Debris-covered glaciers are a particularly large component of the mountain cryosphere in the eastern Nepalese Himalaya:
NASA Earth observatory image of the day 17th October 2010, showing the debris-covered glaciers near Mt Everest.
The Ngozumpa glacier is a debris covered glacier (just out of the image above to the west) and also the largest glacier in Nepal. It was the target of our fieldwork in April. As you can see in the photo below it presents pretty challenging terrain to work on. Access to the glacier is made difficult because of high upstanding moraines around it. Nevertheless we wanted to get onto this glacier and measure how thick the ice is.
View looking roughly northwards up the Ngozumpa glacier towards Gyachung Kang, showing the lumpy debris covered glacier surface lying about 100m below the upstanding lateral moraines.
The motivation for this is twofold: Firstly, the total volume of ice in the Himalaya is poorly known so every data point that can be added helps improve estimates of how much water is stored in glaciers there. Secondly, we need to know the ice thickness in order to be able to apply numerical models of how the glacier will behave.
To measure the thickness of the glacier we set out to use ground penetrating radar. This sends out a low frequency radio wave that penetrates the glacier ice and is reflected back from the glacier bed towards the surface. The time it takes to receive the reflected signal can be converted into a representation of ice thickness. We had both Narod and Dolores radar systems with us, and antennae that could span 2-20MHz frequencies. These are long cable antennae that need to be laid out and moved across the glacer surface.
To my knowledge the only previous measurements on this glacier are from Sarah Thompsons PhD thesis, in which she measured ice thicknesses of 80m just above what is called the Spillway lake, so about 1km from the terminus. This is quite interesting as the maximum depth of this lake is about 30m suggesting that its might still be underlain with lots of ice.
The map on the left shows the Ngozumpa glacier ouline, with the approximate locations of the ground penetrating radar lines measured shown in red (we also did some down-glacier long profiles between these cross-profile lines to help generate a more comprehensive 3D model of the ground beneath the glacier), and the location of the Spillway lake, which is shown in detail on the right with the depths in 2010 (Thompson et al., 2012).
Hamish Pritchard is leading the charge in analysing the ice thickness data as he has already collected similar data from the Langtang region of central Nepal. The radar data from these glaciers is generally quite noisy and it can be difficult to see a strong bed reflector, but by taking a lot of measurements and by measuring extended profiles, the bed is easier to identify.
Thompson, S., Benn, D. I., Dennis, K., Luckman, A. (2012) A rapidly growing moraine-dammed glacial lake on Ngozumpa Glacier, Nepal.Geomorphology,145–146, 1–11