BARRAGE is the structure that is responsible for increasing the height of the watercourse and diverting the water into river intake and further to power intake through which it could be transported to turbines through the tunnel.
The construction of the barrage required the diversion and de-watering of the vicinity wherein the construction was to be done. For serving this motive, the principal watercourse became diverted through a channel. To accomplish that, a cofferdam becomes created from Boulders and muck received from excavation.
COMPONENTS OF BARRAGE
Mainly the Barrage is composed of two abutments and 4 piers joined together via breast walls. The piers and abutment have the provision of a stop log gate at the upstream side and radial gates on the downstream side. The top of the barrage has a provision of a Gantry device whose most important cause for existence is to manipulate the gate openings. The trunnion component at the downstream aspect of the barrage has the principal purpose of wearing the weight coming from the gate assembly. The structure is similarly prolonged through training walls and wing walls on both aspects of banks. The purpose of walls is to present directions to the watercourse coming undiverted via the gates. The Barrage is given support via joining it to the hard strata on each aspect. For this purpose, a drift is constructed.
A cut-off wall is made to prevent the seepage of water from upstream to the downstream side. adjoining to the cut-off wall, NOF (Non- Overflow phase) is provided to attach the deck slab of the barrage to the drift. While constructing the barrage the excavation for drift was done about 17m on the left side and 23m on the right side.
CONSTRUCTION OF BARRAGE:
- Diversion and de-watering of water:- To make the conditions to be suitable for the construction inflowing river, it was necessary to divert and de-water the area. To serve this purpose, Cofferdam was constructed which was made of boulders and muck. Dam proficiently confined the flow of water for approximately the first half-width of the river to the second half of the river so that the construction could be done in the first half. The cofferdam was designed to sustain the discharge of up to 200 cumecs.
- Excavation:- The excavation had to be done in such a manner so that the whole structure remains stable not only for maximum flowing condition. For this reason, it was necessary that the excavation should be done to such an extent so that a hard stratum is achieved.
- Plain cement concrete (P.C.C):- Filling up with P.C.C for the excavated portion provides a strong bond between the hard strata and foundation which ensures structural stability. In barrage, reinforcement details were provided only for a depth of 3m beneath the river bed level. So, the 7m of depth was filled with PCC. Provision of P.C.C creates a seepage-free condition inside the excavated area and provides a hard and leveled surface for concrete.
- Reinforcement and shuttering:- Shuttering is the process of defining the boundaries of structure for concreting purposes using plywoods. Firstly, the formwork is erected and then the reinforcement is tied. Reinforcement of company SAIL TMT of grade Fe500 is used for the construction of barrage.
- Concreting:- For concreting purposes, majorly M20A40 grade concrete was used for the superstructure and a layer of thickness 750 mm of High-Performance Concrete (HPC) was also used.M60 was also used in the basement of the barrage. High-Performance concrete is nothing but the concrete formed by using the extra amount of cement in it.
WORKING OF BARRAGE
As stated above, the purpose of the barrage is to divert the watercourse to the intake structure. This will be achieved by regulating the opening of gates so as to allow the diversion discharge of only 59.6 cumecs.
Two types of gates are installed on barrage. They are:
1. Stop log Gate: These are the gates which are constructed on the upstream side of the barrage. . The main intending purpose of providing stop log gates is for the maintenance of radial gates which are responsible for the diversion of the water. The radial shape provides an extra surface area to the flowing water, hence less pressure will be exerted on gates resulting in less damage. The opening of the gates will be controlled by a gantry machine installed on the top of the barrage. The gate assembly will have heavy-duty wires which will be connected to the gantry machine
2. Radial Gates: These are the main gates on the downstream side that serve the purpose of the storage and regulating water and it helps in diverting water to the intake.
It is given for the protection and maintenance on the concrete bay walls. During the 2013 disaster in the mandakini river, barrage was heavily affected. So In order to fill up the cracks V shaped chipping is done for protection against scouring.
It is provided in barrage in order to prevent scouring. It also serves the function
to hold the sand compacted and densified between two sheet piles in order to increase the bearing capacity when the barrage floor is designed as a raft.