A Functional Relationship between Roughness Coefficient of the Fixed Bed and the Scouring Rate at the Downstream

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Sung Won Park, Jin Hwan Hwang, Jungkyu Ahn

Thursday 2 july 2015

14:20 - 14:35h at Amazon (level 1)

Themes: (T) Water engineering, (ST) River and coastal engineering

Parallel session: 12B. Engineering - River

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Scoured holes at the downstream of bed protections have recently threatened the safeties of the weirs and their foundations in four major river basins in South Korea. Many researchers have made attempts to reveal the dominant factors of the local scour at the downstream of the fixed bed to prevent the progress of the scour hole. Values of the maximum depths of the scoured hole were mostly considered for the design of the bed protections and prevention of the undesirable scour depth. Breusers (1967) suggested that the maximum scour depth, which is normalized with water depth, was increased exponentially with respect to the normalized time scale and the scouring rate was constant. From the previous researches, it was revealed that the scouring rate was not constant and dominant factors of the local scour at the downstream of the fixed bed were flow and turbulence characteristics at the transition of the bed. In this research, the experiments about the local scour at the downstream of the fixed bed were carried out with no sediment supply from the upstream. Acrylic fixed bed and movable sand bed were setup with same height. Bed elevations at each time steps with various flow conditions were measured along the centerline and three-dimensional flow velocities were measured with Vectrino at the transition. Scouring rates were estimated from the development of the maximum scour depth with respect to the flow condition. And the Chézy’s roughness coefficients of the each cases were calculated from the three dimensional velocity at the transition and compared with the scouring rates. From the results, values of the depth averaged relative turbulent intensity, which represents the flow and turbulence characteristics at the transition, were calculated. And the functional relationship between the Chézy’s roughness coefficients from the values of the depth averaged relative turbulence intensity at the transition were analyzed with the scouring rates.