Detailed Geophysical-ERT Investigation for Structural Design of Imja Lake Lowering For Community Based Flood and Glacial Lake Outburst Risk Reduction Project

The Community Based Flood and Glacial Lake Outburst Risk Reduction Project (CFGORRP) intendeds to reduce human and material losses from GLOF from Imja Lake (5010 m) in Solukhumbu District by reducing the GLOF hazard risk in 27 settlements in the downstream valley by reducing Lake Level by more than 3 m through construction of an open channel. As a part of the project the task of “Detailed Geophysical-ERT Investigation for Structural Design of lmja Lake Lowering” was awarded to the MEH Consultants Pvt. Ltd. Kathmandu. The project period was from 30 September to 31 December, 2014. The main objectives of the work were (i) to identify at least 3 most suitable open channel alignments, (ii) to prepare longitudinal and transverse profile using ERT, (iii) to provide geological, geotechnical, geophysical-ERT related inputs to the survey and design thematic team for the selection of appropriate design parameters and safety measures and (iv) and to recommend remedial measures for digging out open channel by more than 3 m for mitigating adverse effects if any on buried ice, seepage and piping etc.

The study was carried out in four phases, namely, desk study, field study, laboratory analysis and office work. The base line information on geophysical, geological and geotechnical results by the previous workers related to glaciers, GLOFs and Imja Lake in Nepal were collected and reviewed during the desk study phase. Field investigation (ERT survey, geological and geo-technical studies) was carried out at the Imja Lake area from 10 October to 5 November, 2014. Laboratory tests were carried out to evaluate the properties of soil and construction material. The office work included data interpretation, preparation of geological maps, cross sections, evaluation of alternative open channel alignments and to recommend appropriate appropritate design parameters and safety measures.

The Imja Lake lies geologically in the Higher Himalaya of Nepal with metamorphic crystalline basement rocks such as gneisses, schists and granite. An E-W extending regional normal fault system (South Tibetan Detachment System) passes from just north of the Imja Lake. Regional seismic data show that this area lies in one of the seismically high hazard zones of the Nepal Himalaya. The minimum peak ground acceleration (PGA) value for this area is 400 gal for 10% exceedace in 50 years.