Syunsuke Ikeda

Bend theory of river meanders. Part 1. Linear development

Instability of the alternate-bar type in straight channels has long been identified as the cause of fluvial meandering. The condition of inerodible sidewalls, however, does not allow a meandering channel to develop. Herein a stability analysis of a sinuous channel with erodible banks allows for delineation of a ‘bend’ instability that does not occur in straight channels, and differs from the alternate-bar instability. In the case of alluvial meanders, the two mechanisms are shown to operate at similar characteristic wavelengths. This provides a rationale for the continuous evolution of alternate bars into true bends such that each bend contains one alternate bar. The same bend instability applies to incised meanders. A mechanism for incised alternate bars which differs from that for the alluvial case appears to operate at different characteristic wavelengths than that of bend instability. Analysis of data suggests that meandering in supraglacial meltwater streams is primarily due to the alternate bar mechanism, whereas the meandering of rills incised in cohesive material and of caves is likely due to the bend mechanism. The meander wavelength of incised reaches of meandering streams is often longer than that of adjacent alluvial reaches. An explanation is offered in terms of bend instability.

Numerical study on turbulent flow and honami in and above flexible plant canopy

Abstract The waving of flexible plant termed honami is caused by large-scale vortices which are induced by inflectional instability of flow field. The movement of plant in turn may affect the turbulent flow field via resistance to flow, turbulence production and dissipation. The present study treats the interaction of turbulent flow and honami employing 2D-LES, in which a concept of “plant grid” is used to treat the movement of plant in addition to the ordinary LES grid. The drag due to the plant is formulated in proportion to the square of the relative velocity between flow and motion of plant in momentum equations for turbulent flow field. In transport equation of turbulent energy, the additional terms of turbulent production and dissipation due to motion of plant are included. The computational result indicates that the honami reduces the periodicity of vortex generation, and the velocity fluctuation, therefore, becomes irregular if compared with the case for which the plant is assumed to be rigid. It was also found that the honami repeats generation and disappearance.

Three-dimensional fully developed shallow-water flow in mildly curved bends

A mathematical model which describes three-dimensional fully developed flow in mildly curved open channel is developed. The side-wall boundary layers are included in the analysis, and the normal component of the secondary current is constrained to be zero at the side walls. It is found that the advective transport of fluid momentum due to the secondary current induces accumulation of primary-flow (streamwise) fluid momentum near the outer bank. The analysis makes it possible to analyze the curvature-induced excess of shear stress at the outer bank. The model is well supported by available laboratory data.

Study on the transport of suspended sediment in an open channel flow with permeable spur dikes

The transport of suspended sediment in rivers with spur dikes is an important issue for bank protection and environmental management. Laboratory experiments were performed to study the characteristics of the flow and the transport of suspended sediment in an open channel with permeable dikes. Firstly, the essentials of these characteristics were studied by comparing results of flume experiments on permeable and impermeable dikes. The influence of the aspect ratio (d/l) of the interval between dikes (d) to the length of dikes (l) on these characteristics was then investigated. In these experiments, the properties of horizontal eddies, turbulence structures, and suspended sediment concentrations were studied. The results show that the development of large-scale horizontal eddies requires some distance in a shear layer for permeable dikes, although they are periodically generated from the first dike in the case of impermeable dikes. The basic structures of horizontal eddies are independent of the aspect ratio in the equilibrium region for permeable dikes. The suspended sediment concentrations for cases of permeable dikes gradually decrease between the several upstream dikes and they approach a uniform distribution in the downstream region, although those in the case of the impermeable dikes are relatively uniform in the downstream direction.

Stable Channel Cross Sections of Straight Sand Rivers

A mathematical model is presented for defining the size and cross section of self-formed straight sand rivers in which sediments are actively transported everywhere as bedload and suspended load. A balance of lateral bedload and lateral diffusive transport of suspended sediment is imposed to maintain a stable channel cross section. Singular perturbation technique is employed to solve the sediment balance equation, from which a relation for stable depth is derived. A resistance formula for dune-covered beds is used to obtain stable width. It is revealed theoretically that the channel center depth relative to sediment size is approximately proportional to S−0.8 for natural sand rivers, in which S is longitudinal slope. The analysis is found to agree reasonably well with available laboratory and field data.

Sediment-associated nutrient transport during floods in a gravel river

For riparian vegetation, sediment-associated nutrient transport during flood is one of the major agencies for controlling the nutrient environments of the soil. This paper describes the sediment-associated nutrient transport during flood in a gravel river by field observation and numerical computation. A field observation was performed to know the effects of flood flow on the soil environment of flood plain in a gravel river. In the observation, the grain size distribution and the chemical composition of the flood plain soils were measured before and after the flood. The results of the observation show that the amounts of particulate nutrients in the flood plain soil decrease during fairly large flood, because the fine sands which include the nutrients abundantly were removed by the flood flow. A series of numerical computations for the transport of suspended sediments and the associated transport of nutrients during flood was performed by varying the peak discharge of the flood. The result of the computation on the reduction of the particulate nutrients in the flood plain soils agrees well with those observed in the field. It was found that the particulate nutrients on the flood plains decrease if the discharge of the flood exceeds about 500 m3/s in the observation area.

Fall velocity of single spheres in vertically oscillating fluids

The settling or rising velocity of a single spherical particle in a vertically oscillating fluid is analysed in terms of the nonlinear perturbation technique. It is demonstrated that the retardation of the particle in an oscillating fluid is caused essentially by the nonlinear nature of the drag force, and the retardation is found to be governed by three independent variables, the Reynolds number of the settling sphere in still fluid, the dimensionless velocity amplitude of the fluid oscillation and the dimensionless frequency of fluid oscillation. It is shown that the existing theories are special cases of the present analysis. The model is tested with Hos data which cover wide ranges of the three independent parameters, and it supports the current theory.

Visualization of behaviors of a propagating flame quenching for hydrogen---air gas mixture

In this research, a flame arrester consisting of a slit structure was experimentally investigated. Experimental data show adequate maximum experimental safe gap (MESG) value for the flame arrester. The flow rate characteristics of the flame arrester were measured and compared with theoretical results. It was made clear that the flow impedance of convergent flow is 20% less than that of divergent flow. The experimental data and theoretical data show good agreement. The performance test by an EN12874 as ‘in-line stable detonation’ flame arrester was examined for a hydrogen–air gas mixture. The experimental data show that the gap was 0.2 times the MESG value in bi-directions for the flame arrester. The quenching and extinguishing processes were visualized by high-speed cameras.Graphical Abstract

Sediment and nutrients transport in watershed and their impact on coastal environment.

Sediment and nutrients yields especially from farmlands were studied in a watershed in Ishigaki island, Okinawa, Japan. The transport processes of these materials in rivers, mangrove, lagoon and coastal zones were studied by using various observation methods including stable isotope analysis. They were simulated by using a WEPP model which was modified to be applicable to such small islands by identifying several factors from the observations. The model predicts that a proper combination of civil engineering countermeasure and change of farming method can reduce the sediment yield from the watershed by 74%. Observations of water quality and coral recruitment test in Nagura bay indicate that the water is eutrophicated and the corals cannot grow for a long time. Based on these observations, a quantitative target of the reduction of sediment and nutrients yield in watershed can be decided rationally.

CHARACTERISTICS OF OSCILLATORY FLOW OVER RIPPLE MODELS

Tanah Lot Temple is situated in Tabanan Regency - Bali, on the coast of the Indonesian Ocean. Due to continuous wave attack, wind force, and weathering of the rock bank where the Temple stands, abrasion has occured which is more and more threatening the existence of the Temple. Considering that Tanah Lot Temple is a sacred place for the Hindu Balinese people and a place of high cultural value, and also an important tourism, steps to save the Temple are imperative. The Central as well as the Regional Authorities, and also the Bali nese community are very much interested in the effort to keep the Temple intact. Measures have been undertaken to protect both the seaside and land-side banks of the Temple rock bank. This paper only discusses counter measures of the sea —side bank of the Temple.