Ryuichi Hirakawa

Effects of Water Surface oscillation on Turbulent Flow in an Open Channel with a Series of Spur Dikes

Spur dikes have been used as hydraulic structures to suppress bank erosion by flood flow, waves generated by ship and to maintain flow depth as a navigation channel. Recently, it’s noticed that they have functions to produce diversified attractive environments in river. In this study, experiments were executed to clarify water and momentum exchange between main flow and non-submerged spur-dikes regions. The results showed that prominent water surface oscillations happened in an arrangement of spur -dikes and the variations obviously correlated with velocity fluctuations, enhanced the water exchange, but did not influence on Reynolds shear stress.

ANALYTICAL STUDY OF THE TOPORAPHICAL CHANGES OF SAND-BAR AND THE DESTRUCTION OF HERBACEOUS VEGETATION DUE TO FLOODING IN THE KITA RIVER

This paper describes the topographical changes of sand-bar and the destruction of herbaceous vegetation on the sand-bar in the Kita River. The sand-bar in the Kawasaka district was excavated with vegetation in 2001 through the special emergency project after the sever disaster due to flooding by Typhoon No.19 in 1997. A flood on Oct. 2004 destroyed again almost all vegetation in a recovery process on the sand-bar. Nowadays, herbaceous vegetation is restored over all the sand-bar because of no flooding since 2008. In the paper, the destruction of herbaceous vegetation by the topographical changes of sand-bar was simulated through the 2-D analysis of the riverbed evolution. It was considered from the simulation that almost all vegetation will be destroyed by the flood over the maximum discharge of 4,000m/s. Moreover, it was found that vegetation will not be destroyed but grow thick by the flood under the maximum discharge of 2,000m/s.

FLOW PATTERN AND SUSPENDED SEDIMENT TRANSPORT AROUND SPUR DIKES IN THE KIKUCHI RIVER ESTUARY

A series of spur dikes was constructed to reinforce an embankment at bends in the lower reach of the Kikuchi River. In recent years, eutrophicated fine sediment accumulation around spur dikes has caused a serious problem in estuary ecosystem. Therefore, Ministry of Land, Infrastructure and Transport has developed the renaturalization project of sand beach in the estuary. In this study, we have particularly investigated flow patterns and fine sediment transport around these spur dikes by using Stream-pro Acoustic Doppler Current Profiler (ADCP) and radio-controlled model boat equipped with Workhorse ADCP in the period of spring tide. The results show that these spur dikes weakened flow and SS transport in the concave, and strengthened them in the convex of the bend. In addition, fine sediment accumulation were discussed through streamwise distribution of SS transport in both cases of flood and ebb tide.

Effects of Relative Groine Height in Bed Configurations and Flow Structure in an Open Channel Flow

A lot of non-overflow groynes that do not sink under the water at the ordinary water flow are used, and it is known to it does working that keeps away the water current form the river bank..On the other hand, when the water level is surpassing of the height of the groyne, it becomes to sink.. In this study, the height of the groyne was made constant, depth was changed systematically, and it aimed to clarify the characteristic of the bed level variation and the flow velocity profile of the flowfield with a single groyne by the laboratory experiment under the condition to assume the Froude number to be 0.2. As a result, the maximum depth of scour has become small as the relative groyne higher. The groyne height when relative height grows, the ratio of the main flow velocity to the mean flow velocity has grown.

IMPROVEMENT ON THE GROWTH ENVIRONMENT OF ENDANGERED SPECIES

Vallisneria higoensis which is an aquatic plant peculiar to Kumamoto Prefecture is on the verge of the present extinction. In this study, it aimed at an improvement of the growth environment of a water plant by making the flow velocity increase using groyne. The indoor experiment considered groyne arrangement. Consequently, it turns out that the one-bank groynes increases the flow velocity of a mainstream part by about 5% compared with a case without a groyne, and it is increasing about 10% in the both-banks groynes. It was admitted by the result of a field survey that the flow velocity in main flow region after both-bank groynes installation was increasing about 11 to 23% compared with before installation. It turns out that 5m distribution expansion was seen at the maximum about growth of aquatic plant, and the area of a aquiherbosa is also expanded.