• Journal of Korean Society of Disaster and Security
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• v.12 no.1
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• pp.57-66
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• 2019

One of the most serious problems with concrete small dams or barriers installed in small/median rivers is the deposit of sediments, especially, in Korea. An effective way to discharge such sediments to downstream is to construct a bypass pipe under the river bed. However, efficiency may become lowered if ripraps are entered into the bypass pipe. Therefore, in this study, we derived the threshold condition for the exclusion of ripraps from the bypass pipe using 3D numerical analysis. Upstream flow of the small dam was assumed to be stationary, and the energy concept was applied to the control volume containing the bypass pipe and its periphery. As a result, when the ratio of the water level difference between upstream and downstream to the diameter of the riprap was approximately equal to 1.2, the threshold condition for exclusion of the stones or riprap from the bypass pipe was affirmatively determined. If the characteristics of the adsorptive sediment adversely affecting the river environment in the future would be taken into account, results from this study are expected to put to practical use in the management of concrete small dam with bypass pipe system.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.37 no.1
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• pp.239-246
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• 2017

There are various transversal structures (small dams or drop structures) in median and small streams in Korea. Most of them are concrete structures and it is so hard to exclude low-level water. Unless drainage valves and/or gates would not be installed near bottom of bed, sediment from upstream should be deposited and also contaminants attached to the sediments would devastatingly threaten the water quality and ecosystem. One of countermeasures for such problem is the bypass pipe installed underneath the transversal structure. However, there is still issued whether it would be workable if the gravels and/or stones would roll into and be not excluded. Therefore, in this study, the conditions to exclude the rip stone which enter into the bypass pipe was reviewed. Based on sediment transport phenomenon, the behavior of stones was investigated with the concepts from the critical shear stress of sediment and d'Alembert principle. As final results, the basis condition (${\tau}_c{^*}$) was derived using the Lagrangian description since the stones are in the moving state, not in the stationary state. From hydraulic experiments the relative velocity could be obtained. In order to minimize the scale effect, the extra wide channel of 5.0 m wide and 1.0 m high was constructed and the experimental stones were fully spherical ones. Experimental results showed that the ratio of flow velocity to spherical particle velocity was measured between 0.5 and 0.7, and this result was substituted into the suggested equation to identify the critical condition wether the stones were excluded. Regimes about the exclusion of stone in bypass pipe were divided into three types according to particle Reynolds number ($Re_p$) and dimensionless critical shear force (${\tau}_c{^*}$) - exclusion section, probabilistic exclusion section, no exclusion section. Results from this study would be useful and essential information for bypass pipe design in transveral structures.

• Proceedings of the Korea Water Resources Association Conference
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• 2017.05a
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• pp.132-132
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• 2017

큰 하상계수를 가지는 국내 중소하천의 특성상 농업용수 등의 수자원 이용을 위해 국내에는 약 10,000개 정도의 하천 횡단구조물이 설치되어 있다. 그러나 느려진 유속으로 인하여 횡단구조물 상류에 유사가 퇴적될 확률이 높아지며, 이러한 유사는 부착된 오염물에 의한 수질 및 하상오염, 좁아진 유수단면적에 의한 통수능 저하로 발생하는 홍수위 증가 등 많은 위험성을 내포하고 있다. 이에 대한 해결책 중의 하나로 저층수 배사관(bypass pipe)을 활용하여 쌓여진 유사를 하류로 흘려보내는 방법이 있다. 저층수 배사관은 횡단구조물 상류와 하류의 하상을 연결하는 관으로써 상류와 하류의 수두차에 의해 흐름 특성이 결정된다. 하지만 배사관 내 사석이 유입되어 배제되지 않는 경우 배사관의 효율이 현저하게 저하될 가능성이 있다. 본 연구에서는 FLOW-3D를 이용하여 배사관내 사석이 유입되었을 때, 어떠한 조건에서 배제되고 배제되지 않는지를 명확히 하고자 한다. 하상이 평탄한 수로$(13.0(L){\times}5.0(B){\times}1.0(H)m)$를 재현하였으며, 배출관의 직경은 10cm로 결정하였다. 사석의 종류는 4가지(3cm, 5cm, 7cm, 9cm)로써 이를 고려하여 격자크기를 0.5cm로 결정하였다. 이는 사석의 구형형상이 충분히 잘 재현될 수 있는 격자크기이다. 모의 수행은 하류의 수위를 변화시켜가며 수행하였다. 저층수 배사관 내 유동 특성이 상류와 하류의 수두차에 의해 결정되기 때문에, 모의의 효율성을 위해 상류의 수위는 일정한 값으로 진행하였다. 모의 결과 수위차가 클수록 배제가 잘되는 경향을 보였다. 입자의 크기와 상하류 수위차를 무차원화 한 변수를 결정하였다. 분석결과 이 변수가 일정 값을 넘어서는 경우 배제가 되며, 반대로 이 값을 넘어서지 못한 경우 배제가 되지 않음을 확인하였다. 향후 다양한 관의 형상에 대해서도 배사관 내 한계조건을 도출하여, 저층수 배사관의 설계 및 시공에 도움이 되고자 한다.