Spillways and Energy Dissipators (Part - 9) - Civil Engineering (CE) PDF Download

Hydraulic Design of Trajectory Bucket Type Energy Dissipator

An upturn solid bucket used when the tailwater depth is insufficient for the formation of the hydraulic jump, the bed of the river channel downstream comprises sound rock and is capable of withstanding, without excessive scour, the impact of the high velocity jet. The flow coming down the spillway is thrown away from toe of the dam to a considerable distance downstream as a free discharging upturned jet which falls into the channel directly, thereby avoiding excessive scour immediately downstream of the spillway. There is hardly any energy dissipation within the bucket itself. The device is used mainly to increase the distance from the structure to the place where high velocity etc. hits the channel bed, thus avoiding the danger of excessive scour immediately downstream of the spillway. Due to the throw of the jet in the shape of a trajectory, energy dissipation takes place by 

• internal friction within the jet,
• the interaction between the jet and surrounding air,
• the diffusion of the jet in the tailwater, and
• the impact on the channel bed. 

When the tailwater depth is insufficient for the formation of the hydraulic jump and the bed of the channel downstream comprises sound rock which is capable of withstanding the impact of the high velocity jet, the provision of a trajectory bucket is considered more suitable as provision of conventional hydraulic jump type apron or a roller bucket involves considerable excavation in hard strata forming the bed. It is also necessary to have sufficient straight reach in the downstream of a skijump bucket. The flow coming down the spillway is thrown away in air from the toe of the structure to a considerable distance as a free discharging upturned jet which falls on the channel bed d/s. The hard bed can tolerate the spray from the jet and erosion by the plunging jet would not be a significant problem for the safety of the structure.

Thus, although there is very little energy dissipation within the bucket itself, possible channel bed erosion close to the downstream toe of the dam is minimized. In the trajectory bucket, only part of the energy is dissipated through interaction of the jet with the surrounding air. The remaining energy is imparted to the channel bed below. The channel bed should consist of sound, hard strata and should be free from laminations, joints and weak pockets to withstand the impact of jet. The design of the trajectory bucket presupposes the formation of large craters or scour holes at the zone of impact of the jet during the initial years of operation and, therefore, the design shall be restricted to sites where generally sound rock is available in the river bed. Special care shall be taken to concrete weak pockets in the bed located in a length of  

Design Criteria - The principal features of hydraulic design of tra jectory bucket consist of determining:

• Bucket s hape,
• Bucket invert elevation, radius or principal geometrical parameters of the bucket, lip elevation and exit angle, trajectory length, and
• Estimation of scour downstream of the spillway. 

The various parameters are shown in Figure 61(c) 

An example of the use of a trajectory bucket is the one provided in the Srisailam Dam Spillway (Figure 64). 

FiGURE 64. Srisailam Dam Spillway

Further details about the design of bucket type energy dissipators may be had from the Bureau of Indian Standards Code IS: 7365-1985 “Criteria for hydraulic design of bucket type energy dissipators” 

Protection of downstream of spillways from scour 

It may be noted that inspite of the provision of the best suited energy dissipator for a specific spillway under the prevailing site conditions, there may be still some energy is expected to be maximum for the trajectory type spillway, followed by the solid and slotted roller buckets and finally the hydraulic jump type stilling basins. In order to protect the downstream riverbed from these undesirable scour, the following types of protection works have been recommended by the Bureau of Indian Standards code IS:13195-1991 “Preliminary design, operation and maintenance of protection works downstream of spillways-guidelines”. 

1. Training Walls at the Flanks of the Spillways- Training walls extended beyond the end-sill of the stilling basins or buckets generally serve to guide the flow into the river channel, protect the wrap-rounds of the adjacent earth dams, river banks or power house bays and tail race channels. To this extent, the training walls are -considered to be downstream protection works.
2. Protective Aprons Downstream of Bucket Lips or End-sills of Stilling Basins -Protective aprons of concrete laid on fresh rock or acceptable strata immediately downstream of bucket lips or end-sill of stilling basin, protect the energy dissipator against undermining due to excessive scour during or after construction of the spillways. A suitable concrete key is normally provided, at the downstream end of the apron. Where the normal river bed level is higher than the end-sill and a recovery slope is , provided, it sometimes becomes necessary to lay a concrete apron on such a recovery slope also for protection.
3. Concrete Blocks or Concrete Filling on River Bed Downstream of Energy Dissipator -Concrete blocks or concrete fillings are sometimes provided on the river bed downstream of energy dissipators to safeguard against excessive scour and prevent further scour.
4. Protective Pitchings on Natural or Artificial Banks Downstream of Spillways Protective pitchings of stone rip rap, masonry or concrete blocks are provided on natural river banks or artificially constructed embankments of diversion channels, power house tail race channels or guide banks, for protecting them against high velocity flows or waves. 

Figure 65 shows the various types of protection works that may typically be used downstream of a spillway.  

Flaunt 65. Different types of protection works downstream of a typical spillway project

The importance of providing protection below a spillway, especially of the trajectory type may be noted from the incidence of deep scour on the downstream of the Srisailam dam spillway. 

Case Study

Srisailam dam spillway (Figure 64) across river Krishna was constructed during 1977-83. It is a 137 m high concrete dam, with 12 spans of 18’3 m x 16’8 m. The river bed is composed of quartzites and shales. In the immediate downstream vicinity of the spillway, there were horizontal shear zones 0’2 m to 0’9 m thick, where the quartzites are crushed and sheared. During the monsoons of 1977 to 1980, the construction stage flood passed over the partially constructed spillway bays, spilling over 7 bays which were at different levels having a maximum difference of level of 23 m. The difference in level between the lip of the ski-jump bucket and downstream rock was about 44 m.  

Shorter throw of the water spilling over the bucket lip, as a cascading flow caused deep scour in the immediate vicinity of the bucket lip. During subsequent floods, the scour holes were concreted and leveled as protective aprons in some part of the spillway. Such aprons were however, subjected to repeated damage and undermining. By April 1985, depth of scour below blocks 11 to 13 reached from 9 m to 22 m below the protective apron. Cavities of undermining below the apron were also present at a depth of 6 to 9 m. 

The protection work consisted of providing an underwater massive concrete block touching the apron and filling the eroded cavities below the apron. The water level at downstream toe varied from the top of existing apron to about 1.5 m below it. 

The scheme involved forming 4 cells with steel cylinder walls and filling concrete in each cell followed by concrete capping. Heavy concrete blocks (approximate 1 metre cube) were placed downstream of the cylinder watls to further protect the rock from the water jump damage. 

Since the construction of the above protection works, the spillway was completed to final levels and crest gates have also been installed. Hydraulic model studies were conducted to evolve an operation of the spillway in such a way that the throw of the trajectory fall further away from the toe of the dam. This together with the protective measures already implemented is expected to prevent further erosion at the toe of the dam.