Optimization of Hydro-Power Potential of Samanalawewa Project

The Ceylon Electricity Board under the Ministry of Power & Energy commenced the construction of Samanalawewa Hydroelectric Project in early 1987. The aim of the project was to harness the hydro electric potential of the Walawe Ganga to produce about 430 million units of firm electrical energy per anum and to add 120 MW power to the national grid. The project consists of a rockfill dam with a crest length of about 530 m and a height of about 100m above the river bed level. Samanalawewa dam is located just downstream of the confluence point of Walawe Ganga and Belihul Oya, one of its main tributaries. The reservoir created by the dam has a total live storage capacity of 218 MCM and the dead storage is about 60 MCM. The U shaped reservoir spreads to a distance of 8km upstream along two limbs of the Walawe Ganga and Belihul Oya. The intake with the waterway system for the powerhouse is located about 6 km upstream of the dam. At this region, the reservoir and a secondary tributary of Walawe, Katupath Oya come nearest to each other and the level difference exceeds 400m. (Fig 1)


Introduction
The Ceylon Electricity Board under the Ministry of Power & Energy commenced the construction of Samanalawewa Hydroelectric Project in early 1987.The aim of the project was to harness the hydro electric potential of the Walawe Ganga to produce about 430 million units of firm electrical energy per anum and to add 120 MW power to the national grid.The project consists of a rockfill dam with a crest length of about 530 m and a height of about 100m above the river bed level.Samanalawewa dam is located just downstream of the confluence point of Walawe Ganga and Belihul Oya, one of its main tributaries.The reservoir created by the dam has a total live storage capacity of 218 MCM and the dead storage is about 60 MCM.The U shaped reservoir spreads to a distance of 8km upstream along two limbs of the Walawe Ganga and Belihul Oya.The intake with the waterway system for the powerhouse is located about 6 km upstream of the dam.At this region, the reservoir and a secondary tributary of Walawe, Katupath Oya come nearest to each other and the level difference exceeds 400m.(Fig 1) The principal feature of the project is to make use of this difference for power generation at a power station on the left bank of Katupath Oya.The plant has been generating power since its commissioning in October 1992, amidst the leakage problem which developed in the reservoir due to the adverse geological conditions prevailing in the Right Bank.The reservoir was operated 30 m below the full supply level until reservoir remedial works were completed in 1999.The reservoir remedial works -Wet Blanketing was successfully completed by lowering ground water level of the right bank ridge to a safe limit even at the full supply level.However, the reduction of main leakage was only 25% and it stands maximum of 2.18 cumecs at full supply level of460 m MSL.It fluctuates with the reservoir level and recorded minimum value was 1.37 cumecs at 428m MSL Ceylon Electricity Board (CEB) has obligations to release sufficient water from the Samanalawewa Dam for downstream water users, which was, estimated about 80 MCM annually.Since the average runoff of the reservoir is 590 MCM, downstream release significantly affect the total energy production of the project.CEB continues to loose about 28 GWH annually due to additional irrigation release from the reservoir as this water does not pass through turbines.This paper propose to establish a system of mini-hydro power plants to recapture the lost energy and to enhance the power potential from the project.

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ENGINEER the construction of Samanala wewa Reservoir, the Uggal-kaltota anicut receives water from a reduced catchment area of 68.0 sq.kms below the Samanalawewa dam due to interception of 342 sq.kms by the dam.
Since the riparian rights of the downstream water users have to be respected, CEB has to issue sufficient quantities of water for irrigation.The average water release from Samanalawewa Dam to the Kaltota irrigation scheme is more than 3.0 cumecs in most of the year.An average flow of 1.62 cumecs is received from natural release of the main leak in the Right Bank of the dam and the balance through the irrigation outlet of the dam.Table 1 shows the annual discharge from the Samanalawewa Reservoir to the downstream from 1993 to 2004.The annual average runoff of the Samanalawewa reservoir is estimated as 598 MCM and average downstream release amount to 91 MCM for the period of 1999 to 2004.

Downstream water requirements.
A continuous downstream release is essential at the Samanalawewa dam for the irrigation requirement and environmental requirement.

Irrigation requirement
Theoretically, natural leakage is sufficient for irrigation requirements in most of the year except during land preparation.However, additional releases through Irrigation Outlet are required due to reach losses and excessive water duty of Kaltota Scheme.

Environmental requirement
At the time of construction of Samanalawewa dam, there was no strict environmental regulations imposed on major development works.Therefore environmentalist attention was not drawn for the cut-off of river flow to the Duvili Falls located downstream of dam.But today, the location has developed as major local tourist attraction in the area.In any event it would be necessary to release about 1.5 cumecs to sustain the beauty of waterfall.

Loss of power potential due to controlled release
The water released from Samanalawewa dam has a very high "Opportunity cost" attached since the option of generating power out of this has to be forgone.As there is no control for natural leakage, value can be attached only for Irrigation outlet releases.
Table 2 gives the average energy storage of one cubic metre of the reservoir for different reservoir levels.The operating head of turbines of the Samanalawewa power station is more than 300m and therefore, reservoir level fluctuation has little impact on energy storage per cubic metre in the reservoir.
The amount of lost energy can be calculated using daily monitoring data of the water release and reservoir level.Table 3 shows the loss of energy from 1999 to 2004.
The average loss of electrical energy for the last six years of operation is 28 GWH and the value could be estimated as RS 200 million at current average selling price of electricity unit.

Method of Recovery of Lost Energy
Reduction of consumption of water for irrigation is one of the options for minimizing the loss of power generation.It is reported that various methods of farming practice such as staggering of land preparation, rotational distribution of water, alternative crops introduction and use of "Madagaskar Method" a method using less water for paddy cultivation have been tried out without much success.
Therefore, it is proposed to operate mini hydropower plants using irrigation water to recover energy as much as possible.Three potential sites have been identified.They are  The water releases to the downstream from the Samanalawewa Dam could be utilized to generate about 28 GWH of electricity energy annually from three mini-hydro plants with combined installed capacity of 5.6 MW and water shall be released back to the Walawe River for irrigation.

Mini-Hydro plant Utilizing Irrigation Release
Samanalawewa dam has an irrigation and bottom outlet installed in one of the diversion tunnel.The steel conduit has a guard valve (butterfly valve) towards the reservoir side and a bifurcation for two gated outlets at the downstream end.The bottom out let can release maximum 70 cumecs and the irrigation valve can discharge maximum about 7 cumecs.It has been observed that the bottom outlet has to keep open 70% of the time to supplement downstream irrigation requirement.
It is proposed to tap the steel conduit before the bifurcation to facilitate an inlet for the proposed mini-hydro plant.This plant can be located inside the diversion tunnel in a 3m wide passage adjacent to the bifurcation block.

Flow Characteristics & Design Flow
Outflow data from the irrigation valve on daily basis is available since impounding in Mayl991.
The main leak appeared in October 1992 and the reservoir remedial works completed towards the beginning of 1999.Since then, a regular pattern of water releases can be observed.Therefore, for the purpose of this study, flow records from 1999 to 2003 were considered.The associated hydrograph of the plant is shown in the Fig. 2 based on daily flow records.The minimum and the maximum discharges are 0.5 cumecs and 3.0 cumecs respectively.The flowduration curve is given in the Fig 3.
In view of the increasing demand for irrigation water at the Kaltota Irrigation Scheme, it is sensible to design the plant for 3.0 cumecs.

Gross Head of the Irrigation Outlet Power Plant
The minimum operation level and the full supply level of the Samanalawewa reservoir are 424 m msl and 460m msl.respectively.The tailrace level of the plant shall be 365.5 m msl Accordingly, the available head for the plant varies between 94.5 m and 60.5m.However, in most of the time the reservoir operates between 430m to 455m msl.
Therefore, for the purpose of design of the plant, the maximum head available was taken as 90m.

Power Plant
The turbine with the spiral casing and draft tube shall be installed on the invert of the diversion tunnel adjacent to the bifurcation block The invert level of the diversion tunnels is 365 m msl.An opening shall be provided for the draft tube by breaking concrete wall, downstream face of the tunnel.The plunge pool water level is about 365m msl.This level can be taken as the tailrace level.
It is recommended to install Tergo Impulse or Francis type, vertical axis, and medium speed turbine.However, a detailed study shall be carried out to select the most suitable type of turbine considering the head variation and flow variation during the detailed design stage.The installed capacity of the plant is estimated about 2.0 MW.   4.

Mini-hydro plant Utilizing Main leakage
Before the conclusion of the reservoir remedial works -wet blanketing, the maximum leakage at full supply level was 2.7 cumecs.This was reduced to 2.2 cumecs in December 1998 after wet blanketing.Therefore, the flow measurements before 1999 was discarded, as they were no longer represent the actual present conditions.The design flow is taken as 2.0 cumecs.

Intake and the Headrace Canal
The intake water level at the leakage outlet is 293m msl.The intake shall be located just downstream of the outlet.It is proposed to transmit the leakage flow along a contour canal.
On the Right Bank.The total length of the canal shall be 700 meters.The design capacity of the canal would be 2.20 cumecs with a velocity of the flow of 1.8 m/s.The entire length of the canal shall be lined to avoid any seepage and transmission losses The slope of the canal shall be 0.001 in order to reduce the sectional area as the canal is running along a steep slope for a distance of 200m.Aqueducts shall be provided in most of the places.However, the construction will not be difficult due to convenient access along the canal route.
A rectangular section (1.80 m X 1.20m ) will be selected with 0.3m freeboard A measuring weir or flume shall be incorporated to the intake structure to continue monitoring of main leak.

Forebay Tank and Spill '
A forebay tank with a large stilling tank and a trash rack shall be provided at the end of the headrace canal.Aside spill shall be incorporated to the forebay tank in order to release the flow during emergency shut off of the machine.

Power House, Penstock and Tailrace
The powerhouse shall be located on the right bank of the Walawe Ganga close to the confluence of Kalunaide Ara with the river at an elevation 350 m MSL.The tailrace canal would be provided and the design water level of tailrace would be 246 m MSL.The estimated length of the penstock is 200 meters.
The Samanalawewa spillway, which was designed for a flood with the return period of 1000 years, has the flood discharge capacity of 3600 cumecs.Due consideration will be given to the possibility of flood waters wash away the plant in a rare event of a 1000 year flood.A detailed analysis should be carried out to select the exact location and the machine chamber elevation of the powerhouse.

Hydrology
The river flow measurements have to be modified In the rainy season, the irrigation release from the Samanalawewa is reduced.Therefore diversion of Baduludena Ara to the fore bay in rainy period has significant impact on the total annual energy produced from the plant.

Head Race Canal
The head race canal shall be located on the Left Bank of the river.It would be a 1600m long contour canal.

Project Cost and The Benefits
The Construction cost of the projects is estimated to be Rupees 550 Millions excluding the consultancy work.The average annual income from the planned project is expected to be Rs.160 million.

Environmental Concerns
The project sites are completely urunhibited area and belongs to forest Department/ Ceylon Electricity Board.The project area is predominantly forestry and range land.Therefore, proposed project does not require relocation of people.Construction of Samanalawewa reservoir involved inundating of 897 ha.whereas the proposed power projects do not require inundating of land.

Conclusion
As explained above, the preliminary study conducted shows that the proposed mini-hydro power project is technically, environmentally and economically feasible.

1. 1
Downstream water releases The dam has been provided Irrigation and low level outlets to release irrigation requirements of the downstream and emptying in case of emergency.This is by means of a 2m diameter steel pipeline extending up to the downstream face of the dam and installed in one of the diversion tunnels (Tunnel 1).At the downstream end of the pipeline a hydraulically driven Jet flow gate with a maximum discharge capacity of 70 cumecs at full supply level is provided for discharge regulation.Irrigation release is facilitated through a small by pass line fitted with a Howell Bunger valve and it has max discharging capacity of 7 cumecs at full supply level.The diverted water for power generation reenters the Walawe Ganga about 20km downstream of the dam.The Kaltota Irrigation Scheme that was completed in early 6Cs is situated 12 kms downstream of the Samanalawewa Dam.It has a command area of 1100 Ha and water is supplied to the scheme from the Uggal Kaltota anicuts in the Walawe Ganga.The two anicuts divert water to both the Left Bank Canal and the Right Bank Canal.Since Eng.Da Wijcsinghc.BSc Eng.. M.TtcUllTD).CEng.. MIE(SL) Consulting Engineer.Nippon Koel Co., Ltd.

Fig 1
Fig 1 Layout nap of Sananalamn Project and LocoMon of PnpoMd.HM-Hydra Plants

3 .
To reduce the need for thermal power plants thereby minimizing the air pollution.

Figure 2 :
Figure 2 : Hydrograph for Daily Irrigation Release

Figure 3 :
Figure 3 : Flow Duration Curve -Irrigation Release to that of Samanalawewa Power Station for the last 6 years = (28.247/259.237)= 11 % ENGINEER The main leakage found to be stable and it fluctuates with the reservoir level.Leakage emerges from the right bank ridge at about elevation 393 m MSL.4.2.1 Flow Characteristics & the Design Flow of the Main Leak The associated hydrographs of the plant are shown in the Fig. 4 based on daily flow measurements.The mean average flow of the leak is 1.74 cumecs.The minimum and the maximum flow are 1.34 cumecs and 2.2 cumecs respectively.The flow -duration curve is given in the Fig 5.

Figure 4 :
Figure 4 : Daily Canal Flow at the Gauging Site (Main Leak) Figure 6 : Hydrograph for the Gauging Site line shall be constructed.The total length of the penstock line would be 550 m.Nine anchor blocks and 56 nos ring supports shall be provided for the penstock.Bifurcation shall be located close to the powerhouse.Diameter of the penstock line would be 1.35mThe powerhouse shall be located on the left bank of the Walawe Ganga close to the confluence of Baduludena Oya with the river at an elevation 253 m MSL.The tailrace canal would be provided and the design water level of tailrace would be 250 Connection to the National Grid 33 kV lines are available at the Samanalawewa Dam site for the plants 1&2 and the third plant could be connected to the 33 kV line at Watawala by a 7 km long transmission line for connection to the national grid..

Table 3 :
Loss of Power generation due to Controlled

Table 4 :
Annual Energy Potential of Proposed Mini-Hydro Plants and comparison of Annual EnergyProduction with Samanalawewa Power Station (Unit; GWh) Percentage of Annual Energy from Mini Hydro Plants