• Journal of Korea Water Resources Association
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• v.49 no.10
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• pp.887-901
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• 2016

Cohesive sediments form flocs through the flocculation process. The size and density of floc are variable whereas those of a fine sediment are always assumed to be constant. The settling velocity, one of main factors of sediment transport, is determined by size and density of particle. Therefore, the flocculation process plays an important role in transport of cohesive sediment. It is of great difficulty to directly measure the density of floc in the field due to technical limitation at present. It is a popular approach to estimate the density of floc by applying the fractal theory. The main assumption of fractal theory is the self-similarity. This study aims to examine the applicability of fractal theory to cohesive sediment in small rivers of Korea. Sampling sediment has been conducted in two different basins of Geum river and Yeongsan river. The results of settling experiments using commercial camera show that the sediment in Geum river basin follows the main concept of fractal theory whereas the sediment in Yeongsan river basin does not have a clear relationship between floc size and fractal dimension. It is known from this finding that the fractal theory is not easily applicable under the condition that the cohesive sediment includes the high content of organic matter.

• Ocean Systems Engineering
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• v.13 no.3
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• pp.313-324
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• 2023

The vertical distribution of suspended sediments in the mangrove-mud coast is complicated due to the characterization of cohesive sediment properties, and the influence of hydrodynamic factors. In this study, the time-evolution of suspended sediment concentration (SSC) in water depth is simulated by a one-dimensional model. The model applies in-situ data measured in October 2014 at the outer station in Cu Lao Dung coastal areas, Soc Trang, Vietnam. In the model, parameters which have influence on vertical distribution of SSC include the settling velocity Ws and the diffusion coefficient Kz. The settling velocity depends on the cohesive sediment properties, and the diffusion coefficient depends on the wave-current dynamics. The settling velocity is determined by the settling column experiment in the laboratory, which is a constant of 1.8 × 10^-4 ms^-1. Two hydrodynamic conditions are simulated including a strong current condition and a strong wave condition. Both simulations show that the SSC near the bottom is much higher than ones at the surface due to higher turbulence at the bottom. At the bottom layer, the SSC is strongly influenced by the current.

• 한국해양학회지
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• v.22 no.1
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• pp.25-33
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• 1987

The transport and fate of fine-grained, cohesive sediments in an estuary were investigated numerically. A numerical model of sediment entrainment, deposition, and transport has been developed by incorporating recent results of laboratory and field investigations. The time-dependent flow fields produced by fiver inflow and semi-diurnal tides, were calculated, and the corresponding distributions of suspended-sediment concentrations were obtained. The time-changes of sediment bed condition due to entrainment and deposition were obtained. The entrained sediments contribute initially to high sediment concentrations in the estuary basin. As the time passes, the suspended-sediment concentrations were much reduced by the seaward transport due to residual currents. The erosional and dipositional areas were appeared to be strongly dependent on the current-velocity fields and sediment properties of the estuary.

• Journal of Korea Water Resources Association
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• v.47 no.8
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• pp.703-715
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• 2014

This study aims to investigating the effect of Schmidt number (${\sigma}_c$) on sediment suspension and hydrodynamics calculation. The range of ${\sigma}_c$ is also studied based on the flux Richardson number ($Ri_f$) and gradient Richardson number ($Ri_g$). Numerical experiments are carried out by 1 dimensional vertical model. Both cohesive and non-cohesive sediments are tested under the conditions of pure current and oscillatory flow. The turbulence damping effect due to sediment suspension is examined considering ${\sigma}_c$ as a constant for the damping effect. The results of this study show the consistent effect of ${\sigma}_c$ on sediment suspension regardless of hydrodynamic condition. It is also found that the model overestimates the flow velocity and turbulent kinetic energy when the damping effect is not considered. Under the conditions of $Ri_f$ and $Ri_g$ causing density stratification, it is known that the vertical mixing of sediment is reasonably calculated in the range of ${\sigma}_c$ from 0.3 to 0.5.

• Fisheries and Aquatic Sciences
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• v.13 no.2
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• pp.167-181
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• 2010

To predict changes in the marine environment of the Beolgyo Stream Estuary in Jeonnam Province, South Korea, where cohesive tidal flats cover a broad area and a large bridge is under construction, this study conducted numerical simulations involving tidal flow and cohesive sediment transport. A wetting and drying (WAD) technique for tidal flats from the Princeton Ocean Model (POM) was applied to a large-scale-grid hydrodynamic module capable of evaluating the flow resistance of structures. Derivation of the eddy viscosity coefficient for wakes created by structures was accomplished through the explicit use of shear velocity and Chezy's average velocity. Furthermore, various field observations, including of tide, tidal flow, suspended sediment concentrations, bottom sediments, and water depth, were performed to verify the model and obtain input data for it. In particular, geologic parameters related to the evaluation of settling velocity and critical shear stresses for erosion and deposition were observed, and numerical tests for the representation of suspended sediment concentrations were performed to determine proper values for the empirical coefficients in the sediment transport module. According to the simulation results, the velocity variation was particularly prominent around the piers in the tidal channel. Erosion occurred mainly along the tidal channels near the piers, where bridge structures reduced the flow cross section, creating strong flow. In contrast, in the rear area of the structure, where the flow was relatively weak due to the formation of eddies, deposition and moderated erosion were predicted. In estuaries and coastal waters, changes in the flow environment caused by artificial structures can produce changes in the sedimentary environment, which in turn can affect the local marine ecosystem. The numerical model proposed in this study will enable systematic prediction of changes to flow and sedimentary environments caused by the construction of artificial structures.

• The Sea:JOURNAL OF THE KOREAN SOCIETY OF OCEANOGRAPHY
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• v.18 no.3
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• pp.122-130
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• 2013

To examine floc characteristics of cohesive bed sediment of the Yeongsan River estuary, a floc camera system has been developed and utilized to observe flocs under varying conditions. In order to validate the floc camera system, sand particles were passed through 88-125 and $63-88{\mu}m$ sieves and observed within the laboratory. Mean grain size and settling velocities were found to be 102 and $56.2{\mu}m$ and 6.7 and 5.9 mm/s, respectively. Artifacts of particles estimated outside of the sieve range are attributed to being imaged out of the depth of focus. However, as mean grain size and settling velocity of each size class were within the confidence interval, the floc camera system was confidently used to examine cohesive bed sediments of Yeongsan River estuary. The bed sediment sample was prepared with a concentration of 0.1 g/L in 0 psu deionized water. The mean grain size, settling velocity and fractal dimension of flocs were $40.6{\pm}0.66{\mu}m$, 14 mm/s, and 2.86, respectively. Experiments were also conducted using different salinities (10 and 34 psu) and sediment concentrations (0.1 and 0.3 g/L). Despite changing these parameters, the mean observed grain size and settling velocities were found to be the same within the error range of the system. The relatively higher values of settling velocity and fractal dimension are considered a result of the sediment containing relatively small concentrations of organic matter. Moreover, consistent floc size over various grain sizes and concentrations may be the result of insufficient turbulence to aggregate flocs.

• Proceedings of the Korean Society of Coastal and Ocean Engineers Conference
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• 1996.10a
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• pp.102-108
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• 1996

연안매립 및 신항만 건설, 해안선 정비사업 등과 같은 대규모 공사는 인근 해역에서의 해수유동의 변화와 해수의 혼탁도 증가 및 퇴적물의 침식과 퇴적에 의한 해저 지형의 변형을 초래하고, 이로 인해 기존 항로의 매몰과 해양생물 생태계 변화 등과 같은 문제점을 유발한다는 것은 잘 알려진 사실이다. 특히 한국의 서해연안 대부분과 남해연안 일부에 분포되어 있는 미세-점착성 퇴적물은 사질성 퇴적물과는 달리 퇴적물에서 부유된 토사 입자의 침강 속도가 아주 작아서, 파랑과 조류 등과 같은 해수유동에서 의해 쉽게 이동되어 현저한 해저 지형의 변형을 초래한다. (중략)

• Proceedings of the Korea Water Resources Association Conference
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• 2008.05a
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• pp.1976-1980
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• 2008

본 연구에서는 만경강 점착성 퇴적물의 표본채취 및 실내실험을 통하여 만경강 점착성 퇴적물의 침강특성이 정량적으로 산정되었으며, 과거 타 지역 퇴적물의 침강특성 산정 결과들과 비교 검토 되었다. 또한 퇴적물의 기본 물리 화학적 특성과 침강특성간의 상관관계 해석을 통하여, 만경강 점착성 퇴적물의 침강특성결과의 타당성이 간접적으로 검토되었다. 침강실험 결과, 부유사 농도가 증가함에 따라 침강속도가 증가하는 응집침강 영역과, 역으로 침강속도가 감소하는 간섭침강 영역이 명확히 나타났으며, 만경강 퇴적물의 부유사농도값이 0.1$W_s$<1 mm/sec의 침강속도 값을 갖는 것으로 확인되었다. 또한 산정된 침강속도는 과거 타 지역과 비교하여 정량적으로 상당한 차이를 보였다.

• Proceedings of the Korea Water Resources Association Conference
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• 2008.05a
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• pp.2166-2169
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• 2008

한강 하구 점착성 퇴적물의 침강속도를 정량적으로 산정하고, 산정된 침강속도의 계절적 변화 해석을 목적으로 침강수주를 이용한 침강 실험이 수행되었다. 침강실험은 1개 정점에서 3계절에 채취된 퇴적물 표본 시료에 대해 계절별로 $5{\sim}6$회씩 총 16회의 실험이 수행되었으며, 또한 퇴적물 자체의 물리 화학적 특성 및 해수 특성에 따른 침강속도의 정성적 변화 특성을 파악하기 위하여 한강 하구 점착성 퇴적물에 대한 물리 화학적 특성 및 해수 특성에 대한 분석이 수행되었다. 본 연구를 통하여 도출되는 한강 하구 점착성 퇴적물 침강속도의 정량적 결과는 향후 한강 하구에 대한 체계적 관리 및 기능 회복을 위한 기술 개발시에 크게 활용될 수 있을 뿐만 아니라, 저면의 침식과 퇴적에 따른 하상변동 해석, 수질 및 퇴적물 오염 절감을 위한 대책 방안 수립 등 다양한 목적으로 크게 활용될 수 있을 것이다.

• Proceedings of the Korean Society of Coastal and Ocean Engineers Conference
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• 2003.08a
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• pp.222-226
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• 2003

대부분의 경우에 하구의 미세-점착성 퇴적물은 보통 무기성 광물과 유기물 및 생화학물의 혼합체이며, 광물 입자들은 주로 점토와 실트로 구성되어 있다. 이러한 혼합체의 침식특성은 사질성 퇴적물과는 달리 입자간의 응집현상에 의한 의해 크게 영향을 받으며, 응집강도는 광물질 구성, 입경분포, 유기물 함량 등으로 묘사되는 퇴적물 자체의 물리ㆍ화학적 기본특성에 따라 크게 변화하고 (Mea, 1986), 특히 저면 퇴적물의 침식 여부는 흐름 전단응력에 의한 저면퇴적물의 저항력 즉, 저면전단강도의 상대적 크기의 차이에 좌우되므로, 그 침식 특성은 저면전단강도 흑은 저면밀도로 묘사되는 저면특성에 따라 크게 변화한다(황규남 등, 2003). 또한 각 해역마다 저면 퇴적물은 퇴적물 공급원, 수동학적 조건, 생태학적 조건 등이 모두 다른 상태에서 형성된 퇴적층이므로, 저면 퇴적물의 기본특성 및 저면특성은 "site- specific" 한 성격을 갖는다. (중략)