• Title/Summary/Keyword: Bed Sediment Transport

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A Case Study of Sediment Transport on Trenched Backfill Granular and Cohesive Material due to Wave and Current

  • Choi, Byoung-Yeol;Lee, Sang-Gil;Kim, Jin-Kwang;Oh, Jin-Soo
    • Journal of Advanced Research in Ocean Engineering
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    • v.2 no.2
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    • pp.86-98
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    • 2016
  • In this study, after the installation of a subsea pipeline, backfilling was performed in the trenched area. During these operations, a stability problem in the subsea pipeline occurred. The pipeline was directly impacted by environmental loading such as waves and currents that were caused by backfill material when scouring or sediment transport and siltation was carried out. Therefore, this study reviewed whether trenching was necessary, and conducted research into an indigenous seabed property that contains granular soil. A study of cohesive soil was also conducted in order to cross-correlate after calculating the values of the critical Shields parameter relevant to elements of the external environment such as waves and current, and the shear Shields parameter that depends on the actual shearing stress. In case of 1), sedimentation or erosion does not occur. In the case of 2), partial sedimentation or erosion occurs. If the case is 3), full sedimentation or erosion occurs. Therefore, in the cases of 1) or 2), problems in structural subsea pipeline stability will not occur even if partial sedimentation or erosion occurs. This should be reflected particularly in cases with granular and cohesive soil when a reduction in shear strength occurs by cyclic currents and waves. In addition, since backfilling material does not affect the original seabed shear strength, a set-up factor should be considered to use a reduced of the shear strength in the original seabed.

Analysis of Flow and Bed Changes by Hydraulic Structure using CCHE2D: Focusing on Gangjeong-Goryeong Weir (수리구조물에 의한 흐름 및 하상변동 연구- 강정고령보를 중심으로 -)

  • Ahn, Jung-Min;Jung, Kang-Young;Shin, Dongseok;Lyu, Siwan
    • Journal of Wetlands Research
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    • v.19 no.2
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    • pp.181-192
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    • 2017
  • Analysis using a numerical model is important to understand the sediment transport mechanism associated with erosion and sedimentation near weirs and other hydraulic structures within riverine systems. The local riverbed change near a hydraulic structure (Gangjeong-Goryong multi-function weir in the Nakdong river) was analyzed in order to examine the effect of hydraulic structures on local bed change. A 2D numerical model (CCHE-2D) was employed to simulate the sedimentation and erosion over a reach (25 km) including the weir. For the calibration and verification of the model, rainfall data from a real event (Typoon 'Ewiniar' in 2006) were used for flow and stage simulation. And the simulated results show a good agreement with the observed data for the whole domain. From the result, it was found that the installation and operation of the weir could aggravate bed changes by typhoon between movable weirs, and which resulted in redistribution of sediment.

On the Sediment Transport Characteristics of the Bottom Turbulent Boundary Layer (저면난류경계층(底面亂流境界層)의 저질이동특성(底質移動特性))

  • Kim, Nam Hyeong;Kiyoshi, Takikawa
    • KSCE Journal of Civil and Environmental Engineering Research
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    • v.13 no.2
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    • pp.267-277
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    • 1993
  • A finite element method(FEM) is presented and applied to the two-dimensional bottom turbulent boundary layer. The time-dependent incompressible motion of a viscous fluid is formulated by using the well-known Navier-Stokes equations and vorticity equation in terms of the velocity and pressure fields. The general numerical formulation is based on Galerkin method and solved by introducing the mixing length theory of Prandtl for eddy kinematic viscosity of a turbulent flow field. Numerical computations of the transport of sediment on an arbitrary sea-bed due to wave motion in the turbulent boundary layer are carried out. The results obtained by the FEM made clear the difference in characteristic features between the boundary layer due to oscillatory flow and the boundary layer due to wave motion.

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Nearshore Sediment Transport in Vicinity of Anmok Harbor, East Coast of Korea. (동해 안목항 주변 연안 토사이동)

  • 김인호;이정렬
    • Journal of Korean Society of Coastal and Ocean Engineers
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    • v.16 no.2
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    • pp.108-119
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    • 2004
  • The breakwater extension at Anmok Harbor has resulted in erosional stresses along the wide range of shorelines immediately south of the harbor. In this study, therefore, the downdrift affects caused by the breakwater extension are investigated through both analytical and numerical approaches. In addition, this study stresses the need of monitoring and analysis system for the effective integrated coastal zone management and shows through the case study of Anmok Harbor how the numerical experiments are accomplished for the coastal zone management. The numerical model system, which predicts the seabed changes obtained from the difference between the rates of sediment pickup and settling due to gravity, is combined with the wave, wave-induced currents, and suspended sediment transport models. A new relationship between the near-bed concentration and the depth-mean concentration, which is required in estimating the settling rates. is presented by analyzing the vertical structure of concentration.

Characteristics of Sediment Transport due to the Construction of Jetty at the Heoya-River Mouth, Ulsan (울산 회야강 하구 도류제 건설에 따른 표사이동 특성)

  • Lee, Seong-Dae;Kim, Bong-Ik
    • Journal of Wetlands Research
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    • v.16 no.4
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    • pp.477-486
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    • 2014
  • Integrated sand control including sediment discharge from hinterland rivers is necessary to maintain coastal sand resources over a long term. In this regard, the following subjects should be considered; efficient ways to transfer discharged sand from a river to the neighboring coast, measures to improve storage efficiency of the discharged sand at the river delta and/or river terrace, measures to prevent the sand resources from being discharged into the deep sea during flooding. From the 1997 to January 2004, the jetty of 156 m length was constructed the Heoya-river mouth to protect the blockade of river mouth. Several tests were carried out to investigate the characteristics of sediment transport and morphological change due to the construction of the jetty at the Heoya-river mouth. Firstly, The sand discharge from Heoya river is quantified by one-dimensional numerical analysis assuming the mixed sand of three different particle diameters. Also the numerical mode system, which predicts the sea bed changes obtained from the Bailard's energy model(1981), was combined with the wave, wave-induced currents and sediment transport models. Then, to understand the changes to the blockade of the river mouth, several aerial photographs were compared, which showed that the changes were significant.

CHARACTERIZATION OF POOL-RIFFLE SEQUENCES IN SOLUTE TRANSPORT MODELING OF STREAMS

  • Seo, Il-Won;Yu, Dae-young
    • Water Engineering Research
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    • v.1 no.3
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    • pp.171-185
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    • 2000
  • A mathematical model to adequately predict complex mixing characteristics of sorptive polluants in natural streams with pools-and-riffes has been developed. In this model, sorption of pollutants onto the bed sediment as well as mass storage and exchange in the storage zones were incorporated into one-dimensional mass balance equatins. The geometric and hydraulic characteristics of the pool-riffle sequences were properly conceptualized. Simulations with parameters of pool-and-riffle streams better fit the measured data in overall shape and peak concentration than simulations with parameters for uniform channels. The analyses on the characteristics of the storage zone model parameters reveal that a linear relationship between the logrithm of the storage zone volume ratio and a function of the friction factor exists. A linear relatiohship might also be tenatively assumed between the logarithm of the dimensionless mass exchange coefficient and the logarithm of the aspect ratio of the storage zone if some of the high values of the dimensionless mass exchange coefficient collected on the successive bed forms are excluded.

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Preliminary Study on the Development of a Platform for the Optimization of Beach Stabilization Measures against Beach Erosion II - Centering on the Development of Physics-Based Morphology Model for the Estimation of an Erosion Rate of Nourished Beach (해역별 최적 해빈 안정화 공법 선정 Platform 개발을 위한 기초연구 II - 양빈 된 해빈 침식률 산정을 위한 물리기반 해빈 지형모형 개발을 중심으로)

  • Cho, Yong Jun
    • Journal of Korean Society of Coastal and Ocean Engineers
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    • v.31 no.5
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    • pp.320-333
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    • 2019
  • In this study, a physics-based 3D morphology model for the estimation of an erosion rate of nourished beach is newly proposed. As a hydrodynamic module, IHFOAM toolbox having its roots on the OpenFoam is used. On the other hand, the morphology model comprised a transport equation for suspended sediment, and Exner type equation derived from the viewpoint of sediment budget with the bed load being taken to accounted. In doing so, the incipient motion of sediment is determined based on the Shields Diagram, while the bottom suspended sediment concentration, the bed load transport rate is figured out using the bottom shearing stress directly calculated from the numerically simulated flow field rather than the conventional quadratic law and frictional coefficient. In order to verify the proposed morphology model, we numerically simulate the nonlinear shoaling, breaking over the uniform beach of 1/m slope, and its ensuing morphology change. Numerical results show that the partially skewed, and asymmetric bottom shearing stresses can be successfully simulated. It was shown that sediments suspended and eroded at the foreshore by wave breaking are gradually drifted toward a shore and accumulated in the process of up-rush, which eventually leads to the formation of swash bar. It is also worth mentioning that the breaker bar formed by the sediments dragged by the back-wash flow which commences at the pinnacle of up-rush as the back-wash flow gets weakened due to the increased depth was successfully duplicated in the numerical simulation.

A Quasi-Steady Model for Sedimentation and Flushing of Reservoirs (저수지 퇴배사 모의를 위한 준정류모형)

  • Choi, Sung-Uk;Choi, Seong-Wook
    • Journal of Korea Water Resources Association
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    • v.45 no.2
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    • pp.217-227
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    • 2012
  • This paper presents a quasi-steady model for numerical simulations of reservoir sedimentation and reservoir flushing. The quasi-steady model is based on the assumption that the flow is steady with time-dependent stream morphology change. This is reasonable because stream morphology changes over a long period, while the flow changes rapidly. The proposed model is first applied to two laboratory experiments for reservoir sedimentation. The channel is shown to be adjusted to new sediment supply at the upstream by changing both the flow depth and slope. Simulated water surface and bed profiles compare favorably to measured data. The model is also applied to reservoir flushing. Good agreement between simulated and measured data is not obtained due to time variation of outflow generated to facilitate the flushing in the experiment. Finally, relationships for equilibrium flow depth and bed slope are proposed and tested through numerical experiments.

Potential of River Bottom and Bank Erosion for River Restoration after Dam Slit in the Mountain Stream

  • Kang, Ji-Hyun;So, Kazama
    • Proceedings of the Korea Water Resources Association Conference
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    • 2011.05a
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    • pp.46-46
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    • 2011
  • Severe sediment erosion during floods occur disaster and economic losses, but general sediment erosion is basic mechanism to move sediment from upstream to downstream river. In addition, it is important process to change river form. Check dam, which is constructed in mountain stream, play a vital role such as control of sudden debris flow, but it has negative aspects to river ecosystem. Now a day, check dam of open type is an alternative plan to recover river biological diversity and ecosystem through sediment transport while maintaining the function of disaster control. The purpose of this paper is to verify sediment erosion progress of river bottom and bank as first step for river restoration after dam slit by cross-sectional shear stress and critical shear stress. Study area is upstream reach of slit check dam in mountain stream, named Wasada, in Japan. The check dam was slit with two passages in August, 2010. The transects were surveyed for four upstream cross-sections, 7.4 m, 34 m, 86 m, and 150 m distance from dam in October 2010. Sediment size was surveyed at river bottom and bank. Sediment of cobble size was found at the wetted bottom, and small size particles of sand to medium gravel composed river bank. Discharge was $2.5\;m^3/s$ and bottom slope was 0.027 m/m. Excess shear stress (${\tau}_{ex}$) was calculated for hydraulic erosion by subtracting the values of critical shear stress (${\tau}_{c}$) from the value of shear stress (${\tau}$) at river bottom and bank (${\tau}_{ex}=\tau-{\tau}_c$). Shear stress of river bottom (${\tau}_{bottom}$) was calculated using the cross-sectional shear stress, and bank shear stress (${\tau}_{bank}$) was calculated from the method of Flintham and Carling (1988). $${\tau}_{bank}={\tau}^*SF_{bank}((B+P_{bed})/(2^*P_{bank}))$$ where $SF_{bank}=1.77(P_{bed}/p_{bank}+1.5)^{-1.4}$, B is the water surface width, $P_{bed}$ and $P_{bank}$ are wetted parameter of the bed and bank. Estimated values for ${\tau}_{bottom}$ for a flow of $2.5\;m^3/s$ were lower as 25.0 (7.5 m cross-section), 25.7 (34 m), 21.3 (86 m) and 19.8 (150 m), in N/$m^2$, than critical shear stress (${\tau}_c=62.1\;N/m^2$) with cobble of 64 mm. The values were insufficient to erode cobble sediment. In contrast, even if the values of ${\tau}_{bank}$ were lower than the values for ${\tau}_{bottom}$ as 18.7 (7.5 m), 19.3 (34 m), 16.1 (86 m) and 14.7 (150 m), in N/$m^2$, excess shear stresses were calculated at the three cross-sections of 7.5 m, 34 m, and 86 m distances compare with ${\tau}_c$ is 15.5 N/$m^2$ of 16mm gravel. Bank shear stresses were sufficient for erosion of the medium gravel to sand. Therefore there is potential to erode lateral bank than downward erosion in a flow of $2.5\;m^3/s$. Undercutting of the wetted bank can causes bank scour or collapse, therefore this channel has potential to become wider at the same time. This research is about a potential of sediment erosion, and the result could not verify with real data. Therefore it need next step for verification. In addition an erosion mechanism for river restoration is not simple because discharge distribution is variable by snow-melting or rainy season, and a function for disaster control will recover by big precipitation event. Therefore it needs to consider the relationship between continuous discharge change and sediment erosion.

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Analysis of the Effect of Dredging and Weirs on Bed Change in the Nakdong River and its Tributary using HEC-6 (HEC-6를 이용한 준설 및 보로 인한 낙동강 본류 및 지류 하상변화 분석)

  • Ahn, Jung Min;Kwak, Sunghyun;Lyu, Siwan
    • Journal of Korea Water Resources Association
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    • v.48 no.9
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    • pp.743-756
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    • 2015
  • It is necessary to evaluate the effect of dredging and weir operation on the flow and long-term bed change for river management. Especially, large scale river treatment project, with dredging or weir installation and operation, can increase the instability of riverbed in tributaries as well as mainstream. This study focuses on the effect of weir installation and dredging on the long-term bed change in Nakdong river (Gangjeong- Goryeong Weir~Dalseong Weir) and its tributary (Geumho river). HEC-6 model has been used to analyze the amount of long-term bed change and sediment transport resulted from the river treatment including dredging or weir installation. From the result, it was concluded that a large scale river treatment can accelerate and increase the long-term bed change both in mainstream and tributary.