Formerly Additional Director
Central Water and Power Research Station, Pune, India
E mail: firstname.lastname@example.org
Mountainous streams with dependable flows and considerable heads are ideally suited for run-of-river schemes with relatively small reservoirs and almost no possibility of long term storage. Such schemes have been effectively commissioned in the north Indian states of Himachal Pradesh, Uttarakhand, Arunachal Pradesh and Sikkim as also in countries likeNepal and Bhutan. However, such streams carry large amounts of sediments. On an average, such rivers carry sediment load of about 1000 ppm, which in some cases may go up to even 10,000 ppm during monsoon floods. Of this, coarser materials settle in the reservoir to reduce its capacity while finer particles travel up to turbines to cause abrasion damage. Desilting basins are generally provided to settle sediment before the flow reaches the turbines.Flushingof the reservoir is carried out to evacuate the coarser material deposited in the reservoir.
Desilting basins are designed such that particles coarser than a specified size (about 0.2 mm inIndia) is settled in the basin and emptied through flushing tunnels at the bottom of the desilting basin. The general features of typical desilting basins are:
- About 90% removal of particles coarser than 0.2 mm size
- Flow through velocity in the basin of about 0.3 m/s.
- Gates on the upstream and downstream for operation and control
- Flushingducts and flushing tunnels for desilting
- Minimum two chambers for ease of selective operation
Figure 1 shows schematic of a dam- desilting basin-power house complex.
It would be obvious that the entire set up of desilting basins would involve huge expenditure, which may even be disproportionate to the cost of the barrage and water conductor system. As for example, one can imagine the cross sectional area of the chamber required for passing the design discharge with a flow through velocity as low as 0.3 m/s.
Even under such circumstances, desilting basins have been provided in many schemes. Unfortunately, the prototype experience has not been encouraging. Turbine runners have been damaged due to abrasion. Fig 2 shows typical view of runner damage.
Figure 2: Abrasion damage to the turbine runners
In view of this, there is serious reconsideration among the civil and power engineers about dispensing with the provision of desilting basins in run-of-river plants.
Although the desilting basins are designed to remove about 90% of the material coarser than 0.2 mm or so, this constitutes only about 30% of the total sediment load. The remaining 70% comprising finer material in the range 0.2mm to 0.075mm pass through turbines. If this is predominantly quartz, then it is capable of causing abrasion damage. Further, the desilting system is designed for a particular discharge (generally the design discharge of the power plant) and sediment concentration. If the discharge is less, reduction in the flow through velocity will cause more deposition of sediment which in turn would overload the flushing capacity. On the other hand, if sediment concentration is more than the design value, more sediment will enter the turbines. In addition, discharge for silt flushing tunnel- of the order of 5-10% of the design discharge would have to be provided separately in the design. It would thus be seen that desilting basins can not be expected to fully solve the problem of sediment management.
Thus, engineers are in favor of dispensing with desilting basins by taking recourse to modification in the basic concepts of planning, design and operational aspects of run-of-river plants.
The first recourse is to treat the reservoir itself as a large desilting basin! Such plants are operated with reservoir at FRL during no-floods periods when the incoming discharges and sediment loads are relatively small and deposition of finer contents can be managed. Thus, relatively silt free water would enter the power plant. During floods, the plant is generally operated with reservoir at MDDL to obviate the possibility of deposition of large amount of sediment in the reservoir. This must be changed to operation at FRL. This would of course induce deposition starting from the river-reservoir junction and move towards downstream in the form of sediment wave. But if the reservoir topography, plant discharge and relevant levels such as spillway crest and intake are favorable, an average flow velocity in the neighborhood of 0.3 m/s can be ensured and in this case also, silt free water would enter the power plant. The deposited sediment can be effectively flushed out of the reservoir at appropriate time during the floods. However, this would require specific planning and design.
Reservoir flushing would be an integral part of the design and operation of these plants. The layout of the dam and power intake as well as the design of spillway would have to be tuned to meet this requirement efficiently. The relative position of the power intake with reference to the spillway is an important design consideration. Figure 3 shows possible layouts with preference. Thus, layouts 1 and 2 would be most effective in flushing operation.
Figure 3: (top)- Preferred layouts of dam-power intake
(bottom)- Section of low level spillway
The design of the spillway requires special consideration. It should be so designed that most of the storage is contained by the gates with the crest of the spillway at or near the river bed level. In addition to allowing an efficient flushing operation, this would also allow placing the sill of the intake at higher levels. A typical section of the low level spillway discussed above is shown in figure 3. For a detailed discussion on these aspects, refer Chapter 13- Spillways for Flood and Sediment Disposal of reference 1.
The above measures would largely ensure relatively silt free water to the turbines. However, there are possibilities that even with operation at FRL, the flow velocity in the reservoir may be somewhat higher than 0.3 m/s or some of the silt content coarser than about 0.2 mm enters the turbines. This would cause abrasion damage to the turbine runners. At some plants, coating of under water parts with Tungsten Carbide in Cobalt Chromium matrix using HVOF process with thickness of coating in the range of 500-600 microns, has given good results. Alternatively, provision of spare runner with the provision of runner replacement gallery in the power house design, could also be explored since the cost of spare runner would be a fraction of the capital cost of the desilting basin complex.
And as a last measure, the plant can be shut down for a day or two during the period of floods carrying large amount of sediment. This situation can then be utilized for flushing operation.
It would thus be seen that desilting basin may be dispensed with in most of the cases with appropriate planning, design and operation of the power plants.
1. Khatsuria, R.M.(2004)- Hydraulics of Spillways and Energy Dissipators- Marcel Dekkers,New York.