Ground penetrating radar survey for risk reduction at Imja Lake, Nepal
Date
2013-03-15
Journal Title
Journal ISSN
Volume Title
Publisher
Center for Research in Water Resources, University of Texas at Austin
Abstract
Description
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.
Keywords
ground penetrating radar, Imja Lake, Nepal, glaciers, glacier lake outburst flood, GLOF, terminal moraine complex