Spillways and Energy Dissipators (Part - 8) - Civil Engineering (CE) PDF Download

Design of bucket-type energy dissipators 

Hydraulic behaviour of bucket type energy dissipator depends on dissipation of energy through: 

• Interaction of two rollers formed, one in the bucket, rolling anti-clockwise (if the flow is from the left to the right) and the other downstream of the bucket, rolling clockwise; or
• Interaction of the jet of water, shooting out from the bucket lip, with the surrounding air and its impact on the channel bed downstream. 

Bucket type energy dissipators can be either: 

• Roller bucket type energy dissipator; or      
• Trajectory bucket type energy dissipator.

The following two types of roller buckets are adopted on the basis of tailwater conditions and importance of the structure:

•  Solid roller bucket, and      
• Slotted roller bucket. 

Figure 59. Roller buckets ; (A) Solid ; (b) Slotted

Roller bucket type energy dissipator is preferred when:      

• Tailwater depth is high (greater than 1.1 times sequent depth preferably 1.2 -times sequent depth), and    
•  River bed rock is sound.  

Trajectory bucket type energy dissipator is generally used when:

• Tailwater depth is much lower than the sequent depth of hydraulic jump, thus preventing formation of the jump;
• By locating at higher level it may be used in case of higher tailwater depths also, if economy permits; and
• Bed of the river channel downstream is composed of sound rock. 

This is shown in Figure 60. 

Figure 60. Trajectory bucket type energy dissipator 

Action of the various types of bucket-type energy dissipators is given below:

Hydraulic Design of Solid Roller Bucket 

An upturn solid bucket is used when the tailwater depth is much in excess of sequent depth and in which dissipation of considerable portion of energy occurs as a result of formation of two complementary elliptical rollers, one in bucket proper, called the surface roller, which is anticlockwise (if the flow is to the right) and the other downstream of the bucket, called the ground roller, which is clockwise.

In the case of solid roller bucket the ground roller is more pronounced and picks up material from downstream bend and carried it towards the bucket where it is partly deposited and partly carried away downstream by the residual jet from the lip. The deposition in roller bucket is more likely when the spillway spans are not operated equally, setting up horizontal eddies downstream of the bucket. The picked up material which is drawn into the bucket can cause abrasive damage to the bucket by churning action. 

For effective energy dissipation in a solid roller bucket, both the surface or dissipating roller and the ground or stabilizing roller, should be well formed. Otherwise, hydraulic phenomenon of sweep out or heavy submergence occurs depending upon which of the rollers is inhibited. 

Design Criteria - The principal features of hydraulic design of solid roller bucket consists of determining: 

• The bucket invert elevation,
• The radius of the bucket, and
• The slope of the bucket lip or the bucket lip angle.

The various parameters are shown in Figure 61 (a). 

FiGURh 61. Sketches for bucket type energy dissipators

An example of the use of a solid roller bucket is the energy dissipator of the Maithan Dam Spillway (Figure 62) 

Figure 62. Maithon Dam spillway

Drawal of Bed Materials - A major problem with the solid roller bucket would be the damage due to churning action, caused to the bucket because of the downstream bed material brought into the bucket by the pronounced ground roller. Even in a slotted roller bucket downstream material might get drawn due to unequal operation of gates. The channel bed immediately downstream of the bucket shall be set at 1 to 1.5 m below the lip level to minimize the possibility of this condition. Where the invert of the bucket is required to be set below the channel general bed level the channel should be dressed down in one level to about 1 to 1.5 m below the lip level in about 15 m length downstream and then a recovery slope of about 3 ( horizontal ) to 1 ( vertical ) should be given to meet the general bed level as shown in Figure 62. Careful model studies should be done to check this tendency. If possible, even provision of solid apron or cement concrete blocks may be considered to avoid trapping of river bed material in the bucket as it may cause heavy erosion on the spillway face, bucket and side training wall. 

In the case of slotted roller bucket a part of the flow passes through the slots, spreads laterally and is lifted away from the channel bottom by a short apron at the downstream end of the bucket. Thus the flow is dispersed and distributed over a greater area resulting in a less violent ground roller. The height of boil is also reduced in case of slotted roller bucket. The slotted bucket -provides a self-cleaning action to reduce abrasion in the bucket.

In general the slotted roller bucket is a improvement over the solid roller bucket for the range of tailwater depths under which it can operate without sweepout or diving. However, it is necessary that specific model experiments should be conducted to verify pressure on the teeth so as to avoid cavitation conditions. In case of hydraulic structures in boulder stages slotted roller buckets need not be provided. Heavy boulders rolling down the spillway face can cause heavy damage to the dents thereby making them ineffective and on the contrary, increasing the chances of damage by impact, cavitation and erosion. 

Hydraulic Design of Slotted Roller Bucket 

An upturned bucket with teeth in it used when the tailwater depth is much in excess of sequent depth and in which the dissipation of energy occurs by lateral spreading of jet passing through bucket slots in addition to the formation of two complementary rollers as in the solid bucket.

In the slotted roller bucket, a part of the flow passes through the slots, spreads laterally and is lifted away from the channel bottom by a short apron at the downstream end of the bucket. Thus the flow is dispersed and distributed over a greater area providing less violent flow concentrations compared to those in a solid roller bucket. The velocity distribution just downstream of the bucket is more akin to that in a natural stream, that is, higher velocities at the surface and lower velocities at the bottom. While designing a slotted roller bucket, for high head spillway exceeding the total head of 50 m or so, specific care should be taken especially for design of the teeth, to ensure that the teeth will perform cavitation free. Specific model tests should therefore be conducted to verify pressures on the teeth and the bucket invert should accordingly be fixed at such an elevation as to restrict the subatmospheric pressures to the permissible magnitude. 

Design Criteria - The principal features of hydraulic design of the slotted roller bucket consists of determining in sequence:

• bucket r adius;
• bucket invert elevation;
• bucket lip angle; and
• bucket and tooth dimensions, teeth spacing and dimensions and profile of short apron.

The various parameters are shown in Figure 61(b) 

An example of the use of a slotted roller bucket is the energy dissipator provided in the Indira Sagar Dam Spillway (Figure 63).

Figure 63. Indira Sagar Dam Spillway