Timing, Causes, and Ecological Impacts of the 1991 Glacial Lake Outburst Flood at Rijieco in the Eastern Himalayas

Abstract. We investigate and reconstruct a publicly lesser-known glacial lake outburst flood that happened at a moraine-dammed proglacial lake – Rijieco (27.963° N, 88.9° E) in the eastern Himalayas, Tibet, China, more than thirty years ago. Satellite images interpret that from 1977 to 1991, the surface area of Rijieco increased from 0.34 km2 to 0.43 km2, along with a reduction of 0.27 km2 in the glacial area, and subsequently dropped to 0.24 km2 after the GLOF. Further inspection of the 1991 images narrows down the date of occurrence of the Rijieco GLOF to between 21 September – 7 October 1991. The most probable triggering mechanism of the GLOF is an avalanche from the south-west part of the glacial lake because the local hydrometeorological data show precipitation of 100 mm higher than the multi-year mean in the preceding two months and an elevated temperature anomaly in the month of the disaster, combined with a sufficient topographic potential of the slope. Based on the field-measured dimension of the dam gap (31 m high and 75 m wide) and local topography, it is estimated that nearly 6 million m3 of impounded water was released during the GLOF. The reconstruction of the outburst flood with the HEC-RAS 2D hydrodynamic model shows that the flood peak discharge at the dam was 2900 m3/s and then attenuated first to around 200 m3/s at the alluvial fan and 45 m3/s at the entrance to the Duoqing lake, an inland lake about 50 km away from the dam. The sudden release of the impounded water and large volumes of entrained sediment shortly expanded the Duoqing water area by 12.2 % and strongly disturbed its margin vegetation. The transported sediments silted an area of 1.0×106 m2 of the channel and flood plain where new vegetation has not yet recovered. The harm of GLOFs to the lake ecosystem in the high-altitude Himalayan region may not be repaired in a short period. This study reveals short-term geomorphic impacts of GLOFs and noticeable but less mentioned long-term ecological impacts on a Tibetan inland lake system.