THESIS ABSTRACT

Abstract

The Ghyapche landslide, situated on the boundary of Wards 3 and 4 of Manthali Municipality in Ramechhap District, is an actively deforming slope failure in the headwaters of Ranajor Khola that has delivered sediment and debris to the Tamakoshi confluence and contributed to recurrent outburst flooding affecting the settlements of Manthali. This study evaluates the causes, temporal evolution, and downstream impacts of the landslide and its associated floods, and identifies mitigation measures through an integrated approach combining time-series remote sensing, terrain analysis in a Geographic Information System, one-dimensional hydraulic modelling, and key informant interviews. An ALOS PALSAR digital elevation model at 12.5 m resolution, processed in the UTM Zone 45N projection, provided the basis for the geospatial analysis, supplemented by field observation and interviews with ward focal persons. Time-series mapping established that the landslide expanded from approximately 0.48 km² in 2003 to 0.63 km² by 2013, since when it has remained broadly stable at between 0.65 and 0.67 km², indicating that the principal phase of expansion preceded the documented flood events. A landslide susceptibility map produced by the Weight of Evidence method, combining slope, aspect, land cover, distance from stream and distance from road, classified the terrain of the catchment, and validation against the mapped 2025 landslide established that 94 percent of the failure fell within the High and Very High susceptibility classes, with an area under the success- rate curve of 0.96. A catchment of 36.96 km² was delineated, and flood inundation was modelled using the Height Above Nearest Drainage method, yielding modelled extents of 0.66, 1.36 and 2.74 km² under low, moderate and high magnitude scenarios respectively. Sampling of these surfaces at eight settlements established that the airport, bus park and Khadbari fall within the modelled inundation, in agreement with reported flood impacts, while the settlements of the upper catchment are exposed instead to landslide encroachment. To examine the flow behaviour governing sediment deposition, a one-dimensional steady-flow model was constructed in HEC- RAS, with discharge transferred from the gauged Likhu Khola by the catchment-area-ratio method. The modelling established that flow is constricted and accelerated at the Khadbari bridge but decelerates sharply into the wide reach immediately downstream, identifying this reach as the zone of preferential sediment deposition, a result corroborated by field evidence of extensive deposition. Key informant testimony recorded year-round movement of the landslide, cumulative displacement and loss of life, the inundation of cultivated land and settlements, and the failure of earlier gabion protection, together with an absence of any early warning provision. The study concludes that instability at Ghyapche is conditioned by steep terrain developed on weak, foliated lithology and is episodically accelerated by monsoon rainfall, while the flooding of Manthali arises principally from sediment-induced aggradation and backwater at the Tamakoshi confluence rather than from direct inundation by Ranajor Khola. A combination of structural protection, regulated management of the deposited sediment, and a community-based early warning system is recommended for the reduction of hazard to the affected settlements.

Keywords

Ghyapche landslide, outburst flood, time-series analysis, Height Above Nearest Drainage, hydraulic modelling, HEC-RAS, landslide susceptibility, Ranajor Khola, Ramechhap