Akihiro Tominaga

Three-dimensional turbulent structure in straight open channel flows

Secondary currents are generated and modified as a result of the anisotropy of turbulence, which is caused by the boundary conditions of the bed, the side-wall and the free surface, as well as the aspect ratio of the channel and the channel geometry. Secondary currents affect the primary mean flow, producing threedimensional structures. Such a generation mechanism for secondary currents in closed and open channel flows can be explained well by using the longitudinal vorticity equation. The secondary motions in open channel flows are quite different from those in closed channel flows.

Secondary Currents in a Straight Channel Flow and the Relation to Its Aspect Ratio

The initiation and maintenance mechanism of multi-cellular secondary currents in a straight wide-river is quite unknown at present. This study investigated experimentally the existence of secondary currents over a flat bed by varying the aspect ratio of channel. It was suggested strongly that an initiation of cellular secondary currents may be motivated by the mutual interaction between the secondary currents and the sand bed.

FIELD MEASUREMENTS OF DISCHARGE AND FLOW STRUCTURES IN A RIVERSIDE CONCAVITY ZONE INDUCED BY TIDAL FLOW IN THE HORIKAWA RIVER

Field observations on characteristics of discharge were conducted in an urban tidal river, the Horikawa River. Velocity distributions and discharges on various cross sections were measured by acoustic Doppler profiler (ADP). The longitudinal distribution of maximum discharge caused by tidal flow was revealed. The velocity structures in a falling stage were different from those in a rising stage. In the falling stage, the surface-layer velocity is higher and the lower-layer velocity is lower than the log-law profile. In the rising stage, the medium-layer velocity indicates maximum value. In a concavity zone of the tidal river, horizontal vortices were observed in both the falling and rising stages. The flow structures in the concavity zone indicate three-dimensional feature. The surface vortex structures were different between falling and rising stage.

FIELD OBSERVATIONS ON CHARACTERISTICS OF SALINITY INTRUSION AND VERTICAL MIXING IN AN URBAN TIDAL RIVER

Field observations on characteristics of salinity intrusion and vertical mixing were conducted in an urban tidal river, the Horikawa River in Nagoya City, in order to understand the effects of stratified flow and reciprocating flow on water quality. We measured time series of vertical distribution of water depth, velocity, salinity, temperature, DO etc. during one tide by using an electromagnetic velocimeter and a multiple water quality meter. The vertical mixing type in the measured region was basically classified as a mild mixing type, but a considerable change of mixing type was observed with time and location. An increase of installed water slightly affects a stratified flow structure at the measuring site. Salt water has lower DO value than that in fresh water and it is moving up and down through a bottom layer during tidal motion.

EXPERIMENTAL STUDY ON HYDRAULICS AND BED PROTECTION EFFECTS OF A TRADITIONAL RIVER STRUCTURE, “SEIGYU” GROIN

A traditional river structure, “Seigyu”, is a kind of spur dikes for protecting riverbank against erosion. Actual examples of Seigyu groins and their effects on bed protection were surveyed in the Ooi River. Experiments on flow structures and bed evolution were conducted in a laboratory flume with Seigyu models. The effectiveness of the groins was recognized on the deceleration of downstream flows and on the prevention from bed scour. The arrangement manner and the placing method of the groins were also investigated. A series of Seigyu groins is effective even for steep-slope flows but the erosion becomes very large beside groins. We compared the bed configuration around Seigyu groins with that around a group of rod roughness. The characteristics of resistance to flow for Seigyu groins is different from that for rod dikes because Seigyu groins have gabions near the bed. This fact makes different effects on riverbank protection.

EFFECTS OF CONFIGURATION AND VEGETATION ON THE MECHANISM OF WATER EXCHANGE IN ARTIFICIAL RIVERSIDE EMBAYMENT

Riverside embayment is expected to provide good habitat for fish in rivers. We measured flow field and water qualities at the artificial embayment zone in the experimental river of the Aquatic Restoration Research Center in order to investigate the mechanism of water exchange in riverside embayment. In a high water stage, flow structures near the embayment were revealed by the measurement. Vegetation on the downstream end of the entrance zone reduces the recirculating flow and this effect is well reproduced by the numerical model. In a normal water stage, the flow pattern in the embayment is affected by the configuration of the embayment. Laboratory experiments were conducted to make clear the effects of vegetation along the entrance of the embayment systematically.

STUDY ON MECHANISM AND CONTROL METHOD OF WATER EXCHANGE IN ARTIFITIAL RIVERSIDE ENBAYMENT

Water exchange in riverside embayment is influenced by two factors. One is the vortex caused by the shear instability in the interfacial region between main flow and embayment. The other is the recirculating vortex in the embayment. The relation between shear-induced vortex and recirculating vortex was investigated by using flow visualization technique, velocity measurement and turbidity measurement. In order to enhance the water exchange, projection structures were mounted on the upstream end and the downstream end of the embayment. The feature of both vortices and the water exchange rate are changed by position and length of the projection structures.

EVALUATION OF H-Q RELATION OF THE SETA RIVER IN 18-19 CENTURIES USING HISTORICALDOCUMENTS

The Seta River is the only one natural outlet river of Lake Biwa. This paper examines the relationship between water level of Lake Biwa and discharge of the Seta River in historical years. Since there are no gauged discharge data in historical years, water level data are transformed into differences of outflow from inflow (Qout-Qin) using the relationship between water level and storage volume (H-V relation) of Lake Biwa. The results indicate that flow capacity of the Seta River was expanded after the dregging in Tempo years (1831-1833), and that there were no notable changes in flow capacity during Meiji Period (1874-1899).

Transformation Process of Flood Waves Passing Compound Channels with Locally Vegetated Flood Plains

Transformation process of flood waves and hydraulic characteristics of unsteady flows were investigated experimentally in compound channels with locally vegetated flood plains. Some noticeable differences between unsteady flow structure in compound channels with and without vegetation were revealed. The shortly vegetated zone in the flood plains causes the sharp decrease of the flow depth in front of that and increases the upstream stage extremely. 2-D depth-averaged numerical calculation technique for unsteady flow with vegetation in flood plains was developed. The calculated results reproduce well the characteristic tendency of the experiments. This calculation method is useful for the flood routing passing such a complex flow field.

Effects of permeable and non permeable protection works against bridge pier scour and their influence on local flow structure

The effects of the protection works against the scour around bridge pier were studied experimentally, taking notice of their permeability. The scour around the pier is sufficiently diminished by the flush surface protection works. There are significant differences between the permeable and non permeable protection works in the scoured bed configuration and the flow structure behind the pier. The scour depth is smaller in the permeable case than the non permeable case. This is attributed to the reduction of the longitudinal vortices behind the pier by permeating flow.