The central part of a check dam was completely removed for the purpose of the biodiversity restoration in a mountain stream in the Shigekura River of the Tone River system. In the section including the removed dam and the natural stream section in which the dam is not installed, restoration of stream environment was considered by measuring and examining change of logitudinal profile of stream bed and depth of water, and distribution and form of pools. The concavo-convex feature of the stream bed was analyzed by spectral analysis. Comparisons were made between the section affected by the presence of the dam (D-2013) and natural section (U-2013). The pool density and the ratio of the cumulative length of the pools to the section length were found to be smaller in the former compared with those in the latter. In the section affected to the dam, there is almost no peak projected by power-spectrum density. This shows that there is no periodicity in unevenness of a stream bed, and the section accordingly does not have the natural form characteristic of a stream. The ratio of the cumulative length of the pools to the section length in the upper section (D-2013-2), i.e. where the dam was removed, is longer than that in the lower section of the removed dam (D-2013-1). Although the big difference was not seen between the upper and the lower sections of the removed dam about the morphological feature of each pool, unevenness of the stream bed with periodicity was observed in the upper section. In the upper section of the removed dam, stream bed fluctuation becomes active and pool is reviving, the section is accordingly approaching the quality of a natural stream by dam removal.
Sediment disasters cause human injuries, loss of life, as well as cultural heritage sites. Among other problems, woody debris hazards have increased due to typhoons and localized torrential rain. In particular, woody debris hazards have eroded river in the upstream region because of removal costs of woody debris from dam reservoirs and damage on dam sites. One of such counter measures is a steel open-type Sabo dams which consist of steel pipes, and they are designed to allow soil and small gravels to pass downstream through the gaps. When the debris flow has occurred, the Sabo dam can capture large rocks by trap mechanism of arch action. According to the report of the Izu-Oshima disaster case, steel open dams have captured woody drifts and sediment without rocks. Some of authors also surveyed the disaster site and noticed that steel open dams captured the woody debris which was intertwined with woody roots and sediment. However, the mechanism has not yet been clarified concerning why steel open dams have trapped the woody debris with roots and sediment without rocks. This paper presents an experimental approach on clarification of blockage of a steel open dam by woody roots and trap mechanism of following sediment by using a straight channel. The experiment sets up the steel open dam model in a straight channel and a mixed sand with driftwood model flow down from the upstream. Sand capturing mechanism and an influence of driftwood shape are observed. Finally, it is pointed out that roots of driftwood show significant effect on capturing ratio of sands.
Kuchino-erabu volcano recently erupted in August 3, 2014 and May 29, 2015, which are considered that the plume discharged from the vent did not form dense pyloclastic flows but dilute pyloclastic surges. Pyloclastic surges travel down slopes in very high speed with very high heat that may cause serious damages to human property and life. In order to manage and mitigate the risks of pyloclastic surges, it is severely required to predict traveling range of pyloclastic surges. In this paper, the authors present analytical model of pyloclastic surge, considering the phenomena as a kind of gravity-driven density currents flowing downward on inclined slopes. The model is applied to the eruptions mentioned above and compared with the actual range of pyloclastic surges. Also the rheology of pyloclastic surge is mentioned counting the computational results.