Depositional landforms


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Depositional landforms

  1. 1. Depositional Landforms •Floodplains •Levees • Braiding •Delta •Alluvial fan
  2. 2. Floodplains • What ? – A wide and flat valley – Alluvium deposits on riverbanks build it up during repeated flooding – Provides fertile alluvium for farming
  3. 3. How? • Form when river overflows its banks during flooding and spread large amount of sediment over the banks • When flood subsides or over, there is a decrease in volume so that river deposits its load. • Repeated flooding result in materials deposited on the riverbanks forming a floodplain
  4. 4. Levees What? It is a natural embankment on the riverbanks Where? Form when coarser alluvium is deposit along the riverbanks due to flooding
  5. 5. How? • Form during flood • The coarser allluvium is deposited near the banks and finer alluvium deposited further from the banks • The accumulation of the coarser alluvium along the river banks formed a natural raised banks called levees
  6. 6. Braiding • A river subdivides into many smaller streams within the channel • Small islands / eyots separate each stream • Eyots are made up of deposited loads such as gravels and coarse sands
  7. 7. How? • Depends on load-discharge relationship • Short period of the year, some rivers carry a heavy load which the rivers have difficulty in removing • When the river level falls rapidly, competence and capacity reduced – channel deposit materials within the channel • Channel become broad and shallow with a low hydraulic radius
  8. 8. (Braided river in southern Alaska.)
  9. 9. EYOTS
  10. 10. (Braided river at junction of Gakona and Copper River, Alaska)
  11. 11. Main Characteristics 1. Small islands or eyots of deposited material within the channel. 2. A network of small intertwining channels converge and diverge around the eyots. 3. At times of high discharge, braided streams carry a large load of coarse material. 4. Braiding is a characteristic of streams with a variable discharge 5. Very common in Semi-arid region where torrential rainfall will lead to temporary stream with high discharge.
  12. 12. Delta What? • A piece of flat land that is made up of sediments deposited at the river mouth • Streams found on the delta known as distributaries
  13. 13. • Flocculation occurs as fresh water mixes with salt water - e.g where a river flows into the sea -(then called brackish water), and is the process by which tiny particles in suspension like clay and silt (which under normal fresh water conditions would likely never fall out of suspension) coagulate with the salt in the water by chemical reaction and become heavy enough to ‘sink’ and be deposited Flocculation
  14. 14. Typical Conditions: • Calm and shallow seawater with gentle slope • River deposits its load at the river mouth • Wave in the sea is weak and sheltered coast • More deposition due to active erosion in the upper course
  15. 15. How does it form? • Form when a river approaches the calm seawater • Its velocity decreases • Starts to deposits its load at the river mouth • Sediment continued to accumulate into the sea- forming delta, which is a flat land
  16. 16. Material deposited: 3 layers 1. Bottomset beds – the lower part of the delta that consists of fines material that can be transported furtherest out to sea 2. Foreset beds – the middle layer where material deposited is slightly coarser 3. Topset beds – the uppermost layer where coarsest particles settle first
  17. 17. • Delta’s can be categorised into three main shapes – Arcuate – Birds Foot – Cuspate Types of Delta
  18. 18. •The most common shape of delta •Characteristics: curving shoreline (smoothed by long shore drift), distinct pattern of drainage (branching of distributaries), and typically more ‘gravely’ deposits . •Example: the Nile Delta Arcuate Delta’s
  19. 19. •‘Fingers’ of deposition build out into the sea along the distributaries channels giving an appearance like a birds claw. Typically with a finer sediment •Example: the Mississippi delta Birds Foot Delta
  20. 20. • A cuspate delta is shaped like a tooth by gentle but regular opposing currents in the water body the river flows into, or longshore drift • Example: The river Ebro delta in Spain Cuspate Delta
  21. 21. Alluvial Fans • Are fan shaped similar to delta but deposited on land • Comes from a stream of a steep mountain course and enters a flatter plain with a low gradient • Sudden change of gradient river lose energy  river starts to deposit its load. • The heavier (coarser) the material will b deposited first then the finer materials