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Geography Lecture 9 (8. Hjulstrom Curve (Fluvial processes (Erosion…

- - - - Largest and finest particles need higher CEV
        Clay and boulders need very high CEV
        Large particles are heavy, and clay has high cohesiveness
    - - Settling velocity increases with particle size
        Large boulders have high settling velocity
        Clay has low settling velocity
    - - Range of velocities at which a particle will be transported is the largest for the finer particles and narrows as sediment size increases
- - - - Hydraulic radius
        
        Ratio of the channel's area to the length of the wetted perimeter
        
        R = A/P
        When R is high, friction is low, velocity is high
        
        Affected by the amount of discharge
        Bankfull = most efficient
        Below bankfull or flood = inefficient
      - Channel slope
        
        Gradient of the stream surface per unit distance
        
        Channel slope decreases downstream
        BUT velocity increases at lower course due to other factors
        
        Volume of discharge increases at lower course
        
        Sediment load is smaller at lower course
        
        Steeper slope = higher velocity
      - Channel roughness
        
        Determined by the channel load and channel surfaces
        
        Channel roughness decreases downstream as load sizes decrease and channel gets smoother
  - - - Bedload
        
        Large rock fragments - roll or slide
        Found near the source
        Bigger rocks moved by saltation (bouncing)
        Smaller rocks moved by traction (rolling)
      - Suspended load
        
        Small particles suspended in water
        Greater turbulence, greater size of particles
        Usually forms the most load
        Amount increases towards mouth of river
      - Dissolved load
        
        Dissolved minerals in water
        Channel water may contain acids that dissolve rocks and carry minerals (limestone and calcium carbonate)
    - - Upper course has more bedload (less energy to erode)
        Lower course has more suspended load
    - - Competence
        
        It is a measure of the largest rock particle the river can transport
        
        Higher velocity or turbulence = higher competence
      - Capacity
        
        It is measure of how much load the river can transport
        
        Higher discharge = higher capacity
      - The calibre and amount of load the river can carry depends on its competence and capacity
  - - - Irregular eddy motions (whirlpools)
        Usually in upper course
        Leads to more vertical erosion
    - - Layers of water following a specific path
        Maximum velocity is usually closer to the surface
        Occurs less frequently, usually in gentler gradients
        Increases competency and capacity, but not erosion
- - - - Depends on volume and elevation
        (Q = A*V)
    - - Depends on volume and speed
        (1/2mv^2)
- - - - Attrition
        
        Material carried in the river knock against each other and become smaller and smoother as they continue to knock into each other
      - Solution
        
        Acids carried in the river may dissolve material from the channel and carry it along
      - Hydraulic action
        
        When turbulent water colludes with the riverbank, the water is forced into the cracks in the riverbank. This compresses the air and may lead to cavitation. This dislodges rock fragments and usually erodes the foot of the riverbank. It may also cause a collapse of the riverbank above as the support has been eroded away
      - Corrasion
        
        Wearing the material of the channel by the load that the river is carrying. This usually causes downcutting and may result in pothole drilling
  - - - Suspension
        
        Fine particles are transported by the water without touching the river channel. More turbulent flows can carry larger particles
      - Saltation
        
        Skipping motion of sand-sized particles. Sand bounces and knock other sand grains which sets off a chain reaction
      - Traction
        
        Coarse bedload particles slide, roll and hop along the river bed. Speed varies, and it can only happen when energy is high
      - Solution
        
        Eroded rock minerals are dissolved and carried away as ions. It needs no mechanical energy and takes place everywhere
        In humid tropics, chemical weathering is efficient, and transport of minerals is very important
  - - - Sudden decrease of gradient
        
        River enters a water body with lower speed
        
        Decrease in volume of water in channel
        
        Increase in friction (mass movement)
        
        River broadens out
        
        Precipitation
        
        Flocculation (clay gathers and deposits)
- - - - Higher GPE, lower KE
        Erodes downwards
        Narrow valleys
    - - Medium GPE, medium KE
        Erodes all ways
        Wide + deep
        Valleys are U-shaped
    - - Low GPE, medium KE
        Highest discharge, lateral erosion
        More deposition
        Flood plains and levees
  - - - Shows changes in height of the river channel from source to mouth
    - - "Ideal long profile"
    - - [find an image]
      - Precise shape is defined by many factors
- - - - Upper course, water flows between boulders and turbulence is high. Bedload is taken up in suspension or rolled around, and this causes downcutting.
    - - When a river meets less resistant rock, the rock will be worn away faster. The water then continues wearing away at the softer rock, eventually forming a waterfall. Turbulence can cause the water to curl back, and erode the bottom of the waterfall. Over time, the waterfall may become less stable and collapse due to the lack of support
    - - Gradient suddenly increases or when the river flows over a series of gently dipping bands of harder rock. It increases turbulence of a river and thus, erosive power
  - - - Land that is susceptible to floods
        Wetted perimeter increases while hydraulic radius decreases
        Friction increases and fertile material is deposited
        Edge of the floodplain is called the bluff line
    - - The largest deposited material from floods forms a natural barrier along the river
        Subsequent low-discharge periods will cause material to be deposited at the riverbed, which increases the risk of flood
    - - Made of fine sediment, deposited in an area of slow moving water
        The meeting of fresh and salt water causes flocculation, which causes clay to coagulate and deposit.
      - Shapes
        
        Arcuate
        Rounded, convex shape
        (River Nile)
        
        Cuspate
        Material is spread evenly
        (Tiber)
        
        Bird's Foot
        Many distributaries
        (Mississippi)
        
        Lacustrine
        When a river enters a lake and drops material. Eventually filling up the lake
        (Kura, Volga)
    - - Similar to delta, but only found at arid mountain feet
        They are also intermittent