The current P-I dam site which will likely be abandoned

Current Punatsangchu I dam likely to be abandoned for a barrage upstream

The current Punatsangchu I dam site is likely to be abandoned as a third party foreign expert has recommended that the Punatsangchu I project abandon its current dam site and instead build a barrage upstream.

This recommendation is the final nail in the coffin of the P-1 dam which was already under a cloud with a geologically weak right bank that threatened the stability of any future dam on the current spot.

The government had appointed this third party foreign expert after two expert Indian agencies, the Central Water Commission (CWC), which is the P-1 project’s civil engineering consultant, and the National Hydro Power Corporation (NHPC), brought in to review CWC’s proposal as an external expert third party did not agree on the ‘factor of safety’ of the right bank to build a dam at the current location.

The NHPC had been hired to review the CWC’s original recommendation to build the dam at the current site after some strengthening measures.

A dam site would have to have a minimum factor of safety of 1 or above as per international standards while the European factor of safety is 1.5 to 1.8.

The CWC in its report and drawings gave a factor of safety of 1.2 to 1.4, however, the NHPC hired to review this CWC report as a third party disagreed with this CWC assessment and said the factor of safety is below 1 and hence not safe enough to build a dam.

The NHPC was saying that CWC’s strengthening measures and drawings for the right bank presented to the Technical Coordination Committee in October and December 2019 is not secure enough for a dam to be built right next to it.

The NHPC instead recommended to build a smaller structure in the form of a barrage 820 meters upstream of the current site.

A dam significantly raises the water level and has storage capacity while a barrage is a less tall structure that focuses more on diverting water flow.

The NHPC’s third party review had to undergo another review by this foreign third party.

This foreign company after its own study and review of the reports of the CWC and NHPC concluded that the current dam site is too risky due to the weakness of the right bank.

The foreign company, like the NHPC, has instead recommended a barrage upstream from the current site. The barrage itself has several options of what type of barrage is to be built.

The barrage is expected to be a much less risky proposition compared to the dam.

A source said the 1,200 MW project power generation is not expected to be significantly affected. The barrage option is expected to save costs compared to the more expensive dam. It will also mean that the highway passing over the dam will not have to be relocated.

The Government of India has been informally informed of the expert party’s review and according to sources both governments are likely to abandon the current dam site and go for the barrage option.

Even though the barrage is built upstream, it will still be able to use the Head Race Tunnel and the Power House which are almost complete. The NHPC had said that the current Head Race Tunnel can be elongated to reach this barrage.

The expert report will be presented to Punatsangchu Project Authority meeting expected to happen in the next one to two weeks.

The NHPC’s criticism was not just restricted to the right bank but it had also felt that the ‘Stilling Basin’ or a depression to reduce the velocity and flow of the water coming out of the Dam’s sluice gates is not adequate.

Normally a stilling basin has hard rock under it but given its absence the proposal was to build a 7-meter concrete raft to take the impact.

The project’s original completion date was November 2016 but with repeated delays especially with regard to the right bank area it got pushed to several new deadlines with the last one being in 2025.

As per the Budget 2020 document P-I hydroelectric is expected to be commissioned in March, 2025 with revised estimated cost of Nu 93.755 bn. The project had started back in 2008 with a budget allocation of 35 bn.

As of March, 2020, Nu 79.453 bn has been disbursed and 86.65 percent of the works had been completed with the main uncompleted works being the dam.

When the first slide on the right bank occurred in July 2013 it was CWC that said it has the capacity to deal with the issue and accordingly rectification measures were done but they failed in the August 2016 slide which again failed in the 2019 slide.

Background

The problem all started in 2008 when the then PHPA MD R.N Khazanchi proposed to change the original dam site to generate more power. The then cabinet agreed in June 2008 based on Khazanchi’s presentation.

A Joint Audit Report of the Royal Audit Authority (RAA) of Bhutan and the Comptroller General Audit of India (CAG) of 2012-13 said that the PHPA, its main consultant WAPCOS and CWC knew that there were geological weaknesses at the right bank area but still went ahead with the tendering of the dam on the same site in 2009.

Instead of conducting further investigations PHPA’s overall consultant WAPCOS, on behalf of GSI, issued a clearance for the project dam bids to be opened on 16th February 2009.

The problem made itself known with a major slide on the right bank in July 2013 which at the time the overall consultant WAPCOS and the civil consultant CWC said could be fixed with Nu 3.5 bn of mitigation measures. However, despite the measures another major slide happened in August 2016 and again despite additional mitigation measures the same area slid on 22nd January 2019.

After this, the government in the 21st PHPA Authority meeting in January 2019 insisted to the Indian consultants to come up with a comprehensive solution.

CWC was tasked to come up with a holistic solution to strengthen the Right Bank without which construction of the Dam on the right bank side could not proceed.  The report was submitted by CWC on 1st October 2019 and detailed drawings later in December of the same year.

PHPA officials at the time said they were confident enough with the report to start concreting of the dam from the stable left bank side. The total dam width is 241 meters the the starting work area in the left side was 51 meters.

One suggestion of the CWC was that the dam on the right bank side be constructed over 288 concrete piles driven into the ground to avoid excavating the shear zone below the right bank side

The Bhutanese side not very convinced with the idea of concrete piles under the dam for the long term and feeling the need for an overall review of CWC’s proposal called for a review in the Technical Coordination Committee meeting

The National Hydro Power Corporation (NHPC) was then called upon to do a third party review of the WAPCOS and CWC proposal.

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