Muneo Hirano

Rapid flows of sand-water mixtures at high concentration in a steep channel

Abstract Nondimensional parameters are derived for determining behavior of rapid flows of sand-water mixtures. From the result, it is found that the ratio of flow depth to sand diameter and sand concentration for the estimate of friction forces acting on a bed, and the ratio of tube diameter to sand diameter and concentration for that of drag forces on a tube are important parameters. Dependence of the drag coefficients on nondimensional tube diameter and concentration is found similar to that of the friction coefficient on relative depth and concentration. For the smaller ratio of a characteristic length to sand diameter and larger sand concentration, intergranular interactions play dominant role. For the larger ratio, the effect of turbulence of the mixture flows becomes major. For smaller concentration, interstitial water becomes important.

Data Analysis of Debris Flow with Soft Computing

The purpose of this study is to develop the analysis method for debris flow with soft computing. The soft computing methods which embrace neural networks and/or genetic algorithms (GA) are expected to solve problems which are defined imprecisely from a complex phenomena.The neural network combined with the GA was introduced into the analysis for the occurrence condition of debris flow. Training data sets to the LVQ network are optimized by the GA. Theprocedure was used to examine a characteristic change in occurrence condition of debris flow. Runoff analysis of debris flow by the use of GMDH was proposed. These methods were applied to the data for the Mizunashi River in the Unzen Volcano. The results led that the soft computing methods are useful for data analysis of the debris flow.

Concentration Profile of Suspended Sediments in Open-Channel Flow at Steep Slopes

Experiments of mixture flow of fine sand and water are made in a steep open channel. Concentration profiles of suspended sand are measured. A two-layer model for the mixture flow is proposed; the lower layer of intergranular interactions and the upper layer of turbulent motion. Using the top of the lower layer as the reference level, the concentration profiles are calculated from the Rouse equation. The calculations are in good agreement with the experiments.

STABILITY MECHANISM AND CONTROL OF LONGITUDINAL VORTEX STREETS

Boundary effects on the stability of the street of longitudinal vortices with their alternately changing direction are investigated by using the vortex filament model. The analysis shows that the longitudinal vortex street is unstable to a infinitesimal disturbance in both the infinite and the semi-infinite regions over a flat bed, whereas it is stable in the finite region bounded by the free surface and the flat bed. These results were recognized to correspond with the behavior of longitudinal vortices experimentally found in both the inner and the outer layers of an open channel flow. Furthermore, arrangements of a vortex generator are investigated to control longitudinal voricies, sand wave and bed load.

Flow Behaviour of Debris Flows in the Mizunashi River, Mount Unzendake

Between June and August, 1992 field measurements of debris flows were obtained at the Mizunashi River, Mt.Unzendake. The instrumented study site was a most downstream reach of the Mizunashi River, which is 25.1m wide and have an average slope of about 2°. Radio currentmeter, ultrasonic water level gauge and hand sampling of flows were used to obtain surface velocity, depth and compositional data for the several debris flows.Sediments in the debris flows were silt, while deposits from the debris flows on the study site were very coarse sand.Sediment concentration for the debris flows ranged from 6.3 to 13.5percent by volume. Nondimensional surface velocity us/u* ranged from 3 to 20.On the basis of these results, the volume of sediment runoff was estimated for the debris flows.