Nepal’s Imja Lake, Khumbu Region, Appears Resilient Against Gorkha Quake
Nepal’s Imja Lake is a typical large glacial lake created by a moraine dam (formed by accumulations of dirt and rocks — commonly cemented by ice — that have been transported and deposited by a glacier). Located near Mount Everest, the local topographic relief is extremely high. Imja Lake was recently the site of an intensive scientific survey supported by the United Nations’ Development Program to re-assess the outburst flood potential of the lake and to design and, this spring, implement a project to lower the lake level and reduce the flood hazard. The project had not been completed when the earthquake struck. The lake is also the site of a planned micro-hydroelectric power plant.
The lake began forming as a series of ponds on the glacier surface around 1960. By the mid 1970’s, the ponds had combined into a single large lake, which has continued to grow since then as the glaciers feeding it have retreated and thinned. The lake’s growth effectively cut off what had been the debris-covered terminus of the glacier, which now stands as an ice-cored end moraine. The moraine’s unconsolidated rock debris and the ice have been unstable, both gravitationally and thermally, for many years. The high lateral moraines, the high bedrock ridges that almost surround the lake, and the hanging glaciers perched above it could be de-stabilized, such as from ground shaking, and trigger an outburst flood. Fortunately, the present lake level is only about 100 feet (30 meters) higher than the area immediately downstream from the end moraine, thereby limiting the volume of water that could be released. The wideness of the end moraine previously had reduced the chances of a sudden outburst, but the growth in the past few years of ponds on the end moraine has weakened the dam, making it less able to contain the lake.
A bathymetric (lake depth) map from the recent survey is shown above, superposed on an EO-1 ALI image dating from before the earthquake. The survey found the lake to be up to 492 feet (149.8 meters) deep and to contain about 75 million cubic meters of water, of which about one-third is readily drainable in the event of a glacier lake outburst flood. The aforementioned lake lowering will reduce the level by only 10 feet (3 meters), which will reduce the outburst flood problem but not eliminate it.
The lake remains very hazardous, so it is with much relief that Imja Lake survived the 2015 Gorkha quake without huge effects visible in the first acquired post-quake satellite image from NASA’s EO-1 satellite (black and white image below). However, further satellite image analysis and boots and eyes on the ground are needed to confirm this.
This work is supported by NASA’s SERVIR Applied Science Team and is in support of the emergency NASA-USGS-Interagency Earthquake Response Team.
Image Credit: NASA GSFC
