• The bed of the channel in which water is flowing, may adopt various shapes depending upon the velocity of flow.
  • At very low velocity of flow, bed of the channel does not move at all.
  • When velocity is slowly increased a stage is reached when the sediment load is just at the point of motion.
  • This stage is known as ‘‘threshold of motion”. If bed is
  • made of fine sand, having particles of less than 2 mm diameter, saw tooth type ripples develop in the bed on slightly further increasing the velocity.
  • This phenomenon can be easily seen in sand at beaches.
  • At still larger velocity, dunes with ripples appear at the bed, which on the further increase on the velocity take the shape of rounded dunes.
  • When velocity is still further increased, the dunes are eliminated and a flat surface becomes available.
  • If velocity is increased still further, sand waves are formed in association with surface waves.
  • When velocity is still further increased.
  • Froude number V gD approaches almost unity and surface waves to become so steep.
  • That they break and the whole wave system gradually moves upwards.
  • The sand waves at this point of velocity are know anti-nodes.
  • Antinodes can form only in open channel flows and not in wind blown sand.
  • Because they require an interaction between the bed.
  • The channel and the water surface for their formation.
Development in the bed of the canal at various velocities of flow.
Development in the bed of the canal at various velocities of flow.