River course and processes

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  • Channel Characteristics and Processes
    • WETTED PERIMETER- the length of bed and banks the river is in contact with
      • EROSION
        • The break up of solid material on the bed of a river
        • HYRAULIC ACTION-  where the water forces its way into small cracks and faults in the rock creating small explosions and breaking the rock apart
        • ATTRITION- When rocks hit together and break apart, making them smaller
        • ABRASION- the sandpaper effect of material carried by the river scraping at the bed and banks
        • Vertical/Lateral
        • The movement of particles from where they are eroded to where they are deposited
        • TRACTION- where large rocks are rolled along the river bed
        • SALTATION- when pebbles are bounced along the river bed
        • SUSPENSION- when small particles of sand etc. are held up by the water pressure and tension and carried by the river
        • SOLUTION- when minerals etc. are dissolved and carried in the river
        • CAPACITY- the total amount of material carried by the river, i.e. the total volume of the load.
        • COMPETENCE- the diameter of the largest particle a river can carry for a given velocity
        • The laying down of solid material on the river bed
        • REASONS FOR DEPOSITION- 1. Reduction in gradient  2. Reduction in discharge  3. Increased calibre (load size)  4. There is shallow water (inside a meander)  5. The river floods and overtops its banks
      • HIGH POTENTIAL ENERGY, fixed by the gradient and altitude in relation to base level
        • The river doesn't have the erosive capacity to get through the hills (spurs), but erodes downwards instead and around the spurs.  It's path therefore is determined by the hills
        • Vertical erosion occurs until sub-aerial weathering causes a V-shap to be formed. Gradient is dependent on the climate, rock type and vegetaion of the area.
      • RAPIDS
        • Develop where the gradient of the river bed increases without a sudden break like a waterfall or where the river flows over a series of gently dipping bands of harder rocks.
        • Rapids increase the turbulence of the river and hence the erosive power
      • POTHOLES
        • Boulders sit in the river bed and create a swirling eddy current as the water flows past as the river isn't strong enough to move the boulders by traction
        • Commonly found under waterfalls or rapids where hydraulic action and abrasion are significant
        • Formed as a result of a change in gradient, a band of resistant rock occuring across the course of a river, the edge of a plateau or the rejuvination of the area giving the river more erosional power
        • Forms a plunge pool underneath, an overhang on the ledge and a gorge as the waterfall moves back
      • KINETIC ENERGY, energy due to movement of water which converts potential to kinetic. Determined by volume of water and slope.
        • Occurs when rivers are carrying vast amounts of sediment and the river's velocity drops so the sediment is deposited as the river can no longer carry such a large load.
        • The river is forced to divide into many small, winding channels that eventually rejoin to form a single channel
        • The sudden drop in velocity may be from a fall in gradient as the river gets closer to the sea
        • When a river's capacity exceeds the channel size, the river bursts it's banks. the flat land either side of the river (the floodplain) will be flooded
        • The increase in wetted perimeter and thus reduction in hydraulic radius means that the friction increase so the velocity decreases. This causes the river to drop any small sand or sllt which often makes the floodplain very fertile land.
      • LEVEES
        • Naturally raised embankments on either side of the river, formed form past flooding depositing material there.
        • Because of the sudden fall in velocity during flooding, the heaviest material (e.g. pebbles and gravel) are dropped first, closest to the river. This material builds up to create a levee.
      • DELTAS
        • When the river reaches the sea, the energy of the sea is absorbed by the slower moving water. This causes it to deposit it's load.
        • The alluvium (deposits) build up on the sea bed until they are higher than the sea, partially blocking the mouth of the river. this causes the river to braid into several channels forming a delta.
      • MEANDERS
        • 1. Lateral erosion begins as the gradient reduces.
        • 2. The river follows the path of least resistance so it's often forced to follow a winding route.
        • 3. The current swings to the outside of the bend and concentrates the erosion there. This is called the helicoidal flow
        • The river flows fastest on the outside of the bend so there a river cliff is formed from erosion.
        • The river flows slowest on the outside bend so deposition occurs, creating a slip off slope
        • At a meander neck, erosion may occur to such an extent that the meander neck is broken. This is now the easiest path for the river so it will take this course.
        • Deposition will eventually separate the river from the original path, creating an Oxbow lake.
    • HYDRAULIC RADIUS=Cross sectional area/wetted perimeter, the higher it is the less friction there is so the more efficient the river is


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