STUDY OF GLACIAL LAKE OUTBURST FLOOD FOR PUNATSANGCHHU HYDROELECTRIC PROJECT BHUTAN

Abstract

A number of hydroelectric (HE) projects in India and Bhutan are being planned in the Himalayan regions. Due to global warming, the glacial lake formation is taking place in the catchment area of the some of the projects. The formation of these glacial lakes is due to accumulation of glacier melt behind the moraine dams formed by landslides or some other phenomenon. Since these are naturally formed dams with highly porous soil material inside the dam body, there is a strong possibility of breaching of these dams either due to piping or due to overtopping, releasing a sudden discharge of significant magnitude, endangering the safety of the HE project being planned. Hence it has become necessary for the project planners and designers to account for the glacial lake outburst flood (GLOF) in conjunction with PMF/SPF input for deciding the spillway capacity of projects located in similar hydro-meteorological regions. The present study pertains to estimate the most critical Glacial Lake Outburst Flood (GLOF) for Puntsangchhu HE Project, Bhutan which should be accounted for along with PMF for fixing the suitable capacity of the spillway. The Punatsangchhu hydroelectric project is a run-of-the-river scheme along the course of the Punatsangchu river, downstream from Wangdue phodrang town in Bhutan. It will have an installed capacity of 1095 MW with an annual average generation of 5566 GWh. According to existing feasibility/DPR studies, a 143.6 metre high concrete gravity dam will be located about 10 kilometre downstream from the Punatsangchu bridge and an underground powerhouse, operating under a head of 329 metres, will be located 18 kilometre downstream from the bridge. There are hundreds of glacial lakes in the catchment area of the project. One of the glacial lake (Lugge) breached in year 1994 resulting a peak discharge of 2550 cumecs at Wangdue Phodrang gauging station, about 100 km downstream of Lugge lake (12 km upstream of proposed Punatsangchhu Project site). The International Centre for Integrated Mountain Development (ICIMOD), August, 2001 Report "Inventory of Glaciers, Glacial Lakes and Glacial Lake outburst Floods, Monitoring and Early Warning Systems in Hindu Kush-Himalayan Region Bhutan" identified 24 glacial lakes as potentially dangerous in Bhutan on the basis of analysis of data using different criteria and the study of topographic maps and satellite images. Out of these, there are 5 potentially dangerous glacial lakes in Mo Chu sub basin and 8 glacial lakes in Pho Chu sub basin of Punatsanchhu basin. The potentially dangerous glacial lakes as per ICIMOD inventory of Mo Chu and Pho Chu sub basins have been marked using Google Earth. Using the distance measurement tool of Google Earth the distance of these glacial lakes has been measured from confluence point of Mo Chu and Pho Chu rivers at Punakha. There is no estimate available for depth of glacial lakes in Bhutan from their water spread areas. However, some estimates are available for glacial lakes in Swiss Alps, as given in "Remote sensing based assessment of hazards from glacier lake outbursts: a case study in the Swiss Alps" by Christian Huggel, Andreas Kaab, Wilfried Haeberli, Philippe Teysseire, and Frank Paul published on NRC Research Press. For the lakes on Pho Chu and Mo Chu sub basins, the lake volume and lake depths have been estimated using empirical relations as available in the above mentioned paper, as under; Mean depth D=0.104 A0.42 The lake volume .V = 0.104 A1.42, where A is the lake area in sq. m. The volume and depth of potentially dangerous lakes on Mo Chu and Pho Chu sub basin estimated using the above relationships. Based on the volume of water stored in the glacial lakes and their distance from the project site a detailed criticality analysis has been carried out. From the criticality analysis the following three GLOF scenario have been found critical for estimating the flood peak for Punatsangchhu H.E. Project; I. GLOF originating from Raphstreng (possible scenario in the near future) discharging a volume of 13.6 million m3 II. Combined Raphstreng- Thorthormi GLOF scenario (possible scenario in the far future) discharging a volume of 53 million m3 III. GLOF originating from Lugge lake discharging a volume of 24 million m3 For the GLOF simulation the dam break module along with the HD module of MIKE 11 mathematical model developed by Danish Hydraulic Institute, Denmark has been used. The outburst flood from the glacial lakes along with the 100 year return period flood has been channel routed through the downstream valley from the glacial lake to the Punatsangchhu project site, to get the desired GLOF estimate