The canal bed is protected by adopting six main devices: 1. Water-Cushion 2. Baffle Wall 3. Biff Wall 4. Deflector 5. Staggered Blocks 6. Friction Block.
Device # 1. Water-Cushion:
When a water cushion is provided below the fall it serves two purposes. Firstly it reduces the intensity of impact of falling sheet of water. Secondly it dissipates the energy of flow.
The water cushion is created by constructing a still water pocket or a pond below the fall (Fig. 17.8). The still water pocket is called a cistern.
Device # 2. Baffle Wall:
It is an obstruction constructed across the canal downstream of the fall. It is in the form of a wall of low height. It heads up water just upstream of it. Thus it tries to create a water cushion on the upstream. Many times when the flow conditions are favourable hydraulic jump may occur. As a result energy of flow is dissipated.
Device # 3. Biff Wall:
It is an end wall of a cistern. It is a vertical wall with a horizontal projection extending in the cistern (Fig. 17.9).
Owing to the projection, the flow of water returns back in the cistern. It creates an obstruction to fast moving water down the fall. As a result energy of flow is dissipated.
Device # 4. Deflector:
It is a short wall constructed at the end of down-stream apron (Fig. 17.10).
This end wall deflects the high velocity flow of water. Due to deflection the velocity of flow in the direction of motion is reduced.
Device # 5. Staggered Blocks:
They are nothing but rectangular blocks or cubes generally made of concrete. They are arranged in staggered position on the downstream horizontal apron (Fig. 17.11). They deflect the high velocity flow in the lateral direction. It provides an obstruction to high velocity flow and the energy of flow is dissipated effectively. They are very commonly used below the falls to dissipate the energy in combination with cistern.
Device # 6. Friction Blocks:
They are also called arrows. They are shaped specially to increase the surface area of contact (Fig. 17.12). These blocks are arranged in staggered position. Due to staggering the high velocity flow is diverted laterally. Thus the water which flows in lateral direction obstructs the high velocity flow in the forward direction. As a result energy of flow is dissipated.
