Shigeki Kobatake

WATERS DATA TREATMENT ON 3-DIMENSIONAL MEASUREMENT OF THE LANDFORM OF THE RIVERBED

3D laser scanner used on this study can not measure the landform under the water. However, the absence of data shows existence of the water in the other. It is important for the elucidation of the deformation mechanism of the riverbed to grasp the landform of the riverbed in detail. In this study, we tried to discriminate the water route from the measured data by using the trimming algorism and reflectivity. The shoal and water route was elucidated by an analysis of the trimming algorism and reflectivity. Some important characteristics of the waters area in measurement data of the landform of the riverbed by the 3D laser scanner were showed

STUDY ON THE DESTRUCTION CRITERION OF RIPARIAN TREES IN A GRAVEL-BED RIVER CAUSED BY FLOOD DISTERBANCE

Recently riparian trees (Robinia pseudo-acacia) in the course of gravel bed-river have extremely increased and enlarged their covered-area over bars and flood plain. Such a situation brings about problems of river management for the environmental aspects as well as the safety of rivers against flood.In this paper, the fall down process and the damage conditions of the riparian trees caused by the 1999, 2001 floods were examined in the segment-1 reach of Watarase River. Especially, from the field surveies after floods, it was clear that the damage, which the floods gave to the trees, could be classified by the scale of the flood external force. Based on the field study and numerical calculation with 2D-flow model, the critical tractive force to be a key index of the scale of flood disturbance was evaluated and results showed that the fall down process of riparian trees is based on the degradation of movable river-bed

A STUDY OF ACCUMULATION OF A LUMP OF FLOATING LOGS ON BRIDGE PIER

The characteristics of the accumulation of a lump of floating logs on bridge pier are investigated by laboratory experiment. The results show that floating logs can be very dangerous for bridge when many floating logs make a lump even if the ratio of span to log length is little. If the floating log is long and the flow rate is calm, the area of a lump of floating logs causes the area of accumulation of a lump of floating logs on bridge pier without the influence of the condition of each floating logs. But the shorter the floating logs become and the larger the flow rate becomes, the smaller the area of accumulation of a lump of floating logs on bridge pier becomes.

ANOTHER ROLE OF SABO DAM

We have started observations of rainfall amount and water stages at Sabo dams in some devastated basins and a natural basin which was chosen as a basin for comparative studies. This paper points out another role of Sabo dams after describing some characteristics of runoff and sediment yield from devastated basins and a forested basin. The obtained results are as follows:1) The ratio of runoff/rainfall during June to November of the devastated basin exceeds that of the forested basin. These phenomena may be occurred by high intensity of evapo-transpiration in the forested basin and high storage capacity of water in group of Sabo dams constructed in the devastated basin.2) The peak specific discharge from the forested basin exceeds that of from the devastated basin for large flood. These phenomena may be also occurred by storage effect (namely, flood regulation effect) of group of Sabo dams constructed in the devastated basin.

Experimental Study on Characteristics of Flood Wave Propagation in a Vegetated Channel

A knowledge of the nature of flood wave propagation in vegetated rivers is very important for predicting the flood routing. Vegetation in rivers often make sudden-contractions and cause high flow resistance at a flood stage. The high water stage and the channel storage of flood appear, due to the backwater effect of vegetation roughness. In this paper, the characteristics of flood wave propagation in a vegetated channel are studied through fundamental experiments with changing the rate of vegetation-covered area which is defined as the ratio of the vegetation-covered area to the non-covered area. Especially, the relationships between the rate of vegetation-covered area and the increasing rate of the maximum water stage, the decreasing rate of the maximum food discharge are investigated.

Experimental Study on Cooling effects of River using Wind Tunnel

The cooling effect of river on atmospheric temperature around river in summer is a well known phenomenon. To elucidate this phenomenon qualitatively, some experiments were carried out in wind tunnel having 90×90 cm cross section. A river channel is simulated by a long squarepipe (12.5×8 cm cross section), and dry ice was set inside of this pipe as cooling source. Both side of this pipe, heat insulating materialcovered with artificial lawn as surface roughness were installed as riverside area. Wind velocity was measured with 2-dimensional hot-film anemometer. Changes of values of Reynolds stress and eddy viscosity, and of theirprofiles were analyzed in combination with experimental conditions of cooling and wind velocity.

Study on the evapotranspiration effects on the daily discharge variations

It is a well known phenomena that the discharge varies as time in a day time by evapotranspiration suction. This phenomena can be observed in a small upper mountainous basin at low flow stage. The precise observations were taken place in two basins having area 0.18km2 and 0.060km2. Each basin shows discharge variations clearly. Numerical simulations based on Richardss equation taking acount of evapotranspiration were carried out and the calculated discharge rate were compared with the observational one. The simulation model can explain simple decrease phenomena, but for complex stage as discharge recovery, the model is not sufficient to explain the phenomena.

ESTIMATION OF SCALE OF SLOPE FAILURE TAKING ACCOUNT OF DISTRIBUTION OF SUB-SURFACE FLOW DEPTH

Stability analysis of slope, based on the assumption of uniform field, cannot estimate two-dimensional scale of failure. On the other side, the depth of sub-surface flow which is highly related to slope failure is never considered uniform at natural slope. Paticularly, at a V-shaped micro configuration of the ground, the rise of sub-surface flow depth must be rapid and the failure will occur first at there. This paper proposes a model which estimates two-dimensional scale of failure occurred at such a not uniform field. The model consists of two parts, one is configuration model as shown in Fig. 1-Fig. 3 and one is stability analysis model. The stability analysis model is based on circular arc method, which introduces cohesion on the side of circular arc in addition to the bottom of the arc.