It is well known that forests have important effects on mitigating flood and drought, and decrease risks of slope failure and surface erosion. This study evaluated how hydraulic properties of forest soils affect such functions of forests. Firstly, we clarified changes in soil hydraulic properties resulting from structure development. For this purpose, we assumed undisturbed forest soils as the structure-developed soils, and soils prepared by crashing secondary soil structure as structure-undeveloped soils. Hydraulic properties of these soils were compared. As a result, the structural development increased effective porosity and saturated hydraulic conductivity. The amount of the increase was large near the soil surface. Therefore, we can say that the soils in the shallower layer were affected more by the structure development, having larger water holding capacity and permeability than the structure-undeveloped soils. By using the observed hydraulic properties, numerical simulations were conducted for modeling hydrological processes in a forested hillslope. Results showed that the structure-developed soils supplied rainwater into the bedrock with milder intensity and stabilized baseflow discharge. This was because the structure-developed soils hold more rainwater than the structure-undeveloped soils due to the high water holding capacity. On the contrary, the structure-developed soils produced greater and rapider saturated through flow than the structure-undeveloped soils, because of the high permeability. This resulted in increase storm runoff for the structure-developed soils. For the structure-undeveloped soils, delayed and dull peaks of storm runoff were resulted to form expanded saturation area in the hillslope. That is, structure-undeveloped soils produced larger soil pore pressures than the structure-developed soils. Consequently, the hillslope with structure-developed soils was less vulnerable to slope failure and surface soil erosion.

Many rockfall protection fences have been constructed along the cliffs and slopes in the mountain areas to prevent the rockfall disasters. The fences which are composed of steel posts, diamond-shaped steel nets and steel wire ropes, are set on the concrete foundation and/or rockfall protection walls. Now, stabilities of the structure have been checked following the design guideline for rockfall countermeasure. However, the guideline has been constituted based on not those dynamic response characteristics but statics. In order to investigate dynamic behavior of the structure and to consider validity of the guideline, the impact tests for prototype fences with 2 m height set on the concrete foundation having cross section of 1 m square were conducted by using about 1-ton steel weight. In this experiment, two types of anchoring systems for the posts were tried: conventional embedding type; and base-plate type, and two loading heights were considered: 1.4 m; and 0.6 m. The results obtained from this study were as follows: (1) even though the foundation cannot ensure the overturning stability following the guideline, all of these ensured experimentally; (2) the stability may be able to be checked by using Ushiro et al.'s equation with coefficient of restitution e=0 in the safety side which was derived by assuming rigid-body motion and using law of conservation of angular momentum; (3) the maximum impact force may be estimated by using the guideline; (4) the weight can be restrained from passing through the nets due to space keeping members even though the wire ropes break out of anchoring device; and (5) the base-plate type posts may be applicable for the practical use.

In this paper, we introduce a new method for detecting debris flow in mountainous rivers using unit vectors of RGB values of stream water surface images. In the Imokawa River in Niigata Prefecture, Japan, turbidity was evaluated relatively using unit vectors of RGB values by comparing turbidity sensor observation data with image analysis results for images of turbid water associated with low or medium level flows. In the Kurodakesawa River in Hokkaido, we examined the validity of debris flow detection using the same analysis of unit vectors of RGB values for images of debris flow and turbid water caused by low or medium level flows, and then, we revealed differences in characteristics between debris flow and turbid water that did not lead to debris flow. We also showed that there is a linear correlation between turbidity and changes in RGB unit vectors, and that there is a unique direction vector for each stream. Furthermore, by setting a plane orthogonal to this direction vector, we proposed a method to set a threshold for not only quantitative evaluation of turbidity, but also a high turbidity zone that would detect the occurrence of debris flow. Currently, quantitative evaluation of turbidity by image analysis is not suitable for use as an instrument for observing turbidity due to its coarse accuracy, but it is effective for use as a monitoring instrument. The results of this study are expected to be used effectively in the future for monitoring debris flow using images from CCTV and other monitoring cameras, such as detecting turbidity of the precursor phenomena of debris flow.

Recent studies suggested that runoff of bedrock groundwater can influence shallow landslides. As a first step towards incorporating the effect of bedrock groundwater runoff into shallow landslide prediction models, we analyzed characteristics and developed models of bedrock spring runoff based on detailed observations of two bedrock springs in a small granitic mountainous catchment. Spring temperature and chemistry indicated that they originate from the same aquifer in deep bedrock. The increment in runoff in response to rainfall events was correlated with the total rainfall amount, and the lag time from the rainfall peak to the runoff peak was more than approximately 1 day. Runoff dynamics were reproduced by the functional models using antecedent precipitation indices with half lives of 156.0 and 211.9 h. These results imply that bedrock groundwater can influence shallow landslides through raising soil mantle groundwater level by its perennial runoff.

Even if seismic intensity or peak ground acceleration (PGA) is close to that of past earthquakes which caused catastrophic sediment disasters, some earthquakes have few coseismic landslides. In order to clarify the reason for this, we compared the spectra of earthquakes with numerous landslides (NLE) and earthquakes with few landslides (FLE) and discussed the influence of periodic characteristics of seismic motion on coseismic landslide occurrences. The compared spectra were Fourier spectra, acceleration response spectra, and velocity response spectra. We compared a series of spectra from short to long period between NLE and FLE, considering distance from the fault/hypocenter, seismic intensity and PGA. As a result, in the short-period component, the spectra of NLE and FLE generally overlapped, making the difference between NLE and FLE unclear. On the other hand, the difference tended to be significant in long-period components, as the NLE spectra exceeded the FLE spectra at periods longer than 1 second. Based on these results, we suggest that it is possible to improve the accuracy of judging the number of coseismic landslides by considering the long-period components of spectra in addition to the seismic intensity and PGA.