moist – very stable Cohesion Water also Increases the Weight of Hillslopes Marcus Landslide (5 million cubic meters) Cohesion - Plant Roots Cohesion - Plant Roots Mazatzal Wilderness Fire (Summer, 2005) Plant Roots (Gone) “Dry Ravel” Cohesion - Clay Clay has very low frictional properties and absorbs several times its normal volume when wet, leading to hillslope instability Black Canyon City Landslide What do Hillslopes DO? Transfer weathered rock to the fluvial systems! Things to Know: What are Hillslope Processes? How do they fit within geomorphology? What is the weathering front? Differences between innie and outie hillslopes An understanding of the different innie, outie, and “innie and outie” hillslope processes including: creep, landslides, rock avalanches, debris flows, mud flows, rockfall, sheetwash, rainsplash, critters and plants Understand what determines the slope of hills and how: cohesion and angle of repose Help: Ask Dr. Douglass