This is in regard to the article titled “The Sinking Dam Site of Bhutan’s Biggest Hydropower Project Under Construction” written by Mr. Yeshey Dorji and published in the Editorial section of March 03, 2018 issue of Business Bhutan.
Please note that there are many factual errors in the article which are grossly misleading. The management would like to provide the following clarifications:
The over topping of Coffer Dam occurred in the monsoons of 2016 and 2017 in PHPA II and not in PHPA I. For a Concrete Gravity Dam, the diversion arrangements comprising the Coffer Dam and Diversion Tunnel (DT) are designed based on the higher of maximum non-monsoon flow observed at Dam site or 25 years return period non-monsoon flow. Accordingly, the diversion arrangements for PHPA II have been designed for 1 in 25 years return period non-monsoon flood corresponding to 1, 118 cumecs: which means that the Coffer Dam is designed to overtop during the monsoon periods. The design compares the cost of sizes of diversion tunnel against damages and delays related to over topping of Coffer Dam during monsoon. For PHPA II, restricting DT to pass 1, 118 cumecs was overall cheaper against over flows in monsoon.
PHPA I, due to different circumstance of Dam Construction, consists of 2 Dam Construction, consists of 2 Diversion Tunnels with capacity of 2,430 cumecs.
The silt carried by the river need not be removed as it will be passed/flushed through the Sluice Gates and Silt Flushing Tunnel back into the river downstream of the Dam. It is the same arrangement as in Chhukha and Tala Projects.
The Full Reservoir Level of PHPA I is 1,202 masl (metres above sea level) and the reservoir rim level is already established. The PHPA I Dam has a gross storage of 8.38 MCM (million cubic meters), while PHPA II Dam has a gross storage of 7.00 MCM. The storage in the already existing Tala Hydropower Plant and Kurichhu Hydropower Plant are 9.80 MCM and 15.70 MCM respectively. Such volumes shall not cause seismic events or weather changes. There are hundreds of reservoirs in the world with more than 100 fold capacities.
The Downstream Surge Chamber (one of the 3 Power House Caverns) in PHPA II had a cave-in on March 03, 2016 where unfortunately 6 workers lost their lives. The remedial/restoration works are under progress.
Punatsangchhu I & II Hydroelectric Project Authority