PRACTICAL ASPECTS OF BED FORMATION

• 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.