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Studies on morphodynamics in shallow rivers with effects of vegetation and large wood using computational models
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| Title: | Studies on morphodynamics in shallow rivers with effects of vegetation and large wood using computational models |
| Other Titles: | 浅水流河川の河床変動に及ぼす植生および流木の影響の解明とその数値解析モデリング |
| Authors: | Kang, Taeun Browse this author |
| Issue Date: | 25-Sep-2018 |
| Publisher: | Hokkaido University |
| Abstract: | Many studies have been conducted to understand the effects of vegetation. However, the effects of vegetation (immobile vegetation type) in curved channels and the effects of large wood (L/D> 10, L: stem length, D: stem diameter; mobile vegetation type) in braiding channels are insufficiently studied. These vegetation types are important factors that can significantly alter bar formation, channel braiding and generation of thalweg in rivers. To expand our understanding of the interaction between bed morphology and both vegetation types, we investigated responses of bed morphology through a numerical model and a laboratory experiment. In the first part, the study on effects of immobile vegetation, simulations were conducted using a two-dimensional, depth-averaged river flow and river morphology model to investigate the effect of vegetation growth and degree of flow discharge on a shallow meandering channel. To consider the effects of these factors, it was assumed that vegetation growth stage is changed by water flow and bed erosion. The non-uniformity of the vegetation growth was induced by the non-uniform and unsteady profile of the water depth due to the irregular shape of the bed elevation and the unsteady flow model reliant on hydrographs to evaluate three types of peak discharges: moderate flow, annual average maximum flow, and extreme flow. To compare the effects of non-uniformly growing vegetation, the change in channel patterns was quantified using the Active Braiding Index (ABI), which indicates the average number of channels with flowing water at a given cross-section, and the Bed Relief Index (BRI), which quantifies the degree of irregularity of the cross-sectional shape. Two types of erosion were identified: local erosion (due to increased flow velocity near a vegetation area) and global erosion (due to the discharge approaching peak and the large depth of the channel). This paper demonstrates that the growth of vegetation increases both the ABI and BRI when peak discharge is lower than the annual average discharge, whereas the growth of vegetation reduces the BRI when peak discharge is extreme. However, under extreme discharge, the ABI decreases because global erosion is dominant. The conclusions from this study help to deepen the understanding of the interactions between curved river channels and vegetation. In the second part, the study on the effects of large wood deposition (mobile vegetation type), the large wood deposition patterns were analyzed in shallow flows, considering the effect of large wood root wad, by means of laboratory experiments and computer simulations. In this part, we first conducted a laboratory experiment to develop the large wood dynamic model. For the computations, we used the depth-averaged two-dimensional model Nays2DH on iRIC to simulate shallow flows. A newly developed large wood simulation model was combined with the shallow flow model. The laboratory tests were performed by changing several hydraulic parameters: flow discharge, channel slope, and anisotropic bed friction. In shallow water with a depth similar to the diameter of large wood, the root wad decreased the draft for wood motion (the depth at which large wood contacts the river bed) by lifting the head of large wood. The experimental results showed that the large wood tends to move toward the side walls and deposit on the bed after passing an obstacle. Computational results reasonably showed that the proposed coupling model reproduced the fundamental and physical aspects of the phenomena. Through the laboratory experiment, we attained reasonable reproducibility of the large wood dynamic model. Then we applied this model to practical experiments: Welber et al. (2013) and Bertoldi et al. (2014), which are based on observation data of the Tagliamento River, Italy. In this study, we modified Exner’s equation to consider the bed morphology with large wood deposition, and we improved the detailed motion of large wood due to transition of projection area and anisotropic bed friction considering three angles: stemwise direction, streamwise direction and wood particle pass direction. From the simulation results, we quantitatively calculated the ABI, BRI, and mean values of the deposition position and deposition angle. We then analyzed the relationship between the bed morphology responses and the wood deposition patterns in terms of the root wad effect and input supply. The proposed model reproduces the prominent features of the flume experiment, indicating that the present numerical approach can clarify and predict the behavior of large pieces of wood in accordance with the bed morphology. |
| Conffering University: | 北海道大学 |
| Degree Report Number: | 甲第13345号 |
| Degree Level: | 博士 |
| Degree Discipline: | 工学 |
| Examination Committee Members: | (主査) 教授 清水 康行, 教授 泉 典洋, 特任教授 今 日出人, 教授 木村 一郎 (富山大学大学院理工学研究部) |
| Degree Affiliation: | 工学院（環境フィールド工学専攻） |
| Type: | theses (doctoral) |
| URI: | http://hdl.handle.net/2115/71820 |
| Appears in Collections: | 課程博士 (Doctorate by way of Advanced Course) > 工学院(Graduate School of Engineering)
学位論文 (Theses) > 博士 （工学）
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