Akihiro Kadota

Three-dimensional structure of space-time correlation on coherent vortices generated behind dune crest

Large-scale coherent vortices such as separated vortices and kolk-boils are often observed and generated behind crests of sand dunes in fluvial rivers. The kolk-boil vortex greatly affects transport of suspended sediment which develops up to the free surface. In the present study, both visualization by dye-injection technique and turbulence measurements with a laser Doppler anemometer were conducted in order that the elemental structure of vortices could be clarified. 3-D space-time structures of the coherent vortices are discussed by evaluating conventional correlation coefficients and also by using conditional sampling techniques. Instantaneous velocity-velocity combinations at different points were simultaneously measured by means of two sets of laser Doppler anemometers and an automatic traversing system. Refined physical models of coherent vortices are proposed in the present study.

Experimental Study on Changes of Bed Configuration Caused by a Permeable Groyne of Stone Gabion

The permeable groyne of stone gabion is recently proposed as countermeasures for the scouring problem and also for suitable ecosystem because it keeps the stable flow and bed conditions near the groyne tip. However it should be considered that typical flow phenomena as same as impermeable groyne such as developments of large-scale vortices still occur far downstream the groyne and the phenomena might affect on the morphology of main channel. Especially in Japanese rivers, these effects will be seen due to sudden change of discharge due to heavy rainfalls so that flow around the groynes change rapidly from submerged and flow conditions. These phenomena are one of the reasons for morphological changes at the downstream of groyne. In the present study, the characteristics of bed configuration downstream of a single permeable groyne are experimentally discussed on effects of the permeability and the scales of groyne and stone gabion as well as the flow conditions such as submerged flow over the groyne. The results show that local scour around the permeable groyne is proved to be significantly small, whereas the bed configuration of the downstream region far from the groyne largely changes forming sand waves.

Sequential Prediction of Daily Groundwater Levels by a Neural Network Model Based on Weather Forecasts

This paper investigates the implementation of an Artificial Neural Network (ANN) model for sequential prediction of daily groundwater levels based on precipitation forecasts. The basic principle of the ANN-based procedure consists of relating previous daily groundwater levels and daily precipitation forecasts in order to predict daily groundwater levels up to seven days ahead. The daily precipitation values up to one week ahead are assumed to be deterministic since meteorological short-range forecasts are generally available. The methodology is applied to the groundwater system of Matsuyama City, Japan. Insufficiency of water is a periodical problem in this city and thus accurate predictions of groundwater levels are very important to improve the water resources management in the region. The excellent accuracy obtained by the ANN model indicates that it is very efficient for the multi-step-ahead prediction of daily groundwater levels. As conclusion, this methodology may provide trustworthy data for the application of models to the sustainable management of Matsuyama’s groundwater system.

Instantaneous-Advective Structures of Large Scale Coherent Vortices Around a Single Groyne

Studies on flow patterns and transport process around groyne-like structures have recently and significantly increased related to improved experimental and computational possibilities. However, only few studies focus on flow in wide flow-area and also variations of water level, especially the case of submerged groynes. To investigate the instantaneous flow patterns around a single groyne, experiments were conducted by means of surface flow visualization technique in the shallow water channel. A Particle Tracking Velocimetry (PTV) method is adopted and implemented in order to calculate velocity fields fast and accurately. A 2D-PTV code was programmed by combining the advantages of FFT and Direct Correlation methods to get accurate results within reasonable time. In addition, Proper orthogonal decomposition (POD) is applied to obtain an impression of the fluctuating flow pattern and its coherent structures. Conditional sampling analysis is also applied to reveal the advection properties of POD decomposed flow pattern. The present study discusses the turbulence statistics; POD decomposed coherent patterns and their advection properties under submerged and emerged conditions. The results of POD pattern and advection clearly highlight the typical fluctuating flow pattern and large-scale motion, respectively.

Turbulence Measurements and Conditional Sampling Analysis on Coherent Vortices in Unsteady Open-Channel Flows over Dune Bed

Large-scale coherent vortices such as separated vortex and kolk-boil vortex are often observed and generated behind the crest of sand dunes in fluvial flooded rivers. The kolk vortex is a strong upward-tilting streamwise vortex and it seems like a tornado motion in the atmosphere. The kolk vortex develops up to the free surface and then becomes a boil. On the contrary, the separated vortex is generated from the crest of dunes then reattaches down to the dune bed surface. The upward motion of boil is mainly generated by the reattachment to the dunes. The behavior of these coherent structures is quite different between the rising and falling stages of the flood period. In open-channel flows such as rivers and estuaries, various coherent vortices are often observed and greatly affect sediment transport. The coherent structures often occur especially in flooded fluvial rivers (Coleman 1969, Kinoshita 1984). It is very important to investigate unsteadiness effects on the unsteady open-channel flows so that the growing process of the dunes and the behavior of the suspended sediment can be explained dynamically. In this study, two velocities at different points were accurately measured by means of the simultaneous use of two sets of laser Doppler anemometers, to analyze conditional space-time correlation and coherent structures in unsteady open-channel flows over dune bed. Also, measurements of concentration by using dye-injection technique and fiber-optic concentration instruments were conducted. The differences of correlation coefficient and dye concentration between the rising and falling stages were discussed.

CHANGE OF VEGETATION AREA ON SAND BARS DEFORMED BY SEVERAL FLOODS IN AN ESTUARY

Large sand bars which had been fixed for a long time in the estuary of the Shigenobu river in Matsuyama city were deformed intensively by several large floods which occurred successively in a short period in 1993. And the bars above sea level at mean tide became diverse, which resulted in the change of vegetation (reed) areaon the bars. This two-dimensional morphological change in the estuary is surveyed and numerically simulated with the finite difference method for the two-dimensional shallow water equations, and the scale of tidal marsh at both low and high tides is discussed from the view point of vegetation area on the bars. The vegetation area four years after the floods changed and diverged according to the deformation of the sand bars above sea levelat the tide of T. P. 1m.