The Rokko mountain streams and their basins have undergone vast changes during their history. Throughout the Meiji (1868-1912), this area was not vegetated. In 1938 and 1967, heavy rainfall and flooding occurred in these causing great landslide and debris flow disasters. Currently the mountains are covered by forest. In this study, wash load was observed and measured at each mountain stream, and turbidity propagation experiments were in a longitudinal direction along the Sumiyoshi River Basin. The relationship between the wash load amount, s(m³/s), and the discharge, Q(m³/s), is expressed by the equation Qs=α Q². The spatiotemporal variations in the value and the reasons for such variation were studied. The following results were clarified and observed : 1) Currently, the alue of α of the mountain stream in the Rokko Mountains varies with every flood, but it does not vary by orders of The value α is expressed as a relationship between the product of the average gradient in the basin, D, and area of the basin, A. It is expected that the relationship will depend on the geology of every region. 2) The current for the generation and loss of wash loads in the mountain streams of the Rokko Mountains are thought to based on the discharge of wash load components in riverbed sediment when stream water rises and the capture of components when the water recedes. 3) When the environments that generate sediment in the basins change and he bare land ratio ra increases, α changes by orders of magnitude. 4) When slope failure and sediment discharge occur, he bare land ratio ra increases. However, when a riverbed gravel wash load capture mechanism is held, α does not Conversely, it is predicted that when the wash load floating capture mechanisms are saturated, α increases.

There have been recent damaged examples of steel open type Sabo dams against large scale debris flow. This was caused due to large rocks carried in the debris flow resulted from torrential rainfall of abnormal weather. This paper presents a safety verification method of steel open type Sabo dams against large scale debris flow (hereafter, called as load level II). First, the load level II should be considered from the topographical point of view at the dam construction point. Then, the load level II is determined by examining the field survey or past large debris flow disasters. Second, a safety verification method of steel open type Sabo dams is proposed against the load level II form the viewpoint of performance-based design. Finally, a safety verification of steel open type Sabo dams is numerically illustrated against fluid force and large rock impact corresponding to the load level II by using a software of ANSYS AUTODYN.

Natural hazard mitigation has been enlarged to incorporate preservation of cultural properties as the objects, in addition to lives and properties. Studies on historical documents have suggested that former generations seem to have followed better ways when it comes to land use, indicating “ancient wisdom.” It is essential to accumulate empirical analyses on how remains perform as witnesses in time of natural hazards. The city of Hiroshima is well known for its chronicle sediment-related hazards, it faced another large sediment-related disaster in 2014 with 77 casualties. There are 384 ruins in the studied area, of which 138 had been wiped out by modern development. Among the remains, nine sites were located in the area flooded by sediment. Examination of the tumuli shows that the impacts were either minor or inexistent. It is safe to say that archeological surveys could provide useful clues both for evacuation planning and for facility installation.

In this paper, we propose a new single index for estimating the magnitude of heavy rainfall events that have a high probability of causing sediment-related disasters（SRD）and examine the efficacy of an early warning system（EWS）based on this index. The single index is derived from a snake line connecting plots of heavy rainfall events on an x-y plane, with the common logarithm of return period at a given soil water index（SWI）on the x-axis and hourly rainfall （HR）on the y-axis. The index（IRPI）is defined as the distance from the coordinate origin to x-y points on the snake line. The IRPI is integrated with SWI, representing the long-term rainfall effect, and HR, indicating the short-term rainfall effect on SRD using the same units（i.e., years）as return period. EWS using IRPI were applied to the western region of Izu-Oshima Island, Tokyo, Japan, where typhoons Wipha（2013）and Ida（1958）caused severe SRDs. The results of this study were as follows：1）The difference between heavy rainfall causing SRD and heavy rainfall that does not occur is to be recognized as a difference of IRPI；2）IRPI can be displayed along a time axis based on the single index, making estimation of the temporal fluctuation of SRD risk during each heavy rainfall event simpler than estimation using an x-y plane without a time axis ; and３）Employing IRPI in an early warning system allows intuitive understanding of the meaning of the critical value, since IRPI uses the same units as return period.