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Among the different types of flood Bhutan experiences, Thimphu is most vulnerable to rainfall-induced floods.

This was shared during the Thimphu thromde’s disaster management committee meeting held on October 16.

Thimphu thrompon Kinlay Dorjee said that although there is no lake at the source of Wangchhu, the disaster management team still needs to prepare, as there are chances of flood due to heavy rainfall.

Thimphu probabilistic flood hazard map prepared with limited data available (Photo courtesy: MoWHS)

He said that if there is continuous rainfall, about four areas are identified along the Wangchhu as risk areas – the Lingkana palace area, Changlimithang, Changjiji football ground, and the Convention Centre. “It is important to research, prepare, and inform the public about flood especially to those who are staying in the risk prone areas.”

Deputy executive engineer of Flood Engineering and Management division with the works and human settlement ministry, Kuenzang Choden, said that a research was done to improve the current flood management in Wangchhu basin through improved flood hazard mapping using global data sets.

The research focuses on how, where, and which places might be affected in case of flooding in the four risk prone areas of Thimphu.

Kuenzang Choden said that the team collected data, and conducted flood frequency analysis before setting up the hydrodynamic models.  “After the hydrodynamic models were set up, the results were analysed and the probabilistic flood hazard map for 100 years return period was produced.”

She said that although the map is not 100 percent accurate, it could be used to predict where the flood might occur, the depth of the flood, and the flood’s reach, so that people could avoid building important structures or build appropriate structures in the areas. “After understanding the uncertainties in the input data and the model itself, I would suggest not to focus only on mapping itself but conduct detailed field investigation,” she said. “The detailed field investigation and flood vulnerability assessment for small streams such as Olarongchu and Chubachu are  underway.”

Kuenzang Choden said that retaining walls are built taking only the amount of water it can withhold into account, and not other things that rivers carry.

She said it is important to not block the natural course of the river. “With the experience in flood management, it is sometimes the small obstructions in river flow path that matters more. It appears to be a small risk but has the potential to cause huge damage,” she said. “Therefore, it is important to create room for rivers by cutting trees that grow in the river path, stabilise slopes near the rivers, and clear other obstructions such as old bridges that are not in use anymore.”

Kuenzang Choden said that it is important to lay critical infrastructures such as water supply and sewerage pipes at locations where there is no risk of flooding or any other hazard. “These infrastructures were however, found to be laid on and across the stream bed and flood plain areas along Chubachu and Olarongchu stream.”

Thrompon Kinlay Dorjee said that to minimise flood impact and according to town planning, no plot is given or structures be built within 30 metres from a river and 15 metres of streams.  However, he said that there are some houses built within the area, which have been relocated.

A member of the committee suggested that Bhutan being a religious country should also have a group from the Zhung Dratshang, and an astrologer from the Pangrizampa monastery to conduct religious rituals during and after disasters.

Karma Cheki

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