Evaluating Factors for Controlling Sediment Connectivity of Landslide Materials: A Flume Experiment

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.

Experimental and numerical modeling to investigate the riverbank’s stability

“…Good results were obtained to predict the stability of the bank. [10] studied the effect of the bed slope and the angle of flow on the transportation of sediment using an acrylic flume by applying different proportions of moisture content The results showed that the collapse volume increases with the increasing the angle of flow and the bed slope when the moisture content was greater than the degree of saturation or less.…”

Overview of Landslide Hydrology

“…This dynamic nature of subsurface hydrology depends on the complex interactions among precipitation inputs, physical properties and heterogeneity of soils and bedrock, local geomorphology, and vegetation and associated biomass. These factors influence the timing of landslides with respect to precipitation inputs and antecedent soil moisture [14][15][16][17], the mass and mode of failure [18], and the extent of runout or transformation of landslides into debris flows [19].Both the infiltration of rainwater and snowmelt and bedrock exfiltration provide the local trigger of these landslides, while drainage and evapotranspiration tend to stabilize hillslopes by rerouting and removing subsurface water. Subsurface hydrology is strongly affected by preferential flow within the soil, substrate topography, and exfiltration from fractures in bedrock [2,13,18,[20][21][22]; the overall regolith moisture regime and recharge rates are influenced by evapotranspiration, soil development processes, soil water-groundwater interactions, and landform aspect and shape [14,16,[23][24][25].…”

“…This dynamic nature of subsurface hydrology depends on the complex interactions among precipitation inputs, physical properties and heterogeneity of soils and bedrock, local geomorphology, and vegetation and associated biomass. These factors influence the timing of landslides with respect to precipitation inputs and antecedent soil moisture [14][15][16][17], the mass and mode of failure [18], and the extent of runout or transformation of landslides into debris flows [19].…”

“…Furthermore, challenges remain associated with landslide runout, i.e., the spatial propagation of landslide sediments. In mountainous terrain, a threshold appears to exist between landslide dam formation in receiving channels and debris flow occurrence associated with topographic conditions, water content, and the lithology from which sediment is derived [19]. Hydraulic modeling studies have reasonably predicted the spatial propagation of pumice debris flows [26].…”