Yoshihisa Kawahara

Flow–vegetation interaction in a compound open channel with emergent vegetation

This study aims at revealing the flow structure in a vegetated compound channel with special emphasis on the interaction process and momentum exchange between the flow in the main channel and the vegetation that fully or partly covers the floodplain. A three-dimensional numerical simulation is carried out with a nonlinear k−ϵ model coupled with a vegetation model to explore the distribution of mean velocity and turbulence structure in the main channel and localized vegetation zones under emergent conditions. Laboratory experiments along with numerical simulations are carried out. Three types of vegetation zones are prepared on a floodplain of one side of the prismatic channel. The first type has an isolated vegetation zone that is 0.9 m long, the second type has two distant vegetation zones, each of which is 0.9 m long and these zones are 0.9 m apart, and the third type has fully covered vegetation zone that is 2.7 m long. The numerical results are compared against experimental observations in terms of mean velocities, showing a fairly good agreement between them.

Integrated Modeling for Inundation Flows in Urban Areas

This study proposes an integrated physically based distributed model for inundation flows. It has four sub-models: a hydrological model for rainfall-runoff, a two dimensional shallow water model for overland flows, a one dimensional model for flows in sewer networks, and a data management model. The 2-D shallow water model is at first tested against the physical experiments about the inundation due to levee breach. Then the model is applied to the inundation caused by the extreme tidal surge in Takamatsu city, a local capital in Japan in 2004. Comparisons with measured data show that the model reasonably describes the inundation process in the urban area and that the sewer network played an important role in draining the sea water.

A new geophone device for understanding environmental impacts caused by gravel bedload during artificial floods

Here, to assess the contribution of gravel bedload on the removal of attached-algae and aquatic plants from a cobble-bed river during small floods, we propose a geophone type method for measuring the local bedload of non-uniform sized gravel. Due to limited peak discharge for focused events during our study, a large fraction of bed material (here cobbles) was immobile and only a small fraction of bed material (sand and gravel) was expected to be transported during the flushing flows we analyzed. The device we developed has a size equivalent to immobile bed material and a shape similar to bed material (rounded cobbles) at the site. The instruments design allows avoidance of disturbances in river bed micro-topography during installation and local bedload transport during floods. A flume experiment was conducted in order to establish an empirical algorithm for estimating the diameter of impacted gravel and, here, discuss uncertainty related to diameter estimations. The proposed method was utilized to quantify gravel bedload in a cobble-bed river during flushing flows. In the text, we also discuss the contribution of measured gravel bedload during flushing flows on the removal of attached-algae (up to a 37% reduction in chlorophyll-a density) and aquatic plants (a reduction of 38% in dry mass per area). Based on time variation for the measured gravel bedload, we also suggest the propagation of a bed-form composed of the fine sediment fraction migrating on immobile larger sediment and implications for the propagation of the fine sediment wave for attached-algae removal. This article is protected by copyright. All rights reserved.

Railway embankment failure due to ballast layer breach caused by inundation flows

A railway embankment constructed on a floodplain is at risk of damage due to flooding flows. The process and critical conditions that lead to railway embankment damage during flooding are not clearly understood, rendering risk estimations impossible and hindering the development of flood-resilient rail systems. For this work, we first reviewed records of railway damage in flood plains and flows through the ballast layer. The breaching process was selected as the focus of our study. We secondly specified the fundamental characteristics of flows through a ballast layer. The critical flow rate per unit width and the minimum upstream water depth required for initiating extensive ballast breaching were experimentally evaluated using a full-scale ballast layer with rails and sleepers constructed using materials originally utilized in actual railways. A two-dimensional flow model was then employed for estimating the flow through a ballast layer that was placed on an impermeable base embankment. A simple ballast breaching model was also employed in order to explore a higher flow rate condition that could not be represented in our experiment due to limited facilities. The breaching pattern represented by the simulation model corresponded to the breaching pattern observed in the experiment. In addition to the above, here, we also discuss the ballast breaching process based on qualitative field records and quantitative experimental results, as well as the ballast breaching process as represented by the simulation.

Experimental and Numerical Study on Inundation Flows in Urban Areas

Hydraulic experiments are carried out in a large channel to make detailed measurement of inundation flows that pass through model buildings. The inundation flows are caused by levee breach and change from supercritical flows into subcritical flows as they flow down. The model buildings are located in supercritical area or subcritical area. Then a refined two-dimensional shallow equation model is applied to the experiments. The present numerical model introduces the porosity to take into account the fraction of the land surface occupied by buildings and adopts the CIP-CLS2 scheme for accurate computation for convective processes. The comparisons between the experiments and the numerical simulations demonstrate that the present numerical model can reproduce the complex flow behavior, indicating its applicability to real inundation flows.

ESTIMATION OF FLOODPLAIN ROUGHNESS AND DISCHARGE HYDROGRAPH USING WATER LEVEL HYDROGRAPHS AND 2-D NUMERICAL SIMULATION

Accurate estimation of discharge hydrograph during flood events has been one of the most important issues in River Engineering. In this study numerical simulations of flood flows in a compound meandering channel are carried out using a 2-d numerical model to discuss the possibility to reproduce the discharge hydrograph only with the information on water levels. It is demonstrated that the numerical simulation with water level hydrographs at upstream and downstream boundaries can well reproduce the discharge hydrograph at the section in the middle reach and hence that the discharge hydrograph at upstream end is not always necessary. It is also shown that since discharge hydrograph is very sensitive to the magnitude of floodplain roughness, even a record of flood discharge at high water stage, not necessarily at the flood peak, may give good estimation of floodplain roughness, leading to highly accurate estimation of discharge hydrograph.

THE INFLUENCE OF WATER HEAT ENERGY USE ON STREAM TEMPERATURE IN URBAN RIVERS UNDER TIDAL VARIATION

To predict the water temperature in lower reaches of urban rivers, field measurements and model development were performed for simultaneous transport of water and heat in a stream. Detailed information on river flow and heat transport under tidal motion was obtained through an intensive measurement in the lower reach of the Ara River. Based on the knowledge of time variations and cross-sectional profiles of the flow and the stream temperature, a onedimensional dynamic model is developed to predict flow and water temperature in tidally affected urban streams. The model takes into account of tidal motion, longitudinal dispersion, water exchange with tributaries, wastewater effluent and the heat exchange between the atmosphere and the stream water. Using the calibrated model, effects of heated water discharge on the stream temperatures due to the district heating and cooling system were estimated for a possible scenario in which the river thermal energy is used through the water heat pump technology.

Application of Reynolds-Stress Model to the Study of Heat Island Structure over a Slightly Inclined Terrain

Abstract The terrain slope is a real-world topographical feature. In this study, the effect of a slight terrain slope on the flow structure is studied in connection with the problem of the urban heat island by a Reynolds-stress model and a modified κ - e model proposed by Viollet. Results from either model indicate that even a slight slope can cause appreciable change in the flow structure. The degree of modification by sloping depends on both the slope and the thermal stratification. As a result, the heat island problem might be lessened due to the presence of a down-slope.