Information on the behavior of debris flow in the initiation zone is important for the development of counter measures. However, only a few observations have been conducted in the initiation zones of debris flows owing to monitoring difficulties. To detect the behavior of debris flow in an initiation zone, we established a monitoring system in the upper Ichinosawa catchment within the Ohya landslide, central Japan. By analysis of video images obtained from field monitoring, flows that appear during sequences of debris flow surges were classified into two primary types : flows comprising mainly cobbles and boulders, and flows comprising mainly muddy water. The velocity of the muddy flows can be evaluated by Manning's equation. Flows comprising mainly cobbles and boulders had higher flow resistance compared to muddy flows, and cannot be evaluated their velocity by Manning's equation. Furthermore, in the case of muddy flows, it seems the flows were turbulent, whereas flows comprising mainly cobbles and boulders were laminar and their boulders slide. Flows comprising cobbles and boulders usually appeared at the front of a surge and are followed by the muddy flows. In each typical debris flow surge, the flow depth was highest during passage of flow comprising cobbles and boulders and the flow velocity was highest at the front of the muddy flow. However, some surges were comprised only one flow type. Grain size of boulders was different amongst debris flow events. Longitudinal grain size segregation of debris flow was not clear in the upper Ichinosawa catchment.

View full abstract

Sediment transport process in mountain streams is not entirely controlled by flow discharge. As a result, the process does not concomitantly follow flow discharge pattern but shows non-consecutive partly independent dynamism. The process has been a subject of extensive studies in theoretical, experimental, as well as observational aspects. Based on direct sampling, experimental bed load equations have been tested to examine the relationship between flow discharge and sediment discharge. The sampling and observation efforts led to the development of more indirect but stable sediment transport monitoring methods (hereafter “indirect method") in recent years. One of the indirect methods, a hydrophone sediment discharge measuring system (hereafter “hydrophone system"), has been intensely studied for the development of practical sediment transport monitoring. Hydrophone systems have been tested quantitatively in relation to bed load equations as well as to directly sampled sediment discharge in 100 and 200 km²-scale river basins. Quantification efforts have been carried out both as site-specific comparison between direct and indirect methods, and as a year-long basin-scale examination utilizing reservoir sedimentation data. These efforts enabled us to take a longer view than a snap shot view of sediment discharge, which is transported in a collective form seen as sediment waves. Examination of the extent to which erosion and flood control structures such as sabo dams transform sediment discharge of floods has been a major practical concern for a long time. Bed load equations can analyze the effects of structures at a given moment but are not adequate to quantify those exerted upon collective forms of sediment discharge. Therefore, indirect measurement, together with flow discharge observation, was conducted in an upstream segment of a mountain stream in order to study the transformation processes and effects of structures. Since direct sampling is not readily possible in the segment, bed load analytical equations developed in a similar mountain stream was applied to quantify the sediment transport process in the segment. The result indicated that sediment discharge flowed down as a wave at a velocity about one-third of that of average stream flow and that the maximum sediment discharge did not grow as much as the aggregated sediment discharge of the waves in the structurally regulated segment of the stream. Flood cases studied here were minor in their intensities. Further observational efforts and analysis are needed to examine whether the quantification method is robust enough and whether the results obtained here hold to a larger flood. This study is a preliminary attempt to introduce an analytical framework that is applicable to larger flood cases.

View full abstract

The sediment discharge and its grain size distribution were measured by the observation facility which installed at Bozudaira sado dam in Yotagiri River of Tenryu River System. The 34 floods which had over 0.5 m depth included maximum 1.76 m were measured from 2001 to 2007, and the characteristics of bedload discharge were mainly considered based on these data. Then the relationship between non-dimensionally tractive force and non-dimensionally bedload discharge from grain size distribution of river bed near the upstream of the observation point have a similar property to the bedload discharge formula such as Ashida, Takahashi and Mizuyama's formula from a quantitative standpoint. But the grain size distribution of bedload was different by the observation and the calculation by bedload discharge formula. These results show there is a possibility that bedload discharge and its grain size distribution are strongly affected by the conditions of the sediment yield and discharge in upper river basin. Therefore the impact of the change of the sediment yield and discharge on the characteristics of bedload discharge was considered from the temporal variation of bedload discharge on the same tractive force. As the result, it was shown that the sediment discharge was increase two or three order after the occurrence of debris flow even if the scale is small, and the bedload larger then about 2 mm of particle size was on a declining trend since the Iijima No.6 Sabo Dam 40 m high was completed in upstream of observation point in February 2006.

View full abstract

The author encountered the Japanese word sharyohyo (sediment staff gauge) in the context of Japanese sediment and erosion control engineering (known as sabo in Japanese) and, through field research and the study of reference materials, was able to confirm more than ten uses of the word in an old stone pillar and old reports. As a result, the author was able to determine that a syaryohyo was a kind of benchmark used for measuring riverbed and hillside elevation. This paper describes an investigation of sediment transport processes in drainage basins conducted with reference to sharyohyo and relevant old reports from the early days of sediment and erosion control engineering, and the filling in of data missing from records from the Meiji Era to the early years of the Showa Era. In addition, the importance of continuity in conducting observations and data collection in the field of erosion control is described.

View full abstract

The history of large-scale sediment disasters since the 1930s in Miyazaki, which is a region of high rainfall in Japan,was studied in order to evaluate frequency of large sediment-movement events, and to examine the relationship between magnitude of sediment movement and rainfall. The large sediment-movement events of amount exceeding 100,000 m³ of sediment have occurred seven times in the fifty one years from 1954 to 2005, and thus the frequency of events is calculated once seven years. Those events were triggered by heavy rainfalls greater than 600 mm of a two-day amount or 1,000 mm of a three-day amount. There are two types of the large sediment-movement events: one is a very large landslide of 10⁵ to 10⁶ m³ of collapsed sediment; and the other is a mass of landslides and debris flows of various sizes,amountingto10⁵ to 10⁶ m³ of transported sediment. Using data of all the landslide disasters in the past forty years from two landslide-prone areas of Miyazaki, the authour examined how landslides increase in number in response to an increase of rainfall. Number of landslides can be regarded as an indicator of magnitude of sediment-movement events. Two types in the relationship between number of landslides and rainfall were found. One is the type that the number of landslides increases rectilinearly in response to an increase of rainfall, and a change of magnitude of sediment movement is continuous from low magnitude to high one. The other is the type that the number of landslides increases remarkably when rainfall exceeds 1,200 mm, and a change of magnitude of sediment movement is discontinuous between low and high magnitudes.

View full abstract