Riverbank Erosion under Boat-Generated Wave Attacks and Proposed Countermeasures for Wave Attenuation

• 資料種別: 博士論文
• タイトル: Riverbank Erosion under Boat-Generated Wave Attacks and Proposed Countermeasures for Wave Attenuation
• 著者・編者: LA, VINH TRUNG
• 著者標目: LA, VINH TRUNG
• 出版事項: 埼玉大学大学院理工学研究科
• 出版年月日等: 2015
• 出版年（W3CDTF）: 2015
• 並列タイトル等: 河岸侵食について航走波による河岸侵食とその対策に関する研究
• タイトル（掲載誌）: 博士論文（埼玉大学大学院理工学研究科（博士後期課程））
• 授与機関名: 埼玉大学
• 授与年月日: 2015-03-24
• 授与年月日（W3CDTF）: 2015-03-24
• 報告番号: 乙第223号
• 学位: 博士(学術)
• 博論授与番号: 乙第223号
• 本文の言語コード: eng
• 対象利用者: 一般
• 一般注記: type:text; Although bank erosion has been the interest of numerous studies, just a little work has been done on the impact of waves, either natural or anthropogenic. It is well-accepted that the bank erosion rate is dependent on the frequency of wave impacts as well as the wave energy relative to soil resistance of riverbank. In heavily navigated rivers and channels, high frequency of ships/boats generates waves constantly attacking towards riverbank and therefore puts banks under the risk of being eroded. In recent years, due to considerably increasing demand of inland waterway transportation served for daily activities such as business, passenger and cargo loading, tourism, etc…a large number of boats/ship have been observed. Especially, with the appearance of high speed boats getting more common, the ones equipped with power engine that move relatively close to the riverbank and generate high waves, wave-induced bank collapse is becoming a severe problem.This study presents three field investigations. The first one was conducted in Arakawa River in Japan. This work is a preliminary stage with an aim to identify whether heavy navigation-generated waves is the main factor causing sediment movement in the river. The results showed that the amount of sediment movement caused by a ship run (15 minutes) is smaller than that caused by tide (7 hours). However, by considering the time consuming, the effect of ship wave on sediment transportation is much greater than tide. And in such a heavily navigated river, this effect should be more impressive. The second one was conducted at five rivers in Ca Mau Province, Southern part of Vietnam. At each site, boat-generated wave and soil of riverbank characteristics were collected. The shear stress induced by boat waves acting on riverbed is excessive the critical shear stress of soil which means bank erosion is occurring. The observation at two sites showed that vegetation can protect riverbank from erosion by attenuating wave energy. Based on the field sites, wave and vegetation characteristics, such a 10 m width for sparse vegetation or an 8 m width for dense vegetation is able to dissipate completely the wave energy and therefore prevent bank erosion due to wave attacks. In the third field investigation, the efficiency of vegetation on wave attenuation was elucidated. Two vegetation species were compared in aspect of wave attenuation capability, including Rhizophora apiculata and Nypa fruticans. The results indicated that wave heights reduced exponentially and about a half of initial incident wave height was reduced in the short distance when propagating through vegetation. Wave reduction is dependent on vegetation porosity, incident wave height, and still water depth. The effect of vegetation porosity, however, is more sensitive compared to the latter. Rhizophora apiculata is better than Nypa fruticans on wave attenuation even though its porosity is greater. It should be due to the special root structure of this mangrove species which acts like a natural net, interrupting and breaking up the wave train effectively. By taking ground slope effect into account, under steeper slope condition compared to previous studies, it is explainable why the wave reduction rate in present study is more impressive. Similar results were obtained in the study of Brinkman (2006) because of relatively close slope condition. The last part of this study discusses about applying artificial porous structure on riverbank protection. It rises from the fact that in some areas, especially in cities where land is limited or soil condition is not allowable, vegetation belt may not be planted and man-made structures are built instead. Experimental study was conducted in combination with 1-D model to establish the relationship between drag coefficient and porosity. The optimal θ to mitigate wave attacks was then determined by a 2-D model taking into consideration not only cross slope behind groins (i) but also the gap between two neighboring groins (G). In cases of i =1/100 and 1/500, the combination of θ = 0.4 and L/G = 5 (L is the stream-wise length of a groin) could bring out the best wave energy reduction. Thus, it is necessary to consider both of i and G for designing stream-wise groins.; ABSTRACT ........................................................................................................................ iACKNOWLEDGEMENT ............................................................................................... iiiCONTENTS ...................................................................................................................... ivLIST OF TABLES .......................................................................................................... xiiiLIST OF SYMBOLS ....................................................................................................... xvLIST OF ABBREVIATION .......................................................................................... xixCHAPTER 1 ....................................................................................................................... 1INTRODUCTION ............................................................................................................. 11.1. Riverbank erosion and suspected reasons ............................................................ 11.2. Literature review .................................................................................................... 81.2.1. Ship/boat-generated wave issue ....................................................................... 81.2.2. Previous studies to cope with wave attacks .................................................... 101.3. Purpose of the study ............................................................................................. 121.3.1. Aim and objectives .......................................................................................... 121.3.2. Outline of Thesis ............................................................................................. 13CHAPTER 2 ..................................................................................................................... 14EFFECT OF SHIP WAVE AND TIDE ON SEDIMENT TRANSPORTATION ..... 142.1. Overview ................................................................................................................ 142.2. Study area and field investigation ....................................................................... 142.3. Equipment deployment and measurement ......................................................... 162.3.1. Measuring point selection ............................................................................. 162.3.2. Experimental equipment and deployment .................................................. 172.3.3. Tide observation ............................................................................................ 202.3.4. Data analysis method ..................................................................................... 252.4. Results and discussions ........................................................................................ 252.4.1. Tidal effect ...................................................................................................... 252.4.1.1. Characteristics of the tide ......................................................................... 252.4.1.2. Time variation of Reynolds stress ............................................................. 272.4.1.3. The particle distribution and weight of sediment collected at the field .... 292.4.2. Ship wave effect.............................................................................................. 322.4.2.1. Time variation of velocity and water depth .............................................. 322.4.2.2. Time variation of Reynolds stress ............................................................. 342.4.2.3. The particle distribution and weight of sediment collected at the field .... 352.4.3. Comparison between tide and ship wave effects on sediment transport .. 36CHAPTER 3 ..................................................................................................................... 38THE EFFECT OF BOAT-GENERATED WAVE ATTACKS ON RIVERBANKSTABILITY ..................................................................................................................... 383.1. Overview ................................................................................................................ 383.2. Study area and field investigation ....................................................................... 383.3. Equipment deploys and cases .............................................................................. 423.3.1. Boat wave generation and measurement ....................................................... 423.3.2. Soil characteristics measurement ................................................................... 433.3.3. Cases ................................................................................................................ 443.4. Numerical simulation for assessing the occurrence of bank erosion ............... 453.4.1. Governing equations ....................................................................................... 453.4.2. Boundary conditions ....................................................................................... 473.4.2.1. Boundary conditions of the waterway ...................................................... 473.4.2.2. Boundary condition of the ship ................................................................. 483.4.3. Model validation by comparing with the field investigation ......................... 483.5. Results and discussion .......................................................................................... 503.5.1. Bank erosion under boat-generated wave attacks ......................................... 503.5.1.1. Waterway conditions in the researched area ........................................... 503.5.1.2. Current situation of riverbank stability .................................................... 533.5.2. Preliminary assessment about bank erosion mitigation by vegetation ......... 56CHAPTER 4 ..................................................................................................................... 59EFFICACY OF RHIZOPHORA APICULATA AND NYPA FRUTICANS ONATTENUATION OF BOAT-GENERATED WAVE UNDER STEEP SLOPECONDITION .................................................................................................................... 594.1. Overview ................................................................................................................ 594.2. Study area and vegetation characteristics .......................................................... 594.3. Methodology and cases ......................................................................................... 624.3.1. Measurement of boat-generated waves and analysis .................................... 624.3.2. Drag force measurements ................................................................................. 634.3.3. Cases ................................................................................................................ 654.3.4. Numerical simulation for determining the slope effect on wave attenuation................................................................................................................................... 664.4. Data collection and analysis ................................................................................. 664.4.1. Wave propagation through Rhizophora apiculata and Nypa fruticansforests ........................................................................................................................ 664.4.2. Effects of vegetation density, incident wave height, and still-water depth onwave attenuation ....................................................................................................... 684.4.3. Comparison of wave attenuation capability of Rhizophora apiculata andNypa fruticans .......................................................................................................... 704.4.4. Model validation and application ................................................................... 714.5. Discussions ............................................................................................................. 724.5.1. Variation of drag coefficient .......................................................................... 724.5.2. Wave attenuation characteristics ................................................................... 744.5.3. Application for vegetation design to deal with wave attacks ......................... 80CHAPTER 5 ..................................................................................................................... 81THE OPTIMAL POROSITY OF STREAM-WISE GROINS FOR WAVEATTENUATION UNDER DIFFERENT CROSS SLOPE CONDITIONS ............... 815.1. Overview ................................................................................................................ 815.2. Experimental setup and procedure ..................................................................... 825.2.1. Experimental apparatus ................................................................................. 825.2.2. Artificial porous structure .............................................................................. 835.2.3. Cases ................................................................................................................ 845.3. Numerical simulation ........................................................................................... 845.4. Results and discussions ........................................................................................ 865.4.1. Wave propagation through the porous structure .......................................... 865.4.1.1. Wave height reduction .............................................................................. 875.4.1.2. Velocity reduction ..................................................................................... 885.4.1.3. Effect of porosity parameter on wave attenuation .................................... 895.4.2. Validation of numerical simulation ............................................................... 905.4.3. Optimal design of porous structure for mitigating the wave attacks ............ 93CHAPTER 6 ..................................................................................................................... 97SUMMARY AND CONCLUSIONS .............................................................................. 97REFERENCES .............................................................................................................. 100; 主指導教員 : 田中規夫
• DOI: 10.24561/00010366; https://doi.org/10.24561/00010366
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• 受理日（W3CDTF）: 2016-04-01T14:32:40+09:00
• 作成日（W3CDTF）: 2015-12-16
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