Abstract
Connectivity of landslide sediment to and within fluvial systems is a key factor affecting the extent of mobilization of hillslope material. In particular, the formation of landslide dams and the transformation into landslide-induced debris flows represent “end members” of landslide sediment mobility. To quantify sediment connectivity, we developed a two-segment flume representing tributary inflow and the main channel. Mobility of sediment was examined by combinations of various topographic factors, such as tributary inflow angle (0 to 90° in 30° increments) and main channel gradient (10° and 15°), as well as water content of sediment (0 to 100% in 20% increments). We also examined differences of mobility among sediments derived from various lithologies (sand and shale, pyroclastic sediment, weathered granite, and weathered sedimentary rock). Mobility of sediment differed, depending on the water content of sediment, particularly less than saturation or greater than saturation. When all types of unsaturated landslide sediments entered the channel at inflow angles of 60° and 90°, substantial deposition occurred, suggesting the formation of landslide dams. At low inflow angles (0° and 30°) in a steep channel (15°), >50% of landslide sediment was transported downstream, indicating the occurrence of a debris flow. The amount of sediment deposited at the junction angle was greater for pyroclastic sediment followed by weathered granite, weathered sedimentary rock, and finally, sand and shale. Our connectivity index suggests that a threshold exists between landslide dam formation and debris flow occurrence associated with topographic conditions, water content, and types of sediment.
Highlights
Sediment connectivity is defined as the linkage and transfer of sediment from sources to deposits within a catchment system, via sediment detachment and transport [1]
Because our study focused on the formation of landslide dams and landslide-induced debris flows, our sediment connectivity index (SCI) is expressed based on the amount of sediment in sections B and E using the following equation: SCI =
Previous studies revealed that inflow angle and channel gradient are major topographic factors influencing sediment mobility (e.g., [15,18,20])
Summary
Sediment connectivity is defined as the linkage and transfer of sediment from sources to deposits within a catchment system, via sediment detachment and transport [1]. The concept of connectivity has been applied to catchment hydrology, sediment transport, and soil erosion by water to understand the spatial and temporal variability of sediment dynamics [6,7]. Gomi et al [8] showed that sediment from hillslopes was directly transported to channels as bedload and suspended sediment due. Water 2019, 11, 17 to its high mobility in headwater catchments affected by recent landslides. Observed a disconnection of sediment in the continuum from hillslopes to the catchment outlet due to temporary deposition within the channel bed. When sediment remains near its source, there is a disconnection between sources and downstream reaches, while high connectivity between sources and downstream reaches occurs when sediment is highly mobilized
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