Mechanism and future risk of slope instability induced by extreme rainfall event in Izu Oshima Island,Japan

Natural Hazards - A volcanic slope in Izu Oshima Island in Japan experienced a profound rain-induced disaster in October 2013. Since this slope had been stable for centuries except for minor...     

Decision-theoretic sensitivity analysis for reservoir development under uncertainty using multilevel quasi-Monte Carlo methods

At various stages of petroleum reservoir development, we encounter a large degree of geological uncertainty under which a rational decision has to be made. In order to identify which parameter or group of parameters significantly affects the output of a decision model, we investigate decision-theoretic sensitivity analysis and its computational issues in this paper. In particular, we employ the so-called expected value of partial perfect information (EVPPI) as a sensitivity index and apply multilevel Monte Carlo (MLMC) methods to efficient estimation of EVPPI. In a recent paper by Giles and Goda, an antithetic MLMC estimator for EVPPI is proposed and its variance analysis is conducted under some assumptions on a decision model. In this paper, for an improvement on the performance of the MLMC estimator, we incorporate randomized quasi-Monte Carlo methods within the inner sampling, which results in an multilevel quasi-Monte Carlo (MLQMC) estimator. We apply both the antithetic MLMC and MLQMC estimators to a simple waterflooding decision problem under uncertainty on absolute permeability and relative permeability curves. Through numerical experiments, we compare the performances of the MLMC and MLQMC estimators and confirm a significant advantage of the MLQMC estimator.     

On meteotsunamis around Tsushima Strait generated by the Baiu front

On July 15, 2009, a meteotsunami occurred over Tsushima Strait and flood damage was recorded on the west coast of Tsushima Island. This study investigated the meteorological systems related to that meteotsunami event using surface observation data, grid-point value data, and a mesoscale meteorological model. The mid-latitude trough in the mid-troposphere approached the moist air of the Baiu precipitation system, and a cold downdraft occurred over a wide area of western Japan. In the western Tsushima Strait, the warm, moist air of the lower atmosphere kept the front of the cold downdraft stationary. Around this area, an atmospheric gravity wave was generated continuously with the wave-conditional instability of the second kind mechanism and propagated toward Tsushima Island. The atmospheric gravity wave generated a pressure disturbance at sea level, which produced the meteotsunami. The frequency of the pressure disturbance using the numerical model had a peak in the 5- to 20-min period range, which corresponded with the peak frequency range of the observed tidal records.     

Effects of length and application rate of rice straw mulch on surface runoff and soil loss under laboratory simulated rainfall

Forest land affected by deforestation yields high soil and water losses.Suitable management practices need to be found that can reduce these losses and achieve ecological and hydrological sustainability of the deforested areas.Mulch has been found to be effective in reducing soil losses;straw mulch is easy to apply,contributes soil organic matter,and is efficient since the day of application.However,the complex effects of rice straw mulch with different application rates and lengths on surface runoff and soil loss have not been clarified in depth.The current paper evaluates the efficiency of rice straw mulch in reducing the hydrological response of a silty clay loam soil under high intensity and low frequency rainfall events(tap water with total depth of 49 mm and intensity of 98 mm/h)simulated in the laboratory.Surface runoff and soil loss at three lengths of the straw(10,30,and 200 mm)and three application rates(1,2,and 3 Mg/ha)were measured in 50 cm(width)×100 cm(length)×10 cm(depth)plots with disturbed soil samples(aggregate soil size<4 mm)collected in a deforested area.Bare soil was used as control experiment.Runoff volume and erosion were significantly(at p<0.05)lower in mulched soils compared to control plots.These reductions were ascribed to the water absorption capacity of the rice straw and the protection cover of the mulch layer.The minimum runoff was observed for a mulch layer of3 Mg/ha of straw with a length of 200 mm.The lowest soil losses were found with straw length of10 mm.The models developed predict runoff and erosion based on simple linear functions of mulch application rate and length,and can be used for a suitable hydrological management of soil.It is concluded that,thanks to rice straw mulch used as an organic soil conditioner,soil erosion and surface runoff are significantly(at p<0.05)reduced,and the mulch protection contributes to reduce the risk of soil degradation.Further research is,however,needed to analyze the upscaling of the hydrological effects of mulching from the plot to the hillslope scale.