Kazuyoshi Hasegawa

STRUCTURE OF STEPS OF TRANSVERSE RIBS COMPOSED WITH COBBLES IN MOUNTAIN STREAMS

Transverse ribs are popular existences in a mountain river, those are composed of the transverse lines of large cobbles across a stream. For intension to apply the rib-step morphology to a fish-way, knowledge is desired on the configuration structures of rib-steps. Present paper describes results of field survey for the Gunbetsu River to clarify plan forms, rows of cobbles etc. of the ribs, and also deals with configuration features of experimentally formed ribs in a laboratory channel. From the both results, followings were found: Plan form of a transverse rib is not necessarily straight but curved line with S-shape or M-shape is more popular. The former transverse ribs are formed by comparatively large cobbles, while the latter transverse ribs are composed of the cobbles with various diameters. Correlation with respect to the diameters of adjacent cobbles is not strong, which shows the order of cobble diameters is rather random.

EXPERIMETAL STUDY ON BIFURCATED CHANNEL CHANGES AND SEDIMENT TRANSPORT IN A MOUNTAIN RIVER

Movable bed experiments with large-scale bed forms and bifurcated channels were conducted by using a steep slope channel and heterogeneous bed materials in order to investigate mechanisms of stream channel change and sediment transport in a mountain river. From the experimental results and observation, it was found that a bifurcated channel in a mountain river has unstable mechanism such a mainstream changes alternately from one to another at the nodal point of bifurcated channel, and that bed evolution and sediment discharge in the lower reach are obviously influenced by the mainstream alternations. For interpretation of the mechanism, a simple mathematical model was introduced, which was derived from the sediment continuous equation. Self-excited change of a mainstream due to the change of sediment transport rate at the nodal point was shown by the model without considering influence of alternating bar migration.

EXPERIMENTAL STUDY OF SEDIMENT TRANSPORT ON ARTIFICIAL AND SPONTANEOUS RIB BEDS

Experiments of sediment transport were performed to investigate the characteristics of transportation on rib beds by using artificial ribs and spontaneously formed ribs. The transported sand from the head of channel was heaped in a pool to form a ridge by the flow with submerged jet in the case of artificial rib beds. The ridge in the pool was reduced in volume by the flow when the sediment supply from the upper pool reduced or ceased. Thus accumulation wave of sand slowly traveled to downstream with damping the height. On the other hand, the low flow on the spontaneously formed ribs ran through the crevices of a step to transport the sediment in a bed load state. So little stagnation of sediment transport system between them depended on the depth of the pools.

DEVELOPMENT OF ESTIMATION METHOD OF WATER INFLOW INTO DAM RESERVOIR BASED ON FILTERING THEORY

The inflow into dam reservoir is estimated from the change of hydrostatic water level. It is difficult to obtain the estimation of inflow enough to use for practical dam control using moving average process which is adopted as smoothing method for water level data by almost dam reservoir. The aim of this study is to develop a new estimation method of hydrostatic water level and of inflow into dam reservoir. In this paper, the new smoothing method of water level, which is constituted of the combination of notch filter (NF) and low-pass filter (LPF). In the new method, NF and LPF is used for eliminating a seiche and other random noise, respectively. In the case of Kanayama Dam Reservoir, it would be able to obtain the estimated inflow, in which the power of seiche is cut off enough to use for practical dam control if it is possible to admit 20 minutes time lag of flood peak in the estimated inflow.

LATERAL DIFFUSION OF SUSPENDED SOLIDS AND SCOURING AND DEPOSITION OF FINE SAND ON THE FLOODPLAIN DURING FLOODS

The concentration of suspended solids was measured at the Mukawa Bridge located 2.6km upstream of the Mukawa River mouth, and at the Hobetsu Bridge located 40.7km upstream of the river mouth. The lateral diffusion of suspended solids was measured by comparing the concentration of suspended solid at the center of river course and shores. Suspended solids are transported toward the center from the shore in the early flood stage and afterward move to the shore from the center. Observations of sediments on the floodplain were conducted before and after flood at the Mukawa River to understand the relationship between suspended solids and sediment on the floodplain. The scouring of the sediment on the floodplain and the deposition of the particles, which have the same size as suspended solids on the floodplain, was verified. From these investigations, it is shown that suspended solids and the sediment are interchanged during floods.

Field Measurement of Velocity Profiles of a Flood Flow and Analysis of the Results

This study presents the observation result of vertical velocity profiles found in a small flood of the Mukawa River in Hokkaido in August 1996. The result shows that the velocity profiles in the rising stage of the flood deviate from the logarithmic law and have a steep velocity gradient; the profiles in the peak stage of the flood follow the log-law; the profiles in the falling stage again increase the velocity gradient.The characteristic property was analyzed together with experimental unsteady flow data, by using 2-D momentum equation with the parabolic type eddy viscosity coefficient. The theoretical velocity profile derived from the equation, which has a form of Legendre differential equation, well agreed with the observation data. From the fact, it was concluded that the deviation of the velocity profile of the flood from the log-law depends on the effect of flow acceleration of the flood.

Flow properties found in a deep open experiment channel with dense vegetation bank

This paper presents the measurement results of turbulent velocity fields for flows in a deep open experiment channel with dense vegetation. The flows showed the strong 3-dimensional properties that the maximum flow directional velocity appears near the 1/3 water depth from the bottom and Reynolds stress become negative over the 2/3 water depth area. For the cause of the phenomena, transport of low velocity fluid by strong boils to go upward from the bottom near vegetation boundary was suggested from the distribution of turbulent flow energy.

Analysis of water surface oscillation in a dam reservoir using the data of water level from the multiple water gauges

It is important to know the exact amount of water flowing into a dam reservoir for operating a dam. Usually, this inflow is estimated from a data of the water level. Accuracy of measurement of water level necessarily reflects the accuracy of estimation of inflow into the reservoir. But, data of water level include many kind of noise, for example, seiche, wind wave, swell, and so on. In this paper, we analyzed the data of water level obtained from multiple water gauges installed in Jozankei Dam Reservoir. We found characteristic of water surface oscillation such as several kind of seiche, swell, and wavy displacement by wind. We could confirm that the wavy displace-ment of water surface caused by wind gives much error to estimate the inflow into the reservoir.

Lateral bedload transport for bank materials with wide distribution of size considering the slope effect

Lateral bedload function for fractional particle sizes on the tilting side slope was developed by following Parkers method. On the slope with near angle of repose, tangential component of the gravity force acts on the surface particles in equivalent intensity comparing with the tractive force. Thus the full non-linear equation for a particle motion was solved numerically. Thickness of the bedload layer was given from extending Ashida Michiue equation to consider the gravity effect and to use Egiazaroff equation for critical tractive force of fractional particles. Summation of the lateral sediment discharge for each fractional particle size calculated from the product of the moving particle velocity and bedload layer fairly well agree with the observed data obtained from large scale bank erosion experiments using volcanic ashes for the bank material.

Measurments of Lateral Momentum Diffusion Coefficient in Open Channel Flows with Groins

Coefficients of lateral momentum diffusion in open channel flows influenced by a series of groins were obtained from the detailed velocity profiles which were measured by LDV. The coefficients are not constant in lateral direction rather distribute with a peak in diffusion area of a groin. Magnitude of the coefficients are comparable or more less to compare with that of vertical momentum diffusion. It was found that the coefficients calculated from log-velocity distribution along the oblique line stretching from the shear layer of a groin correspond to the directly measured data.