SEDIMENTATION TRANSPORT AND DESIGN OFCHANNELS

In a channel bed made of fine sand, when the velocity is gradually increased, the sequence of bed

formation is—saw-tooth ripples-dunes with ripples-dunes-flat surface sand waves and anti-dunes.

* When a few particles on the bed just start moving, the condition is known as critical condition.

* A regime channel or a stable channel is one in which neither silting nor scouring takes place.

* Critical velocity in a channel is the mean velocity which will just keep the channel free from silting

or scouring.

* Kennedy formula for critical velocity is

Vo = 0.55 m y

0.64.

where y = depth of flow

m = critical velocity ratio

Lacy differentiated between true regime, initial regime and final regime.

* According to Lacy

1. Natural silt transporting channels have a tendency to assume a semi-elliptic section.

2. The coarser the silt, the flatter is the semi-ellipse.

3. Silt factor f is given by

f =

where V is in m/sec and R hydraulic mean depth in m.

4. Wetted perimeter

P =

where Q is discharge in m3

/sec and P is in m.

5. The sediment is kept in suspension by vertical components of the eddies which are generated on

the channel bed as well as on the sides of the channel.

6. Score depth R¢ is given by

R¢ = 0.473 (Q/f)

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