21 March, 2010 - The department of geology and mines (DGM) has a proposal to divert the Phochu and Mochu rivers to protect Punakha dzong in case of a glacial lake overflow flood (GLOF) and cyclone floods.

It proposes moving the confluence of the two rivers more than 300 m downstream.

“We’ve sent to the dzongkhag an estimate of Nu 508,310 to survey and study the river basin area,” said Ugyen Wangda, chief geologist of DGM. If approved, construction would take around three years.

The present defenses of the Punakha dzong, built after the 1994 floods, can withstand only 18 million cubic metres (cu m), according to DGM. “The proposed diversion will protect the dzong from even a 53 million cu m flood, if the Lunana lakes of Rapstreng and Thorthormi burst,” said Ugyen Wangdi.

The proposal came up in a May 2009 home ministry meeting after the GLOF false alarm in Punakha, when an underground lake in Tshojo glacier had burst. “The main concern was on how to evacuate the holy relics of the Ranjung Kharsapani and the Machen to a safer place,” said Ugyen.

The main danger is from the headwater of Phochu, where there is a chance of GLOF, if the Rapstreng and Thorthormi lakes overflow into each other, according to DGM.

“We plan to divert the Phochu two or three kilometres before the dzong and take it over the paddy fields, which are currently on the left banks,” said Ugyen.

A kilometre long and three metres broad giant wall could be built further away from the dzong’s current wall. The enlarged area, behind the wall, is proposed to be converted into a large park. “If there is GLOF and the river overflows its banks, there should be no impact as the river basin will be very large,” said Ugyen Wangdi.

For Mochu, the primary concern is flooding as the river exceeded the 1994 flood levels during cyclone Aila. Another concern is a backflow of water from the more violent Phochu to Mochu. “The plan is to dam the Mochu at Zomlingthang, three kilometres upstream, and divert it away from the dzong, which would mean the current parking and highway would get affected, and so the highway would have to be sent above the old Punakha town,” said Ugyen Wangdi.

He said that a one kilometre wall would be built farther away from the current dzong wall. A wall would also be built on the highway side to prevent erosion.

DGM will also be assisted by experts from JICA. “If in Norway a country can hold back and divert the ocean, then I’m sure in Bhutan we can divert a river,” said Ugyen Wangdi.

The diversion would also be to avoid zigzagging the rivers as it leads to erosion and also ensure that the rivers meet smoothly at a gentle angle, instead of crashing into one another.

The Punakha dzongda Kuenzang N. Tshering said, “Though it’s possible from the Phochu side, the Mochu side will be more difficult, as the traditional bridge, highway and parking will go and there is little space.” He said the dzongkhag would have to consult the people who might be affected.