Ground penetrating radar survey for risk reduction at Imja Lake, Nepal
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This study presents observations of the structure of the terminal moraine complex at Imja Lake. Detailed ground penetrating radar (GPR) surveys were conducted at Imja Lake. The lake and the surrounding Imja glacier have been described in the previous section. This paper should contribute to the understanding of the structure of the terminal moraine and the distribution of ice in the core of the moraine. The formation of glacier lakes in the Nepal Himalaya has been increasing since the early 1960s. Accompanying this increase in the number and size of glacier lakes is an associated number of GLOF events. The Khumbu region of Nepal (which includes the Dudh Koshi basin) is regularly mentioned as an area particularly prone to GLOF events and containing important sites for possible GLOF risk reduction projects (especially in the Imja Khola). Imja lake in the Khumbu is often mentioned as a potentially dangerous glacier lake (PDGL) and its GLOF risk has been investigated for more than 20 years (Armstrong, 2010). In May and September 2012, the authors visited the lake and observed the rapid rate of change of the terminal moraine complex. They performed ground penetrating radar surveys of most of the terminal moraine complex and mapped the ice core of the moraine. Imja Lake is currently the focus of several groups in an effort to reduce the risk of a GLOF posed by the increasing lake level. The presence or absence of ice in the core of the terminal moraine complex is of critical importance in designing a risk reduction program for the lake. This work has used Ground Penetrating Radar (GPR) to investigate the internal structure of the moraine complex in order to map out the ice thickness in critical areas. The results of the GPR survey show that there is extensive ice present in the core of the terminal moraine complex at Imja Lake (see Figure 8). The thickest areas of ice are in the moraine near the western end of the lake on the northern side of the lake outlet. The ice in this region is several tens of meters thick and up to fifty meters thick in some places. Along the northern and southern sides of the lake outlet, the ice is between ten and twenty-five meters thick. In some portions of the moraine on the southern side of the outlet the ice thickness is up to forty meters. Extensive seepage of water from the terminal moraine was observed in two locations during visits to the lake in September 2011, May 2012, and September 2012. GPR transects above and below the site of seepage show the presence of ice above the seep and much less ice below the seep. Seepage of water through the terminal moraine is an indication of potential weakness in the moraine and a possible site of future moraine failure. Recent work has been initiated by the United Nations Development Programme to develop an Imja Lake Risk Reduction Program. One of the primary methods suggested for reducing risk associated with the lake is to reinforce and deepen the outlet channel so that it can lower the lake level up to 3 meters below the current level. This project involves making excavations of the outlet channel and the construction of a diversion channel on the southern side of the outlet. The results presented here indicate that there may be ice present in the moraine in the vicinity of the excavations being considered in this project. Excavation activities that encounter ice in the moraine material may cause weakening of the ice resulting in increased water seepage and erosion of the moraine. Therefore, it is recommended that additional GPR surveys be conducted in this area accompanied with Electrical Resistivity Tomography (ERT) surveys. The ER surveys will be able to more definitively indicate the presence of ice in the moraine as well as the degree of water saturation of the moraine material.