Phurpa Lhamo  

Once confirmed, the construction of a barrage to replace the dam at Punatsangchhu Hydroelectric Project I (PI) will take at least four years to complete.

Dam construction has been stalled for a while now, and an independent study has shown that a barrage is feasible around 2.5km upstream.

According to the Druk Green Power Corporation (DGPC) managing director (MD), Dasho Chhewang Rinzin, if a decision is made in 2022 to proceed with the barrage option, the PI project could be operational by early 2026.

“The construction of a barrage with the tunnel joining the barrage to the already built usable component of the dam could take up to four years from the start date,” he said.

The DGPC is currently providing support and backup to an international hydropower consultancy group ‘Stucky’ to prepare a detailed project report (DPR) on the construction of a barrage.

In July of this year, Minister of Economic Affairs Loknath Sharma said that the government had conveyed to the Indian government that a barrage could replace the dam at PI.

The decision to construct the barrage came after the right bank of the dam experienced multiple landslides. The project witnessed its first slide in July 2013, followed by a slide in August 2016, and later again in January 2019.

In 2013, the project’s consultant, the Centre Water Commission (CWC) of India, submitted a holistic report on the dam. The CWC filed its report on strengthening measures in October 2019.

The CWC stated that the safety factor was 1.2 to 1.4.

The National Hydroelectric Power Corporation (NHPC) reviewed the report and had differing views on the report. The NHPC has pointed out flaws in the design, stating that the safety factor was insufficient.

The NHPC vetted the report and disagreed with it, because the corporation’s estimate showed a safety factor below 1.

According to international standards, for a dam to be built, the safety factor should be at least one.

The issue remained unresolved, so the government initiated an independent review, which is currently led by the DGPC.

While the DPR for the barrage was to be submitted in June 2021, the submission of the DPR has been postponed to February 2022.

Dasho Chhewang Rinzin said that while the preliminary design report submitted in March 2021 confirmed that a barrage was feasible, Stucky recommended further detailed geophysical and geotechnical investigations for design and cost optimisation through specialised agencies, especially covering the barrage footprint project components.

“Since many of these additional geotechnical investigations could not be taken up during the 2021 monsoon months due to the very high river discharge, the submission of the DPR had to be postponed to February 2022,” he said.

What is a barrage?

According to Dasho Chhewang Rinzin, a barrage is similar to a dam of lower height and, therefore, meant fewer demands in terms of the technical requirements for founding on the riverbeds.

He said that a barrage acts like a dam to divert the water from the river into the tunnels, but without the storage capacity. “Therefore, in a very true sense, the barrage is a pure run-of-the-river scheme and would generate power in keeping with the actual river discharge. Under exigencies, some of the water available for generation may have to be spilled in the winter months. With the planned barrage, the project would lose its flexibility for daily peaking generation.”

The dam meant a storage capacity that would give the project several hours of daily peaking generation capability in the winter months.

The dam also ensured flexibility in operating the powerhouse, which would have ensured that spillage of water during exigencies in the winter months was limited.

Kuensel learned that as of December last year, Nu 28 billion (B) has been spent on dam construction.

It was also learnt that the estimated cost for a barrage construction is Nu 16B. However, around Nu 4.9B worth of the assets created under the dam package could be utilised. The structures include desilting chambers and the connecting head-race tunnel, but the dam pit area will have to be abandoned.

While it has been speculated that the power generation will decrease with the construction of the barrage, Dasho Chhewang Rinzin said that all hydropower projects in Bhutan that have been built or that are under construction are considered as run-of-the-river schemes with some limited storage capacity for daily peaking in the winter months.

“With the adoption of the barrage option, due to non-availability of the flexibility of operation during the lean discharge months, the loss in generation due to exigencies when there could be spillage of water is estimated at about a maximum of 1 percent of the total estimated design annual generation of 5,544MU,” he said.

He stressed that while an impact on generation is expected with the barrage option, the decrease in power generation will be minimal.

Meanwhile, additional specialised geotechnical investigations are also ongoing.

Dasho Chhewang Rinzin said the specialised core drilling, including cone penetration tests and cross-hole topography, have started after the contractor mobilised from India and the work is expected to be completed by December of this year.

“The input from these investigations will be critical in arriving at the most optimised design and cost solution for the barrage option,” he said.

Edited by Tashi Dema