For the first time in two decades, a key and controversial phase in the construction of the Tehri dam has begun. The gates of two diversion tunnels have been shut to form a reservoir, officials said. And the bridge that connects Tehri to the rest of Uttaranchal has been made out of bounds. Work on the two main diversions tunnels, T-3 and T-4, now carrying the waters of the Bhagirathi, started two days ago. Out of the total 16 gates, 11 gates were in place, partially blocking the flow of the river. Tehri Tehri Hydro Development Corporation (THDC) authorities said all the gates of the tunnels were blocked by Wednesday, marking the completion of one of the most important phases of the 2,400 MW Tehri Hydro Electric Project. ‘‘No actual flooding will take place right now because the level of the water is not much,’’ said Radha Raturi, district magistrate, Tehri. In this phase, the dam would reach a level of 636 metres. Two colonies are going to be submerged — Valmiki colony with 30 families, Suman colony with 5 families. ‘‘All these people have been rehabilitated in New Tehri,’’ said Raturi.