Contribution of landslides and ordinary monsoonal high flows to instantaneous hydro-geomorphic readjustments of gravel river channel and sediment fluxes

Category Other
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Fort, Monique۱; Arnaud-Fassetta, Gilles۲; Cossart, Etienne۳
Holding Date 08 September 2008

In tectonically active contexts such as the Himalayas, the confinement of mountain valleys favours various mass-wasting features (debris-slide, earth flow, debris flow, rock slide) that are commonly triggered by abundant, monsoon-related rainfall. The landslide runout may be the cause of short-lived dams and lakes that play a major role in the overall process of sediment transfer, with manifold hydro-geomorphic effects. Upvalley, these short-lived dams force local, temporary aggradation and storage of sediments, and cause rapid backwater flooding, thus threatening and eventually inundating the nearby settlements. Downvalley, dam overtopping and/or breaching out instantaneously increases the density and competence of the flow, hence its morphogenic efficiency, leading to severe bank erosion, and reducing considerably the residence time of sediments in these temporary, spatially limited traps.
To document the influence of landslides and ordinary monsoonal high flows on sediment fluxes, we applied a sediment budget approach to a selected case study (Kali Gandaki valley, Nepal Himalayas). On the basis of geomorphic survey and mapping, we reconstructed the extent of landslide deposits, established the cross section of the landslide mass in its valley and characterized the material. With the use of a DEM within a GIS, we assessed the geometry of the lake reservoir created upstream from the landslide dam, the volume of trapped sediments, and the volume of pre-landslide terraces. Eventually, the geometry of post-depositional, erosional features was used to calculate the volume of the eroded landslide mass. In the absence of hydraulic measures, we had to reconstitute both the discharge and stream power of the river. From cross-sections and morpho-sedimentary indicators (alluvial break through deposits perched above the current talweg), values of peak discharge and specific stream power were calculated upstream of the dam-lake and downstream of the landslide dam, in accounting for the progressive breaching of the landslide dam. These values were then compared with peak discharge and stream power during both the dry and wet seasons. In conclusion, it is suggested that short-lived dams and their failure are the most common and efficient mode of erosion and sediment transfer that control sediment fluxes outward from the mountain zone.