• Title/Summary/Keyword: braided channel

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Numerical Experiments of Vegetation Growth Effects on Bed Change Patterns (식생생장 영향을 고려한 하도변화에 대한 수치모의)

  • Kim, Hyung Suk;Park, Moon Hyeong;Woo, Hyo Seop
    • Ecology and Resilient Infrastructure
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    • v.1 no.2
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    • pp.68-81
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    • 2014
  • In this study, the numerical simulation regarding the process and characteristics of topography change due to the vegetation recruitment and growth was carried out by adding the vegetation growth model to two-dimensional flow and sediment transport models. The vegetation introduction and recruitment on the condition for developing an alternate bar reduced the bar migration. The vegetated area and channel width changes were more significantly influenced by changes in upstream discharge rather than the duration of low flow. When the upstream discharge decreased, the vegetation area increased and the channel width decreased. The vegetation introduction and recruitment on the condition for developing a braided channel significantly influenced the characteristics of topography changes. In the braided channel, vegetation reduced the braided index, and when the upstream discharge decreased significantly, the channel topography was changed from the braided channel to the single channel. The vegetation area decreased as the upstream discharge increased. The channel width decreased significantly after the vegetation was introduced and it also decreased as the upstream discharge decreased. It was confirmed through the numerical simulation that a decrease in flood discharge accelerated the vegetation introduction and recruitment in the channel and this allowed to confirm its influence on the characteristics of topography changes qualitatively.

Lake-like Swamps Formed in Floodplain on Middle Reach of Nakdong-gang River (범람원에 형성된 호소성 습지에 관한 연구 - 낙동강 중류를 사례로 -)

  • Cheon, Mi-Yeon
    • Journal of the Korean association of regional geographers
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    • v.14 no.1
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    • pp.27-39
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    • 2008
  • The propose of this research paper is to clarify the formative process of the lake-like swamps formed in channels of Nakdong-gang river and its tributary, Gumho-gang river. In order to understand the formative process of lake-like swamps, I compared topographical maps of three years(1918, 1962, 2005), analyzed the outline change of swamps, and draw their cross-sectional view and analyzed the horizontal distribution of particle size. Results of analysis are as follow. First, Saegang swamp and Geumgang swamps are residual swamps which are formed in abandoned distributary channels of braided reach. In braided reach, tributaries wind freely to create a new channel or to eliminate a old channel. Second, the Ssiksil swamp and Nat swamp are yazoo-type swamps. Their stream channels run parallel to the natural levees, as it blocks the waterway of a effluent tributary flowing from backward mountains to main stream. Third, the Secheon, Hochon and Jinchon swamps are backswamp behind natural levee. As main stream overflows, natural levee and back swamp are formed. Later flowing water of next bigger flood enter through natural levee into backswamp, create a channel following backswamp.

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A Study on the Landforms Near of Mooseom Village, Naeseongcheon (내성천 무섬마을 인근의 하천 지형 특성에 대한 연구)

  • Kim, Jong Yeon;Shin, Won Jeong
    • Journal of The Geomorphological Association of Korea
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    • v.26 no.3
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    • pp.1-17
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    • 2019
  • Naeseongcheon is Korea's representative sand stream, and it is one of the regions where the dynamic changes of various river topography developed in the sand bed can be observed. Most of drainage area near of the river channel are formed with Daebo granite, and the granite weathering zone is developed at the surface of hill. Due to the massive input of sediment flux, braided channel reaches are found some of the area. However, the results of the study shows that the alluvial layer is very thin in some reaches. In addition, bedrock or weathered materials, including the Tors are exposed at the channel beds. On the other hand, during the flood, a considerable amount of sediment was introduced, causing the massive sediment to be close to 1m thick. In addition, despite the short distance, large changes in the particle size and sorting of the sediment were observed. Vegetation, on the other hand, has been shown to have a significant effect on the development of the overall channel bed topography, as reported in previous studies. In small floods or low water levels, vegetation's protection role of the surface is predominates, but in large flood conditions, herbaceous loss at the surface of the point bars, accelerating the erosion of surface.

Development of Dry/Wet Algorithm for 2-Dimensional Flow Analysis (2차원 흐름해석을 위한 마름/젖음 알고리듬의 개발)

  • Kim, Sang-Ho;Han, Kun-Yeun;Choi, Seung-Yong;Oh, Hyun-Uk
    • Proceedings of the Korea Water Resources Association Conference
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    • 2008.05a
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    • pp.624-628
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    • 2008
  • Two-dimensional flow analysis is a way to provide good estimates for complex flow features in flow around islands and obstructions, flow at confluence and flow in braided channel. One of difficult problems to develop a two-dimensional hydraulic model is to analyze dry and wet area in river channel. Dry/wet problem can be encountered in river and coastal engineering problems, such as flood propagation, dam break analysis, tidal processes and so on. The objective of this study is to develop an accurate and robust two-dimensional finite element method with dry/wet technique in complex natural rivers. The dry/wet technique with Deforming Grid Method was developed in this study. The Deforming Grid Method was used to construct new mesh by eliminating of dry nodes and elements. The eliminated nodes and elements were decided by considering of the rising/descending velocity of water surface elevation. Several numerical simulations were carried out to examine the performance of the Deforming Grid Method for the purpose of validation and verification of the model in rectangular and trapezoidal channel with partly dry side. The application results of the model were displayed reasonable flow distribution.

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STRATIGRAPHIC ARCHITECTURE OF FLUVIAL SEQUENCES IN THE NORTHWESTERN PART OF KYONGSANG BASIN

  • Jo H. R.;Chough S. K.
    • 한국석유지질학회:학술대회논문집
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    • autumn
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    • pp.47-56
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    • 2000
  • The northwestern part of Kyongsang Basin largely comprises interbedded sandstone and mudstone with local conglomeratic deposits in the basin margin, representing marginal alluvial fans and fluvial depositional environments. The non-marine successions are divided into successive stratigraphic units, each of which is unique in facies assemblages and architecture of sandstone bodies. Two stratigraphic units, i.e., Sinpyong-Anpyong and Jotap units are examined in terms of stratigraphic architecture and its causative processes. Detailed architectural analysis reveals that the channel systems of Sinpyong-Anyong unit were of braided patterns, whereas those of Jotap unit were dominated by small-scale bedforms. The difference in fluvial styles can be attributed to the changes in amount and caliber of sediment load and water discharge, which might have been ultimately governed by basin tectonics, climate, and base level. Along with the marked change in fluvial style, the two successive units show repeated expansions of distal, water-logged floodplains and lacustrine facies in the basal and uppermost parts of Sinpyong-Anpyong unit, where the proportion of channel sandstone bodies is relatively low. These stratigraphic intervals are succeeded by the sequences with proximal, well-drained floodplain facies and relatively coarser-grained channel sandstone bodies of higher proportion, reflecting the progradation of proximal systems (the middle part of Sinpyong-Anpyong unit and Jotap unit). The overall stratigraphic architecture can be ascribed to the fluctuations in accommodation space and sediment supply induced by repeated basin subsidence.

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Lateral Migration Features of the Alluvial Channels in Hapyeong Intake Station, Nakdong River (해평취수장 부근에서 충적하천의 저수로 이동 특성)

  • Jang, Chang-Lae;Lee, Gang-Man;Kim, Gye-Hyun
    • Journal of Korea Water Resources Association
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    • v.41 no.4
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    • pp.395-404
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    • 2008
  • It is important to understand the complex, various migration features of the alluvial channels for river engineering. In this study, the morphological changes and migration features of alluvial channels were investigated by analyzing the aerial photographs of active channels between 1972 and 2004 in Hapyeong Intake Station, Nakdong river. The lower channels were migrated from left bank to right bank and showed the features of braided channel in 2004. The instability of lower channels was increased due to the increased channel slope and width. The sinuosity of lower channels was decreased with time. As time increased, the increasing rate of lower channel and lateral migration rate were decreased. As a result of meso-scale regime analysis by using bankfull discharge, multiple row bars were developed, and suspended sediment load was governed in the flow as a sand bed channel.

Study on Types and Distributional Properties of Abandoned Channels in Korea (우리나라의 구하도 유형과 분포 특성)

  • Lee, Gwang-Ryul
    • Journal of the Korean Geographical Society
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    • v.46 no.3
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    • pp.304-318
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    • 2011
  • The types, distributional and shape properties of abandoned channels in Korea are classified and examined. There are 409 abandoned channels in Korea and they can be classified into the 6 types such as the incised meander cutoff, stream piracy, distributary, artificial incised meander neck-cutoff, channel straightening of free meander and distributary streams. The abandoned channels by incised meander neck-cutoff showing the most frequency with 266, have the topographical properties of high altitude and steep relief and frequently distribute in Gangwon and Gyeonbuk Province and in the upper reaches of main stream of Nakdong-River, and Dal-River and Pyeongchang-River, tributaries of Namhan-River. The most frequency areas of abandoned channels by distributary, channel straightening of free meander and distributary streams are the lower reaches of Namhan-River and main stream in the lower reaches of Nakdong-River, Mangyeong-River and Yeongsan-River, and Geumho-River, respectively. The abandoned channles by incised meandering neck-cutoff and stream piracy are relatively used as farmlands or forests due to the high altitude and relative altitude from the river bed.

Structure and Physical Property of the Crust of Mid-west Korea: Analysis of Sedimentary Basins in the Namyang and Tando Areas, Kyeonggi Province, Korea (한반도 중서부 지각구조와 물성 연구: 경기도 화성군 남양 및 안산시 탄도지역에 분포하는 퇴적분지의 분석)

  • Park, Sung-Dae;Chung, Gong-Soo;Jeong, Ji-Gon;Kim, Won-Sa;Lee, Dong-Woo;Song, Moo-Young
    • Journal of the Korean earth science society
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    • v.21 no.5
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    • pp.563-582
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    • 2000
  • Two Cretaceous(80-90 Ma) non-marine sedimentary basins, Namyang and Tando Basins, are distributed in the Namyang area, Hwaseonggun and in the Tando area, Ansanshi, Kyungki Province, Korea. The Namyang and Tando Basins are composed of 10 facies, which are pooped into 5 facies associations(FA). FA I consists of massive conglomerate facies, normally graded conglomerate facies and reversely graded conglomerate facies, which is interpreted to have been formed by laminated sandstone facies, massive conglomerate facies(channelized), which is thought to have been formed by sheet flow, stream flow and suspension sedimentation in an alluvial/braided plain environment. FA III consists of massive mudstone(pebbly) facies, laminated mudstone facies, massive sandstone facies and is interbedded by channel-fill conglomerate. It is interpreted to have been deposited by suspension settling during flooding and channel-fill deposition in a floodplain environment. FA IV consists of massive conglomerate facies, normally graded conglomerate facies, massive sandstone facies, normally graded sandstone facies, and laminated sandstone facies and is interbedded with mudstone facies. It is thought to have been deposited by debris flow and turbidity current in a fan-delta environment. FA V consists of massive mudstone facies, laminated mudstone facies, laminated sandstone facies and is interbedded by massive conglomerate bed. It is thought to have been formed by suspension sedimentation and low-density turbidity current in a lake. In the Namyang Basin FA I is distributed in the eastern and southern margin of the basin, FA II in the middle part of the basin as north-south tending band. and FA III in the western part. In the Tando Basin FA II is distributed in the middle part of eastern margin and in the northwestern margin, FA IV in the southwestern part, and FA V in the central part. Correlation of the facies associations shows that FA I and II in the Namyang Basin are distributed in the lower to middle part of stratigraphic sequence and FA III in the upper part of the sequence whereas FA II and IV in the Tando Basin are in the lower to middle part and FA V in the upper part of the sequence. These patterns of facies associations distribution suggest that the Namyang Basin was developed as an alluvial fan and alluvial/braided plain at first and then evolved into a floodplain whereas the Tando Basin was developed as a fan-delta and alluvial/braided plain at first and then evolved into a lake environment.

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Numerical simulation of channel changes of braided river with sediment supply changes (유입유사량 변화에 의한 망상하천의 지형변화 수치모의 분석)

  • Jang, Chang-Rae
    • Proceedings of the Korea Water Resources Association Conference
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    • 2016.05a
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    • pp.51-55
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    • 2016
  • 유입유사량의 변화에 의한 하도의 지형변화를 파악하는 것은 하천을 복원하거나 유지하는데 중요하다. 본 연구에는 2차원 수치모형을 이용하여 상류에서 유입되는 유사량의 변화에 의하여 망상하천의 저수로 변화와 유사의 분급 특성을 분석하였다. 망상하천의 변화를 파악하기 위하여 하폭은 2.8 m이고, 수로 길이는 50 m로 설정하였다. 하상경사는 0.0125, 유량은 $0.014m^3/sec$로 설정하였다. 상류에서 공급되는 유사량은 평형상태의 유사량에 대하여 50%, 70%, 90%, 100%로 감소시켰다. 상류에서 공급되는 유사량이 감소할수록, 하상고는 감소하고 하상토 입경은 증가하였다. 또한 하도 단면에 대한 저수로의 수를 나타내는 망상하천 지수(Braiding Index)는 감소하였다. 이것은 하도의 저수로 수가 감소하는 것을 나타낸다. 유입 유사량이 감소할수록, 하도의 기복도가 감소하며, 이것은 하도의 역동성이 감소하는 것을 의미한다.

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Effects of Reduced Sediment Dynamics on Fluvial Channel Geomorphology in the Jiseok River (유사계의 역동성 감소가 지석천 충적하도의 지형변화에 미치는 영향)

  • Ock, Gi-Young;Lee, Sam-Hee
    • Journal of Korea Water Resources Association
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    • v.45 no.5
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    • pp.445-454
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    • 2012
  • The present study aims to investigate the long-term channel morphological changes derived from channelization, embankment and levee construction works in unregulated fluvial channel of the Jiseock River. Analyses of aerial photographs taken past (Year 1966) and recent (Year 2002) showed the temporally remarkable changes in channel planform such as channel shape, bar migration, vegetation encroachment in bar. During the period, the natural single threading changed into braided types together with decreasing sinuosity by 9.2%, increasing vegetation occupied bar ranged 97% of total bars area. Because such channel morphological changes are closely similar to those in dam downstream channels, we assume that both/either flow regime alteration and/or sediment transport discontinuity may be critical for the fixed channel and spread of vegetated bars even in unregulated river without dam reservoir upstream. We found more reduced frequency and magnitude of flooding water level comparing with past, but no significant alteration of inter annual water level variation. Bed material has been coarsened by 4~5 times and the riverbed has been degraded in overall channel but aggraded locally in conjunction reach of tributaries. The results indicates that reduced sediment dynamics in fluvial channel which derived by bed material coarsening, river bed degradation and unbalanced sediment transport capacity between tributary and mainstem can be a causal factor to trigger channel morphological changes even in unregulated rivers.