USING LICHENOMETRY TO ASSESS LONG TERM GLOF AND LANDSLIDE FREQUENCY IN THE NEPAL HIMALAYA

Glacial lake outburst floods (GLOFs) and landslides are major hazards in and downstream from glaciated areas in the Nepal Himalaya and many other mountainous areas, and it is possible that global warming will increase GLOF frequency. However, in the Nepal Himalaya, little is known of the frequency of GLOFs prior to 50 years ago, making it difficult to assess the long term hazard they pose. In light of this, the objectives of our study are to develop the first lichen apparent growth curve for the region and use lichenometry to date pre-historic GLOF events as well as major landslides in the area.

The first stage of the study was carried out in May 2009 near Kyanjin Gompa in Langtang Valley, Nepal, with the purpose of developing a lichen apparent growth curve for Rhizocarpon geographicum for the area. R. geographicum is commonly used for lichenometry in alpine areas because it is easily identified, long-lived and slow-growing. The lichen apparent growth curve empirically relates lichen diameter to age and allows for dating the time of surface exposure of rocks. To develop the apparent growth curve, diameters of R. geographicum lichen were measured on boulders at two sites that local people recalled having been covered by ice blocks from Khyimjung Glacier until 1959 and 1982. Lichen diameters were also measured on boulders downslope from Kyanjin Gompa on two debris flows that had been previously dated at 1611 ± 135 and 1474 ± 163 using ¹⁰Be. The apparent growth curve that best fits the data is y = 51.11(1 – exp(-0.0458x)) + 0.1620x, where y is maximum lichen diameter (mm) and x is time (years). Application of the apparent growth curve to lichen diameters measured on boulders deposited by a landslide that dammed the Langtang River above Kyanjin Gompa dates the landslide to 1683, and lichen diameters on boulders deposited by a large landslide that occurred along the Langtang River above Langtang Village yield an age of 1904, and lichen diameters on the end moraine of Khyimjung Glacier date its most recent advance to 1966. In addition, lichen diameters on ancient mani walls (Buddhist sacred walls) suggest that the walls are cleaned of lichen every 10-20 years on average.

Future work will include improving the calibration of our apparent growth curve, broadening the geographic application of the apparent growth curve and dating GLOF deposits.