Iwasaki Toshiki(Faculty of Engineering　Civil Engineering　Natural Disasters and Adaptation)｜ Hokkaido University Researchers (Hokkaido University Researchers Directory)

Name

AuthorName

Position

Affiliation

Research Field

Iwasaki Toshiki

Faculty of Engineering　Civil Engineering　Natural Disasters and Adaptation

Associate Professor

Last Updated :2025/04/25

■Researcher basic information

Research Keyword

numerical simulation

morphodynamics

river engineering

hydraulic engineering

Research Field

Social infrastructure (civil Engineering, architecture, disaster prevention), Hydroengineering

Educational Organization

Bachelor's degree program, School of Engineering

Master's degree program, Graduate School of Engineering

Doctoral (PhD) degree program, Graduate School of Engineering

■Career

Career

Jun. 2019 - Present
Hokkaido university, Graduate school of engineering, Associate professor

Jun. 2016 - May 2019
Civil engineering research institute for cold regions, River engineering, Researcher

May 2014 - May 2016
University of Illinois Urbana-Champaign, Department of civil and environmental engineering, Research scholar

Apr. 2013 - May 2014
Hokkaido university, Graduate school of engineering, Postdoctoral research fellow

Educational Background

Apr. 2010 - Mar. 2013, Hokkaido university, Graduate school of engineering, 環境フィールド工学部門

Apr. 2008 - Mar. 2010, Hokkaido university, Graduate school of engineering, 北方圏環境政策工学専攻

Apr. 2006 - Mar. 2008, Hokkaido university, School of engineering, civil engineering

Apr. 2001 - Mar. 2006, Tomakomai national college of technology, civil engineering

Committee Memberships

Dec. 2024 - Present
釧路自然再生協議会土砂流入・水循環小委員会, 委員長代理

Dec. 2024 - Present
北海道河川審議会, 委員

Oct. 2024 - Present
鵡川・沙流川流域委員会, 委員

Jul. 2024 - Present
天塩川音威子府地区治水対策技術検討会, 委員

Jun. 2023 - Present
土木学会水工学委員会, 河道管理小委員会委員

Jun. 2023 - Present
北海道開発局, 千代田実験水路実験検討委員会委員

Aug. 2022 - Present
北海道開発局, 新桂沢ダムモニタリング部会委員

Jun. 2021 - Present
土木学会, 水工学委員会水工学論文集編集委員

Jun. 2021 - Present
土木学会, 水工学委員会委員

Nov. 2019 - Present
北海道開発局, 後志利別川リバーカウンセラー

Mar. 2018 - Present
土木学会水工学委員会, 基礎水理部会部会員, Others

Dec. 2022 - Mar. 2024
北海道開発局, 北海道地方における流域治水のあり方検討会委員

Jun. 2021 - May 2023
土木学会, 土木学会論文集B部門編集幹事・幹事長

May 2021 - May 2023
土木学会北海道支部, 講演担当チーフ幹事

Jun. 2021 - May 2022
土木学会水工学委員会, オンライン小委員会幹事

Oct. 2020 - Mar. 2021
北海道開発局旭川開発建設部, 忠別川における河道の質的整備に向けた検討WG 委員

Jul. 2018 - Sep. 2020
IAHR-APD 2020 Local Organizing Comittee member

Oct. 2019
北海道開発局, 後志利別川河川水辺の国勢調査アドバイザー委員

■Research activity information

Awards

Nov. 2019, Committee on Hydroscience and Hydraulic Engineering, Best paper award of annual conference on Hydraulic Engineering JSCE　　　　　　　　　　　　　　　
Toshiki Iwasaki

May 2018, 北海道開発局, 北海道開発技術研究発表会寒地土木研究所長賞　　　　　　　　　　　　　　　
岩崎理樹

Jun. 2015, 土木学会, 土木学会論文奨励賞　　　　　　　　　　　　　　　
岩崎理樹

Mar. 2013, 土木学会水工学委員会, 水工学論文奨励賞　　　　　　　　　　　　　　　
岩崎理樹

Mar. 2010, 土木学会北海道支部, 土木学会北海道支部年次講演会優秀学生講演賞　　　　　　　　　　　　　　　
岩崎理樹

Mar. 2008, 北海道大学土木工学科, 廣井賞　　　　　　　　　　　　　　　
岩崎理樹

Mar. 2006, 苫小牧工業高等専門学校環境都市工学科, 近藤賞　　　　　　　　　　　　　　　
岩崎理樹

Papers

Morphological response of gravel bed rivers near a knickpoint: Effect of bars on dynamic equilibrium river profile
Soichi Tanabe, Toshiki Iwasaki, Yasuyuki Shimizu
Earth Surface Processes and Landforms, Wiley, 13 Aug. 2024, [Peer-reviewed]
English, Scientific journal, Abstract

Geomorphological evolution is one of the main factors that increases flood damage in small or medium rivers located in upstream river reaches. These types of flood damage have been increasing at knickpoints where the riverbed slope and river width change abruptly and are likely to cause non‐equilibrium conditions for sediment transport during floods. Therefore, it is important to understand the non‐equilibrium morphological response at the knickpoint and the resulting new dynamic equilibrium state under given external forces. The effects of two‐dimensional (2D) morphological features on the dynamic equilibrium riverbed profile, however, have not been specifically studied because the methods currently in use for calculating equilibrium profiles are based on zero‐ or one‐dimensional (0D or 1D) modeling. Here, we perform numerical calculations using the 2D morphodynamic model iRIC‐Nays2DH to clarify the dynamic equilibrium profile and the process of reaching a dynamic equilibrium state. We also use an existing 1D model to show the 2D effect in the dynamic equilibrium state. To understand this, we set up three channels: slope transition point, width transition point, and both the slope and width transition point. 1D results show a constant slope profile in channels with constant width and upward‐convex profiles in channels with width expansion at the equilibrium state, owing to the adjustment of the difference in sediment transport volume in the two reaches with different widths by changing the slopes. In contrast, the 2D results show that the alternate bars create a small autogenic knickpoint even in the straight channel and significantly dampen sediment deposition at the width expansion point, as seen in the 1D model result. This was because the bars' shape increased the volume of sediment transport because the shape of the bars concentrated flow. These results suggest that 2D morphological features, such as fluvial bars, play a significant role in the equilibrium riverbed profile.

Assessment of risks associated with development of river meandering under climate change using a physics-based free-meandering model
Shigekazu Masuya, Takuya Inoue, Toshiki Iwasaki, Riho Kido, Kohei Ogawa, Yasuyuki Shimizu
Environmental Fluid Mechanics, Springer Science and Business Media LLC, 21 Jun. 2024, [Peer-reviewed]
English, Scientific journal, Abstract

Recent increases in heavy rainfall events, which may have resulted from climate change, have caused various natural water- and sediment-related disasters. A typical sediment-related disaster in a steep gravel-bed river is extreme bank erosion and subsequent meander development, damaging residential areas and infrastructure along the river. Assessing the bank erosion rate and its future changes under climate change is essential to prevent such severe disasters. Here, we propose a simple but useful framework for this purpose using a physics-based numerical model of free meandering and a large dataset of flood hydrographs generated from climate and runoff models. First, numerical experiments on meandering development were conducted using a two-dimensional morphodynamic model of free meandering. The results indicate that meander dynamics could reach a dynamic equilibrium state under several hydraulic and channel geometry conditions, and the characteristics of the simulated meandering channel were reasonably consistent with the field data. The meander amplitude of this state was positively correlated with the steady discharge; therefore, the increase in flow discharge may have an important effect on river dynamics under climate change. To quantify this effect, we combined a simple predictor of meander amplitude derived from a numerical experiment with a flood hydrograph dataset for current and future climatic conditions. The results suggest that the meander amplitude increased significantly for the same probability of occurrence; therefore, the risk of riverbank erosion due to river meandering will increase significantly under future climate conditions.

DISCUSSIONS OF BAR GROWTH RATE AND DEVELOPMENT OF BAR HEIGHT ESTIAMATION METHOD
Takuya INOUE, Toshiki IWASAKI, Tamaki SUMNER, Yuki HIRAMATSU, Toshihisa OZEKI, Shunichi SASAKI, Tatsuro TOMINO, Riho KIDO
Japanese Journal of JSCE, 80, 16, n/a, n/a, Japan Society of Civil Engineers, Feb. 2024, [Peer-reviewed]
Scientific journal

PRACTICAL DEVELOPMENT OF A RISK EVALUATION METHOD FOR EROSION TYPE LEVEE BREACH BASED ON THE EXISTING RIVER CONDITIONS ~ CASE OF DAMAGE IN THE CHIKUMA RIVER IN THE OCTOBER 2019 FLOOD ~
Yuji TODA, Toshiki IWASAKI, Atsuko MIZOGUCHI, Ryota ISOBE, Katsuhisa SUZUKI, Aika BANNO, Yuika NAKAJIMA, Norio TANAKA, Koichi FUJITA
Japanese Journal of JSCE, 80, 7, n/a, n/a, Japan Society of Civil Engineers, 2024, [Peer-reviewed]
Scientific journal

Morphodynamic equilibrium of alternate bar dynamics under repeated hydrographs
Dai Huang, Toshiki Iwasaki, Takashi Yamada, Yuki Hiramatsu, Satomi Yamaguchi, Yasuyuki Shimizu
Advances in Water Resources, 104427, 104427, Elsevier BV, Mar. 2023, [Peer-reviewed]
English, Scientific journal

Effect of Sediment Supply on Morphodynamics of Free Alternate Bars: Insights from Hydrograph Boundary Layer
Huang Dai, Toshiki Iwasaki, Yasuyuki Shimizu
WATER, 13, 23, MDPI, Dec. 2021, [Peer-reviewed], [International Magazine]
English, Scientific journal, Sediment supply plays an essential role in river morphology. However, the specific impact of sediment supply on river morphology is not apparent. According to the hydrograph boundary layer (HBL) concept, upstream riverbed changes caused by the imbalance between sediment supply and the capacity can propagate only a limited length and have a negligible effect on the riverbed beyond such a short length. We performed a two-dimensional morphodynamic calculation to test the concept of HBL, which was proposed under a one-dimensional simulation, meaning that the concept of HBL is still valid for plane changes in river morphology. We employed an unsteady flow with equilibrium or constant sediment supply in a straight, modeled gravel-bedded channel with an unerodible bank to simulate alternate bar morphodynamics. The results show that regardless of the sediment supply condition, the alternate bar features formed downstream of the HBL are considerably similar. This suggests that sediment disturbance at the upstream end has a negligible effect on the mobile-bed dynamic processes, including alternate bar formation and development downstream of the HBL.

Recent advances on bedform research and application: Process-based to machine learning
Sanjay Giri, Amin Shakya, Mohamed Nabi, Suleyman Naqshband, Toshiki Iwasaki, Satomi Yamaguchi, David C Froehlich, Biswa Bhattacharya, Yasuyuki Shimizu
EGU General Assembly Conference Abstracts, Apr. 2021
English

The Role of Large-Scale Bedforms in Driftwood Storage Mechanism in Rivers
Takara Okitsu, Toshiki Iwasaki, Tomoko Kyuka, Yasuyuki Shimizu
WATER, 13, 6, MDPI, Mar. 2021, [Peer-reviewed], [Corresponding author], [International Magazine]
English, Scientific journal, The quantification of driftwood deposition in rivers is important for understanding the total budget of driftwood at the watershed scale; however, it remains unclear how such driftwood storage in rivers contributes to the overall system because of the difficulties in undertaking field measurements. Herein, we perform numerical simulations of driftwood deposition within an idealized river reach with a sand-bed, to describe the role of large-scale bedforms, more specifically, alternate bars, multiple bars, and braiding, in driftwood storage in rivers. The numerical model we propose here is a coupling model involving a Lagrangian-type driftwood model and an Eulerian two-dimensional morphodynamic model for simulating large-scale bedforms (i.e., bars and braiding). The results show that the channel with a braiding pattern provides a wide area with enhanced capacity for deposition of driftwood, characterized by exposed mid-channel or in-channel bars, leading to high driftwood storage. The alternate bar is also a large bedform representing a sediment depositional element in rivers; however, because of the narrow exposed bar area and its downstream-migrating feature during floods, the alternate bars seem to contribute less to driftwood deposition in rivers. This suggests that the role of multiple bars and braiding is critically important for the driftwood deposition in rivers.

EXPERIMENT ON EFFECT OF POSITIONAL RELATIONSHIP BETWEEN IN-CHANNEL SUPPLIED SAND AND SANDBARS ON THE TRANSPORT PROCESS
平松裕基, 山口里実, 岩崎理樹
土木学会論文集 B1(水工学)(Web), 77, 2, 2021

EXPERIMENTS ON THE EFFECT OF RIVERBANK VEGETATION ON THE CHANNEL MORPHOLOGY - THE CASE THAT THE ROOTS ARE SHORT RELATIVE TO THE BANK HEIGHT -
Satomi YAMAGUCHI, Tomoko KYUKA, Takaaki MINAMI, Yasuyuki SHIMIZU, Toshiki IWASAKI
Journal of Japan Society of Civil Engineers, Ser. B1 (Hydraulic Engineering), 77, 2, I_721, I_726, Japan Society of Civil Engineers, 2021
Scientific journal

A storm-induced flood and associated nearshore dispersal of the river-derived suspended ¹³⁷Cs
Yusuke Uchiyama, Natsuki Tokunaga, Kohei Aduma, Yuki Kamidaira, Daisuke Tsumune, Toshiki Iwasaki, Masatoshi Yamada, Yutaka Tadeda, Takashi Ishimaru, Yukari Ito, Yutaka W. Watanabe, Ken Ikehara, Miho Fukuda, Yuichi Onda
Science of the Total Environment, 2021, [Peer-reviewed], [International Magazine]
English, Scientific journal, Accidental leakage of radionuclides from the Fukushima Nuclear Power Plant (FNPP1) took place in the aftermath of the catastrophic tsunamis associated with the Great East Japan Earthquake that occurred on March 11, 2011. Significant amount of radionuclides released into the atmosphere were reportedly transported and deposited on land located near FNPP1. The Niida River, Fukushima, Japan, has been recognized as a terrestrial source of highly contaminated suspended radiocesium adhering to sediment particles in the ocean through the river mouth as a result of hydrological processes. Remaining scientific questions include the oceanic dispersal and inventories of the sediments and suspended radiocesium in the ocean floor derived from the Niida River. Complementing limited in situ data, we developed a quadruple nested 3D ocean circulation and sediment transport model in an extremely high-resolution configuration to quantify the transport processes of the suspended radiocesium. Particularly, we investigated the storm and subsequent floods associated with Typhoon 201326 (Wipha) that passed off the Fukushima coast in October 2013, and subsequently promoted precipitation to a considerable extent and associated riverine freshwater discharge along with sediment outfluxes to the ocean. Using in situ bed sediment core data obtained from regions near the river mouth, we conducted a quantitative assessment of the accumulation and erosion of the sediments and explored the resultant suspended radiocesium distribution around the river mouth and nearshore areas along the Fukushima coast. We identified three major accumulative areas, near the river mouth within an area < 1 km, around the breakwaters in the north of the river mouth, and along the southern coastal area, while offshore and northward transports were minor. The present study clearly exhibits substantial retention of the land-derived radiocesium adsorbed to the sediments in the coastal areas, leading to possible long-term influences on the surrounding marine environment.

Three-dimensional antidunes coexisting with alternate bars
Takuya Inoue, Yasunori Watanabe, Toshiki Iwasaki, Junichi Otsuka
EARTH SURFACE PROCESSES AND LANDFORMS, 45, 12, 2897, 2911, WILEY, Sep. 2020, [Peer-reviewed]
English, Scientific journal, Antidunes are fluvial bedforms that form in rivers with supercritical flows. The water surface over antidunes is strongly in phase with the bed surface, and the water surface is amplified to produce large surface waves. Many experimental studies have addressed antidunes; however, the shapes of three-dimensional antidunes in a wide channel with alternate bars have not yet been appropriately understood. In this study, we experimentally investigated the streamwise and transverse length scales of antidunes under conditions with a large width-depth ratio. Our experimental results provide evidence for the coevolution of antidunes and free alternate bars, and show for the first time that the development of free bars greatly alters the three-dimensional shape of water surface waves over antidunes. In the absence of free bars in a wide channel, multiple longitudinal wave trains form, and the number of wave trains counted in the transverse direction increases with increases in the width-depth ratio. However, the presence of free bars affects the local flow characteristics, resulting in a decrease of the number of wave trains in the transverse direction. Therefore, we propose a simple model for predicting the reduction in the number of wave trains by combining two previous theories for antidunes and free bars. Results obtained by the model were found to largely agree with experimental observations. (c) 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd

FLUME EXPERIMENTS AND NUMERICAL COMPUTATION ON REVETMENT FAILURE DUE TO BED DEFORMATION IN MEANDERING CHANNEL
奥田醇, 清水康行, 久加朋子, 岩崎理樹
土木学会論文集 B1(水工学)(Web), 76, 2, 2020

A case study of flood modeling with adaptive mesh refinement　　　　　　　　　　　　　　　
Wentao Gong, Yasuyuki Shimizu, Toshiki Iwasaki
22nd Congress of the International Association for Hydro-Environment Engineering and Research-Asia Pacific Division, IAHR-APD 2020: "Creating Resilience to Water-Related Challenges", International Association for Hydro-Environment Engineering and Research (IAHR), 2020, [Peer-reviewed]
English, International conference proceedings, In this study, we propose an urban flood modeling framework using Adaptive Mesh Refinement (AMR) technique, which has been developed as an effective way of reducing the computational cost but guaranteeing the computational accuracy. AMR is able to refine or de-refine the computational grid resolution in space and time when necessary. We implement the AMR method to a two-dimensional shallow water flow model, which is solved by using the separation method and constrained interpolation profile (CIP) technique, for flood simulation. The proposed model is then applied to a case of urban flood in Sapporo, Japan. We investigate various refinement criteria to investigate the performance of flood modeling with AMR (i.e., accuracy and efficiency). The fixed fine grid model and fixed coarse grid model are also tested as a comparison with the AMR. The results show that the AMR saves the 47%-68.2% computational time with a reasonable accuracy of flood characteristics.

Numerical Simulation of Colored Sand Transport Process Using Bedload Tracer model
山田嵩, 岩崎理樹, 矢部浩規
寒地土木技術研究, 806, 2020

ESTIMATION OF CHANNEL WIDTH OF A RIVER UNDER CLIMATE CHANGE
舛屋繁和, 井上卓也, 岩崎理樹, 清水康行
河川技術論文集, 26 (CD-ROM), 2020, [Peer-reviewed]
Scientific journal

The role of saltwater and waves in continental shelf formation with seaward migrating clinoform
Toshiki Iwasaki, Gary Parker
Proceedings of the National Academy of Sciences of the United States of America, Jan. 2020, [Peer-reviewed], [Invited], [Lead author], ［Internationally co-authored］, [International Magazine]
English, Scientific journal

Advances in computational morphodynamics using the International River Interface Cooperative (iRIC) software
Shimizu Yasuyuki, Nelson Jonathan, Ferrel Kattia Arnez, Asahi Kazutake, Giri Sanjay, Inoue Takuya, Iwasaki Toshiki, Jang Chang-Lae, Kang Taeun, Kimura Ichiro, Kyuka Tomoko, Mishra Jagriti, Nabi Mohamed, Patsinghasanee Supapap, Yamaguchi Satomi
EARTH SURFACE PROCESSES AND LANDFORMS, 17 Jul. 2019, [Peer-reviewed], [Invited], ［Internationally co-authored］, [International Magazine]
Scientific journal

Bedrock-alluvial streams with knickpoint and plunge pool that migrate upstream with permanent form
Zhang Li, Iwasaki Toshiki, Li Tiejian, Fu Xudong, Wang Guangqian, Parker Gary
SCIENTIFIC REPORTS, 9, 16 Apr. 2019, [Peer-reviewed], ［Internationally co-authored］, [International Magazine]
Scientific journal

INVESTIGATIONS OF THE BEDLOAD SEDIMENT TRANSPORT IN A NATURAL RIVER USING IC-TAG MONITORING TECHNIQUE AND NUMERICAL SIMULATIONS
濱木道大, 岩崎理樹, 井上卓也, 清水康行
河川技術論文集, 25, 2019, [Peer-reviewed], [Domestic magazines]
Scientific journal

MEASUREMENTS OF BED TOPOGRAPHY, WATER SURFACE PROFILE AND VELOCITY FIELD UNDER THE PRESENCE OF TRIANGLE-SHAPE WATER SURFACE WAVE
井上卓也, 岩崎理樹, 矢部浩規
土木学会論文集 B1(水工学)(Web), 75, 2, 2019, [Peer-reviewed], [Domestic magazines]
Scientific journal

ON THE UPSTREAM-MIGRATING SAND BARS
岩崎理樹, 矢部浩規
土木学会論文集 B1(水工学)(Web), 75, 2, 2019, [Peer-reviewed], [Lead author, Corresponding author], [Domestic magazines]
Scientific journal

NUMERICAL SIMULATIONS ON THE EVOLUTION OF REAL-SCALE MEANDERING RIVERS
舛屋繁和, 井上卓也, 岩崎理樹, 清水康行
土木学会論文集 B1(水工学)(Web), 75, 2, 2019, [Peer-reviewed], [Domestic magazines]
Scientific journal

Fundamental study on the performance of blocks for closure of river levee breaches
岩崎理樹, 岡部博一, 島田友典, 矢部浩規, 米元光明
寒地土木技術研究 : 国立研究開発法人土木研究所寒地土木研究所月報 : monthly report, 786, 11, 20, 土木研究所寒地土木研究所, Nov. 2018
Japanese

Numerical simulation of bedload tracer transport associated with sand bar formation, bank erosion, and channel migration
Toshiki Iwasaki, Satomi Yamaguchi, Hiroki Yabe
E3S Web of Conferences, 40, 05 Sep. 2018, [Peer-reviewed], [Lead author, Corresponding author]
International conference proceedings, © The Authors, published by EDP Sciences, 2018. An understanding of bedload transport processes is an essential research goal for better prediction of river morphology and morphodynamics as well as the transport and fate of sediment-bound materials in river systems. Passive tracer particles have been used widely to monitor bedload transport processes in rivers by measuring the spatiotemporal distribution of the bedload tracers. Here, we propose a numerical model for reproducing the transport of bedload tracers in river systems, more specifically, the behaviours of bedload tracers under the influence of complex river morphodynamics. A two-dimensional morphodynamic model is combined with a flux-based bedload tracer model with use of the active layer approach. The model is applied to a laboratory experiment that demonstrates the transport processes within the channel of bedload tracers supplied from the floodplain. The numerical model effectively reproduces the main features of the experiment, namely, the bedload tracers supplied from the floodplain due to bank erosion deposit onto sand bars developed within the channel. Because the sand bars cause a very long residence time of the bedload tracers within the bed, the transport speed of the tracers is slowed significantly under the influence of bar formation and channel migration.

A study on the 2016 August flood and levee breaches in the Sorachi River, Hokkaido, Japan
Jun Okuda, Yasuyuki Shimizu, Tomoko Kyuka, Toshiki Iwasaki, Yoshiaki Ishida
Proceedings - International Association for Hydro-Environment Engineering and Research (IAHR)-Asia Pacific Division (APD) Congress: Multi-Perspective Water for Sustainable Development, IAHR-APD 2018, 2, 1007, 1014, 01 Sep. 2018, [Peer-reviewed]
International conference proceedings, © Proceeding of the 21st LAHR-APD Congress 2018. All rights reserved. There was a record breaking rainfall by Typhoon Lionrock over eastern Hokkaido in August 2016. Two levee breaches were caused by this rainfall leading to inundation disaster at Ikutora region in Minami Furano town. According to a field survey, the inundation disaster started from an upstream point at first, then the flood flow went through a flood plain and reached a downstream levee, finally overtopping to a river channel caused the second breach at downstream point. Since the flood started during midnight, a detailed explanation of the flood's behavior and a procedure of breaches remain unclear. In this study, we have investigated the river channel deformation and the levee erosion using a 2-dimensional numerical analysis. The computed results showed good agreement with field survey data regarding the inundation area, breach points, and the river channel deformation. Especially, the starting time of the inundation in the numerical analysis agreed with expected time estimated by field survey reports. The computational results indicate that with the increase of discharge, the development of a point bar pushed the channel near the upstream of the breach point toward outer bend of the channel meander. This channel migration may cause severe levee erosion. In the same time, the water level went up and overtopping flow happened. It suggests that both the bank erosion caused by the channel migration and overtopping flow caused the large-scale levee breach. Regarding the downstream breach, the results showed that the breach was caused by an overtopping flow from the floodplain to the river channel. which was expected by the field survey.

NUMERICAL SIMULATION OF BEDLOAD TRACER TRANSPORT BY CONSIDERING VERTICAL TRACER DISPERSION
濱木道大, 岩崎理樹, 井上卓也, 清水康行
水工学論文集 Annual journal of Hydraulic Engineering, JSCE, 63, I_955, 960, 土木学会, 2018, [Peer-reviewed]
Japanese, Scientific journal

COMPUTATIONS OF THREE-DIMENSIONAL ANTIDUNES IN RIVERS
岩崎理樹, 井上卓也, 音田慎一郎, 矢部浩規
水工学論文集 Annual journal of Hydraulic Engineering, JSCE, 63, I_643, 648, 土木学会, 2018, [Peer-reviewed], [Lead author, Corresponding author]
Japanese, Scientific journal

EXPERIMENTS ON THE BLOCK STABILITY UNDER THE PRESENCE OF TRIANGLE-SHAPE WATER SURFACE WAVES IN RIVERS
IWASAKI Toshiki, INOUE Takuya, YABE Hiroki
Journal of Japan Society of Civil Engineers, Ser. B1 (Hydraulic Engineering), 74, 4, I_1207, I_1212, 公益社団法人土木学会, 2018, [Peer-reviewed]
Japanese, Scientific journal,  In this study, we investigate the effects of the triangle-shape water surface waves induced by the river antidunes on the stability of concrete block, which is a modelled river revetment work. The water surface waves cannot develop over a fixed flat bed even if we obtain a hydraulic condition, which allows the development of surface waves under a movable bed. The comparision of the block stability experiments between fixed and movable bed under same hydraulic condition therefore obtain the destabilizing effect of the surface wave on the block. The experimental results shows that the presence of the triangle-shape water surface wave significantly destabilizes the block installed on the bed because of the occurance of high velocity induced by the waves and an upward flow from the trough to crest of the antidunes.

INFLUENCE OF SEDIMENT DYNAMICS IN RIVER CHANNEL ON THE CHANNEL CHANGE
YAMAGUCHI Satomi, KYUKA Tomoko, SHIMIZU Yasuyuki, IZUMI Norihiro, WATANABE Yasuharu, IWASAKI Toshiki
Journal of Japan Society of Civil Engineers, Ser. B1 (Hydraulic Engineering), 74, 4, I_1153, I_1158, 公益社団法人土木学会, 2018, [Peer-reviewed]
Japanese,  Large-scale bank erosion caused by channel change during flood often causes serious disasters in gravel bed rivers. In this study, we focused on the influence of sediment dynamics in river channel on the channel change inducing bank erosion. From the results of sediment balance in the Pekerebetsu River and the Otofuke River during a flood, we showed the possibility that sediment supply from the bank affect stream channel change to expand bank erosion in the river. We carried out the experiments on the influence of sediment supply from the bank on the channel change, and showed that sediment supply from the bank can be a cause of channel change inducing large-scale bank erosion.

EXPERIMENT AND NUMERICAL SIMULATION OF ADVECTION-DISPERSION OF BEDLOAD TRACERS
HAMAKI Michihiro, IWASAKI Toshiki, INOUE Takuya, SATO Daisuke, SUMNER Tamaki, SHIMIZU Yasuyuki
Journal of Japan Society of Civil Engineers, Ser. B1 (Hydraulic Engineering), 74, 4, I_1033, I_1038, 公益社団法人土木学会, 2018
Japanese, Scientific journal,  Understanding and predicting of the bedload transport processes in rivers are of great importances for better sediment management works. In this study, we propose a numerical model to understand the bedload transport processes by simulating passive bedload tracer particles. A movable bed experiment with the use of colored bedload tracers is performed to understand the advection-dispersion processes of the tracers, i.e., how the tracer particles transport and disperse over the movable bed. We numerically reproduce observed behaviors of the bedload tracers by using a flux-type tracer transport model combined with a two-dimensional bed evolution model. The experimental results suggest that the bedload mixing process associated with a small-scale bedform dominates the advection-dispersion of tracers. The appropriate determination of the active layer thickness by considering this vertical bedload mixing process is necessary to reproduce the tracer advection speed.

Hydrogeomorphological differentiation between floodplains and terraces
Qina Yan, Toshiki Iwasaki, Andrew Stumpf, Patrick Belmont, Gary Parker, Praveen Kumar
Earth Surface Processes and Landforms, 43, 1, 218, 228, John Wiley and Sons Ltd, 01 Jan. 2018, [Peer-reviewed], ［Internationally co-authored］, [International Magazine]
English, Scientific journal, Floodplains and terraces in river valleys play important roles in the transport dynamics of water and sediment. While flat areas in river valleys can be identified from LiDAR data, directly characterizing them as either floodplain or terraces is not yet possible. To address this challenge, we hypothesize that, since geomorphic features are strongly coupled to hydrological and hydraulic dynamics and their associated variability, there exists a return frequency, or possibly a narrow band of return frequencies, of flow that is associated with floodplain formation
and this association can provide a distinctive signature for distinguishing them from terraces. Based on this hypothesis we develop a novel approach for distinguishing between floodplains and terraces that involves transforming the transverse cross-sectional geometry of a river valley into a curve, named a river valley hypsometric (RVH) curve, and linking hydraulic inundation frequency with the features of this curve. Our approach establishes that the demarcation between floodplains and terraces can be established from the structure of steps and risers in the RVH curves which can be obtained from the DEM data. Further, it shows that these transitions may themselves be shaped by floods with 10- to 100-year recurrence. We additionally show that, when floodplain width and height (above channel bottom) are normalized by bankfull width and depth, the ratio lies in a narrow range independent of the scale of the river valley. Copyright © 2017 John Wiley &
Sons, Ltd.

Modeling of the Advection-Dispersion of Bedload Tracers
岩崎理樹, 川村里実, 矢部浩規
寒地土木技術研究 : 国立研究開発法人土木研究所寒地土木研究所月報 : monthly report, 774, 2, 10, 土木研究所寒地土木研究所, Nov. 2017
Japanese

Numerical simulation of large-scale bed load particle tracer advection-dispersion in rivers with free bars
Toshiki Iwasaki, Jonathan Nelson, Yasuyuki Shimizu, Gary Parker
JOURNAL OF GEOPHYSICAL RESEARCH-EARTH SURFACE, 122, 4, 847, 874, AMER GEOPHYSICAL UNION, Apr. 2017, [Peer-reviewed]
English, Scientific journal, Asymptotic characteristics of the transport of bed load tracer particles in rivers have been described by advection-dispersion equations. Here we perform numerical simulations designed to study the role of free bars, and more specifically single-row alternate bars, on streamwise tracer particle dispersion. In treating the conservation of tracer particle mass, we use two alternative formulations for the Exner equation of sediment mass conservation: the flux-based formulation, in which bed elevation varies with the divergence of the bed load transport rate, and the entrainment-based formulation, in which bed elevation changes with the net deposition rate. Under the condition of no net bed aggradation/degradation, a 1-D flux-based deterministic model that does not describe free bars yields no streamwise dispersion. The entrainment-based 1-D formulation, on the other hand, models stochasticity via the probability density function (PDF) of particle step length, and as a result does show tracer dispersion. When the formulation is generalized to 2-D to include free alternate bars, however, both models yield almost identical asymptotic advection-dispersion characteristics, in which streamwise dispersion is dominated by randomness inherent in free bar morphodynamics. This randomness can result in a heavy-tailed PDF of waiting time. In addition, migrating bars may constrain the travel distance through temporary burial, causing a thin-tailed PDF of travel distance. The superdiffusive character of streamwise particle dispersion predicted by the model is attributable to the interaction of these two effects.

THE EFFECT OF CHANNEL WIDTH AND CHANNEL BED SLOPE ON LEVEE BREACH PROCESSES
SHIMADA Tomonori, WATANABE Yasuharu, OKABE Kazuhiro, IWASAKI Toshiki, NAKASHIMA Yasuhiro
Journal of Japan Society of Civil Engineers, Ser. B1 (Hydraulic Engineering), 73, 4, I_1345, I_1350, Japan Society of Civil Engineers, 2017
Japanese, It is important mitigating flood damage by overflow from a levee breached. The mechanism of levee breach has not been clarified. This study aims to clarify the below point, the effect of channel width and channel bed profile on levee breach processes by using a numerical model to simulate levee breaches based on results of the experiments. The results of the simulation are as follows: In the case of steep slope channel, levee breach widening is dominant. In the case of mild-slope channel, river bed erosion is dominant.

CLASSIFICATION OF LEVEE BREACH PROCESSES ACCORDING TO CHANNEL BED SLOPE AND CHANNEL WIDTH
SHIMADA Tomonori, WATANABE Yasuharu, OKABE Hirokazu, IWASAKI Toshiki, NAKASHIMA Yasuhiro
Advances in River Engineering, 23, 411, 416, Japan Society of Civil Engineers, 2017
Japanese, It is important mitigating flood damage by overflow from a levee breached. The mechanism of levee breach has not been clarified. This study aims to clarify the below point, the effect of channel bed profile and channel width on levee breach processes by using a numerical model to simulate levee breaches based on results of experiments using the Chiyoda Experimental Channel. The results of the simulation are as follows: In the case of steep slope channel, levee breach widening is dominant. In the case of mild- slope channel, river bed erosion is dominant.

THE EFFECT OF CHANNEL WIDTH AND CHANNEL BED SLOPE ON LEVEE BREACH PROCESSES
島田友典, 渡邊康玄, 岡部博一, 岩崎理樹, 中島康博
水工学論文集 Annual journal of Hydraulic Engineering, JSCE, 61, Ⅰ_1345, 1350, 土木学会, 2017
Japanese

INVENTORY ANALYSIS OF SUSPENDED RADIOCESIUM DERIVED FROM NIIDA RIVER DURING A TYPHOON-INDUCED FLOOD EVENT
UCHIYAMA Yusuke, ADUMA Kohei, ODANI Sachika, IWASAKI Toshiki, TSUMUNE Daisuke, KAMIDAIRA Yuki, SHIMIZU Yasuyuki, ONDA Yuichi
Journal of Japan Society of Civil Engineers, Ser. B2 (Coastal Engineering), 73, 2, I_685, I_690, 公益社団法人土木学会, 2017
Japanese,  Niida River is well know to supply highly contaminated suspended 137Cs originated from the Fukushima Nuclear Power Plant to the ocean. We examine the oceanic dispersal and inventories of the sediments and suspended 137Cs in the ocean floor derived from Niida River, using the quadraple nested JCOPE2-ROMS 3D circulation model in a very high resolution configuration, coupled with a 3D multi-class sediment transport model, the iRIC-Nays 2DH river sediment model, the SWAN spectral wave model, and a static 137Cs absorption model. We focus on the storm and flood event associated with Typhoon 201326 (Wipha) passed off the Fukushima Coast, which provoked an enomous amount of riverine fluxes, to quantitatively assess the accumulation and erosion of the sediments and resultant suspended 137Cs inventories around the river mouth and nearshore areas.

THE EFFECT OF CHANNEL WIDTH AND CHANNEL BED SLOPE ON LEVEE BREACH PROCESSES
SHIMADA Tomonori, WATANABE Yasuharu, OKABE Kazuhiro, IWASAKI Toshiki, NAKASHIMA Yasuhiro
Journal of Japan Society of Civil Engineers, Ser. B1 (Hydraulic Engineering), 73, 4, I_1345, I_1350, 公益社団法人土木学会, 2017
Japanese,  It is important mitigating flood damage by overflow from a levee breached. The mechanism of levee breach has not been clarified. This study aims to clarify the below point, the effect of channel width and channel bed profile on levee breach processes by using a numerical model to simulate levee breaches based on results of the experiments. The results of the simulation are as follows: In the case of steep slope channel, levee breach widening is dominant. In the case of mild-slope channel, river bed erosion is dominant.

MODELING THE THREE-DIMENSIONAL RIVER ANTIDUNES: IMPLICATIONS FOR THE TRIANGLE-SHAPE WATER SURFACE WAVE TRAINS
IWASAKi Toshiki, INOUE Takuya, YABE Hiroki
Journal of Japan Society of Civil Engineers, Ser. B1 (Hydraulic Engineering), 73, 4, I_901, I_906, 公益社団法人土木学会, 2017
Japanese,  This study presents a modeling of three-dimensional river antidunes using a Boussinesq model with a non-equilibrium bedload transport model, and discuss the relation of this three-dimensional bedform instability to the triangle-shape water surface wave trains. We performed a linear stability analysis of the three-dimensional antidunes, showing that the proposed model in part reproduces the three-dimensionality of the antidunes. The dominant longitudinal and transverse wave numbers predicted by the analysis are reasonably in agreement with the characteristics of the triangle-shape water surface wave trains, although the model tends to overestimate the wavelength of the water surface waves.

Numerical Simulation of Effects of Sediment Supply on Bedrock Channel Morphology
T. Inoue, T. Iwasaki, G. Parker, Y. Shimizu, N. Izumi, C. P. Stark, J. Funaki
JOURNAL OF HYDRAULIC ENGINEERING, 142, 7, ASCE-AMER SOC CIVIL ENGINEERS, Jul. 2016
English, Scientific journal, Natural bedrock rivers exhibit diverse erosional morphologies. Although the formation of alternate bars on bedrock has been noted in previous studies, the influence of these alternate bars on bedrock erosion has not been clarified. In this study, the authors propose a model for bedrock-alluvial channels that reproduces both bar formation and erosional morphology. In addition, the authors report on numerical simulations to evaluate the influence of sediment supply on the state of the bed at and over the bedrock surface. The numerical results illustrate the formation of different morphologies for different supply rates. When the sediment supply rate is close to transport capacity, mixed alluvial-bedrock alternate bars form. These bars are analogous to purely alluvial alternate bars. A meandering thread of alluvial material migrates downstream over a uniformly eroding bedrock surface. When the sediment supply rate is well below capacity, however, multiple incisional troughs (grooves) form on the bedrock surface, and alluvial cover tends to concentrate in the troughs.

Numerical simulation of bar and bank erosion in a vegetated floodplain: A case study in the Otofuke River
Toshiki Iwasaki, Yasuyuki Shimizu, Ichiro Kimura
ADVANCES IN WATER RESOURCES, 93, 118, 134, ELSEVIER SCI LTD, Jul. 2016, [Peer-reviewed]
English, Scientific journal, Recent studies suggest that braided river could be single-thread channel by colonization of riparian vegetation; however, this kind of mutual interactions between physical and ecological processes in rivers are still poorly understood. Here we investigate the development of meandering channel in a river, which was originally braided and currently vegetated, the Otofuke River in Japan. The significant morphological processes of this river during a destructive flood event was studied using a two-dimensional morphodynamic model. Using well-calibrated parameters, this model qualitatively reproduced observed morphological changes such as the co-development of sand bars, bar-induced meandering and a chute cutoff. We find that for vegetated channels, meandering could maintain moderate sinuosity; in contrast, in the absence of riparian vegetation, bar-induced meandering channels could become braided. This suggests that distinct meandering channels could be a fundamental channel morphology in the originally braided, but currently vegetated river; however, the simultaneous occurrence of the chute cutoff and meandering indicates that this channel could not be a fully-developed high amplitude meandering channel. (C) 2015 Elsevier Ltd. All rights reserved.

The international river interface cooperative: Public domain flow and morphodynamics software for education and applications
Jonathan M. Nelson, Yasuyuki Shimizu, Takaaki Abe, Kazutake Asahi, Mineyuki Gamou, Takuya Inoue, Toshiki Iwasaki, Takaharu Kakinuma, Satomi Kawamura, Ichiro Kimura, Tomoko Kyuka, Richard R. McDonald, Mohamed Nabi, Makoto Nakatsugawa, Francisco R. Simoes, Hiroshi Takebayashi, Yasunori Watanabe
ADVANCES IN WATER RESOURCES, 93, 62, 74, ELSEVIER SCI LTD, Jul. 2016, [Peer-reviewed]
English, Scientific journal, This paper describes a new, public-domain interface for modeling flow, sediment transport and morphodynamics in rivers and other geophysical flows. The interface is named after the International River Interface Cooperative (iRIC), the group that constructed the interface and many of the current solvers included in iRIC. The interface is entirely free to any user and currently houses thirteen models ranging from simple one-dimensional models through three-dimensional large-eddy simulation models. Solvers are only loosely coupled to the interface so it is straightforward to modify existing solvers or to introduce other solvers into the system. Six of the most widely-used solvers are described in detail including example calculations to serve as an aid for users choosing what approach might be most appropriate for their own applications. The example calculations range from practical computations of bed evolution in natural rivers to highly detailed predictions of the development of small-scale bedforms on an initially flat bed. The remaining solvers are also briefly described. Although the focus of most solvers is coupled flow and morphodynamics, several of the solvers are also specifically aimed at providing flood inundation predictions over large spatial domains. Potential users can download the application, solvers, manuals, and educational materials including detailed tutorials at www.-i-ric.org. The iRIC development group encourages scientists and engineers to use the tool and to consider adding their own methods to the iRIC suite of tools. Published by Elsevier Ltd.

Sensitivity of free bar morphology in rivers to secondary flow modeling: Linear stability analysis and numerical simulation
Toshiki Iwasaki, Yasuyuki Shimizu, Ichiro Kimura
ADVANCES IN WATER RESOURCES, 92, 57, 72, ELSEVIER SCI LTD, Jun. 2016, [Peer-reviewed]
English, Scientific journal, A number of numerical models have been proposed to understand and simulate fluvial river morphodynamics; however, it is somewhat unclear whether all the models are able to consistently simulate flow-bed instability phenomena. This study investigates the sensitivity of free bar morphology in rivers to secondary flow models used in depth-averaged models using linear stability analyses and numerical simulations. Both the linear analyses and numerical simulations suggest that under certain hydraulic conditions, an equilibrium-type secondary flow model, which has been widely used in river morphodynamic models, fails to generate a finite wavelength and bar mode, allowing the inception of bars of infinitely short scale and infinitely high mode. Using a nonequilibrium-type secondary flow model avoids the un-physical formation of these incipient free bars, and gives better solutions regarding finite amplitude bars. Since free bars are essential, intrinsic river morphological features, the findings of this study can be applied to a wide range of river morphodynamic calculations. (C) 2016 Elsevier Ltd. All rights reserved.

COASTAL DISPERSAL OF RIVER-DERIVED SUSPENDED RADIONUCLIDES DUE TO A FLOOD EVENT AROUND THE MOUTH OF NIIDA RIVER, FUKUSHIMA
YAMANISHI Takafumi, UCHIYAMA Yusuke, IWASAKI Toshiki, SHIMIZU Yasuyuki, TSUMUNE Daisuke, MISUMI Kazuhiro, ONDA Yuichi
Journal of Japan Society of Civil Engineers, Ser. B2 (Coastal Engineering), 72, 2, I_757, I_762, 公益社団法人土木学会, 2016
Japanese,  We examine oceanic dispersal of the iRIC-Nays2DH-evaluated sediment and suspended 137Cs influxes from Niida River, Fukushima, with a particular attention to the first flood event in late May of 2011 after the Fukushima nuclear accident. Alongshore suspended 137Cs transport occurs assymetrically, comprising storm-driven southward transport confined in the shallow area due to shoreward Ekman transport associated with strong northerly wind, followed by northwestward widespread transport under mild southerly wind condition. About 70 % of the Niida River-derived suspended 137Cs remains near the mouth for 20 days after the flood event. Nevertheless, our model results as well as an observation suggest that the area is dominated by erosion as for high bed shear stress all the time, thus suspended radionuclides are redistributed to dissipate away in long term.

ANTIDUNE INSTABILITY ASSOCIATED WITH A DEPTH-AVERAGED MODEL WITH AN ENTRAINMENT-BASED MODEL
INOUE Takuya, IWASAKI Toshiki, ONDA Shinichiro
Journal of Japan Society of Civil Engineers, Ser. A2 (Applied Mechanics (AM)), 72, 2, I_565, I_574, 公益社団法人土木学会, 2016
Japanese, Dynamics of river bedforms which is an interfacial instability phenomenon between turbulent open channel flow and movable bed significantly affect the flow structures and sediment transport dynamics in rivers, so that this have been an classical and important research topic in the field of river engineering. Here, we perform a linear stability analysis for several combinations of flow and sediment transport submodels, specifically, a shallow flow model and a Boussinesq type depth-integrate model for hydrodynamic model, and an equilibrium and a non-equilibrium bedload transport model for sediment mass conservation model. We then investigate how the submodels adopted determine the linear stability and the wavelength of river bedforms. The results show that the combination of the Boussinesq model and the non-equilibrium bedload transport model can explain the antidune instability and its finite wavelength observed in several published experiments, whereas, use of each submodel alone is insufficient to explain the finite wavelength of antidunes.

Dynamic meandering in response to upstream perturbations and floodplain formation
F. Schuurman, Y. Shimizu, T. Iwasaki, M. G. Kleinhans
GEOMORPHOLOGY, 253, 94, 109, ELSEVIER SCIENCE BV, Jan. 2016
English, Scientific journal, River meandering results from spatially alternating bank erosion and bar growth. Recent flume experiments and theory suggest that a continuous inflow perturbation is a requirement for sustained meandering. Furthermore, flume experiments suggest that bar-floodplain conversion is an additional requirement. Here, we tested the effects of continuous inflow perturbation and bar-floodplain conversion on meander migration using three numerical morphodynainic models: a 1D-model, and two 2D-models with one of them using adaptive moving grid. We focused on the interaction between bars and bends that leads to meander initiation, and the effect of different methods to model bank erosion and floodplain accretion processes on meander migration. The results showed that inflow perturbations have large effects on meander dynamics of high-sinuosity channels, with strong excitation when the inflow is periodically perturbed. In contrast, inflow perturbations have rather small effect in low-sinuosity channels. Steady alternate bars alone are insufficient to cause high-sinuosity meandering. For high-sinuosity meandering, bar-floodplain conversion is required that prevents chute-cutoffs and enhances flow asymmetry, whilst meandering with chute-cutoffs requires merely weak floodplain formation, and braiding occurs without floodplain formation. Thus, this study demonstrated that both dynamic upstream inflow perturbation and bar-floodplain conversion are required for sustained high-sinuosity meandering. (C) 2015 Elsevier B.V. All rights reserved.

NUMERICAL SIMULATION OF TRACER ADVECTION-DIFFUSION UNDER THE INFLUENCE OF FREE BARS
IWASAKI Toshiki, PARKER Gary, SHIMIZU Yasuyuki
Journal of Japan Society of Civil Engineers, Ser. B1 (Hydraulic Engineering), 71, 4, I_877, I_882, Japan Society of Civil Engineers, 2015
Japanese, This paper presents numerical simulations of tracer particle transport both in the absence and presence of free bars. The transport and diffusion of tracer particles is affected by both the probabilistic motion of sediment particles and by the river bed geometry associated with free bars. We compare results of numerical models using an equilibrium (flux-based model) and a non-equilibrium (entrainment-based model) conservation formulation for river bed variation associated with bedload transport. The results show that 1) the entrainment-based morphodynamic model successfully reproduces free bar formation, 2) the flux-based morphodynamic model fails to simulate tracer diffusion in the absence of bars, and 3) free bar formation enhances streamwise tracer diffusion.

APPLICATION OF THE KMR MODEL WITH SPATIAL GEOMETRIC SCALE ON COMPUTATIONS OF RIVER MORPHODYNAMICS
KIMURA Ichiro, YAMATSUTA Ryo, IWASAKI Toshiki, SHIMIZU Yasuyuki
STRUCTURAL ENGINEERING / EARTHQUAKE ENGINEERING, 3, 1, 174, 183, Japan Society of Civil Engineers, 2015
English, A multilevel grid-type turbulent shallow flow model combined with an equilibrium sediment transport model (KMR-MB: Kinematic Mesh Reconstruction for Movable Bed) is proposed. The KMR method is a kind of multilevel grid model with dynamic refinement that combines grid cells to efficiently capture the unsteady flow phenomena. Saito et al. (2012)^1),2) have applied the KMR approach to open channel flows around a bridge pier and have shown the advantage of the model. They used the maximum value of the strain and rotation parameters for the threshold for quadtree and 3×3 grid divisions. We have extended the model for computations of morphological phenomena in rivers by incorporating an equilibrium sediment transport model as well as a bed continuity equation, and used new criteria for grid cell refinement. We used an average curvature of the bed surface as the criteria for refinement and combined quadtree-type grid cells. The present model has been applied to simulate the alternate bar formations. The experimental result by Akahori et al. (2011)³⁾ was used to validate the model. The comparison between numerical and experimental results showed that the present KMR-MB can excellently simulate the phenomena with less CPU time than those computed with the fixed grid.

A STUDY ON EROSION MECHANISMS OF AGRICULTURAL LAND BY FLOOD FLOWS OF ABU RIVER AT YAMAGUCHI-SHIMANE HEAVY RAIN IN JULY 2013
AKAHORI Ryosuke, AKAMATSU Yoshihisa, IWASAKI Toshiki, INUI Ryutei, NAGANO Hiroyuki, KAMITSURU Shogo
Journal of Japan Society of Civil Engineers, Ser. B1 (Hydraulic Engineering), 71, 4, I_1471, I_1476, Japan Society of Civil Engineers, 2015
Japanese, Semimountainous areas in Yamaguchi and Shimane Prefectures suffered severe damage brought by floods on July 2013. The damages in these areas were characterized by the vulnerability of their backlands that consists of agricultural lands. In order to investigate the mechanisms of land erosion on these areas where existing high-resolution data was not prepared, post-disaster RTK-GPS surveys and numerical simulations were conducted for Abu River in Yamaguchi. The surveys were aimed at providing the high-resolution topographic datasets of these regions in order to evaluate the influences of micro-topography on channel incisions by the flood flow. The results of the numerical calculations of movable bed-evolution well reproduced the process of the channel incisions on these areas, and it implies that the micro-topography is one of the most influential factors on erosion and channel formation processes on such semimountanious regions.

A STUDY ON EROSION MECHANISMS OF AGRICULTURAL LAND BY FLOOD FLOWS OF ABU RIVER AT YAMAGUCHI-SHIMANE HEAVY RAIN IN JULY 2013
AKAHORI Ryosuke, AKAMATSU Yoshihisa, IWASAKI Toshiki, INUI Ryutei, NAGANO Hiroyuki, KAMITSURU Shogo
Journal of Japan Society of Civil Engineers, Ser. B1 (Hydraulic Engineering), 71, 4, I_1471, I_1476, 公益社団法人土木学会, 2015
Japanese, Semimountainous areas in Yamaguchi and Shimane Prefectures suffered severe damage brought by floods on July 2013. The damages in these areas were characterized by the vulnerability of their backlands that consists of agricultural lands. In order to investigate the mechanisms of land erosion on these areas where existing high-resolution data was not prepared, post-disaster RTK-GPS surveys and numerical simulations were conducted for Abu River in Yamaguchi. The surveys were aimed at providing the high-resolution topographic datasets of these regions in order to evaluate the influences of micro-topography on channel incisions by the flood flow. The results of the numerical calculations of movable bed-evolution well reproduced the process of the channel incisions on these areas, and it implies that the micro-topography is one of the most influential factors on erosion and channel formation processes on such semimountanious regions.

NUMERICAL SIMULATION OF TRACER ADVECTION-DIFFUSION UNDER THE INFLUENCE OF FREE BARS
IWASAKI Toshiki, PARKER Gary, SHIMIZU Yasuyuki
Journal of Japan Society of Civil Engineers, Ser. B1 (Hydraulic Engineering), 71, 4, I_877, I_882, 公益社団法人土木学会, 2015
Japanese, This paper presents numerical simulations of tracer particle transport both in the absence and presence of free bars. The transport and diffusion of tracer particles is affected by both the probabilistic motion of sediment particles and by the river bed geometry associated with free bars. We compare results of numerical models using an equilibrium (flux-based model) and a non-equilibrium (entrainment-based model) conservation formulation for river bed variation associated with bedload transport. The results show that 1) the entrainment-based morphodynamic model successfully reproduces free bar formation, 2) the flux-based morphodynamic model fails to simulate tracer diffusion in the absence of bars, and 3) free bar formation enhances streamwise tracer diffusion.

Computational modeling of Cs-137 contaminant transfer associated with sediment transport in Abukuma River
T. Iwasaki, M. Nabi, Y. Shimizu, I. Kimura
JOURNAL OF ENVIRONMENTAL RADIOACTIVITY, 139, 416, 426, ELSEVIER SCI LTD, Jan. 2015
English, Scientific journal, A numerical model capable of simulating the transfer of Cs-137 in rivers associated with transport of fine sediment is presented. The accident at Fukushima Dai-ichi Nuclear Power Plant (FDNPP) released radionuclides into the atmosphere, and after fallout several radionuclides in them, such as radiocesium (Cs-134, Cs-137) and radioiodine (I-131) were adsorbed on surface soil particles around FDNPP and transported by surface water. To understand the transport and deposition of the radioactive contaminant along with surface soil particles and its flux to the ocean, we modeled the transport of the Cs-137 contaminant by computing the water flow and the associated washload and suspended load transport. We have developed a two-dimensional model to simulate the plane flow structure, sediment transport and associated Cs-137 contaminant transport in rivers by combining a shallow water flow model and an advection-diffusion equation for the transport of sediment. The proposed model has been applied to the lower reach of Abukuma River, which is the main river in the highly contaminated area around FDNPP. The numerical results indicate that most Cs-137 supplied from the upstream river reach with washload would directly reach to Pacific Ocean. In contrast, washload-oriented Cs-137 supplied from the upstream river basin has a limited role in the radioactive contamination in the river. The results also suggest that the proposed framework of computational model can be a potential tool for understanding the sediment-oriented Cs-137 behavior in rivers. (C) 2014 Elsevier Ltd. All rights reserved.

A NUMERICAL MODEL FOR SIMULATING TRANSPORT OF RADIOACTIVE CONTAMINANT BY SEDIMENT TRANSPORT IN RIVERS
IWASAKI Toshiki, SHIMIZU Yasuyuki, KIMURA Ichiro
PROCEEDINGS OF THE JAPANESE CONFERENCE ON HYDRAULICS, 70, 4, I_1267, I_1272, Japan Society of Civil Engineers, 2014
Japanese, A numerical model for calculating the behavior of radioactive contaminant associated with the sediment transport in rivers has been proposed in this study. The equations for simulating the concentration of radioactive contaminant of the sediment has been derived from the governing equation of the sediment transport model. The proposed model has been applied to a simple river confluence to understand the mixing process of the sediment and the radioactive contaminant in the confluence point. The numerical results provide a useful data for discussing the relation between the bed evolution process and the behavior of radioactive contaminant. In addition, the model can contribute to estimate the origin of the sediment by using the relationship between the simulated sediment diameter and the concentration of the radioactive contaminant.

A NUMERICAL MODEL FOR SIMULATING TRANSPORT OF RADIOACTIVE CONTAMINANT BY SEDIMENT TRANSPORT IN RIVERS
IWASAKI Toshiki, SHIMIZU Yasuyuki, KIMURA Ichiro
Journal of Japan Society of Civil Engineers, Ser. B1 (Hydraulic Engineering), 70, 4, I_1267, I_1272, 公益社団法人土木学会, 2014
Japanese, A numerical model for calculating the behavior of radioactive contaminant associated with the sediment transport in rivers has been proposed in this study. The equations for simulating the concentration of radioactive contaminant of the sediment has been derived from the governing equation of the sediment transport model. The proposed model has been applied to a simple river confluence to understand the mixing process of the sediment and the radioactive contaminant in the confluence point. The numerical results provide a useful data for discussing the relation between the bed evolution process and the behavior of radioactive contaminant. In addition, the model can contribute to estimate the origin of the sediment by using the relationship between the simulated sediment diameter and the concentration of the radioactive contaminant.

Multi-scale modelling of river morphodynamics
M. Nabi, S. Giri, T. Iwasaki, I. Kimura, Y. Shimizu
RIVER FLOW 2014, 1253, 1259, CRC PRESS-TAYLOR & FRANCIS GROUP, 2014
English, International conference proceedings, We developed a morphodynamic model for generation and migration of dunes under shallow flow approximation. The flow was calculated by two-dimensional depth-averaged equations and the bed shear stress was found by fitting a log-low function at each grid cell. The shear stress then was adjusted by adding slope effect. As the bed features have smaller spatial scales than the flow, a two-grid approach is adopted. The flow is solved on a coarse grid and then the flow field is interpolated on a finer grid, on which the bed morphology is calculated. This technique enabled us to solve the morphological process for large scale applications. A parameterized relation is applied for bedload sediment transport and the bed evolution is updated using Exner equation. Two cases with different flow conditions are simulated and compared with experimental results and field measurements. The computed results show good agreement with experiments and field measurements.

Numerical study on secondary currents of the second kind in wide shallow open channel flows
R. Suzuki, I. Kimura, Y. Shimizu, T. Iwasaki, T. Hosoda
RIVER FLOW 2014, 179, 187, CRC PRESS-TAYLOR & FRANCIS GROUP, 2014
English, International conference proceedings, The secondary current of the second kind, which is initiated by the turbulence anisotropy, is a classical topic in fluid engineering and has been studied by many researchers with wide variety of methods. However, we have still controversy on this phenomenon, in particular, the secondary currents in open channels with larger aspect ratio (=B/h > 5, B: channel width, h: water depth). This paper investigated the fundamental characteristics of the secondary current of the second kind in wide shallow open channel flows through the comparison between 3-D (three-dimensional) URANS (Unsteady Reynolds Averaged Navier Stokes Equations) computations and results of laboratory experiments performed by Blanckaert et al. (2010). A second order non-linear k-epsilon model proposed by Ali et al. (2007) was employed as a turbulence model.
First, we tried to carry out the computations under the same conditions of the laboratory tests by Blanckaert et al. We employed three cases in rectangular open channels with different depths for validating the numerical model. The computational results are generally in good agreement with the experimental results not only the cross-sectional secondary flow patterns but also patterns of Reynolds stresses and turbulence kinetic energy. The experimental results showed that the periodic vortices with streamwise axes are generated regularly in the cross section except the vicinity of side walls, and their diameter is almost same as the water depth. It means that the larger vortices are generated in case of larger depth. The same tendency was well simulated by the present computations.
It has been pointed out in previous papers that the secondary currents of straight open channels are initiated by the surface vortex and the bottom vortex near the side walls, and the secondary current becomes weaker as the distance from the side wall increases. It has been reported that the clear secondary current is not generated at the region where the distance from the side wall is larger than 2.5 h (e.g., Nezu et al., 1985). However, such damping of vortex strength was not observed in the present computations as well as the experiments by Blanckaert et al. In order to check the effect of side walls to the secondary current, we performed the computation under the aspect ratio = 100. In this case, the secondary currents are again generated clearly in the whole channel and no damping effect is observed in the middle part. The result was also same even if a periodic boundary condition is adopted in the lateral direction to neglect completely the effect of side walls. Those results imply an important finding that the surface and bottom vortices near the side walls, which have been considered to drive the secondary current cells, are not a necessary source for generating secondary currents in entire cross section of a shallow straight open channel.

Modelling of the initiation and development of tidal creek networks
Toshiki Iwasaki, Yasuyuki Shimizu, Ichiro Kimura
PROCEEDINGS OF THE INSTITUTION OF CIVIL ENGINEERS-MARITIME ENGINEERING, 166, 2, 76, 88, ICE PUBLISHING, Jun. 2013, [Peer-reviewed]
English, Scientific journal, The physical and numerical modelling of the initiation and development process of tidal creek network in tidal environments is presented herein. An attempt was made to reproduce the developing process of tidal creeks on a small experimental flume by considering the scaling rules of flow and sediment transport associated with tidal flows between the real scale and an experimental flume. In the experiments, the initial stage of channel evolution and the complex channel network configuration were successfully modelled, and results show that the similar network geometry observed in tidal environments can be reproduced at an experimental scale. Numerical simulations of initiation and development of tidal creeks were also undertaken using a two-dimensional-based morphodynamic model. The results indicated that the proposed numerical model simulated the geometry of channel network and its development process observed in the experiment reasonably well. In addition, a sensitivity analysis of grid resolution was carried out to show how grid resolution affected the computational results. The results implied that a grid size which can resolve the finer-scale channels that form in the initial stages of network development is required to simulate the evolution of tidal creeks adequately.

APPLICATION OF KMR WITH EFFECT OF SPATIAL SCALE OF GEOMETRY TO COMPUTATIONS ON RIVER MORPHODYNAMICS
木村一郎, 山蔦涼, 岩崎理樹
水工学論文集土木学会水工学委員会編, 57, 1_643, 648, 土木学会, 2013
Japanese

A CONSIDERATION OF CONTROLLING FACTOR ON FORMATION AND DEVELOPMENT OF TIDAL CREEK
IWASAKI Toshiki, OKUDERA Ryota, SHIMIZU Yasuyuki, KIMURA Ichiro
Journal of Japan Society of Civil Engineers, Ser. B1 (Hydraulic Engineering), 69, 4, I_1093, I_1098, 公益社団法人土木学会, 2013
Japanese, This paper presents a consideration of formation mechanism of tidal channel networks. We employed a simple mathematical model to explain the flow and bed evolution in intertidal zones. By assuming the regular periodic tidal wave and the simple bed geometry which has constant slope, we derived an advection - diffusion equation for the bed evolution. This equation has a Peclet type non dimensional number which expresses the ratio between advection and diffusion. We investigated this Peclet number in the experimental results conducted under the different hydraulic conditions. The results show that in the conditions with small Peclet number, which means that the diffusion effect is relatively strong, clear channel network can not develop on the bed surface. On the other hand, in large Peclet number conditions, clear and complicated channel network tends to develop.

APPLICATION OF KMR WITH EFFECT OF SPATIAL SCALE OF GEOMETRY TO COMPUTATIONS ON RIVER MORPHODYNAMICS
KIMURA Ichiro, YAMATSUTA Ryo, IWASAKI Toshiki, SHIMIZU Yasuyuki
Journal of Japan Society of Civil Engineers, Ser. B1 (Hydraulic Engineering), 69, 4, I_643, I_648, 公益社団法人土木学会, 2013
Japanese, A multi-level grid type turbulent shallow flow model combined with an equilibrium sediment transport model (KMR-MB: Kinematic Mesh Reconstruction for Movable Bed) is proposed. KMR method is a kind of multi-level grid model with dynamic refinement and combining grid cells to capture efficiently unsteady flow phenomena. Saito et al. (2012)1) have applied the KMR approach to open channel flows around a bridge pier and shown the advantage of the model. They used the maximum value of the strain and rotation parameters for the threshold for quad-tree and 3 x 3 type grid divisions. We have extended the model for computations of morphological phenomena in rivers by incorporating an equilibrium sediment transport model as well as a bed continuity equation, and employing new criteria for grid cell refinement. We employed an average curvature of the bed surface as the criteria for refinement and combining quad-tree type grid cells. The present model has been applied to simulate the alternative bar formations. The experimental result by Akahori et al (2011)2) was used for validation of the model. The comparison between numerical and experimental results showed that the present KMR-MB can simulate excellently the phenomena with less CPU time than the fixed grid computations.

A MORPHODYNAMIC MODEL BASED ON A SHALLOW FLOW MODEL AND AN EQUILIBRIUM BEDLOAD TRANSPORT MODEL FOR CALCULATING BED EVOLUTION IN RIVERS
IWASAKI Toshiki, SHIMIZU Yasuyuki, KIMURA Ichiro
Journal of Japan Society of Civil Engineers, Ser. B1 (Hydraulic Engineering), 69, 3, 123, 134, 公益社団法人土木学会, 2013
Japanese,  The performance and stability of a numerical model which consist of a shallow water flow model and an equilibrium bedload transport model for calculating bed evolution in rivers is discussed. Such system equations which are generally adopted for calculating reach-scale river bed evolution have a hyperbolic feature. This fact has implied that upwind schemes to the space derivative of bedload transport in Exner equation are useful numerical methods to avoid bed instabilities in calculating the propagation of a small disturbance. Whereas, a bed diffusion effect which is essentially included in bedload dominated bed evolution has been modelled in equilibrium bedload transport models by considering the local slope effect to the bedload transport. The relation between the numerical and physical diffusion effect to the bed evolution should be investigated to understand the performance and stability of the model. A hyperbolic differential equation controlling the bed evolution was derived from the linearized system equations to understand the numerical characteristic of the propagation of the bed disturbance. We defined a Péclet number, Pe, as the ratio between the advective effect which controls the propagation of the bed disturbance and the diffusive effect associated with the bed slope effect. The effect of physical diffusion of the bed exceeds the effect of numerical diffusion by the upwind scheme to the bedload transport in the condition which Pe < 2. The series of computations which focus on the propagation of hump type bed disturbance were conducted. The numerical results show that the calculation of propagating the bed disturbance can be physically stable without the upwind scheme when the grid size which satisfies Pe < 2 is used. The grid size which is required such physically stable computation depends on how the bed slope effect is modelled in the bedload transport model.

AN INFLUENCE OF MODELLING OF SECONDARY FLOWS TO SIMULATION OF FREE BARS IN RIVERS
IWASAKI Toshiki, SHIMIZU Yasuyuki, KIMURA Ichiro
Journal of Japan Society of Civil Engineers, Ser. B1 (Hydraulic Engineering), 69, 3, 147, 163, 公益社団法人土木学会, 2013
Japanese,  This paper investigates an influence of the modelling of secondary flows to the formation and development of free bars in rivers by comparing the results of the numerical simulation, the linear stability analysis and the experiments. Free bars which are developed by a physical instability phenomenon between turbulent flow and bed surface of rivers are one of fundamental river morphologies. Theoretical studies by the linear stability analysis have successfully shown that two dimensional morphodynamic models for calculating the river bed evolution can explain the formation of free bars. However, three dimensional flow structures, for instance, secondary flows caused by the meandering flows with increasing the bar height, have also important roles in the dynamics of free bars on a nonlinear level. Although quasi-three dimensional flow models can be a powerful tool to include the three dimensionalities of flows on the framework of two dimensional approaches, the applicability of such models to the dynamics of free bars has not been well verified yet. The results of the linear stability analysis suggested that the model of secondary flows which is assumed under the fully developed stage allows developing the bar with unrealistically high mode. This high mode bar tends to develop under the hydraulic conditions in which the effect of transverse slope for the sediment transport direction is relatively weak. Such instability can be physically suppressed by applying a model of secondary flows which includes the effect of development of secondary flows. The results of numerical simulation shows that proposed model which considers the development of the secondary flows can reasonably reproduce the incipient of free bars which is indicated from the linear stability analysis as well as the equilibrium bar waveheight in the experiments.

AN EXPERIMENT AND NUMERICAL SIMULATION OF BEDROCK-EROSION CAUSED BY BEDLOAD ON BEDROCK
OIKAWA Shin, IWASAKI Toshiki, YAMAGUCHI Satomi, SHIMIZU Yasuyuki, KIMURA Ichiro
Journal of Japan Society of Civil Engineers, Ser. B1 (Hydraulic Engineering), 30, 2, 11, 26, Japan Society of Civil Engineers, 01 Nov. 2012
English, This paper presents the fundamental studies on the bed degradation process of bedrock induced by the bedload transport over the bedrock. We performed the laboratory experiments focusing on the dependency of the grain size and the strength of bedrock for the erosion velocity of bedrocks. The experimental results show that the grain size and strength of bedrock can affect to the erosion velocity equation of bedrocks. In addition, the movement of the sediment particles over bedrock which has constant side inclining bed was measured to understand the direction of sediment over the bed. We also carried out the numerical simulation of bedrock erosion process by the sediment transport over bedrock to understand the performance of the model proposed by us. The comparison between laboratory experiment and numerical simulation shows that the proposed model overestimates the effect of local slope for sediment transport direction over the bedrocks.

INSPECTION OF FLOOD CONTROL EFFECT OF BIRDS POINT-NEW MADRID FLOODWAY ON MISSISSIPPI RIVER
TANAKA Toshiyuki, SHIMIZU Yasuyuki, NELSON Jonathan, IWASAKI Toshiki
Journal of Japan Society of Civil Engineers, Ser. B1 (Hydraulic Engineering), 68, 4, I_1033, I_1038, 公益社団法人土木学会, 2012
Japanese, The Great Mississippi Flood of 1927 was the most destructive river flood in the history of the United States. After the flood in 1927, Mississippi River and Tributaries Project has carried out and changed the engineering policy. One of the biggest projects in MR&T project is the constriction of Birds Point-New Madrid Floodway. In 2011, May, Mississippi River had flooded dramatically and floodway had used which have not been used since 1937. By using floodway smoothly, explosion of levee had executed which was including in Birds Point-New Madrid Floodway operation measure. On our research, we evaluate the effect of the floodway and reproduce the flood by using numerical simulation. By our simulation, floodway has the effect to reduce the water level 1.5m~3m. We also simulate the characteristics of the floodway.

NUMERICAL SIMULATION ON DEVELOPMENT OF TIDAL CREEK WITH SEEPAGE FLOW
IWASAKI Toshiki, SHIMIZU Yasuyuki, KIMURA Ichiro
Journal of Japan Society of Civil Engineers, Ser. B1 (Hydraulic Engineering), 68, 4, I_979, I_984, 公益社団法人土木学会, 2012
Japanese, This study presents the numerical simulations of initiation and development process of tidal creek network by using two-dimensional morphodynamic model including the seepage ﬂow model, and also discusses the eﬀect of seepage ﬂow for development of tidal creek network. A two dimensional shallow ﬂow equation and two dimensional saturated seepage ﬂow model based on Darcy law were combined to simulate ﬂow ﬁeld both on and in the bed. The numerical tests with and without seepage ﬂow model were carried out to understand the eﬀect of seepage ﬂow for development of tidal creek in simpliﬁed conditions. The result shows that, by including seepage ﬂow model, the channel network tend to become complex one, which has a lot of small scale channels. In addition, the channel width and depth increases by the very shallow ﬂows induced by water supply from inside the bed. This indicates that the seepage ﬂow from inside the bed can have dominant eﬀect for morphodynamics on tidal areas, especially in case of subjecting the dry and wet condition in the bed surface.

CONSIDERATIONS ON A CALCULATION MODEL FOR A REAL SCALE EXPERIMENT ON FRONT OVERFLOW LEVEE BREACH
ZENNO Hiroki, IWASAKI Toshiki, SHIMIZU Yasuyuki, KIMURA Ichiro
Journal of Japan Society of Civil Engineers, Ser. B1 (Hydraulic Engineering), 68, 4, I_865, I_870, 公益社団法人土木学会, 2012
Japanese, It is important from the engineering point of view to predict the collapsing process of river embankment and behavior of overtop flow for reducing disaster during flood. We performed some considerations on refinement of a two-dimensional computational model based on shallow flow equations to predict more accurately the front overflow levee breach phenomena. We tried to introduce a levee erosion model instead of sediment transport model to evaluate the deformation of the levee. In addition, intermittent behavior of the bank collapse is modeled. A calibration of the numerical model is made through the comparison with field experimental data. Recently, a real-scale experiment on a front levee breach was carried out at the Chiyoda Experimental Channel in Hokkaido, Japan. We performed the computation under the same conditions in the experiment. The computational results showed that the present refined model can predict fundamental aspects of the front levee breach phenomena. breach phenomena.

Numerical simulation on bed evolution and channel migration in rivers
Toshiki Iwasaki, Yasuyuki Shimizu, Ichiro Kimura
RIVER FLOW 2012, VOLS 1 AND 2, 673, 679, CRC PRESS-TAYLOR & FRANCIS GROUP, 2012, [Peer-reviewed]
English, International conference proceedings, This paper presents the numerical simulation of sand bar evolution with bank erosion in the Otofuke river, Hokkaido, Japan. A great flood occurred in the Otofuke River, at Hokkaido region in September 6th, 2011 have caused the significant bed evolution and channel migration. As a result, a part of the river embankment was destroyed. To protect such damage of the river embankment during the flood, the complex interaction between flow, bed evolution and bank erosion in rivers have to be investigated. In this study, the flow and bed deformation of the Otofuke river during the flood was examined by using a depth-averaged two-dimensional morphodynamic model. The numerical result indicates that the alternate sand bars developed in low water channel at peak discharge and these sand bars creates the clear alternative thalweg. By the concentration of flows in the thalweg, the significant channel migration might occur during falling stage of discharge. This implies that unsteadiness of discharge is a one of key role to bank erosion process in case of co-existing of sand bars.

Computations of real scale experiment on levee breach with a 2D shallow flow model　　　　　　　　　　　　　　　
Hiroki Zenno, Toshiki Iwasaki, Yasuyuki Shimizu, Ichiro Kimura
34th IAHR Congress 2011 - Balance and Uncertainty: Water in a Changing World, Incorporating the 33rd Hydrology and Water Resources Symposium and the 10th Conference on Hydraulics in Water Engineering, 491, 498, International Association for Hydro-Environment Engineering and Research (IAHR), 2011
English, International conference proceedings, Flood in rivers is a common disaster all over the world. If a levee breach happens, it sometimes causes a fatal disaster. In addition, many buildings, urban facilities, lifelines, etc. are seriously damaged. Detailed mechanism of a levee breach has not been clarified yet. Therefore, it is important to predict the collapsing process of riverbank and behaviour of overtopping flow for reducing flood damage. We applied a two-dimensional shallow flow computational model to levee breach phenomena caused by overflow and the performance of the model was elucidated. Numerical model calibration is made through the comparison with a field experimental data. Recently, a real-scale experiment on a levee breach was carried out at the Chiyoda Experimental Channel in Japan. We performed the computation under the same conditions in the experiment. The computational results showed the excellent agreement with experiment for simulating levee breach phenomena.

Geomorphological characteristics of female and male-type meanders　　　　　　　　　　　　　　　
Kozue Tanaka, Yasuyuki Shimizu, Satomi Yamaguchi, Toshiki Iwasaki, Ichiro Kimura, Gaku Tanaka
7th IAHR Symposium on River, Coastal and Estuarine Morphodynamics(CD-ROM), 7, 1824, 1831, 2011, [Peer-reviewed]
English, International conference proceedings

Geometric features of tidal channel networks
IWASAKI Toshiki, WATANABE Yasunori, OKUDERA Ryota, SHIMIZU Yasuyuki, KIMURA Ichiro
Journal of Japan Society of Civil Engineers, Ser. B2 (Coastal Engineering), 67, 2, 951, 955, 公益社団法人土木学会, 2011
Japanese, The complex channel networks have dominant roles for flow, sediment and nutrient transport on salt marshes and tidal flats, and therefore provide rich habitat structures. This study discusses the fundamental geometric properties of tidal channel network by statistical analysis to their channel length and planimetric shapes. We employed channel networks which delineated from experimental and numerical result and also satellite image of real field. It is found that the probability density distribution of channel length can be described by a lognormal distribution, and network in experimental and real scale have similar properties for channel length. The results also indicate that the difference of tidal conditions change the characteristics of tidal channel network configuration.

HISTORICAL TRANSITION OF RIVER TRAINING WORKS AND INUNDATION CHARACTERISTICS IN SAPPORO
TANAKA TOSHIYUKI, SHIMIZU YASUYUKI, KIMURA ICHIRO, IWASAKI TOSHIKI
Journal of Japan Society of Civil Engineers, Ser. B1 (Hydraulic Engineering), 67, 4, I_895, I_900, 公益社団法人土木学会, 2011
Japanese, The urbanization in Sapporo was started about only 140 years ago. Before the urbanization, the topography of most part of Sapporo was wetland and not suitable for living and cultivation. Intensive river training works have changed this area dramatically for relatively short period, and made Sapporo as one of the biggest cities in Japan with a population of 1.9 million. In this study, the history of river training works and the transition of inundation characteristics in Sapporo are investigated through old literatures and numerical simulations. First, the historical process of river training constructions was clarified by inspecting old documents and found out some key works. Then numerical simulations of flood were performed in three cases, namely, original topography, after construction of Toyohira-River- Bank and after construction of new Toyohira River Channel. The comparison of simulation results elucidated the relation between river training works and change of inundation behavior.

EXPERIMENTAL AND NUMERICAL STUDY ON DEVELOPMENT OF TIDAL CREEK NETWORK
IWASAKI Toshiki, SHIMIZU Yasuyuki, KIMURA Ichiro
Journal of Japan Society of Civil Engineers, Ser. B1 (Hydraulic Engineering), 67, 4, I_859, I_864, 公益社団法人土木学会, 2011
Japanese, This paper presents the laboratory tests on developing process of tidal creek network in the small flume and also validation of numerical model which can replicate the morphodynamic evolution of tidal channel and its geomorphic features. The experimental conditions fully satisfied the kinematic, dynamic and sediment transport similarities between the real and experimental scale. The channel network, which has the similar channel characteristics such as ratio between the channel width and depth observed in real field, was reproduced in the experimental flume. This study also conducted the numerical simulations with experimental conditions for validating the physically-based two-dimensional morphodynamic model. The results showed that the proposed morphodynamic model can simulate reasonably the geometry of channel network and its developing process observed in the experiment.

SIMULATION REAL SCALE EXPERIMENT ON LEVEE BREACH USING 2D SHALLOW FLOW MODEL
ZENNO Hiroki, IWASAKI Toshiki, SHIMIZU Yasuyuki, KIMURA Ichiro
Journal of Japan Society of Civil Engineers, Ser. B1 (Hydraulic Engineering), 67, 4, I_853, I_858, 公益社団法人土木学会, 2011
Japanese, Flood in rivers is a common disaster all over the world. If a levee breach happens, it sometimes causes a fatal disaster. In addition, many buildings, urban facilities, lifelines, etc. are seriously damaged. Detailed mechanism of a levee breach has not been clarified yet. Therefore, it is important to predict the collapsing process of riverbank and behavior of overtop flow for reducing damage. We applied a two-dimensional shallow flow computational model to levee breach phenomena caused by overflow and the performance of the model was elucidated. A calibration of the numerical model is made through the comparison with field experimental data. Recently, a real-scale experiment on a levee breach was carried out at the Chiyoda Experimental Channel in Hokkaido, Japan. We performed the computation under the same conditions in the experiment. The computational results showed the excellent performance for simulating levee breach phenomena.

AN EXPERIMENT AND NUMERICAL SIMULATION OF SLURRY-EROSION CAUSED BY BEDLOAD ON BEDROCK
OIKAWA Shin, IWASAKI Toshiki, YAMAGUCHI Satomi, SHIMIZU Yasuyuki, KIMURA Ichiro
Journal of Japan Society of Civil Engineers, Ser. B1 (Hydraulic Engineering), 67, 4, I_751, I_756, 公益社団法人土木学会, 2011
Japanese, The bed degradation is going rapidly in the upper part of the Ishikari River recently, owing to exposed bedrock erosion caused by bedload on the bedrock. In order to estimate the erosion rate of bedrock, we performed an experiment by a circular channel flume with artificial bedrock made of plaster. We applied the erosion rate estimated by the present experiment to a numerical simulation of bed deformation on the bedrock by using horizontal 2D flow model and the bedload layer model. The result of the simulation shows that gut pattern appeared in the simulated results which is very similar to the experimental result. It was found that the bed degradation was progressed by the erosion caused by sediment transport itself.

Computations of bed deformation and sediment sorting in meander channel using depth-averaged morphodyamic model
Toshiki Iwasaki, Yasuyuki Shimizu, Ichiro Kimura
International Journal of River Basin Management, 9, 3-4, 237, 245, 2011
English, Scientific journal, This paper discusses the bed evolution and the grain sorting process in meandering channel by using a two-dimensional (2D) based morphodyamic model. In river bends, secondary currents, which are three-dimensional (3D) flow structures caused by a balance between centrifugal force and pressure gradient, has important roles for flow structure and the sediment transport. Therefore, a numerical model, which can reproduce this 3D flow structure, is required to describe the morphodynamic phenomena in meandering rivers. This study employed the depth-averaged 2D shallow water equations’ integrated effect of secondary currents of the first kind for flow calculations. This flow model can consider the effect of lag between the developments of secondary current and streamline curvatures. In addition, we employed the equilibrium sediment transport model and a multiple layer model for simulating the sediment transport and sorting phenomena due to non-uniform sediments. This study applied the different kind of model for evaluating the secondary current and sediment transport to a meandering channel with simple-rectangular sine-generated curve. We examined the applicability of models and morphodynamic process in a meandering channel through the comparison between the experimental and numerical results. The result shows that the correlation for secondary currents for flow structures and sediment transport is important to reproduce the bed evolution and sorting phenomena in river bends. © 2011 International Association for Hydro-Environment Engineering and Research.

現地観測と数値解析によるTidal Creekの形成過程に関する考察
岩崎理樹, 清水康行, 木村一郎, 田中岳
水工学論文集(CD-ROM), 53, ROMBUNNO.125, Feb. 2009, [Peer-reviewed]
Japanese, Scientific journal

SIMULATION OF MORPHOLOGY IN THE TIDAL ENVIRONMENTS
MASUYA Shigekazu, SHIMIZU Yasuyuki, IWASAKI Toshiki
PROCEEDINGS OF HYDRAULIC ENGINEERING, 52, 607, 612, 公益社団法人土木学会, 2008
This study aims to develop a physically-based morphodynamic model in order to reproduce incision of tidal creek networks as well as evolution of geomorphology in the tidal environments. The process of incision and long-term geomorphologic evolution on tidal flats and marsh is of practical significance from the environmental view point. Generally, it is postulated that tidal creeks are developed as a result of ecomorphologic processes of tidal system driven by tidal energy. Some previous investigations have provided significant insight to this problem. However, there is still lack of a physically-based modeling approach that can replicate the initiation and evolution process of this phenomenon. In this study, physically-based model was applied to a narrow area referred to as tidal flats in order to provide quantitative and qualitative evaluation of its reproducibility. Calculation was conducted for different time scale up to 350 days. Calculation results revealed that the incision of tidal network and its initial development process can satisfactorily be reproduced by a physically-based computational model.

Numerical simulation of the tidal creek with vegetation growth
IWASAKI Toshiki, SHIMIZU Yasuyuki
Journal of applied mechanics, 11, 745, 752, 公益社団法人土木学会, 2008
Japanese, In this paper, eco-morphological model is proposed to simulate morphology at tidal environment with vegetation growth. Two numerical experiments such as vegetated scenario and unvegetated scenario were performed to clarify influence of vegetation on morphological evolution. This model was applied to a square tidal flat and calculations were conducted for 350 days. Computational results show that a complicated channel network is formed on tidal marsh and natural levees were appeared around the channel network in both cases. In addition, the vegetation propagated along the natural levees. In vegetated scenario, sediment deposition was enhanced by effect of vegetation on natural levee in comparsion with unvegetated scenario. Therefore, these results indicated that tidal channels are formed more clearly due to vegetation.

Other Activities and Achievements

大規模アンサンブル降雨流出計算データを利用した河道侵食被災リスク評価
岩崎理樹, 電力土木, 430, 2024

Modelling long-term equilibrium bar characteristics: A case study of the Chubetsu River
高橋祐貴, 岩崎理樹, 土木学会北海道支部論文報告集(CD-ROM), 80, 2024

Study on morphological evolution of small-sized rivers under climate changes
大河内駿, 田鍋颯一, 岩崎理樹, 土木学会北海道支部論文報告集(CD-ROM), 80, 2024

A study on simple extraction of treetops and their distribution in riparian forests
宮本豊郎, 奥山瑛人, 岩崎理樹, 土木学会北海道支部論文報告集(CD-ROM), 80, 2024

Numerical calculation of morphological evolution of the Jinzu River
島屋有希, 岩崎理樹, 土木学会北海道支部論文報告集(CD-ROM), 80, 2024

Experimental study on the driftwood transport and deposition under the formation of sand bar and braiding channel
南郁慧, 岩崎理樹, 土木学会北海道支部論文報告集(CD-ROM), 80, 2024

Verification of HLL-based river morphodynamic model
児玉武史, 岩崎理樹, 土木学会北海道支部論文報告集(CD-ROM), 80, 2024

Study on the transport materials in rivers: A case study of the Shiribeshitoshibetsu river
浜山千英, 岩崎理樹, 土木学会北海道支部論文報告集(CD-ROM), 80, 2024

流路形態が流木の移動,堆積に与える影響に関する実験的検討
樋口智哉, 南郁慧, 岩崎理樹, 土木学会全国大会年次学術講演会(Web), 79th, 2024

河畔林の樹頂点分布に関する現地調査とポイントパターン分析
宮本豊郎, 奥山瑛人, 岩崎理樹, 土木学会全国大会年次学術講演会(Web), 79th, 2024

土砂供給の非平衡性に起因するBedload Sheetの形成と交互砂州動態への影響
田鍋颯一, 岩崎理樹, 土木学会全国大会年次学術講演会(Web), 79th, 2024

豊平川におけるプラスチックごみに関する現地調査
大屋壱世, 岩崎理樹, 土木学会全国大会年次学術講演会(Web), 79th, 2024

流量変動の繰り返しに伴う砂州の形態変化に関する実験的研究
中根瑞希, 岩崎理樹, 土木学会全国大会年次学術講演会(Web), 79th, 2024

洪水時に河川表層を流下する浮遊物の動態把握に関する研究:後志利別川を例として
浜山千英, 岩崎理樹, 土木学会全国大会年次学術講演会(Web), 79th, 2024

Effects of Graded Sediment Model on the Morphodynamic Calculation of graded sediment bed
有田圭吾, 岩崎理樹, 土木学会北海道支部論文報告集(CD-ROM), 79, 2023

Field observation of Microplastics in Ishikari and Toyohira Rivers
大屋壱世, 岩崎理樹, 土木学会北海道支部論文報告集(CD-ROM), 79, 2023

Numerical simulation of advection and dispersion of bedload tracer particles in braided channel
橋詰竜人, 岩崎理樹, 土木学会北海道支部論文報告集(CD-ROM), 79, 2023

Sensitivity of graded sediment model to the morphodynamic calculaiton of bed and grain size in gravel bed rivers
有田圭吾, 岩崎理樹, 土木学会全国大会年次学術講演会(Web), 78th, 2023

The characteristics of morphological changes of riverbed at bed slope and channel width transition point
田鍋颯一, 岩崎理樹, 清水康行, 土木学会全国大会年次学術講演会(Web), 78th, 2023

Experiment and Numerical Simulation on Transport Process of Sediment Supplied in River Channel
平松裕基, 川村里実, 岩崎理樹, 大串弘哉, 寒地土木技術研究, 827, 2022

Influence of water discharge change due to dam construction on river channel geometry
高橋祐貴, 岩崎理樹, 清水康行, 土木学会北海道支部論文報告集(CD-ROM), 78, 2022

The flood risk analysis in small streams: A case study of the Pekerebetsu River
大河内駿, 高橋広大, 岩崎理樹, 清水康行, 土木学会北海道支部論文報告集(CD-ROM), 78, 2022

Comparison of Computational Fluid Dynamics for different grid types
若原巧実, 清水康行, 岩崎理樹, 土木学会北海道支部論文報告集(CD-ROM), 78, 2022

Field observation of micro-plastics in the Ishikari River
久保田航平, 岩崎理樹, 清水康行, 土木学会全国大会年次学術講演会(Web), 77th, 2022

Study on the geometric features of tidal channels in intertidal areas
伊藤大河, 岩崎理樹, 清水康行, 土木学会北海道支部論文報告集(CD-ROM), 77, 2021

Effects of Bank Strength by Vegetation on Meandering Channel Dynamics
南郁慧, 久加朋子, 山口里実, 清水康行, 今日出人, 岩崎理樹, 土木学会北海道支部論文報告集(CD-ROM), 77, 2021

Numerical simulation of upstream-migrating antidunes
中誠, 岩崎理樹, 清水康行, 土木学会北海道支部論文報告集(CD-ROM), 77, 2021

Effect of variability of multiple hydrographs on the channel deformation
村上大知, 岩崎理樹, 清水康行, 土木学会北海道支部論文報告集(CD-ROM), 77, 2021

A numerical model of free bar morphodynamics using HLL-type scheme
岩崎理樹, 数値流体力学シンポジウム講演論文集(CD-ROM), 35th, 2021

Long term channel process under various patterns of sequential hydrographs
村上大知, 高橋広大, 清水康行, 岩崎理樹, 数値流体力学シンポジウム講演論文集(CD-ROM), 35th, 2021

土砂トレーサーモデルによる着色砂投入実験の再現計算
山田嵩, 矢部浩規, 岩崎理樹, 土木学会年次学術講演会講演概要集(CD-ROM), 75th, 2020

河川におけるプラスチックゴミの移動と堆積に関する調査と実験
久保田航平, 岩崎理樹, 清水康行, 土木学会北海道支部論文報告集(CD-ROM), 76, 2020

洪水氾濫に伴う油流出解析モデルの開発
高橋広大, 岩崎理樹, 井上卓也, 大串浩一郎, 清水康行, 土木学会北海道支部論文報告集(CD-ROM), 76, 2020

2011年タイ洪水における蒸発散の影響
宮原望, 岩崎理樹, 清水康行, 土木学会北海道支部論文報告集(CD-ROM), 76, 2020

令和元年台風19号及び台風21号による広域災害に関する総合研究
二瓶泰雄, 仲江川敏之, 中北英一, 竹見哲也, 山田朋人, 三隅良平, 飯塚聡, 鈴木真一, 柳瀬亘, 立川康人, 田中茂信, 佐山敬洋, 田中智大, 朝位孝二, 清水義彦, 前野詩朗, 田中仁, 吉谷純一, 田中規夫, 泉典洋, 矢野真一郎, 森脇亮, 赤松良久, 内田龍彦, 重枝未玲, 岩崎理樹, 小山毅, 長谷川兼一, 西嶋一欽, 藤本郷史, 毛利栄征, 前田健一, 岡村未対, 卜部厚志, 森口周二, 蝦名裕一, 松四雄騎, 王功輝, 竹林洋史, 鈴木素之, 田島芳満, 佐々木淳, 信岡尚道, 森信人, 有川太郎, 鈴木崇之, 下園武範, 松井正宏, 小林文明, 畑山満則, 牛山素行, 佐藤健, 梶谷義雄, 自然災害科学総合シンポジウム要旨集, 57th, 2020

高速流による高水敷侵食に関する水理実験
岩崎理樹, 井上卓也, 矢部浩規, 土木学会年次学術講演会講演概要集(CD-ROM), 74th, 2019

混合砂河床における砂州の数値計算に関する考察
岩崎理樹, 数値流体力学シンポジウム講演論文集(CD-ROM), 33rd, 2019

アメリカ地球物理連合秋季大会2017(AGU Fall Meeting 2017)参加報告
岩崎理樹, 山崎秀策, 寒地土木技術研究 : 国立研究開発法人土木研究所寒地土木研究所月報 : monthly report, 778, 31, 33, Mar. 2018
土木研究所寒地土木研究所, Japanese

三角波による河川管理上のリスク:ブロックの安定性を例として
岩崎理樹, 井上卓也, 矢部浩規, 国土交通省国土技術研究会(Web), 2018, 2018

COMPUTATIONS OF THREE-DIMENSIONAL ANTIDUNES IN RIVERS
岩崎理樹, 井上卓也, 音田慎一郎, 矢部浩規, 土木学会論文集 B1(水工学)(Web), 74, 5, 2018

NUMERICAL SIMULATION OF BEDLOAD TRACER TRANSPORT BY CONSIDERING VERTICAL TRACER DISPERSION
濱木道大, 岩崎理樹, 井上卓也, 清水康行, 土木学会論文集 B1(水工学)(Web), 74, 5, 2018

2016年アメリカ地球物理連合大会(American Geophysical Union Fall Meeting)と地形学に関する研究集会Gilbert Clubへの参加報告
岩崎理樹, 島田友典, 井上卓也, 西原照雅, 水垣滋, 寒地土木技術研究 : 国立研究開発法人土木研究所寒地土木研究所月報 : monthly report, 766, 64, 67, Mar. 2017
土木研究所寒地土木研究所, Japanese

札内川を事例とする植生の流路変動への影響
渡邉健人, 久加朋子, 山口里美, 清水康行, 泉典洋, 岩崎理樹, 石田義明, 土木学会北海道支部論文報告集(CD-ROM), 73, 2017

MODELING THE THREE-DIMENSIONAL RIVER ANTIDUNES : IMPLICATIONS FOR THE TRIANGLE-SHAPE WATER SURFACE WAVE TRAINS
岩崎理樹, 井上卓也, 矢部浩規, 水工学論文集 Annual journal of Hydraulic Engineering, JSCE, 61, Ⅰ_901, 906, 2017
土木学会, Japanese

三角状水面波列がブロック安定性に与える影響に関する実験
岩崎理樹, 井上卓也, 矢部浩規, 土木学会年次学術講演会講演概要集(CD-ROM), 72nd, 2017

スリランカにおける河川解析ソフトウェア、iRIC講習会の実施とIAHR-APD2016への参加報告
岩崎理樹, 寒地土木技術研究 : 国立研究開発法人土木研究所寒地土木研究所月報 : monthly report, 762, 60, 62, Nov. 2016
土木研究所寒地土木研究所, Japanese

破堤氾濫解析へのiRIC活用事例について
濱木道大, 山内孝昭, 井上卓也, 清水康行, 岩崎理樹, 土木情報学シンポジウム講演集, 39, 2014

Numerical Simulations of Forced Bars in Meandering Rivers with Spatial Width Variations
井上卓也, 岩崎理樹, 清水康行, 伊藤丹, 寒地土木研究所月報, 726, 2013

河床における岩盤上の流砂の集中度合に関する実験的検討と数値シミュレーション
及川森, 岩崎理樹, 山口里実, 清水康行, 井上卓也, 水工学論文集(CD-ROM), 56, 2012

Courses

Academic Communication III　　　　　　　　　　　　　　　
北海道大学工学部
Oct. 2024 - Present

Computational Continuum Mechanics　　　　　　　　　　　　　　　
北海道大学工学部
Oct. 2024 - Present

防災工学演習　　　　　　　　　　　　　　　
北海道大学工学部
Oct. 2024 - Present

水理学２　　　　　　　　　　　　　　　
北海道大学工学部
Sep. 2023 - Present

環境と人間：建築と都市を考える　　　　　　　　　　　　　　　
北海道大学
Apr. 2023 - Present

社会基盤と国土政策　　　　　　　　　　　　　　　
北海道大学工学部
Apr. 2023 - Present

応用数学I　　　　　　　　　　　　　　　
北海道大学工学部
Apr. 2021 - Present

Physics I　　　　　　　　　　　　　　　
Hokkaido University
Apr. 2020 - Present

Water Resource Management　　　　　　　　　　　　　　　
Hokkaido University, Graduate School of Engineering
Oct. 2019 - Present

一般教育演習フレッシュマンセミナーブラドボク　　　　　　　　　　　　　　　
北海道大学
Apr. 2021 - Sep. 2024

土木工学総合プロA　　　　　　　　　　　　　　　
名古屋大学工学研究科
May 2024 - May 2024

水理学演習I　　　　　　　　　　　　　　　
北海道大学工学部
Apr. 2020 - Sep. 2023

Hydraulics I　　　　　　　　　　　　　　　
School of Engineering, Hokkaido University
Apr. 2020 - Sep. 2023

Affiliated academic society

American geophysical union

JAPAN SOCIETY OF CIVIL ENGINEERS

Research Themes

機械学習と力学モデルの融合による流域スケール流木予測管理技術の構築
科学研究費助成事業
Apr. 2024 - Mar. 2028
木村一郎, 磯部公一, 岩崎理樹
日本学術振興会, 基盤研究(B), 富山大学, 24K00985

急流河川における降雨パターンを考慮した樹林化動態解明と流路変動・侵食リスクの検討
科学研究費助成事業
Apr. 2023 - Mar. 2027
久加朋子, 山野井一輝, 川村里実, 岩崎理樹
日本学術振興会, 基盤研究(B), 富山県立大学, 23H01519

河床変動解析における不確実性評価と現場への適用に関する技術検討　　　　　　　　　　　　　　　
河川砂防技術研究開発河川技術・流域管理分野（指定課題）
May 2024 - Mar. 2026
岩崎理樹, 泉典洋, 戸田祐嗣, 溝口敦子
国土交通省, 河床変動解析における不確実性評価と現場への適用に関する技術検討

局所・流域スケールの視点に基づく洪水時橋梁被害リスク評価の展開
科学研究費助成事業基盤研究(A)
05 Apr. 2021 - 31 Mar. 2025
二瓶泰雄, 渡邉健治, 川尻峻三, 岩崎理樹, 柏田仁
日本学術振興会, 基盤研究(A), 東京理科大学, 21H04577

気候変動後の流量発生分布を考慮した砂州の波高増大・固定化リスクの評価手法の提案　　　　　　　　　　　　　　　
河川砂防技術研究開発河川技術・流域管理分野（地域課題）
Apr. 2022 - Mar. 2024
国土交通省, Coinvestigator

河川におけるプラスチックごみに関する量的・質的特性の解明　　　　　　　　　　　　　　　
河川基金助成事業
Apr. 2022 - Mar. 2023
公益財団法人河川財団, Principal investigator, 2022-5211-001

Vulnerability assessment of water and sediment-related disasters by using large-scale water, sediment transport, and morphodynamic model
Grants-in-Aid for Scientific Research Grant-in-Aid for Early-Career Scientists
Apr. 2020 - Mar. 2023
岩崎理樹
今後予期される気候変動を踏まえた河道の脆弱性を評価するために，北海道を対象とした大量サンサンブル降雨―流出計算データを解析し，移動床計算に重要と考えられるハイドログラフパターンについて，K-shapeクラスタリング手法により検討した．これにより，数百という多数の流量データから，重要と考えられる十数の流量ハイドログラフのパターンとして特徴化することが可能となった．クラスタリングから得られた代表ハイドログラフを移動床計算の境界条件として用い，河床－河岸侵食に関しても起こりうる可能性をより網羅的な外力条件から評価可能な手法について開発できた．
上記，短期的河道脆弱性評価に使用可能な1洪水データに加えて，より中長期的な河道脆弱性を検討するために，アンサンブルメンバーごとの連続流量ハイドログラフを確率水文統計し，長期的に起こりうる条件をより網羅的に検討した．急流河川を対象とした移動床計算の結果，現在気候条件においては平均的な外力条件において，比較的堤防内で流路変動が収まる状態であっても，気候変動後の条件においては，最小流量シナリオにおいても，堤防が高い割合で流路変動に伴う侵食の影響を受けることを明らかにした．
昨年度検討した給砂条件の影響について，代表的な沖積河道地形の一つである交互砂州へ与える影響を二次元移動床計算から検討した．その結果，給砂条件の違いが下流に与える影響は比較的給砂条件を変化させた境界付近に強く表れる一方で，その領域より下流に形成される交互砂州へ与える影響は小さいことが明らかとなった．
高精度で安定的な移動床計算を実現するための計算スキームとして，近似リーマン解法の一つであるHLL系のスキームを浅水流モデルと河床変動モデルを合わせたシステム方程式に適用したモデルを構築し，自由砂州の実験結果と比較することでその精度について検証した．
Japan Society for the Promotion of Science, Grant-in-Aid for Early-Career Scientists, Hokkaido University, 20K14831

大量アンサンブル降雨―流出計算データを利用した流路変動解析による河道被災リスクの定量評価手法の構築　　　　　　　　　　　　　　　
河川砂防技術研究開発河川技術部門
May 2020 - Mar. 2022
岩崎理樹,清水康行,泉典洋,今日出人,山田朋人,久加朋子,星野剛,山口里実,井上卓也
国土交通省水管理・国土保全局

浮体混在型河川災害の予測・減災モデルの構築とiRICへの実装　　　　　　　　　　　　　　　
Apr. 2020 - Mar. 2022
北海道河川財団, Coinvestigator

混濁流による高流砂階のベッドフォームの堆積構造解明と堆積モデルの構築　　　　　　　　　　　　　　　
科学技術研究費補助金基盤研究（B）
Apr. 2018 - Mar. 2021
横川美和
文部科学省, Competitive research funding

Comprehensive study on wide-area disasters due to typhoon no.19 and 21 in 2019
Grants-in-Aid for Scientific Research Grant-in-Aid for Special Purposes
Dec. 2019 - Mar. 2020
二瓶泰雄, 中北英一, 竹見哲也, 山田朋人, 三隅良平, 飯塚聡, 鈴木真一, 仲江川敏之, 柳瀬亘, 立川康人, 田中茂信, 佐山敬洋, 田中智大, 朝位孝二, 前野詩朗, 田中仁, 吉谷純一, 田中規夫, 泉典洋, 矢野真一郎, 森脇亮, 赤松良久, 内田龍彦, 重枝未玲, 岩崎理樹, 清水義彦, 小山毅, 長谷川兼一, 西嶋一欽, 藤本郷史, 毛利栄征, 前田健一, 岡村未対, 卜部厚志, 森口周二, 蝦名裕一, 松四雄騎, 王功輝, 竹林洋史, 鈴木素之, 田島芳満, 佐々木淳, 信岡尚道, 森信人, 有川太郎, 鈴木崇之, 下園武範, 松井正宏, 小林文明, 畑山満則, 牛山素行, 佐藤健, 梶谷義雄
令和元年東日本台風（台風19号）とその2週間後の大雨（台風21号と低気圧）による広域災害の被害状況・メカニズムの全容を明らかにすることを目的とし，多角的・総合的に多くの分野の専門家からなる研究グループを結成し，調査研究を継続して行った．ここでは，①台風・低気圧による豪雨・流出現象の気象学的解明，②広域にわたる洪水氾濫災害を引き起こしたメカニズムの解明，③多発した土砂災害・土構造物破壊におけるメカニズムの解明，④強風や高潮による被害実態の解明，⑤住民の避難行動の実態と防災情報の伝達状況の把握，の5つのテーマと主に対象とした．その結果，台風19号による豪雨に，山岳部は勿論，平野部でもseeder-feeder機構が貢献していることを，XRAINによる立体観測情報，ひまわりによる観測情報により，seeder-feeder機構を表現した数値モデルを用いて示した．2019年台風19号による豪雨発生機構について，気象庁メソ客観解析データを用いて，降水活動に係る大気安定度や水蒸気場の解析をし，本豪雨発生時には，最近の他の豪雨事例と比較して，湿潤絶対不安定が持続して形成されていたことが分かった．また，洪水氾濫による建築被害に関しては，長野市内の水害被害住宅を対象に，復旧後の室内環境に着目して現地調査を実施している．復旧には数ヶ月の期間を要するが，復旧手順は現地の判断で実施している場合が多く，客観的な情報が整備されていない．その一助となるべく，限られた事例ではあるが，復旧手順とともにそれに伴う環境改善効果を評価しながら知見を蓄積し，環境改善を測るために室内の浮遊真菌濃度や部位の含水率等を計測した．さらに，風水害に関しては，台風に伴って発生した竜巻の影響が大きかったため，台風と竜巻発生の関係を過去の気象資料から抽出し整理した．
Japan Society for the Promotion of Science, Grant-in-Aid for Special Purposes, Tokyo University of Science, 19K24678

土砂トレーサーを利用した土砂動態－地形変動過程の理解と予測　　　　　　　　　　　　　　　
科学技術研究費補助金若手研究
Apr. 2017 - Mar. 2020
岩崎理樹
文部科学省, Principal investigator, Competitive research funding

Formation of cyclic steps due to surge-type turbidity currents: Flume experiments and applications for rock records
Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (B)
01 Apr. 2015 - 31 Mar. 2018
Yokokawa Miwa, IWASAKI Toshiki, OHISHI Yuri, OHATA Koji, HUGHES CLARKE John, PARKER Gary, TALLING Peter, CARTIGNY Matthieu
Recent field observation revealed that there are abundant “cyclic steps” (CS) formed by turbidity currents (TC) in the submarine channels and their vicinity. Here we did the first experiments on the formation of CS due to the “surge-type” TCs, and showed that the “surge-type” TCs can form CS easier compare with the continuous TC which has equivalent unit-time discharge. Experimental TCs, which is mixture of plastic particles and salt water, were released to the 7-m-long flume at Osaka Institute of Technology, with the surge durations 3, 5, 7 seconds. 130-140 surges were released into the flume for each case, and 5-6 CS were formed.
Based on these experimental results, analytical analysis and numerical simulation were performed. In addition, we applied the experimental results to rock records. We made a depositional model for the sediment waves observed in turbidites in the Miyazaki Group, Kyusyu, Japan, based on the extensive long-distance drawing of the sedimentary structures.
Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research (B), Osaka Institute of Technology, 15H03739

感潮域における持続可能な環境保全手法の確立に向けた地形変動予測モデルの開発
科学技術研究費補助金特別研究員奨励費
Apr. 2010 - Mar. 2013
岩崎理樹
[1]水路網の形成・発達に及ぼす主要因を抽出する目的で,潮汐周期,振幅,領域長などのパラメータを変化させた異なる水理条件下における実験を行い,水路網形成に対する水理パラメータ依存性について検討した.水路網の形成・発達を定性的に評価することで,その形成過程に関する考察を行うと共に,Horton則による水路網形状定量化手法により,水路数といった水路網の形状特性に関する検討を行った.得られた結論を以下に列挙する.1)潮汐による流れが強いほど,水路網が形成されづらい一方,相対的に流れが弱い場合は細かな水路が発達し,一水路網が複雑となることが示唆された.2)水路化には侵食と堆積の両方が重要であるが,両者の物理メカニズムは異なる可能性がある.3)位数1の水路の確率密度分布形状より,位数1の水路の長さスケールは潮汐周期が長くなるほど小さくなる傾向にある.また,時間の経過と共に確率密度分布はその形状を変化させる.
[2]異なる水理条件の下で実施された室内実験により得られる水路網形状の相違を生み出す要因について簡易的な数学モデルを用いて考察を行った.一定勾配である単純地形及び規則的な潮汐を想定し,この条件における流れ一地形変動を簡易に表現可能な数学的モデルを構築した,このモデルから地形変動を表す方程式が移流拡散型の方程式を導出し,この方程式から導出される移流拡散度合いの比を表すパラメータ,ペクレ数を定義した.このペクレ数を各実験ケースで求めて結果と比較し,以下の結論を得た.1)岸沖方向のペクレ数が大きい場合,水路が直線する一方,小さい実験では水路が蛇行する傾向が見られた.2)横断方向の移流拡散の度合いを表すパラメータが小さい領域では,水路網が形成されず,大きい領域では複雑な水路網が形成される傾向がある.これは,横断方向の拡散の影響が強い場合,不安定な成分である水路が拡散するためと考えられる.
文部科学省, 特別研究員奨励費, 北海道大学, Principal investigator, Competitive research funding, 10J02689

Iwasaki Toshiki

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