Yuichiro Fujita

CONSIDERATION ABOUT ONE-DIMENSIONAL RIVERBED CHANGE ANALYSIS BASED ON OBSERVATION DATA

Based on observation data in a large-scale reservoir for power generation, Sakuma reservoir, 1D riverbed variation analyses are carried out and their results are compared with actual data, mainly sediment transport states derived from those of cross sectional survey and grain size distribution. The analyses are applied only to a winterspring season including an artificial enhancement period of sediment transport in order to eliminate ambiguity of the upstream boundary condition of sediment supply. The result, prove that 1D riverbed variation analysis is able to express only a tendency of the actual states and insufficient quantitatively, and that settings of riverbed grain size distribution, sediment transport width, etc. had to be considered carefully.

STUDIES ON SEDIMENTATION IN A LARGE-SCALE RESERVOIR FOR POWER GENERATION

The initial total storage capacity of 327 Mm3 of Sakuma dam reservoir in 1957 has been reduced to 213 Mm3 in 2002 due to sedimentation of 114 Mm3. In order to reduce flood damage in the upstream area caused by the sedimentation, attempts to lower the riverbed is being continued by dredging and flushing with river water. However, riverbed has not been lowered sufficiently, hence more efficient measures are required. So, accumulated reservoir data of flow discharge, cross sectional surveying and grain size distribution were examined to clarify actual state of the sedimentation. Results of the examination show that sediment concentration is decreasing clearly implying that a flood cannot convey the same amount of sediment as it could before. Another results of sediment movement were compared with those evaluated from 1-D analyses of non-uniform flows, indicating the efficiency of the sediment flushing. Accumulation curves of volumetric changes of cross sections were obtained for individual grain sizes, which can express sediment movement in the reservoir properly.

A TWO LAYER FLOW ANALYSIS OF DEBRIS FLOW CONSIDERING VERTICAL DISTRIBUTION OF VELOCITY AND CONCENTRATION

Governing equations of debris flow include correction factors due to vertical distributions ofvelocity and sediment concentration which vary corresponding to bed slope. In this study, the correction factors in governing equations of a two layer model for debris flow proposed by the authors were examined under theequilibriumstate of the flow over a wide range of bed slope. Numerical simulations were carried out on simple conditions to examine effects of the correction factors on erosion and deposition process of debris flows. The term of sediment flux in a mass conservation of the two layer model modified by the correction factor influences stronglyon the erosion and deposition processes of debris flows.

FIELD STUDY ON FISH HABITAT EVALUATION ACCOUNTING FOR THE CONTINUITY OF RIVER CHANNEL SYSTEM

In consideration of life history of fishes, it is necessary to investigate the fish habitat structure not only in main rivers but also in various scales of tributaries and drainage channels connected them. Weirs and abrupt drops in river system could be discontinuity points which divide fish habitats, obstructing fish migration along river channels. Hence, we conducted a field investigation of the environmental characteristics in the Ijira river, a tributary of the Nagara river, and proposed an index expressing the difficulties in runs of fish prevented by these discontinuity points. Evaluating this index and the grade of three type of vegetation covers and introducing these values into a GIS, we showed tentative evaluation maps for fish habitat quality in the Ijira river.

NUMERICAL EXPERIMENTS ON SEDIMENT DISCHARGE OF DEBRIS FLOW BY A 1-D MODEL

Sediment discharge hydrograph by debris flows, peak discharge in particular, must be estimated to design countermeasures against debris, such as check dams and guide walls. As this phenomenon has a high unsteadiness, effects of pressure gradient and inertia terms in basic equations which are closely related to non-stationariness are examined by numerical experiments under simple conditions. A debris flow model proposed by the authors were employed, and the kinematic wave and the dynamic wave calculations were carried out, by using first an equation for erosion-dcposition ratc of a conventional type and second those added with pressure and inertia terms. Kinematic wave produced fairly similar results to those by dynamic wave except for wave shapes around peaks and recession stage. The pressure term introduced into erosion-deposition rates caused prominent undulations in hydrographs which were mitigated very much by the inertia term.

Experiments on Flow Characteristics around Rich-in-Nature Type of Revetments

Recent river channel improvement projects in Japan are almost occupied by richin-nature river works. River engineers are compelled to be conscious to nature in and of rivers. However, philosophy of nature concerning to river has not been discussed before the rich-in-nature improvement project and fundamental functions of such works have not been studied as well. In this study, rich-in-nature types of revetment were discussed and hydraulic model tests were carried out to investigate flow characteristics and hydraulic functions of a step type of revetments. Secondary currents as observed in channels with compound cross sections appeared in each step. Toe protection with boulders showed a great effect of velocity reduction.

Bed Scour Profiles in Downstream Reaches of Various Types of Bed Protection Wroks

This paper reports characteristics of local scour formed in downstream reaches of various types of bed protection works. Four types of bed protection works are used in experiments under a dynamic equilibrium condition to investigate time related changes in scour profiles, maximum scour depths and locations. The scour processes are also discussed in comparison with bed shear stress distributions measured in the maximum scour hole.In addition, field observations of scour profiles were carried out at a downstream reach of hollow-squared blocks in the Manganji River in Hyogo Prefecture, and the observed scour profiles and velocity distributions were similar to those in the experiments.

Turbulent Structure of Open Channel Flows with Sudden Change in Bottom Roughness

Before and behind the bed protection works in natural rivers, the bed roughness abruptly changes from smooth to rough and rough to smooth. This paper deals with the water surface profile and turbulent structure of open channel flows with sudden change in bottom roughness experimentally and theoretically. Changes in vertical distribution of sediment concentration under the fully suspended condition are also discussed. The transition of flow depth was found to be mainly depend on the bed slope and classified into three types of flow patterns. In the downstream reach after roughness change, the velocity and turbulent Property distributions vary with development of the internal boundary layer, in w hich the transfer of turbulent energy by convection and vertical diffusion is dominant factor in the turbulent energy budget. The calculated results of the k-e and sediment transport models agree well with the experimental ones.