Unit 1 - Essays - Physical Question UPDATED Flashcards
With the aid of examples, discuss the view that velocity is the most important influence on sediment deposition in a river.
Paragraph 1: Velocity
Define velocity clearly (speed of water flow).
Reference and briefly explain Hjulström curve clearly:
Explain key points clearly:
High velocity (>100 cm/s): sediments stay suspended, minimal deposition.
Low velocity (<30 cm/s): sediments deposited as flow reduces.
Example 1: Mississippi River Delta
Low velocity near mouth causes fine sediment deposition (silt/clay).
Delta formation clearly shows velocity’s role.
Temporal variation:
Seasonal rivers (e.g., Sahara Desert’s Wadis).
Flood (high velocity) vs drought (low velocity) impacts clearly explained.
Paragraph 2: Sediment size
Explain clearly that sediment size strongly impacts required velocity for deposition:
Large particles (gravel, pebbles): high velocity needed (>150 cm/s).
Small particles (clay/silt): stay suspended at lower velocities due to cohesion.
Spatial example: Nepal’s Kali Gandaki River
Upper course (high velocity) deposits larger stones, gravel.
Lower course (slow velocity) deposits finer materials like silt/clay.
Scale: Clearly mention lab flume experiments:
Consistent velocity shows different deposition patterns for gravel vs. sand vs. clay.
Clarify that sediment size independently affects deposition regardless of velocity variations.
Paragraph 3: Gradient (channel slope as influencing factor)
Clearly explain gradient’s direct relationship with velocity:
Steeper gradient → higher velocity → less deposition.
Gentle gradient → lower velocity → increased deposition.
Example clearly detailed: Colorado River, USA
Steep gradient through Grand Canyon: little deposition.
Gentle gradient downstream near Yuma: significant deposition forming floodplains.
Temporal/human factor: Clearly explain human impact through dams.
Aswan High Dam, Egypt, reducing downstream sedimentation due to altered gradient and velocity.
Paragraph 4: External conditions (discharge, vegetation, chemistry)
Explain clearly why external factors can sometimes dominate deposition patterns despite velocity:
Vegetation: Slows velocity, traps sediment, enhances deposition.
Example: Sundarbans Mangrove Forest (Bangladesh), vegetation captures sediment creating landforms.
Chemical composition (flocculation): High salt content causes sediments to clump together, settle faster, independent of velocity.
Example: Yellow River’s high salt content accelerates deposition irrespective of changes in velocity.
Emphasize spatial importance: external factors vary considerably depending on environment and local conditions, significantly influencing deposition at specific locations.
Conclusion
Clearly summarize that while velocity is indeed crucial, sediment size, gradient, and external conditions independently and interactively affect deposition patterns.
Reinforce the importance of considering variations across different locations, scales, and time periods.
