Showing papers by "Bingchen Liang published in 2019"

Extreme significant wave height of tropical cyclone waves in the South China Sea

Abstract: . Extreme significant wave heights are assessed in the South China Sea (SCS), as assessments of wave heights are crucial for coastal and offshore engineering. Two significant factors include the initial database and assessment method. The initial database is a basis for assessment, and the assessment method is used to extrapolate appropriate return-significant wave heights during a given period. In this study, a 40-year (1975–2014) hindcast of tropical cyclone waves is used to analyse the extreme significant wave height, employing the peak over threshold (POT) method with the generalized Pareto distribution (GPD) model. The peak exceedances over a sufficiently large value (i.e. threshold) are fitted; thus, the return-significant wave heights are highly dependent on the threshold. To determine a suitable threshold, the sensitivity of return-significant wave heights and the characteristics of tropical cyclone waves are studied. The sample distribution presents a separation that distinguishes the high sample from the low sample, and this separation is within the stable threshold range. Because the variation in return-significant wave heights in this range is generally small and the separation is objectively determined by the track and intensity of the tropical cyclone, the separation is selected as a suitable threshold for extracting the extreme sample in the tropical cyclone wave. The asymptotic tail approximation and estimation uncertainty show that the selection is reasonable.

Experimental Investigation on Characteristics of Sand Waves with Fine Sand under Waves and Currents

Abstract: A series of physical experiments was conducted to study the geometry characteristics and evolution of sand waves under waves and currents. Large scale bedforms denoted as sand waves and small bedforms represented by ripples were both formed under the experimental hydrodynamic conditions. Combining the experimental data with those from previous research, the characteristics of waves and currents and measured sand waves were listed. Small amplitude wave theory and Cnoidal wave theory were used to calculate the wave characteristics depending on different Ursell numbers, respectively. The results show good agreement between the dimensionless characteristics of sand waves and the dimensionless wave characteristics with a smaller wave steepness. When the wave steepness is large, the results seem rather scattered which may be affected by the wave nonlinearity. Sand wave steepness hardly changed with bed shear stress. A simple linear relationship can be found between sand wave length and wave steepness. It is easy to evaluate the sand wave characteristics from the measured wave data.

Numerical Investigation of Local Scour with Inclined Piles

Abstract: Du, S.; Liang, B., and Lee, D.Y, 2019. Numerical investigation of local scour with inclined piles. In: Lee, J.L.; Yoon, J.-S.; Cho, W.C.; Muin, M., and Lee, J. (eds.), The 3rd International Water Safety Symposium. Journal of Coastal Research, Special Issue No. 91, pp. 161-165. Coconut Creek (Florida), ISSN 0749-0208.In coastal and offshore engineering applications, foundations of breakwaters, offshore wind turbine foundations and piles are often damaged by local scour induced by currents or waves, which will threaten people's property and life security. The present study focuses on the local scour around cylinder piles, which are inclined toward upstream, downstream, and lateral. A vertical pile together with the three types of inclined piles was investigated by a numerical model in clear-water scour condition. Each type of the inclined pile was simulated for five inclination angles ranging from 15° to 45°. The angle of inclination is between the pile axis and the bed normal. By solving the Reynolds Averaged Navier–Stokes (RANS) equations, steady currents around the piles are simulated with the validated model. The results show significant differences in scour and deposition for the vertical and inclined piles. Sediments transport induced by the horseshoe vortex, contracted streamlines and the wake vortices were analyzed. Upward flow and downward flow in front of the pile were seen quite contrary between downstream inclined pile and upstream inclined pile. Local scour in lateral inclined pile was found moving toward to the inclined side. Maximum scour depth for inclined piles decreases in different degrees compared to the vertical pile.

Study of the threshold for the POT method based on hindcasted significant wave heights of tropical cyclone waves in the South China Sea

Abstract: . An assessment of extreme significant wave heights is performed in the South China Sea (SCS), which is crucial for the coastal and offshore engineering in this area. Two significant factors influencing the assessment are the initial database and the assessing method. The initial database is a basic for assessment, and the assessing method is used to extrapolate appropriate return significant wave heights based on this database during a period. In this study, a 40-year (1975–2014) hindcasted significant wave height of tropical cyclone waves is adopted as the initial database. Based on this database, the peak significant wave height of every tropical cyclone wave is directly extracted as the initial sample; the independent and identically distributed assumption is satisfied; and the interference for the selection of the sample is avoided. The peak over threshold (POT) method with the generalized Pareto distribution (GPD) model is employed to extract the sufficiently large and high sample for model estimation. The peak excesses over a sufficiently high value (i.e., threshold) are fitted; thus, the return significant wave heights are highly dependent on the threshold. To determine the unique threshold for the POT method, characteristics of tropical cyclone waves are researched. The research results reveal that the separation value shown in the distribution of the initial sample is suitable for sampling in the SCS. Because the separation value is within the stable threshold range and the asymptotic tail approximation and estimation uncertainty are reasonable, the selected threshold is suitable and the corresponding return significant wave height is reliable.

Method for selecting wave height threshold

Abstract: The present application relates to a method for selecting a wave height threshold, based on a target location, original wave height samples are acquired, and a wave height threshold interval are obtained; combined with wave height threshold and based on GPD and specified return period, a design wave height Hsi,j corresponding to each threshold within the wave height threshold interval is calculated according to the sample wave height values which are greater that the threshold; a difference is calculated to obtain a stable threshold interval, and a threshold within the stable threshold interval is selected as a reasonable threshold for design wave height estimation In this method, the stability of estimated values can be determined directly and objectively to realize wave height threshold selection