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3.3 Fluvial Processes in The Tropics (EROSION (Hydraulic Action :evergreen…

- - - - Sediments transported by traction and saltation and are moving close to/along the bed of the river (coarser materials: gravels, cobbles and boulders)
        
        Moves intermittently by sliding, rolling and hopping along the bed (all 3 are usually simultaneous)
        
        Affected by velocity, degree of turbulence, amount and character of bed material
        
        Rate of movement is much slower than water flow as particles move intermittently (at regular intervals)
        
        Moves singly or in groups, where larger grains move faster
        
        Bedload constitutes a small fraction of the total sediment load of most rivers
      - Saltation : smaller grains
        
        A combination of traction and suspension and transported at lower flow velocities
        
        Particles first uplifted by turbulent forces and then transported along arc-shaped trajectories where gravity then pulls them towards the channel bed where they impact other particles in turn setting them in motion
      - Traction : larger rocks
        
        Rolling or sliding of rocks along river bed during turbulent flows
        
        A zone of slow water at the bed due to friction of the channel is created and as water flows over the particle, the flow velocity ↑ rapidly and particle is rolled up in a spiral motion (momentum of fast water is transmitted to the slower moving boundary water)
        
        Leads to the collision of sediments and abrasion of the channel → smoother, rounder particles
        
        Near the source of the stream where slope is steep and coarse sediments are available through mm
    - - Suspension
        
        Transportation of fine materials like silt or clay, held up by turbulence within the body of water
        
        Once particles are entrained, little further energy is required to transport them and they are carried at the same speed as the water
        
        Reduces the inner turbulence of the flowing water, ↓ frictional losses and making the channel more efficient
        
        Carried near the stream bed, and lesser nearer to surface
        
        Forms the greatest part of the total load and amount increases towards the mouth of river (end of river to ocean)
- - - - Process where tiny bubbles of air implodes in fissures and cracks of the channel banks due to pressure changes in fast flowing waters
        
        Occurs when there is extreme turbulence in rapids, waterfalls or floods
        
        The resulting shock waves weaken bank materials (esp if they are unconsolidated sands and gravels), eventually leading to bank collapse
        
        Can damage solid rock and is accompanied by abrasion
        
        Only in rock-cut channels or upper courses where bedrock is exposed
    - - Removal of loose material from alluvial channels by the impact of water alone
        
        More common in middle and lower courses and most effective in areas of non-cohesive material
        
        Observed on meander banks where high velocity flow occurs close to the outer bank
        
        Occurs on the river bed or banks and involves a lift and drag force
        
        Lift force caused when water is accelerating over protruding grains, and when flow velocity ↑, critical erosion velocity/entrainment velocity is reached (where forces going to move particles are exactly balanced with resisting forces of friction)
        
        An ↑ in velocity beyond CEV would result in particle movement
        
        Drag force carries the lifted particles downstream
        
        Particles usually sand or fine gravel, only unconsolidated sediments and little effects on hard resistant rocks
    - - Process in which sheer force of water smashes bedrock without any aid from particles
        
        Common in rock-cut channels or upper courses