Steepland terrain is a characteristic of the Pacific Rim which must be better understood to mitigate and reduce risk associated with the cascade of sediment through short, steep, energetic catchments. The natural laboratory of New Zealand’s East Coast ranges provides examples to improve the understanding of linkages between slopes and channels in the upper part of steepland river systems at this critical point in the sediment cascade. These examples illustrate an array of processes contributing abundant sediment in concert with intrinsic and extrinsic controls conditioning catchment connectivity. Once generated from hillslopes, the residence time of alluvium is contingent upon the efficacy of erosion and transfer processes moving sediment along the conveyor. Impacts of point sources of sediment delivery may be long-lived.
In this study, a probabilistic analysis to predict landslide runout based on an inventory of snowmelt-induced landslide disasters with consideration of differences in the topographic features of travel paths was conducted. The inventory included 76 landslides that occurred during snowfall/snowmelt periods between 1947 and 2012. These landslides were divided into three groups based on a topographic classification using GIS analysis. The probability of a landslide traveling for a specific distance and its traveling ratio were examined for each group by fitting the empirical cumulative distributions to exponential distribution functions. The fitted exponential distribution functions and outcomes of the estimations were very similar for landslides traveling along floodplain (FP) and slope (SL), despite differences in the slope gradients of the travel paths. The probability that landslide runout would exceed the criteria of the Sediement Disaster Prone Areas (SDPAs) was <1% for the FP and SL groups. In contrast, landslides traveling along headwater channel (HC) were likely to travel further than the criteria, particularly in landslide-initiated debris flows. Even if their travel distances was restricted to the movement of displaced mass by initial landslides, HC landslides tended to travel further than the other landslide groups.
While the sediment disasters in the mountain area usually occur as multi-modal type, most of the existing warning systems or disaster prevention plans only consider single hazard. This study integrates rainfall-infiltration, slope stability, water discharge, and sediment runoff model to simulate the multi-modal sediment disaster on a basin scale. The results indicate that river discharge and sediment runoff are obviously affected by rainfall pattern, and had significant contribution to the change of riverbed elevation. In addition, this study proposes a new warning indicator (Critical water content, Wcr), which is based on IRIS (the Integrated Rainfall-Infiltration-Slope stability) model and multiple regression analysis, for assessing slope stability. It can swiftly predict which slope and when it collapses on a basin scale. The results not only offer the verification of the disaster prevention plan but also provide the foundation of developing the multi-modal disaster warning system.
We evaluated bedrock groundwater movements on a weathered granite hillslope based on hydrological and hydrochemical observations and tracer experiments. The bedrock groundwater level in the lower part of the hillslope responded rapidly to rainfall. Although the bedrock groundwater level in the middle part of the hillslope fluctuated much more slowly than the duration of rainfall events, the groundwater level of the bedrock groundwater zone rose sharply and reached the upper level of the weathered bedrock after a large rainfall event. This rapid rise of bedrock groundwater was induced by water stored in soil and/or by water in a shallow weathered bedrock layer moving through a cracked and/or fractured bedrock zone. There were several groundwater movement layers within the weathered bedrock in the middle part of the hillslope. Moreover, there were different groundwater movement flow paths from the middle part of the hillslope to the soil and bedrock layer at the bottom of the hillslope with hydraulic conductivities calculated to be in the order of 10-4 to 10-3 cm/s.