• Journal of Korea Water Resources Association
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• v.52 no.spc2
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• pp.801-810
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• 2019

This study analyzes the impact on geomorphic changes downstream due to alternate bars developed weir upstream through laboratory experiments. The disturbance, such as a spur in the side wall, of the flow at the inlet of the channel triggers the development of alternate bar upstream at the beginning of the experiment, and gradually moved downstream with keeping their shapes over time. The bed in the downstream of weir in the mid of channel scoured due to the scarcity of sediment inflow because weir upstream traps it. Moreover, bar migration speed decreases as the bars approaches to the weir with time. However, as time increases, the alternate bars upstream migrate over the weir, and sediment in the eroded bed of the weir downstream are deposited. The phase of the bar upstream changes oppositely after passing through the weir. The phase of the bar downstream changes rapidly as the shape of alternate bar is clear upstream, which is affected by the strong disturbance. The phase of bar changes, and the bar migration speed decreases gradually with time, and finally stopped due to forcing effects on the bar by the disturbance. The faster the reaction of alternate bar with a long spur, the larger the bar height formed downstream and the shorter the bar length. This means that the larger the forcing effect of bar, the more it affects the bar migration. In addition, although the size of the alternate bar increases over time, the bar doesn't migrate downstream and a forced bar is generated.

• Proceedings of the Korea Water Resources Association Conference
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• 2021.06a
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• pp.116-116
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• 2021

자연 하천에서의 부유사 농도 계측은 주로 재래식 채집방식을 활용한 직접계측 방식에 의존하여 비용과 시간이 많이 소요되며 점 계측 방식으로 고해상도의 시공간 자료를 측정하기엔 한계가 존재한다. 이러한 한계점을 극복하기 위해 최근 위성영상과 드론을 활용하여 촬영된 다분광 혹은 초분광 영상을 통해 고해상도의 부유사 농도 시공간분포를 측정하는 기법에 대한 연구가 활발히 진행되고 있다. 하지만, 다른 하천 물리량 계측에 비해 부유사 계측 연구는 하천에 따라 부유사가 비균질적으로 분포하여 원격탐사를 통해 정확하고 전역적인 농도 분포를 재현하기는 어려운 실정이다. 이러한 부유사의 비균질성은 부유사의 입도분포, 광물특성, 침강성 등이 하천에서 다양하게 분포하기 때문이며 이로 인해 부유사는 지역별로 다양한 분광특성을 가지게 된다. 따라서, 본 연구에서는 이러한 영향을 고려한 전역적인 부유사 농도 예측 모형을 개발하기 위해 실내 실험을 통해 부유사 특성별 고유 분광 라이브러리를 구축하고 실규모 수로에서 다양한 부유사 조건에 대한 초분광 스펙트럼과 부유사 농도를 측정하는 실험을 수행하였다. 실제 부유사 농도는 광학 기반 센서인 LISST-200X와 샘플링을 통한 실험실 분석을 통해 계측되었으며, 초분광 스펙트럼 자료는 초분광 카메라를 통해 촬영한 영상에서 부유사 계측 지점에 대한 픽셀의 스펙트럼을 추출하여 구축하였다. 이렇게 생성된 자료들의 분광 다양성을 주성분 분석(Principle Component Analysis; PCA)를 통해 분석하였으며, 부유사의 입도 분포, 부유사 종류, 수온 등과의 상관관계를 통해 분광 특성과 가장 상관관계가 높은 물리적 인자를 규명하였다. 더불어 구축된 자료를 바탕으로 기계학습 기반 주요 특징 선택 알고리즘인 재귀적 특징 제거법 (Recursive Feature Elimination)과 기계학습기반 회귀 모형인 Support Vector Regression을 결합하여 초분광 영상 기반 부유사 농도 예측 모형을 개발하였으며, 이 결과를 원격탐사 계측 연구에서 일반적으로 사용되어 오던 최적 밴드비 분석 (Optimal Band Ratio Analysis; OBRA) 방법으로 도출된 회귀식과 비교하였다. 그 결과, 기존의 OBRA 기반 방법은 비선형성을 증가시켜도 좁은 영역의 파장대만을 고려하는 한계점으로 인해 부유사의 다양한 분광 특성을 반영하지 못하였으며, 본 연구에서 제시한 기계학습 기반 예측 모형은 420 nm~1000 nm에 걸쳐 폭 넓은 파장대를 고려함과 동시에 높은 정확도를 산출하였다. 최종적으로 개발된 모형을 적용해 다양한 유사 조건에 대한 부유사 시공간 분포를 매핑한 결과, 시공간적으로 고해상도의 부유사 농도 분포를 산출하는 것으로 밝혀졌다.

• Korean Journal of Ecology and Environment
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• v.45 no.1
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• pp.129-138
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• 2012

We investigated an ecological impact of drought simulated by discharge depletion on the water quality and benthic diatom community in the indoor experimental channel. As artificial substrates slide-glass was installed in acrylic channel for 16 days. Channels were supplied continuously with eutrophic lake water with a discharge rate of 6 L $min^{-1}$ in duplication during the colonized period. And then during the discharge depletion period, three discharge rates were provided: NDF (No depletion of flow rate (Control): 6 L $min^{-1}$), LDF (Low depletion of flow rate: 3 L $min^{-1}$) and HDF (High depletion of flow rate: 1 L $min^{-1}$). Environmental factors in the water, such as suspended solid, Chl-$a$ and nutrients concentration, were measured with periphytic algae including AFDM (ash free dry matter), Chl-$a$ concentration and cell density at 1-day intervals. Light intensity increased significantly with discharge depletion (F=229.5, p= 0.000). $NH_4$-N concentration was highest at HDF. Suspended solid in outflowing water decreased at HDF (88%), LDF (97%) and NDF (99%), compared to inflowing water (100 %). Chl-$a$ in substrates increased more than two times at LDF and HDF than NDF (F= 8.399, p=0.001). Also AFDM and benthic diatom density increased significantly at LDF and HDF than NDF (F=9.390, p=0.001; F=6.088, p=0.007). In all experimental groups, $Aulacoseira$ $ambigua$, $Achnanthes$ $minutissima$ and $Aulacoseira$ $granulata$ were dominant species accounting for greater than 10% of benthic diatom density. The most dominant species, $A.$ $ambigua$ was highest at LDF, followed by HDF and NDF (F=8.551, p=0.001). In conclusion, the effect of drought simulated by discharge depletion in an artificial stream ecosystem caused significant changes on water quality and benthic diatom biomass. This result provides a useful data to understand the effect of draught on stream ecosystem in situ.

• Journal of Korea Water Resources Association
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• v.50 no.1
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• pp.55-64
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• 2017

This study presents a numerical model for simulating turbidity currents using the ULTIMATE scheme. For this, the layer-averaged model is used. The model is applied to laboratory experiments, where the flume is composed of sloping and flat parts, and the characteristics of propagating turbidity currents are investigated. Due to the universal limiter of the ULTIMATE scheme, the frontal part of the turbidity currents at a sharp gradient without numerical oscillations is computed. Simulated turbidity currents propagate super-critically to the end of the flume, and internal hydraulic jumps occur at the break-in-slope after being affected by the downstream boundary. It is found that the hydraulic jumps are computed without numerical oscillations if Courant number is less than 1. In addition, factors that affect propagation velocity of turbidity currents is studied. The particle size less than $9{\mu}m$ does not affect propagation velocity but the buoyancy flux affects clearly. Finally, it is found that the numerical model computes the bed elevation change due to turbidity currents properly. Specifically, a discontinuity in the bed elevation, arisen from the hydraulic jumps and resulting difference in sediment entrainment, is observed.

• Korean Journal of Ecology and Environment
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• v.44 no.1
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• pp.75-84
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• 2011

We examined the effect of the turbid water on the periphytic diatom community in an artificial stream system. The artificial stream was constructed with transparent acryl and composed of four channels. Each channel ($20\;cm{\times}200\;cm{\times}40\;cm$) was supplied continuously with eutrophic lake water. In order to the freely colonize and grow diatoms, artificial substrate was installed with commercial slide glass soaked in 1% agar. Prior to introducing turbid water, the artificial stream was operated with lake water for 6 days to permit the propagation of diatom community on the substrates. The turbid water prepared with sediment sieved with ${\varphi}$ $64\;{\mu}m$ at $2\;g\;L^{-1}$ (final concentration, 300 NTU) was provided daily for 50 minute duration. The experiment was conducted for 7 days with manipulated experimental condition of light ($50{\sim}80\;{\mu}mol\;m^{-2}s^{-1}$, light:dark=24:0), temperature ($10{\pm}1^{\circ}C$), and flow rate ($0.31\;cm\;s^{-1}$). Sampling and analysis were conducted daily for water quality and diatom. Turbidity of the water varied 162.2~173.2 NTU during the experiment. After introduction of turbid water, DO, pH and TN were decreased, while SS and TP increased significantly. A total of 14 genera and 47 species of diatoms was observed on the artificial substrates during the experimental period. Of these, Navicula appeared to be a most dominant genus with 10 species, followed by Cymbella (6 species), Fragilaria (6 species) and Gomphonema (5 species). Achnanthes minutissima was the most dominant species (>70% of total frequency) in both control and treatment experiments. Increase in diatom abundance lasted for three days since turbid water introduction, after that they gradually decreased by the termination of the experiment. These results suggest that frequent supply of highly-concentrated turbid water significantly decreases the periphytic diatom community, and retard the recovery of the stable food-web within the stream.