Hydrotopics

May 23, 2012

Desilting basins: Are they indispensable?

Filed under: Uncategorized — Rajnikant Khatsuria @ 12:01 pm

R.M.Khatsuria

Formerly Additional Director

Central Water and Power Research Station, Pune, India

E mail: rmkhatsuria@rediffmail.com

 

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.

Figure 1: Typical layout of Dam-Desilting basin 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.

 Reference:

 1. Khatsuria, R.M.(2004)- Hydraulics of Spillways and Energy Dissipators- Marcel Dekkers,New York.

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May 6, 2012

Discharge characteristics of spillways and barrages silted up to crest

Filed under: Uncategorized — Rajnikant Khatsuria @ 5:17 am

R.M.Khatsuria

Formerly Addl. Director

Central water and Power research Station, Pune, India

E mail: rmkhatsuria@rediffmail.com

 

All other factors remaining the same, coefficient of discharge of a spillway decreases as the height of the crest relative to the head on the crest (P/Hd) decreases. The crest shape, though, has an influence to some extent as seen from figure 1. The elliptical shape retains some superiority over other shapes. However, trying to estimate the reduction in the coefficient Cd corresponding to the zero approach depth, viz. P=0, from figure 1 would not yield any result. The flow conditions substantially change as the approach depth diminishes to zero. No specific reference is available in the literature. Theoretically, a level broad crested weir should have a value of Cd =1.706 or C=0.577 in the equation Cd=2/3 (2g)0.5 C.

 

Figure 1:  Effect of approach depth on coefficient of discharge

This issue was particularly addressed by B.D.Suryavanshi, then Director, Irrigation Research, MP, India, in 1972. He conducted studies on a 1:24 scale model of a 7m high ungated weir which was getting silted up. In his experiments, in addition to the original bed, he also simulated a hypothetical condition whereby the weir was silted up fully up to the crest level. It was found that the coefficient of discharge in the equation q=Cd H3/2, of 2.143 corresponding to the original bed condition (P/Hd =1.33) decreased to a value of 1.96 for the bed silted up to the crest (P/Hd =0), a reduction of about 9.5%. Both the above values were worked out considering the head due to the velocity of approach. The corresponding C values were 0.725 and 0.663 respectively, considerably higher than theoretical. The head over the crest, however, reduced from 5.27 m for the original bed condition to 4.52 m corresponding to the silted bed condition, for the design discharge of 28 cumec. It is surmised that the free over fall just downstream of the level crest may be the reason for increase in C beyond the theoretical vale of 0.577.

 A noticeable finding from the above studies was that the afflux upstream of the reach increased in comparison to that caused by the original bed condition. Also, the maximum additional afflux was caused at a section up to which silting was extended. Therefore, if a reservoir behind a low dam or barrage is likely to be silted up in future, the additional afflux should be considered and provided for in the design itself. The results are shown in figure 2.

 

Figure 2: Effect of siltation upstream on flow profiles

In the above case, although the reservoir was silted up to the spillway crest, a free over fall in the downstream contributed to a better value of C. However, in the case of low height barrages, silted up to the crest level, such a free over fall would not be available as the downstream portion would not be too deep. In such a case, C value of 0.577 would be applicable, provided the downstream water level is not more than about 80 percent of the upstream level. For higher submergence, weir formula can not be applied and discharge should be calculated assuming fully developed open channel flow and corresponding velocity profile.

 A particular case of interest would be a low height barrage with crest piers and bridge. When silted up to the crest level, it would virtually be the case of a bridge with the intervening spans filled with level bed of concrete. This case offers calculations involving bridge pier losses. All the possible combinations of upstream and downstream water levels, shown in figure 3, are amenable to calculations by application of US Army Corps of Engineers Hydraulic Design Criteria charts 010-6 to 010-6/5.

Figure 3: Various combinations of upstream and downstream water levels

 

 

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