• Journal of Korea Water Resources Association
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• v.42 no.8
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• pp.619-629
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• 2009

The objective of this study is to estimate bed-resistance in gravel-bed rivers using the equivalent roughness height($k_s$). We calculated the friction factor(f) with the measured data from 8 domestic gravel-bed rivers and investigated the size distributions of the bed materials. The averaged $k_s$ in each cross-section, which is determined under the hypothesis that the vertical velocity distribution follows the logarithmic law, is compared with the reach $k_s$ which is calculated with the cumulative grain diameter distribution curve of bed materials. Moreover, the applicability of existing formulae, such as Strickler type equations, is examined by comparing with Manning's n value converted from the $k_s$. According to the results, the reach $k_s$ proves to be a good indicator of representative characteristic of bed materials in a reach, and the Manning's n based on the reach $k_s$ is appropriate for practical estimation of the bed-resistance, for RMS errors between calculated and measured Manning's n is less than 0.003. The correlation between the $k_s$ and specified bed-material size($D_i$) is very low, so it is difficult to select a proper one among the existing empirical equations.

• Proceedings of the Korea Water Resources Association Conference
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• 2021.06a
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• pp.310-310
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• 2021

지난 수년간 하천 인근에서 홍수로 인하여 다양한 피해가 발생하고 있다. 이러한 홍수피해를 경감하기위해 구조적 비구조적 대책들을 세우고 있으며, 중요한 비구조적 대책 중의 하나가 홍수경보시스템을 구축하는 것이다. 일반적으로 홍수경보시스템을 구축하기 위하여 홍수경보기준지점의 수위를 설정하며 이에 대응하는 한계유량을 산출하고 GIUH 강우-유출모형을 통하여 한계유량에 대응하는 경보강수량을 산정하는 방식을 택하고 있다. 특히 한계유량을 산출하는 경우, 다양한 연구에서 Manning 공식을 통하여 한계유량을 산출하고 있다. 이에 대한 적정성을 비교하기 위해 본 연구에서는 HEC-RAS모형을 통하여 한계유량을 계산하였고 Manning식에서 나온 값과 비교하였다. 비교결과 Manning식에서 산출된 한계유량은 과다한 경보 강수량 값을 채택하고 기존 설계강수량에 비해 매우 큰 값임을 확인할 수 있었다. 이에 비해 HEC-RAS의 한계 유량값은 적정한 경보강수량 값을 제시하였고 연평균알람기준에도 적정함을 알 수 있었다. 본 연구 결과를 통해, 현재 다양한 하천사업이 이루어져 대부분의 하천의 측량이 이루어진 상황에서 기존의 Manning식에 의한 한계유량 산출보다는 HEC-RAS를 통하여 한계유량을 산정해야하는 것이 보다 적정해 보인다.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.32 no.2B
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• pp.139-147
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• 2012

This study is derived relationships of the resistance coefficients of Darcy-Weisbach and Manning for flow resistance and the dimensionless velocity using many field measurements for 1,875 rivers consist of sand 179, gravel 992, cobble 651 and boulder 53 channels in natural rivers, respectively. The relationships of power law forms are developed as a function of flow discharge, friction slope, and relative submergence by the regression and the semi-empirical method. The measurements distribution of Manning resistance coefficients by the Box-Whisker Plots show the values which ranges from 0.004~0.151 for sand, 0.008~0.250 for gravel, 0.015~0.327 for cobble, 0.023~0.444 for boulder in natural rivers, respectively. Relationships of these semi-empirical and resistance coefficients will be useful to give information in hydraulic engineering.

• Journal of Korea Water Resources Association
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• v.45 no.2
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• pp.137-149
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• 2012

Field measurements of resistance to flow are analyzed for 739 rivers vegetated with grass (281 channels), shrubs (150 channels) and trees (308 channels). The measured distribution of Manning roughness coefficients ranges from 0.015~0.250 for grass, 0.016~0.250 for shrubs, 0.018~0.310 for trees. Significant trends are obtained between Darcy-Weisbach (or Manning roughness coefficients) and flow discharge, friction slope, and relative submergence. The regression equations for Darcy-Weisbach and Manning roughness coefficients in vegetated rivers are: $f_{veg}=0.436Q^{-0.363}$, $f_{veg}=3.305S_f^{0.508}$, and $n_{veg}=0.061Q^{-0.124}$, $n_{veg}=0.144S_f^{0.199}$, $V=5.3(h/d_{50})^{1/8.3}{\sqrt{ghS_f}}$, $\sqrt{8/f}(=V/u*)=5.75log(5h/d_{50})$, respectively. These semi-empirical relationships should be useful for hydraulic engineering practice.

• Journal of Korea Water Resources Association
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• v.48 no.4
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• pp.291-298
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• 2015

The aim of this study is that a theoretical formula for estimating the one-dimensional longitudinal dispersion coefficient is derived based on a transverse distribution equation for the depth averaged stream-wise velocity in open channel. In "Part I. Theoretical equation for stream-wise velocity" which is the former volume of this article, the velocity distribution equation is derived analytically based on the Shiono-Knight Model (SKM). And then incorporating the velocity distribution equation into a triple integral formula which was proposed by Fischer (1968), the one-dimensional longitudinal dispersion coefficient can be derived theoretically in "Part II. Longitudinal dispersion coefficient" which is the latter volume of this article. SKM has presented an analytical solution to the Navier-Stokes equation to describe the transverse variations, and originally been applied to straight and nearly straight compound channel. In order to use SKM in modeling non-prismatic and meandering channels, the shape of cross-section is regarded as a triangle in this study. The analytical solution for the velocity distribution is verified using Manning's equation and applied to velocity data measured at natural streams. Although the velocity equation developed in this study do not agree well with measured data case by case, the equation has a merit that the velocity distribution can be calculated only using geometric data including Manning's roughness coefficient without any measured velocity data.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.20 no.6
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• pp.532-539
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• 2008

Kwon et al.(2008) carried out a hydraulic experiment in order to evaluate Manning coefficient, which implicates flow resistance due to bottom friction as well as drag caused by the squared piers higher than water depth and arranged with equal intervals, under the flow condition with a constant drag coefficient, $Re>10^4$. And, based on the equation of equilibrium, they proposed a formula for the equivalent resistant coefficient including empirical drag interaction coefficient obtained by using the experimental results. In this study, the hydraulic experiment was simulated using FLOW-3D, a 3-dimensional computational fluid dynamic code. The computations were compared with the experiment results as well as the semi-theoretical formula, and the comparisons show a good agreement. From the agreement, it was confirmed that when flow resistance bodies were higher than water depth, Manning n value increases with 2/3 power of water depth as shown in the theoretical formula and that drag interaction coefficient was dominated by their intervals.

• Journal of Korea Water Resources Association
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• v.44 no.7
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• pp.553-563
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• 2011

According to the improvement of computer's performance, the development of Geographic Information System (GIS), and the activation of offering information, a distributed model for analyzing runoff has been studied a lot in recently years. The distribution model is a theoretical and physical model computing runoff as making target basin subdivided parted. In the distributed model developed by this study, the volume of runoff at the surface flow is calculated on the basis of the parameter determined by landcover data and a two-dimensional diffusion wave equation. Most of existing runoff models compute velocity and discharge of flow by applying Manning-Strickler's mean velocity equation and Manning's roughness coefficient. Manning's roughness coefficient is not matched with dimension and ambiguous at computation; Nevertheless, it is widely used in because of its convenience for use. In order to improve those problems, this study developed the runoff model by applying not only Manning-Strickler's equation but also Chezy's mean velocity equation. Furthermore, this study introduced a power law of exponential friction factor expressed by the function of roughness height. The distributed model developed in this study is applied to 6 events of fan-shape basin, oblong shape test basin and Anseongcheon basin as real field conditions. As a result the model is found to be excellent in comparison with the exiting runoff models using for practical engineering application.

• The Journal of the Korea Contents Association
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• v.12 no.7
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• pp.445-452
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• 2012

This study is used to analyze the distribution of resistance factors and the relationships of flow resistance with the field measurements which consist of the total 2,604 rivers for 1,865 bed material in natural channels and 739 vegetation in vegetated channels. Resistance factor relationships and distribution range of Manning roughness coefficients and Darcy-Weisbach friction coefficients by the regression analysis are derived from the power law form as a function of flow discharge and friction slope with bed materials and vegetations in natural and vegetated rivers, respectively.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.28 no.6B
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• pp.627-634
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• 2008

In a numerical simulation of open channel turbulent flows using RANS (Reynolds averaged Navier-Stokes) equations model equipped with VOF (Volume of Fluid) scheme, the determination of wall roughness for wall function was studied. The roughness constant, based on the law-of-the-wall for flow on rough walls, obtained by experimental works for pipe flows is employed in general wall functions. However, this constant of wall function is the function of Froude number in open channel flows. Thus, the wall roughness should be determined by taking into account the effect of Froude number. In addition, the wall roughness should be corresponding to Manning's roughness coefficient widely used for open channels. In this study, the relation between wall roughness height as an input condition and Manning's roughness coefficient was investigated, and an equation for effective wall roughness height considering the characteristics of numerical models was proposed as a function of Manning's roughness coefficient.

• Proceedings of the Korea Water Resources Association Conference
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• 2009.05a
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• pp.1775-1780
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• 2009

개수로 마찰흐름 특성에 관한 연구로서 유량조사사업단(2006)의 우리나라 3대강 유역 하천 관측자료와 Jarrett(1984)의 미국 Colorado지역 하천의 관측자료로 부터 흐름특성을 구분하여 개수로 흐름의 마찰특성 또는 조도계수의 변이특성을 분석하였다. 기존 지수형 완난류 마찰계수 산정식(유동훈과 이민호,1997)의 비례상수 $\alpha$에 조도계수 n을 도입하여 실무에서 보다 편하게 적용할 수 있는 새로운 산정식을 개발하였다. 유량조사사업단의 관측자료 분석에 있어서 유량 관측시 관측지점의 수면경사 및 하상경사의 미관측으로 하천정비기본계획상에 제시된 지형으로부터 관측지점의 하상경사를 추정하였으며 1차적으로 에너지경사는 하상경사와 동일하다고 가정하였다. 이러한 가정하에 하천정비기본계획상에 제시된 Manning의 조도계수를 인용하여 관측유속과 계산유속을 비교8 분석하였다. 또한 전통적 산정식인 Ganguillet & Kutter(1869)식과 Manning(1889)식으로 부터 산정된 유속과의 비교를 통하여 조도계수 n을 추정하였고 추정된n을 도입하여 새로 개발된 지수형 마찰계수 산정식의 적용성을 입증하였을 뿐만 아니라 기존 조도계수 산정의 문제점을 제시하였다.