Role of debris flow and glacier on sediment evacuation in steep watersheds: a case study from the Seti River, central Nepal

Abstract:

A tragic debris flow occurred in the Seti River of central Nepal on Saturday, May 5 of 2012 and resulted in death of more than 50 people and caused great loss of property. The Seti River is a trans-Himalayan river near the Pokhara in central Nepal. The Seti River collects water from the Mardi Khola (Khola means small river) and the Seti Khola, with headwaters on the southern edge of the Tibetan plateau. The Mardi Khola confluences to the Seti River in northeastern of Pokhara. Two tributaries of the Seti River show widely different condition of sediment evacuation. The Seti Khola is substantially occupied by glacier on its upstream and has thick (up to tens of meters) terrace deposits and wide valley form. Glacial cover of upper stream may have caused multiple high magnitude/low frequency event. On the other hand, the adjacent river, the Mardi Khola shows typical, V-shaped valley form with little fill terrace but a few of narrow, rock-cut strath terraces.

This research has double-folds. We would firstly evaluate the role of High-Magnitude Low-Frequency event (e.g. debris flow) on the process of sediment evacuation on the basis of cosmogenic ¹⁰Be and ¹⁴C concentrations of sediments collected before and after the debris flow in the Seti Khola. Catchment wide denudation rate (CWDR, 10³-10⁵ yr) using cosmogenic nuclides in fluvial sediment can help us constrain denudation rate at intermediate time scales between thermochronologic data (>10⁶ yr) and sediment discharge (10⁰-10² yr).  Secondly, we would appraise the relative role of glacier with CWDR and morphologic analysis of the two, tributary valleys formed mainly by glacier and river, respectively. Moreover, most of the present model of catchment-wide erosion rate using single cosmogenic nuclide of ¹⁰Be tends to underestimate the effective denudation rate, because single nuclide of ¹⁰Be with long half-life (1.36 Ma) is difficult to detect any change caused by burial shorter than the effective time scale of ¹⁰Be. Thus, we would provide more effective CWDR considering the occasional storing time of sediments with the use of a pair of in-situ ¹⁴C (5.7 ka) and ¹⁰Be analysis.