Picture Courtesy @NipunPerera Photography
Randenigala Hydro Electric Embankment Dam was constructed back in 1986 which supports the largest reservoir under the accelerated Mahaweli Development program launched back in 1978.
The embankment dam is known to be a rock filled earth dam consisting a clay core, which was constructed in purpose of generating power and sustaining irrigational activities in the Mahaweli River catchment.
Constructions were mainly carried out with a joint venture between the German Government and Sri Lankan Authorities (Alzgitter Consult GmbH, Agrar-Und Hydro-Technik GmbH and Electro watt Engineering Services Ltd and CECB )and since the initiation of the operations, the power plant at Randenigala has been accumulating 428 GWh annually (Capacity 126 MW) for the electricity grid in Sri Lanka.
Hydro Electric Embankment Dam
As mentioned in above, Randenigala dam and the reservoir are constructed to fulfill the motives of, generating power, and sustaining agricultural activities in the Mahaweli-Catchment.
This Hydro Electric Embankment Dam is located 19km downstream from the renown, 'Victoria' Concrete Arch Dam.
The characteristics of the dam are as followed,
Construction Specifications and Sequence
When the joint venture between CECB and the alzgitter Consult GmbH, Agrar-Und Hydro-technik GmbH and Electrowatt Engineering Services Ltd,
handed over the dam to the Mahaweli Authority back in 1986, the dam was ensured to sustain 100 years, without any major maintenances.
Unlike its’ companion 19 km upstream, The 'Victoria' Concrete arch dam,
rock filled earth dam in Randenigala has recorded a lesser number of rectification and preservation drives due to its sustainability and the stability in utilized material.
Investigations
Prior to the initiation of the construction sequence, many investigations were carried out to asses, the
Quality of the Bed Rock
Its' Permeability
Integrity
and Fractures.
Past records indicate that 31 bore holes were driven for the investigation purposes, varying at a depth from 40m to 200m and at an inclination, ranging between (0 - 50) degrees.
Including those, two exploratory adits were also contrived on either bank in the core trench promoting further more investigations.
General Layout of the Randenigala Project, Gunawardhena A.U. , 2001
Diversion Tunnel
To proceed with the dam constructions, the flowing water was required to be diverted from its' natural path. In Randenigala project, a tunnel in a length of 410 m was constructed in the left strand, achieving this purpose.
The tunnel was constructed with a concrete base layer, for water to be flowed, effectively.
Sketch of the Diversion Tunnel (Not to Scale)
Constructions of the Diversion Tunnel
Coffer Dams
Coffer dams are constructed in order to divert and prevent water from accessing the Excavation site. In the interim, 3 coffer dams were contrived at Randenigala project for the convenience of the proceedings.
Since the coffer dams are temporary structures fabricated using rock and soil, dam construction needed to be continued along the the maintenance of the coffer dams accounting with their high erosion rates as well.
Constructions of the Coffer Dams
Construction of the Dam
Dam construction was initiated on the foundation which was driven up to the bedrock. The clay core was initially placed allowing for the consolidation and settlements, in a layered manner adapting with proper compaction techniques.
Then the 2 Gneisses layers on each face of the dam, were also constructed simultaneously with the clay core, acting as the filter layer and a protective barrier for the core.
Cross Section of the Dam (Not to scale)
Constructions of the Dam
Features of the Reservoir
Source - Explore Sri Lanka
Randenigala dam creates the Randenigala Reservoir which consists a surface of 13.5 km2 and water storage up to 861,000,000 m3, and held its' dynasty as the largest reservoir in the Mahaweli Catchment up until the completion or the 'Kulasinghe Reservoir' , A.k.a. Moragahakanda Reservoir back in 2018.
Reservoir receives its’ intake mainly from tropical Precipitation (1 250 mm – 3 000 mm, annual rainfall) along with the inflow from Victoria Reservoir through Mahaweli stream and some sub streams in the catchment as well.
Unlike any other reservoir located in Sri Lanka, Randenigala is the fulcrum for a flourished Sanctuary known as 'Victoria-Rantambe-Randenigala Sanctuary' identified in the vicinity.
With the minimum human interactions recorded in the region, an efficacious relationship between wild life and aquatic life is managed throughout for 4 decades since the completion of the dam.
Electricity Generation
Power Station
Being a principal reservoir in the Mahaweli Development Program, the power generation station at Randenigala was expected be at 525 GWh annually at its’ full capacity.
2 turbines (Francis Type) have been installed in the power station, each having the capacity of 63 MW. (Combined – 126 MW) which closely accumulates up to 20% of the total electricity demand in the country.
Water reaches the turbines and accumulates to the downstream afterwards via a high pressure tunnel constructed in the right stride of the dam.
Sketch of the Power Station and High Pressure Tunnel (Not to Scale)
However, since the reservoir water needs to be utilized for agricultural undertakings as well, usually the power generation tops up at about 428GWh annually at the power station.
Furthermore, since the vernissage of the dam back in 1986, depth of the reservoir has been dwindling significantly, due to the silt and sediment accumulation at the reservoir bottom, overtopping the estimated numbers drastically (2020). By cause of this depth reduction and the changes occurred in the rainfall patterns, the power station has not been able to operate at its’ maximum capacity over the past couple of decades and the secondary improvement of 100 GWh was never carried out.
Agricultural Activities
Water discharged at the outlet of the Randenigala power station, is headed to the Minipe Weir after running through the Rantambe Dam and power generation center which is located 2.6 km downstream.
At the weir, water is being diverted to the Minipe Right Bank Canal which nourishes the Ulhitiya Reservoir and the new weir located 250 m upstream divert the water to the old Minipe Left Bank Canal.
With the construction of the supply canal system, the cultivators (especially in the dry region) gained the opportunity to continue their agricultural activities during the both seasons and in addition, they gained the opportunity to
improve their harvest
expand the cultivating possibilities and
adapt to new cultivation practices
from the continuance conveyance as well.
References
A. C. De S. Hewavisenthi (1992) Mahaweli Water Resources Project, Water International, 17:1, 33-43, DOI: 10.1080/02508069208686126
PERMEABILITY OF RANDENIGALA DAM FOUNDATION, by A.U. Gunasekera
ENGINEERING GEOLOGY OF RANDENIGALA HYDRO POWER PROJECT SITE by A.U. Gunasekera
Comments