Slopes and Mass Movements - Lecture Notes | GEOG 2051, Study notes of Geography

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Chapter 13: Slopes and Mass Movements  Slope - curve in incline surface that represents the boundary of a landform o Steepness - how quickly the elevation increases o Angle - 0 to 90 degrees o Gradient - Tangent (rise/run) x 100 = a percent o Forces  Gravity F g = the force of gravity  Friction  When one object slides across another, some of the energy is lost and it makes it more difficult for the object to move  Particles in motion have to overcome the element, the amount of friction depends on the surface of the particles  Cohesion  Water sticks together in droplets because of forces attracting them together  Why you choose wet sand for a sand castle  Increase slope stability and allow them to maintain steeper angles  Water causes slope to be unstable  Weight force o Makes it less stable o Water controls the type of slopes of mass movement that is going on and it controls how much sediment can be moved  Lubricant o Reduces friction and allows movement o Makes it less stable  Buoyancy o Supports material weight o Reduces friction o System  Inputs - Weathering  Outputs - Transport  Inputs are caused by weathering; over time the slope builds up and becomes steeper and steeper  Outputs are caused by material that gets transported off of the slope through mass movement.  Develop towards Equilibrium = Angle of repose  Slopes will tend to move to an equilibrium involving their steepness  The steepest angle that a given material can maintain against gravity  The angle at which a material can stand on its own o Slope elements  Waxing slope  Increase of the slope  Inputs  Starting from the top, to wax means to increase, tends to act at the source area  Free face/fall face  Transport zone  Immediately below the waxing slope, steepest portion of the slope  Waning/Debris slope  Where debris collects and is taken out of the system 1  Deposition zone  Debris – in between free face and waning slope, where a lot of the material going to the free fall area to the waning slope  Waning – slope is getting less and less steep, where material tends to accumulate  Mass Movement o Types  Classified by speed & water content  Falls/avalanches  Fastest forms of mass movement  Only slowed down by friction  Do not require water but it may act as a trigger  In a rock fall, the material completely leaves contact with the slope, falling directly through the air; only thing to slow it down is the friction of the air it is moving through  Avalanches reach high speeds of 70-80 mph o Difference is it retains some type of contact with the slope o Rocks bouncing down the hill with friction of land to slow down the fall o A little slower than a true fall because the slope is not as steep  Slides  Intermediate speeds and water content  Material for the most part moves as a cohesive chunk moving as a unit down the hill  Vegetation stays intact  Flows  Involves a large amount of water, like a river of mud into gravel and rock  Can reach rapid speeds  Materials maintain continuous contact with the ground  Mixture of earth materials and water that gush down a slope  Creep  Slowest movement, not something that one will notice – mm or cm/year  Produces evidence over a large amount of time  Thawing, drying, or cooling of the surface sediment causes it to contract with the help of gravity  The combination of expansion and contraction of slope sediment is the gradual downward movement of the slope; it creeps  Slope Development Influences: o Rock strength - The stronger the rock, the steeper the slope it will be able to maintain o Climate  Arid - Very angular, slow rates of development  Humid - Temperature slope, more weathering, develops quicker o Process  Weathering limited  How sediment moves off the slope, it is being transported faster than it can be replenished  Greater output  Slow rate of weathering  parallel retreat  Transport limited 2  Groundwater - Some portions provided to the river channel, slowest pathway  Drainage patterns o Dendritic  Tree-like  Top right to bottom left  Consistent  Very energy efficient  Gathers stuff from an area and bringing it to a point  Allows river system to cover drainage basin with smallest channel o Parallel  Top right to bottom left (downhill)  Consistent  Steep slopes, stronger downslope trend o Radial  Out from center  High elevation in middle  Tributaries falling down a mountain in all directions o Trellis  Many ridges; occurs in heavily-folded topography  Stream capture  One tributary captured water from one valley and shifted it over to another one (piracy) o Deranged - Doesn’t really seem to have a logical explanation disturbed landscape  Flow in channels o Water sources  Variability o Flow regimes - how persistent the flow of water is over time  Ephemeral  Water flows through the channel once every several years; decades  Intermittent - Water only flows in the channel for part of the year  Central California  Perennial - Year-round flow  Mississippi river o Discharge  The volume of water flowing past a point in a river channel per unit of time  Measured in cubic feet per second of cubic meters per second  Q = Width x Depth x Velocity o Exotic streams  Natural, anthropogenic  Sediment transport o Competence - The largest grain size capable to be moved o Capacity - Total amount of sediment it is capable of carrying o Erosion - Picking up sediments from the bed of the river o Translocation - Moving it to another location o Deposition - Putting it back down o Solution  Dissolved load o Suspension 5  Larger, can range in size from clays to silts and finer portions of sand sized grains, particles held up by the turbulence of water  Suspended load o Bedload - Particles maintain some degree of contact with the bed  Saltation - Hop along the ground in a series of bounces  Traction - Maintain continuous contact with the bed in a rolling fashion  Channel types o Braided  Large sediment supply  Large variability in flow  Mountainous areas common to find these  Multiple channels  Water flow changes season to season  Tasman River, NZ o Meandering  Faster flow on outside  Slower flows on inside  Single channel  Less dynamic  When water flows into a curve, the same amount has to flow out o Meander migration  As water goes around a curve one side travels faster causing more erosion, the other side is going slower and sediment is being deposited because of less velocity and less distance to travel o Point bar  Land floor slowly building up, where soil is being deposited  Depositional zone on inside of meander curve o Oxbow lake - Meander river cut off  False River o Flood plain  Formed from a meandering channel o Levee  Form of protection built by people, some are natural  When water escapes over banks, deposition occurs o Yazoo tributary  Named after Mississippi’s Yazoo River  Flows along floodplain near by until they reach a levee that has a channel they can reach o Meander scar - Only visible from above, left behind by meandering channels  Stream gradients - Changes over time, plays important role, determines how much energy the river has to do work o Base level - Bottom of drainage basin where river empty outs o Potential energy  Energy of position, energy comes from objects’ position relative to other things o Kinetic energy o Longitudinal profile  Curve of slowly decreasing steepness down to the base level o Graded stream - Rivers tend toward the equilibrium gradient 6 o Nickpoint  Where we find waterfalls into terraces  Generates a lot of turbulence  Waterfall o Entrenched meanders  Colorado Plateau  Meander curves that have been cut deep down  Can be created during stream rejuvenation o Alluvial terraces - Series of steps or terraces that run on either side of the river valley  Deltas - Depositional features created where a river enters a larger body of water, i.e.: the ocean: velocity drops, capacity drops, and it deposits all the sediment, forming the deltas o Sediment supply - Deltas that are controlled by sediment o Wave energy - Sediment inputs relatively low; long beaches o Tidal flows  Triangle  Area dominated by multiple distributaries  Fly river  Tidal range can reach very deep o Delta characteristics  Found at the mouth of a drainage basin, depositional features created by rivers deposits of sediment, triangular  Water velocity drops at the mouth of the river, the rivers competence and capacity drops  Flooding o Statistical prediction o Recurrence interval o Hydrograph  Chart that shows change and discharge over time o Lag time  The amount of time that it takes for the inputs (rain) to filter through the whole system into the river where your peak discharge occurs (time between peak rain and peak flood) o Influence of drainage basin characteristics on hydrograph  Can cut off specific areas to get water through quickly and reduce lag time Chapter 15: Aeolian Process and Deserts  Requirements o Sediment size o Moisture – little or none; arid is ideal o Vegetation – little or none  Areas affected o Agricultural areas – survival o Sandy coasts – tourism o Loess – areas that are under-grained by loess (fine grain and sand) o Ergs – deserts  Sediment transport o Air vs. water density o Air is 1000x less dense than water, therefore less effective at moving sediment o No solution transport in getting air 7  Waves o Height - Difference in elevation between the crest and the trough o Length - Distance between crests o Period - Time it takes for a wave to travel one wavelength o Steepness - Same as hill, if slope change in elevation over distance, wave height/wavelength o Generation – where waves come from  Fetch - The distance of open water that is available for wind to blow across  Wind speed  Duration o Orbital motion  Path of water particles, in the shape of a circle  As you move deeper and deeper the wave orbits get smaller  Determines the difference between offshore and nearshore o Shoaling transformation  Sequence of wave that occur as a wave moves near shallow water  Change in height, length, steepness o Breaking o Wave refraction  Bending of crest; different depth = different speed  Energy distribution at shore  Big areas – energy decrease  Small areas – high energy  Longshore currents - flow along the shore that are generated by waves that approach the shoreline at an angle o Beach drift - longshore currents are parallel to coastline, this is why you drift parallel from shore  Tsunami o Marine earthquakes/landslides o Shallow water (very long, low height) o Large fault underwater on one of the plates are rapidly uplifted and then the ripples occur  Primary/secondary coasts o Primary – landforms created by terrestrial processes o Secondary – have been substantially modified by marine processes  Erosional features o Cliffs & wave-cut platforms  Waves cut slopes more and more o Terraces o Arches & Stacks  Depositional features o Beaches  Storm- fair-weather profile changes  Extremely dynamic, trying to find and equilibrium  Fair-weather: long period of swell laves o Spits - Extensions of shoreline from longshore transport, parallel o Tombolo’s - Perpendicular to shoreline; a bridge as a longshore transport shadow zone o Barrier islands  Sub-environments 10  Formation - Spit extension  Rollover - Entire form gets pushed  Washover fan  Peat outcrop 11