The last structure at the Želivka Water Treatment Plant, where drinking water is collected before going on its way to consumers, is the regulating reservoir. Of course, its name is no accident – its water level interacts with the water level in the Jesenice reservoir, 51.97 km away. Do you remember your physics? Yes, these are “communicating vessels”!
Water from the regulating reservoir flows down into a drop shaft through a 1600 mm hole (fitted with a shut-off valve), and then without any pumping continues along the water supply tunnel all the way to the Jesenice Reservoir on the south-eastern edge of Prague. The supply tunnel also ends with a valve chamber that can be shut off. From there, two more 1600 mm pipes each connect to one of the reservoir tanks, which have a capacity of 100,000 m3 (i.e. the total volume of water they hold is 200,000 m3).
The tunnel lies at a depth ranging from 8 m to 150 m below the surface, and passes under 11 towns and villages, 34 streams, 29 roads, 2 railway lines and 2 rivers, the Blanice and the Sázava, which it gets under using inverted siphons. Its construction also involved a number of other structures required for the work (e.g. mining and ventilation shafts, connecting shafts known as “winzes” and structures enclosing the inverted siphons). Following some adaptations and retrofitting, most of these are still in use for the current operation of the tunnel.
The flow rate in the tunnel, and therefore the amount of water produced, depends on the need for drinking water supply in Prague. Water from the Želivka accounts for about 75% of the drinking water consumed in the capital city. The maximum possible flow rate through the supply tunnel is 6,700 l/s, with an average of around 2,200 l running through it every second.
This is one of the ten longest water supply tunnels in the world, and building it was one of the most demanding projects out of the entire complex of water management structures. The mined part of the structure – the tunnel itself – has a circular cross-section with a diameter of 2,640 mm and a length of 51.07 km. The concrete lining, which forms the inner surface of the tunnel, is 200–350 mm thick. Its construction began at the end of 1965 and took seven years. The mining work was carried out simultaneously on 14 sections, which were subsequently joined together.
... during operation the entire length of this tunnel is completely filled with water, and the maximum water flow rate is 6.7 m3/s, which is enough to fill more than 40 bathtubs in a second?
If you’d like to learn more, continue to the other parts of the treatment plant...
