BRIEF DESCRIPTION OF THE ENGINEERING WORKS

 

1.      Power Generation

 

In the design of structures and reservoirs, the power potential of the rivers was considered as well as the supply of water for irrigation. To determine the power potential, studies were carried out for the proposed structures along the Mahaweli Ganga and its tributaries, with approximate estimates of effective flow, of the installed capacity of power plants and of their annual energy output. These preliminary studies showed that, in all, 58 irrigation and power units could be constructed, 29:purly for power generation and 18 purely for irrigation, including anicuts and new tanks.

 

The potential for power development of all these units would be 955 MW, with an annual of 4,720 kWh greatly exceeds the prospective demand for power in the whole country Y*for the next 20 years. in view of this, designs for hydro-electric stations were carried out only for multipurpose units, the parameters of which are given in Table Vlll-1.

 

TABLE VIII - I

Power Characteristics of Multipurpose Hydro-Units

 

                                        Power Capacity in MW                    Annual Power Output in mln. kWh

 Hydro-Unit                      -------------------------------         -----------------------------------------

Firm              Installed                           Firm                 Mean

 

1    Polgolla                 ..        27                   36                                239               26222         

2.   Bowatenna ..         ..        20                   44                                171                   210
3.   Moragahakanda    ..        19                   40                                165                   206
4.   Victoria

. .             ..        49                 120                                430                   570
5.   Randenigala          ..        31                   75                                268                    345
6.   Kotmale - .           ..        51                 105                                448                   580
7.   Upper Uma Oya   ..        11                   25                                95                    126
8.   Lower Uma Oya   ..        13                   30                                113                   148
9.   Taldena ..  ..                    5                     14                                  48                     72
10. Pallewe1a .           ..         4                    10                                  35                     50
11. Heen Ganga          ..        3                      7                                   25                     35

 

                Total ..               233                   506                             2,037                2,611

 

As can be seen from the above Table, the most important units are those of Victoria, Randenigala and Kotmale, with a total energy output of 1,500 million kWh per year, or nearly 60 percent of the total for the whole Scheme.

2. Proposed Water Distribution System ²

 

The key water supply units of the Scheme will be Victoria/Randenigala, providing 1,392,000 Bert feet of useful water, and Kotmale/Moragahakanda, providing 1,369,000 acre feet of useful water. These units were therefore designed in detail. The Victoria/Randenigala units will serve as the main source of irrigation for the Mahaweli Ganga and Maduru Oya basins, while the Kotmale/Moragaha­kanda units will be the most important source for the irrigation systems in the north-central part of the country.

 

The distribution of water is planned as follows:

 

1        The total hydropower potential was somewhat reduced in the second stage of the survey, as against the first, with finalization of runoff parameters and selection of more economic parameters of the proposed electric stations.

 

2         systems considered by the project are discussed in Appendix VII..

 

Irrigation systems in the central basin of the Mahaweli and the Maduru Oya basins systems D-I, D-2, C, B, E) will be supplied with water through the Mahaweli right bank (R.B.) anti Mahaweli left bank (L.B.) canals, from the diversion structure of Minipe on the Mahaweli Ganga. These canals will be augment en route from reservoirs on tributaries of the Mahaweli. Systems F and 0 will be supplied with water from the Kalu Ganga- reservoir.

 

The lower basin (system A) will receive water from the diversion structure at Kandakadu.

 

Irrigation systems in the north-central part of the country will be supplied by two routes: from The diversion structure at Polgolla on the Mahaweli, through a tunnel to Sudu Ganga and the Polgolla­ Weli Oya (P. K.) canal, to the Kala Oya basin (system H) ; and from the diversion structure at Elahera on the Amban Ganga. through the NCR (North-Central part) canal to the other irrigation systems.

 

The hydro-units at Polgolla, Randenigala

, Victoria

and Moragahakanda. as well as the trans basin canal, will be operated as a joint water supply system.

 

The useful release from Moragahakanda unit will thus be augmented by the units at Kotmale and Polgolla through the Sudu Ganga, which flows into the Amban Ganga. from 561,000 acre feet (from its own catchments) to 1 ,396.000 acre feet per year.

Randenigala hydro-unit receives an additional supply of water from Victoria

hydra-unit above it. Practically speaking, the regulated discharge at Randenigala is the sum total effect of both these units. At the same time, the operational regimes of both Victoria and Randenigala depend on Polgolla hydra-unit; thus as the flow diverted at Polgolla to Moragahakanda increases, the regulated yield at Victoria and Randenigala abates, and vice versa.

 

The study of alternatives has established that the optimum diverted discharge from Polgolla should be fixed at a maximum of 2,000 cubic feet per second, lessening considerably in the dry periods. With this discharge, the maximum quantity of regulated flow is provided in the system of the above hydro-units.

 

During critical years, it will be necessary to increase the water supply by an estimated 110,033 acre feet per year to the NCP canal which feeds the irrigation systems in the north-central part of the country. This water can be obtained from Randenigala reservoir through the Left Bank

canal, from which water would be lifted to the NCP canal by a small pumping station (consuming up to a maximum of 5 MW) on the canal after the Amban Ganga crossing.

 

The final water management balance differs from that prepared in the first stage of the survey mainly in the substantial decrease of the volume of water to be pumped from the Left Bank

canal to the NCR canal (110,000 acre feet versus 634,000 acre feet). It is proposed to obtain this reduction by increasing the area of irrigation in the Mahaweli basin and reducing the proportion of paddy rotation in the north-central part of the country.

 

3.   Irrigation and Drainage

Canals

 

Irrigation and drainage canals for blocks of 5,000 acres were designed individually using large scale maps (of no less than 1: 10,000 wherever available) or by reconnaissance tracings. The irriga­tion and drainage network inside the 5,000 acre blocks was designed for the first phase of development on the three standard plots, (see pages 34 & 35), which have representative proportions of paddy and other crops. The total area of these plots is about 15,000 acres. For other phases this network was estimated by pro rata indicators derived in the first stage of the survey, by way of visual surveys in the field and on one standard plot.

 

Improvements to existing irrigation systems will increase water supplies by strengthening the canal network and providing water distribution structures where necessary, with water measuring devices wherever possible. Small-scale village irrigation systems will be absorbed into the new large irrigation blocks.

 

Transbasin canals

 

The alignment of the transbasin canals passes through complicated topographical and geological conditions, crossing a considerable number of drainage lines. They will heed to be lined through their entire length and provided with the structures necessary to control and operate the water dis­tribution.

 

The north - central part (NCP) canal begins at the diversion structure at Elahera on Amban Ganga and continues north to cross the water divide between the Mahaweli basin and the watersheds of the rivers in the north-central part of the country. The NCP canal commands all the systems in this region. excepting system H. Its length is 100 miles; its maximum conveyance capacity at the head, 2.070 cusecs.

 

Left Bunt (L.B) tonal begins at the Minipe anicut on the Mahaweli Ganga and follows the course of the existing Minipe canal to cross Heen Ganga. The latter canal will be reconstructed to suit the purpose. Continuing in a northerly direction, the left bank canal discharges into the Kaudulla tank. The length of the canal is 89.5 miles its maximum discharge, 1,140 cusecs.

 

The Right Bank

(RB.) canal originates from Minipe anicut and, going in a north-easterly direc­tion, crosses three large streams, the Logall Oya, Badulu Oya, and Heppola Oya. It then discharges into the Ulhitiya Oya river, on which there is to be a dam which will also serve as a level-crossing of the canal. From the Ulhitiya reservoir, the canal leads into a tunnel which crosses the water divide between the basins of Mahaweli Ganga and Maduru Oya, to release the diverted discharge into the Maduru Oya reservoir. The canal trace was finalized in the course of the detailed investigations in the second stage of the survey. The Right Bank

canal will be 28.5 miles long, carrying 1,000 cusecs at maximum discharge.

 

The Po/go//a-Ku/a (PK) Oya canal starts from Bowatenna hydro-unit on the Amban Ganga, crosses the water divide between the basins of Amban Ganga and Kala Oya by a tunnel and continues in a northerly direction to supply water to the Kandalama tank. The length of the PK. canal is 9 miles. its maximum discharge, 750 cusecs.

 

Drainage canals

 

Drainage channels and controls arc planned to provide cross drainage under irrigation canals and field channels to natural drainage lines in the irrigated areas. ~Drainage facilities have to be provided in areas already irrigated as well as in the land to be developed. Some measure of flood control will he obtained by storage in the reservoirs, but on the coastal plains of the Mahaweli Ganga, embankments will have to be built to protect agricultural land from inundation. The embankments have been designed to provide protection against floods of 20 years’ frequency, with a flood plain half a mile wide, assuming the river channel is properly cleared and maintained. They will have to be at least 10-12 feet (3-4 metres) high.

 

4.    Flood Control

 

Flood control in the urban and rural areas subject to inundation under the present natural flow regime of the Mahaweli will be provided by absorbing flood peaks in the reservoirs at Kotmale and Randenigala and in other irrigation and power units on tributaries. With the construction of the Kotmale dam, the inundation of 5.2 square miles of urban areas will be prevented and damage will be considerably reduced. The absorption of flood peaks in the Randenigala reservoir and on the tributaries of the Mahaweli Ganga will reduce the cost of the embankments necessary in the coastal plains.