Kulekhani Review 2013

Title : Presentation of a structural model of Kulekhani dam exhibiting the siltation problem in Indrasarovar reservoir and promising solutions.

1.     Introduction:

Nepal has high potential of hydro-electricity. From the water resources present we cannot deny the fact that Nepal can be a leading hydro-power developer in the soon future, if attempted in that direction. There are few perennial rivers which are formed by the melting of the glacier in the Himalayas. These rivers flow throughout the year with significant volume. However, the amount of water in the perennial river also decreases substantially in the dry seasons as compared to wet seasons.

 ROR hydro-power project can be constructed in the perennial rivers. ROR type hydro-power project being relative less costly and easy to construct has been the most developed type of hydro-power project. During the wet season, the electricity production in ROR hydro-power plants is impressive and the electricity requirement is, to some extent, is fulfilled. In contrast to that, in dry season, the amount of water in the rivers decreases considerably, this affects the electricity production. As a result of which Nepal is suffering from electricity crisis.

  Reservoir type hydro-power project is another type which can be constructed in perennial type as well as non-perennial rivers. Dam is constructed in the appropriate river, this decreases the outflow rate of discharge by required amount creating a reservoir upstream.  Water can be collected in reservoir and can be used during dry season i.e Kulekhani hydro-power project which is the only reservoir type project in Nepal till date. In addition to hydro-power generation, dams are constructed for irrigation, flood control etc. and there some are multipurpose dam which fulfils all the above purpose.

If the reservoir type of hydro-power projects are given much focus and are developed in the near future, electricity crisis is sure to be a past thing. Along with the construction of new reservoir type project attention must also be given to the existing Kulekhani hydro-power projects, as the siltation has threatened its future. If this problem is not taken care of, electricity crisis is sure to be prolonged.

2.     Objectives:

Ø  To share knowledge about the Kulekhani Hydropower Project.

Ø  To reflect the Kulekhani Hydropower project’s role in production of electricity in Nepal.

Ø  To share the knowledge of structural feature of Kulekhani dam.

Ø  To expose the siltation problem in Indra sarovar reservoir lake.

Ø  To purpose the solution to the existing siltation problem.

Ø  To act as a bridge between layman and innovation in Hydropower Engineering.

Ø  To increase awareness and importance of civil engineering project such as ‘Kulekhani Hydropower Project’.

Ø  To unveil the importance of civil engineering in the overall development of the our country. 

Ø  To respond the query that the audience have concerning Kulekhani Hydropower project.

3.     Kulekhani  I hydropower project:

Kulekhani hydropower project is the only reservoir type hydro power project of Nepal. It was constructed as the five years’ project by the government of Nepal between 1977 and 1982 under the financial assistance of World Bank, The Kuwait Fund, OPEC Fund, the UNDP and the Overseas Economic Cooperation Fund (OECF) of Japan.

The total project cost of the project was US$ 117.843 million and accordingly the project was transferred to NEA with a capital cost approximately NRs. 155 crores. The telemeter system installed for rainfall , water level measurement and indication cost around NRs. 13.74 crores. The cost of road, check dams and inclined tunnel are about 23 crores.

 It’s has the installed capacity of 60 MW having two units each producing 30MW each. Initially it was designed as peaking power station but at present it has been serving as emergency stand by power station also due to the power crisis in the country. It creates the Indra Sarowar reservoir which has an area of 2.2 sq km. Kulekhani dam is an 114m high rockfill dam.  It is named after Kulekhani village development committee in Makwanpur District in the Narayani Zone of southern Nepal. The water after losing certain head from kulekahni I hydropower project is used in kulekhani II hydropower project. Kulekhani III hydropower project is under construction which also uses the same water again after losing some head in Kulekhani II.

Sailent features of the project

Type : Reservoir
Rated net head : 550 meter
Design discharge : 12.1 m3/s
Headrace tunnel : 6233 m, 2.3 m diameter
Penstock pipe : 1324 m long, 2.1 – 1.5m diameter steel pipe
Installed capacity : 60 MW
Turbine type and numbers : Pelton, 2 sets
Rated speed : 600 rpm
Type of generator : Vertical shaft, Synchronous
Capacity : 35 MVA
Rated voltage : 11 KV
Power transformer : 11/66KV, 3 phase, 35 MVA, 2 Nos
Average annual generation : 211 GWh (Primary energy 165 GWH and secondary energy 46 GWH)
Catchments area : 126 sq. km.
Civil construction start : 1977 A.D.
Commissioning date : 14th May, 1982

4.     Kulekhani dam

The Kulekhani dam is a rock fill dam with the impervious core slightly inclined upstream. It has maximum height of 114m. Its crest length is 397m and the width is 10m. The upstream slope is 1:2.35 steepened upto 1:2. The slope is steepened near the top of the dam to allow the sufficient camber for the post construction settlement. The downstream slope is 1:1.8. The crest level is at 1534m with the free board of 4m during the highest regulation level. 

The axis of the dam is skewed about 45 degree to the river channel. The rock formation at the Kulekhani dam site consists of the interbedded schist, phyllite and quartzite, striking generally in a direction sub-parallel to the dam axis dipping upstream at an angle of 30 to 40 degrees. The left abutment is quite steep with the slope of 1:2/3 and the right abutment with the slope of 1:2.

According to the ICOLD definition, the rock fill dams are the ones, which contain more than 50% of the volume of the rock fill obtained from the rock quarry site or any site containing the natural stones and the boulders. The typical rock fill  dam consists of the main central core, which is impervious in nature generally formed from moraine core or the silty clay etc., filter zones of the sandy gravel, transition zones that is mainly from the fine blasted rock and the supporting rock fill that is generally from the blasted rock. Riprap structures are mostly used for the protection of the upstream and the downstream slope. In the upstream slopes, the embankment dams are mainly protected by the riprap for the effects of the wave action, surface runoff, floating debris, and weathering and in Norway; it is mainly due to the icing problems. In the downstream slope, most of the rock fill dams are vulnerable to failure by overtopping and the stability problems in the slope. In that case, the riprap could be the reliable measure for the safety of the dam.

The main dam body is a zonal rock fill dam containing the following main zones:

·         main inclined core

·         random zone

·         rock quarry zone

·         the filter zone, and

·         Upstream rip rap 

Spillway

The spillway is located adjacent to the dam embankment on the left abutment side. It consists of the gate controlled and uncontrolled section. The gate-controlled section, in immediate adjacent to the dam embankment is controlled by two gates, each 9m wide and 11.5 m high as shown in Figure 2.3. The uncontrolled section consists of a side channel spillway having the length of 65m and extending to upstream. The crest level of the uncontrolled section is at 1530m, which is the full supply level of the reservoir. The uncontrolled section of spillway works after the water level exceeds 1530m .The water from both spillway discharges down the slope of 1:2 and tapers in width from 33m at the top to 21m at the bottom.

The uncontrolled side channel section has the capacity of the 800m3/s with the reservoir level at 1534m which is the crest level of the dam embankment. It is capable of discharging the 100 years flood without the control gates into operation. This existing spillway passes this substantial flood through it even when the spillway gates are not opened which is the excellent safety measure. 

5.     Failure of rockfill dam

The movement or the collapse of the dam is such that it is not able to retain any water causes the failure of the dam. This failure induces the lot of water to release downstream causing the risk in the property and people downstream. Two types of failure are categorized by the ICOLD (1974).   

Failure by overtopping:

Schintter (1979) summarized that out of the analysis of 216 dams, 34 % dams failed only due to the overtopping and one third of the total dam failures is by the overtopping. The prominent reason of the failure is the inadequate spillway discharge capacity besides the malfunctioning of the equipment and the errors in the operational management. Spillway discharge capacity is one of the important factors that help the dam to pass the floods during the extreme flood conditions or the probable maximum flood condition. The reservoir water level is influenced by the spillway discharge. 

The discharge increases as the function of the (3/2) power of the reservoir water head over the spillway crest elevation. For the gated spillway, the discharge will be function of the reservoir head at the centerline of the opening to the power of ½. Generally ungated spillways are not the most likely to fail to pass the adequate discharge even the severe conditions arises such as the earthquake damage, power failure, broken communication, or any conditions during the extreme failure. The failure occurs mostly with the gated spillway. In case of the gated spillway any operational or mechanical failure is expected, then the full capacity of the spillway will not be available and then that might bring the severe conditions of the overtopping. 

In the accidental cases like the closing of the gates during the power failure, the radial gates may be overtopped and might cause the problems regarding the flow instability and the vortices as mentioned in Kjellesvig.

Depth and duration of the overtopping:

 The probability that the dam failure will occur during the overtopping is more dependent on the depth and the duration of the overtopping. [9] The failure of the dam is more site specific and dependent on the zone and the details of the dams. The provision of the downstream protection and plastic materials in the downstream slope helps to make it more resistive to the erosion and thereby preventing the failure. 

Failure by the leakage:

 Due to the insufficiency in the permeability and the filter criteria in the rockfill dams, the leakages may be occurred progressing into the pipe during the erosion in the embankment dams. The embankment filters becomes not able to stop the erosion of the particles and the hydraulics of the flow during erosional process. 

General condition on which failure depends:

·         Orientation of the longest axis of the stones with the slope of the dam

The proper orientation increases the strength of the rip rap and prevents the disruption of the velocity by reducing the shear stress and the velocity of the flow. The sliding of the protection which is also another scenario during the overtopping, it can also be prevented by designing the toe stones and interlocking the toe stones. Generally in the rockfill dam, the failure starts from the toe so the protection and the interlocking of the toe stones eventually help in the prevention from failure. 

·         Diameter od stones

Another major observation is the increase in the strength of the rip rap with the use of the bigger diameter stones. From the results obtained during the tests show that the unit failure discharge increases with the sizing of the stones. The use of the bigger stones gives the better strength for the rip rap.

·         Downstream slope

In the random placement of the dam, the tests conducted with two different placement of the rip rap; one is with the existing Kulekhani dam which has high coefficient of the uniformity and other with the random placement of the dam for the same downstream slope with low coefficient of uniformity shows that the failure discharge at the one with high coefficient of uniformity is found to be less in relative to the one with the low coefficient of the uniformity. This shows that the gradation of the rip rap affects the stability of the downstream slope.

·         Diameter of stone

The strength of the rip rap can be increased with the use of the bigger diameter stones. From the results obtained during the tests show that the unit failure discharge increases with the sizing of the stones. The use of the bigger stones gives the better strength for the rip rap.

6.     Siltation problem in Kulekhani

The power generation capacity of the Kulekhani Hydroelectricity Project, the only storage-type plant in the country, has been found decreasing in recent times, due to the silt deposited at the bottom of the artificial lake.

The drop in water storage capacity of the 92 megawatt project (Kulekhani I and Kulekhani II) has reduced power generation by 63.3 million units in recent times, according to Rabindra Mahaseth, chief of the Kulekhani I Hydroelectricity Project. Kulekhani I (60MW) and Kulekhani II (32 MW) generate power from the water stored in the reservoir and contribute to the national grid. The reduced capacity of the reservoir to hold water is likely to affect the overall power supply system, particularly the load-shedding hours in the Kathmandu valley and the surrounding areas.

The water stored in the reservoir used to contribute to generate around 210.1 million units of power. However, only 140 million units are generated from the plants now. Every year, the water sources feeding the reservoir bring along boulders, stones, mud and sand which get deposited at the bottom of the reservoir. The deposition of silt and other substances that are brought by the water movement from the sources is reducing the water holding capacity of the reservoir.

During the initial years of operation, the Kulekhani reservoir had the capacity to hold around 850.3 million cubic metres water, which has dropped to 590.9 cubic metres. It is estimated that the bottom surface of the reservoir has risen 13 meters above  in last three decades. The reservoir has the capacity to store water up to 1,530 metres ASL.

7.     Sedimentation and its counter measures

The deposition of finely divided soil and rock particles upon the bottom of stream and river beds and in reservoirs is termed as siltation or sedimentation. All streams and rivers carry sediment - particles of clay, silt, sand, or gravel. When streams or rivers enter a reservoir, the cross-sectional flow area increases, water velocity decreases and sediments begin to deposit. The largest sediment particles settle farthest upstream, followed by progressively smaller and smaller particles downstream. The deposition process forms a delta in the headwater area of the reservoir that extends further into the reservoir as deposition continues (see Figure 1.1). Over time, the delta can essentially fill the reservoir with deposited sediments and eliminate the benefits for which the dam was built.

 Effects of Reservoir Sedimentation

In addition to the economic cost of replacing storage capacity, reservoir sedimentation also has negative impacts on other project benefits. They are:

1. Reduction in upstream navigational bridge clearance.

2. Increase upstream flood levels.

3. Increase in upstream water table elevations, thus encouraging marsh growth and loss of land.

4. Increase in water losses due to evaporation and transpiration.

5. Blockage or clogging of upstream water intakes.

6. Rise in base levels on tributary streams and a subsequent increase in aquatic growth.

7. Reduction of water supply for irrigation, industry, and recreation.

8. Reduction of flood control benefits.

Due to the countless disadvantages, sediment management has become an immediate attention. Sediment management in reservoirs is largely classified into the three approaches:

 (1) To reduce sediment inflow to reservoirs.

(2)To route sediment inflow so as not to accumulate in reservoirs, and

(3) To remove sediment accumulated in reservoirs.

8.     Sediment bypass tunnel

Sediment bypass tunnels are an effective means to decrease or even stop the reservoir sedimentation process. By routing the sediments past the reservoir into the tailwater during flood events, sediment accumulation in the reservoir due to both bed-load and suspended-load can be minimized. The second advantage of sediment routing gaining importance is ecological sustainability. River bed erosion downstream of a dam is significantly reduced and the morphological variability increases. Only sediments supplied from the upstream river reach are trans- ported through the bypass tunnel, so that sediments already accumulated in the reservoir are not removed. The sediment concentration in the tailwater of the dam is not affected by the reservoir itself and therefore of natural character.

Sediment bypass tunnel design 

A sediment bypass tunnel generally consists of a guiding structure in the reservoir, an intake structure including a gate, a short and steeply-sloping acceleration section, a long and gently-sloping bypass tunnel section, and finally an outlet structure into the tail water . The discharge enters the tunnel as a free surface flow if the intake is located at the reservoir head. For this intake type the tunnel invert level is located at the river bed level. To generate supercritical flow conditions downstream of the gate, the discharge has to be accelerated by a short and steep acceleration section. The discharge enters the tunnel intake in pressurized flow conditions, if the intake is located further downstream in the reservoir. The tunnel invert level is then located below the river bed resulting in a certain excess energy head, so that free surface flow occurs down- stream of the gate. Due to the relatively high energy head the flow velocity beyond the gate is high and no acceleration section is required.

Abrasion problem

A severe problem affecting all sediment bypass tunnels is the hydro-abrasion of the tunnel invert due to the combination of high flow velocities and a high sediment transport. Depending on the geological conditions of the catchment and the hydraulic design of the bypass tunnel, the abrasion effect on the tunnel invert differs. High quartz content and large mean grain diameters contribute to high abrasion damages. Further, the mode of sediment transport in the tunnel is of relevance, as the design on the tunnel invert differs for various transport modes. Depending on the tunnel slope and the amount of transported sediment, single grains may roll or slide over the tunnel invert.

To counter abrasion, linings of steel or plates of granite, or even molten basalt can be used. Moreover, selected concretes such as micro-silicate concrete, roller-concrete, steel-fiber concrete, polymer-concrete and standard concrete (for reference) are tested. In Japan, several materials have been also tested in case of Asahi dam. These materials show better performances against abrasion damages than conventional concrete, while it will need more study to use them on wider areas because these works are very much expensive. In Japan, from the view points of initial construction cost and easy maintenance, selecting high strength concrete and preparing enough abrasion depth on top of necessary tunnel invert depth is recommended at the moment.

9.     Conclusion

Kulekhani dam’s operation since 1982 has been milestone in the hydro-electricity production in Nepal. Its failure can have critical consequences. At present it recommended that rehabilitation of Kulekhani dam to be done with proper orientation of riprap and further research to be carried out in this topic. The present condition of the Kulekhani is acceptable unless it is hit by the mechanical failures during probable maximum flood but in any case the overtopping might be very vulnerable leading to the failure.

For over 30 years continuous operation, the reservoir capacity of Kulekhani dam has decreased by about 25% of its initial capacity as a result of siltation. The siltation can be prevented, diverted or cured by various method. Reforestation in the upstream of the river and the site susceptible to erosion and sediment trapping can be best preventing act.

Sediment bypass tunnel by extensive study, first should be checked for feasibility and if it is possible, it could be most promising solution to the siltation problem, though expensive at first. It increases the life of the project significantly.

Dredging and dry excavation can be carried out for the already settled sediments depending upon the money and time available.