Lacey regime theory | Different types of bed forms – Fluid

What is Regime Channel?

A channel is said to e a regime channel when it reaches a stable condition. A better method for the design of the regime channel was developed by Lacey.

According to Lacey regime theory, the regime channel’s dimension is considered to attempt an equilibrium state called regime. He also differentiated between the initial regime and the final regime condition.

The initial regime condition is achieved shortly after the channel is put into operation. The canal starts to adjust its bed either by silting or by scouring even though the width of the channel may not charge. The canal then may appear to have attained stability.

But in actuality, this is not the final state of equilibrium (or stability). Hence, this represents the initial regime condition. Finally, due to the continuous action of flowing water, the water overcomes the resistance of banks and sets up a condition in which the channel adjusts its complete section when the final regime condition is achieved. 

After the analysis of a large number of data in permanent irrigation channels, Lacey gave the empirical relation for regime channels.

Lacey regime theory | Different types of bed forms - Fluid

Analyzing all these equations, the following equation for regime velocity is obtained.

Silt factor (f) for different particle size and material

S.N.Material Diameter (mm)f
1Small boulder 64-2566.12-9.75
2Coarse gravel8-644.68
3Fine gravel4-82
4Coarse sand 0.5-21.44-1.56
5Medium sand 0.25-0.51.31
6Fine sand 0.06-0.251.1-1.3

Different types of Bed Forms

A plane bed with no sediment movement is possible, only when the stress ( normal shear stress) in the channel is less than critical stress. As the value of shear stress increase, the motion of particles starts when water starts to flow in such bed, different bedforms are formed.

Depending upon the type of flow, sediment, fluid characteristics, the bed will undergo different levels of deformation and motion. The different forms of the bed are.

  1. Plane bed with no sediments
  2. Ripples
  3. Dunes
  4. Transitions
  5. Anti Dunes

1) Plane bed with no sediments

At this condition, there is no case of sediment movement. The actual shear stress is less than the critical shear stress, so the bed will remain plane. The friction offered to the flow is resisted by the particles only.

2) Ripples

If the shear stress of the channel is increased either by increasing discharge or by increasing slope, the particles of the bed will start to move and very soon the bed will be covered by a pattern which I known as ripples. The flow of the sediment will be rolling and sliding.  

3) Dunes

As the shear stress in the channel bed is gradually increased, the size of the ripples gradually becomes larger. Then, a different form of bed arises which is known as dunes. At the higher value of shear stress, the ripples disappear leaving behind the dune patterns from the bed.

Lacey regime theory | Different types of bed forms - Fluid

4) Transitions

Further increase in shear stress, after the formation of the dune, will lead to a transition state. The undulation in the bed starts to disappear to achieve a plane bed surface.

The sediment transport rate will be considerably higher than the dune phase. The flow will be in a subcritical state with the value of the fraud number close to unity.

5) Anti dunes

If the shear stress is further increased beyond the transition phase, the symmetric sediment wave slowly starts to move upstream. The wave gradually grows deeper and then breaks. The bedform at this state is called anti-dunes. At this stage, the flow will be supercritical.

Lacey regime theory | Different types of bed forms - Fluid

I hope this article on the “ Lacey regime theory” remains helpful for you.

Happy Learning – Civil Concept

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