Tatsuhiko Uchida

Nonhydrostatic Quasi-3D Model Coupled with the Dynamic Rough Wall Law for Simulating Flow over a Rough Bed with Submerged Boulders

AbstractA reliable depth-integrated model that is suited for capturing the complex flow found with an array of submerged boulders over a permeable rough bed can be an attractive tool for river management. This paper presents a new nonhydrostatic quasi-three-dimensional (3D) calculation method called the general bottom velocity computation of the fourth-degree polynomial equation for vertical distribution of velocity coupled with the dynamic wall law (GBVC4-DWL). The computational domain of the GBVC4-DWL method considers the presence of the boulders, a vortex layer on the bed surface, and a roughness layer under the bed surface. The novelty of this study is that it introduces a dynamic rough wall law for the bottom boundary condition that includes the continuity and momentum equations for vortex and roughness layers to evaluate the nonequilibrium flow induced by the boulders. Comparisons of the results of the present method with previous experimental results and results of previous calculation methods indi...

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 3D flow calculation around a structure in gravel bed rivers by using non-hydrostatic depth-integrated model with dynamic rough-wall law

A reliable depth-integrated model is required for practical application to rivers. This paper presents a non-hydrostatic depth-integrated model of the general Bottom Velocity Computation (BVC ) method that employs dynamic wall law (BVC-DWL) to calculate 3D flow calculations around a structure in gravel bed rivers. We applied the BVC-DWL method to a local 3D flow around a structure measured in a field experiment using an enhanced measurement method with the Acoustic Doppler Current Profiler (ADCP). The BVC-DWL method is validated for local 3D flow around structures in gravel bed rivers though comparisons with measured data, demonstrating the advantages of introducing the DWL instead of the conventional equilibrium wall law. This paper presents the non-equilibrium flow structures near the gravel bed around a structure in the calculation results, and discusses the limitations of the conventional rough-wall law and the necessity of introducing a dynamic rough-wall law. method for non-equilibrium fluid motions near the gravel bed, as indicated above for a 3D calculation method. One reason for this is that most previous depth-integrated models, including quasi-threedimensional models, have been simplified with the assumption of the hydrostatic pressure distribution (Ishikawa et al., 1986; Fukuoka et al., 1992; Jin & Steffler 1993; Yeh and Kennedy, 1993). Recently we developed a new quasi-three-dimensional calculation method without a shallow-water assumption such as hydrostatic pressure distribution. This new approach is called the general Bottom Velocity Computation (BVC) method, in which equations for velocity and pressure distributions in the vertical direction are solved with depth averaged velocity equations (Uchida & Fukuoka, 2012, 2014). The BVC method has been validated for several flows including rapidly varied flows over a structure and three-dimensional flows around a non-submerged structure where the horseshoe vortex is prominent (Fukuoka & Uchida, 2013). However, it is still challenging to calculate the velocity distribution near the bed for the 3D flow around structures by using the depth-integrated model including the BVC method. The key to overcoming this issue is assumed on the bottom boundary conditions for depth-integrated equations of the BVC method. Uchida et al. (2014) proposed the dynamic wall law for rough bed (DWL) by introducing continuity and momentum equations for vortex and

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.

STUDY ON THE CRITICAL CONDITIONS REGARDING SLIDING FAILURE OF FOOT PROTECTION WORKS DUE TO BED SCOURING

Foot protection works are commonly installed in the outer banks of curved channels to protect the foundations of revetment structures from bed scouring. Since sliding failure of foot protection works can cause a disaster, it is important to establish a design method concerning the width of foot protection works which avoids slippage against local scouring. Yet, a conventional design method in which the width of foot protection works is decided by geometric conditions without take into account dynamic considerations has not been assessed. The purpose of this study is to clarify the mechanism of the deformation and sliding failure of foot protection works due to bed scouring while taking into account dynamic considerations. And, we develop a method of measuring the forces which make foot protection works slide down to calculate the width of foot protection works.

RELATIONSHIP BETWEEN FLOW AND SHEAR STRESS ACTING ON ERODED SURFACE OF THE BANK

Cross sectional shape of eroded riverbank consisting of cohesive soil becomes overhanging form. The most of these bank erosion mechanisms has been clarified, but shear stress which is the important index has not been sufficiently examined yet. The aims of this study are to explain the quantitative relation between the shear stress and the flow. For this purpose we measure directly the shear stress acting on the riverbank. It is shown that the shear stress fluctuates greatly near the water surface, and near the bottom the large shear stress acts by momentum exchange due to the secondary flow in the eroded bank.

DIRECT MEASUREMENT OF THE SHEAR STRESS DISTRIBUTION ON BANK DUE TO FLOW

To explain the riverbank erosion, it is important to determine quantitatively the shear force distribution acting on it. In this study, we use a multi component load cell to measure directly the shear force on the bank. In this way, we obtained the vertical distribution and the depth-average of the shear force acting on the bank. The vertical shear stress distribution was compared to Reynolds stresses near the bank by using LDV. As a result, the usability of the method is high in the direct measurement of the shear force, because the measured values of vertical shear stress distribution were similar to the Reynolds stress distribution near the bank. The slot existing the measured bank portion and surroundings was found to have negligible effects on the flow and measured values.