Water overflowing above the coffer dam in P II

Punatsangchu II overflows coffer dam

The heavy monsoon downpour lead to the Punatsangchu river overflowing the coffer dam into the dam site work, that was underway on the other side.

The P II coffer dam diverts the river away from the dam site on the other side to a single diversion tunnel.

What has raised eyebrows is that this is the second time this is happening as the river had overflowed the coffer dam last year too.

This has raised concerns on not only the damage it would cause to the dam construction site but also the delay in terms of pumping out the water and clearing the debris left behind by the swollen river.

There are also questions being raised on the height of the coffer dam and why only one diversion tunnel was built for P II compared to the two diversion tunnels in P I.

Last year it took the project around one month to pump out the water and clear the debris.

A PHPA official said that this year too it would take around a month or less to pump out the water and clear the debris left behind.

This would mean that in the meantime no work can be done for the same period on the flooded dam site.

The PHPA official, however, clarified that the coffer dam has been built in such a way to allow for an overflow in the case of a strong flood. He said the technical parameter for the check dam was the highest once in 25 years non monsoon flood.

The P II check dam is designed to hold 1,400 cumecs water flow which was exceeded in the last few days leading to the overflow.

A source of the current overflow issue is down to the Detailed Project Report (DPR) which had decided on only one diversion tunnel. The PHPA official said that the DPR looked at how deep the dam excavation would be for the P II dam and also its role in the project completing on time.

Another factor the official said was that it was cheaper to allow the water to occasionally overflow than build a second diversion tunnel.

He said that this year it may take less than a month given that some concreting work had been done on the dam site.

On if there would be a delay in the project the official said that since the powerhouse was already delayed after a cavern caved in and would require rectification the one month or less delay in the dam site would not matter.

The power house stabilization stabilization works still needs to be finalized based on a final Norwegian Geological Institute (NGI) report on the rectification measures.

The NGI report which deals with rectification measures on the P I right bank and the P II power house would also give the final completion date for both projects. Both projects are currently delayed due to the rectification works required on their geologically weak areas.

P 1 which was last rescheduled to complete by July 2019 was moved to the last quarter of 2021. The project was originally supposed to be complete by November 2016 before the right bank slip was found.  P II which was supposed to be last complete in December 2018 was moved to September 2019. PII was originally supposed to be completed in 2017.

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