Sediment discharge monitoring can be used to detect sediment disasters in upstream areas in order to enable prompt countermeasures. Sediment disaster signals should be easily differentiated from ordinary sediment discharge. This study examined the application of the geospatial interface for the Water Erosion Prediction Project (GeoWEPP) for the assessment of baseline sediment discharge in a mountain watershed to allow early detection of upstream sediment disasters. When compared to detailed observation-based data from the Agatsuma watershed, GeoWEPP successfully reproduced continuous sediment discharge in subwatersheds of varying size, topography, and land use. GeoWEPP parameter settings corresponded to actual conditions and processes involved in water and sediment dynamics. Additionally, the introduction of a restrictive layer and improved settings for evapotranspiration rate were critical for predicting surface runoff and subsequent surface erosion in hillslope sections. The depth to non-erodible layer was important for determining the overall sediment discharge at the study site, as it was the only parameter that varied over time and could not be obtained from the actual depth of bed material. This parameter should be interpreted as a conceptual index that represents the spatial distribution of bed material and its erodibility, and requires an initial adjustment period. Therefore, GeoWEPP calculations should eliminate the first year in order to obtain optimal results in a mountain watershed with a significant depth of bed material in a channel section.
A combined penetrometer-moisture probe (CPMP) was used to determine the structures of layers of volcanic deposits with differing permeability on the volcanic hillslopes of Izu-Oshima Island, Japan. Here, heavy rainfall in 2013 caused shallow landslides due to the hydrogeomorphological structure of the soil mantle. In these volcanic areas that are prone to landslides, accurate determination of the detailed structure of deposit layers with differing permeability is crucial as it would enable us to predict the depth at which pore water pressure could increase and where slip surfaces could be formed. Profiles of volumetric water content, θ, measured using the CPMP, successfully detected strata of deposits consisting of pure and alternating layers of loess, ash, and scoria regardless of the measurement date, point, and depth. Those different types of deposits clearly showed the different order of values of saturated hydraulic conductivity, Ks. We also demonstrated that Ks profiles of interlayered deposits could be determined using an identical relationship between θ, measured by the CPMP, and Ks for all types of deposit on the slope within a particular range of soil moisture, excluding extremely wet or dry conditions. This is because the variations in θ values for each deposit type according to the quantity of antecedent precipitation were small, except in saturated or nearsaturated conditions. However, for determining strata deposit types, CPMP profiles of penetration resistance, Nc, may be considered a supplementary index.
A large-scale sediment disaster occurred in Aranayake, Kegalle District, Sri Lanka, on 17 th, May, 2016, due to the heavy rainfall from the 15th May, 2016. The damage from the heavy rainfall is as follows (DMC (Disaster Management Centre) Situation Report as at 9 : 00, 13 th Jun, 2016). The number of deaths is 31 people, and 96 peoples missing at Aranayake. Therefore to understand the overall disaster situation of the sediment disaster area, we conducted an aerial survey by using a Sri Lankan Air Force chopper on 22 May and field survey on 8th to 9th and 21st to 22nd Jun as JICA (Japan International Cooperation Agency) Survey team. Further, we conducted numerical simulation to understand processes of this sediment disaster. In this report, we compiled two kinds of survey results and a proposal about the improvement of sediment disaster countermeasures in the future.