Thesis or Dissertation Sediment-Loading Processes in a Forested Catchment Influenced by Slope Failure

Hossain, Md. Motaleb

The “slope failure” in my study means the surface failure and deep landslide on catchment slope and the bank collapse in riparian regions, which occur under rainfall, snowmelt or earthquake. Dynamic processes of landslide on catchment slope have been investigated by many geologists, geomorphologists and geophysicists, but studies on subsequent fluvial sedimentation processes, connected to sediment load of river, are very few in the world. My study area, the Oikamanai River catchment, eastern Hokkaido, is forested (ca. 90% area), but accompanied by two tephra layer (Tarumae Ta-b in 1667 and Shikotsu Spfa-1, 40,000 years ago) in forest soils and the Neogene sedimentary rocks with active faults, which tend to frequently produce surface failure and deep landslide, respectively, under heavy rainfall or snowmelt. In order to understand and predict such disastrous phenomena and subsequent fluvial sedimentation in the catchment, I monitored water level and water turbidity of river in the Oikamanai River catchment in rainfall seasons of 2011 – 2014, and modeled observed discharge and sediment load time series. The turbidity and river-stage monitoring were performed at two sites along the river, and thereby, I distinguished rainfall runoff events with slope failure from those without slope failure, and identified the seasonal sediment source in the upper catchment, since there more slope failure runoff events occurred with higher sediment yield. The monitoring at the two sites also revealed the net sediment deposition between the two sites. A semi-distributed model, ArcSWAT2012, and a lumped model, the tank model, coupled with power function, were applied to simulate discharge and sediment load time series, obtained in 2011 to 2014. In ArcSWAT2012, the total basin area (62.47 km2) was divided into three sub-basins (subbasin 1, subbasin 2, subbasin 3), as sub-basins into hydrological response unit (HRU) based on elevation, soil type, land use and slope classes that allow a high level of spatial detail simulation. In this study I have used the dataof discharge, Q (m3/s), suspended sediment concentration (SSC; C, mg/L) and sediment load, L (kg/s) for non-frozen period of April 2011 to November 2014, weather data of 2008 to 2014, and soil data. Soil water content and groundwater storage in soil layers could change every year because the amounts of snowfall and snowmelt are different. So I have utilized my models at annual base. As a result, both SWAT and the tank model plus power function reasonably simulated daily mean discharge and sediment load time series. The simulations results indicate that most of the sediment input in this catchment originates in the western subbasin 2 and sediment deposition occurs between two sites; the same scenario was seen in observed monitoring data also. At present, the interpretation of the quantitative results of sediment load is not yet so satisfactory, because of lack of model parameterization at a local scale in the SWAT and tank model. These results from the fact that the information on hydrological structures of soil and bedrock and behaviors of surface flow or subsurface flow is not sufficient. Hence, studies on discharging and sediment-eroding processes on and below the catchment slope (slope hydrology and sedimentology) are essential to understand the sediment loading processes on catchment scale.
ⅱ, 65
Hokkaido University(北海道大学). 博士(理学)

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