The Calculation Of Forces On A Pile Exerted By Internal Solitary Waves On The Northern Slope Of The South China Sea

Internal solitary waves(ISWs)occur frequently in the northern South China Sea(SCS).With the characteristics of large amplitude and strong current,the ISWs cause severe damage to the ocean structures like piles.In order to avoid the fracturing of piles under the limit action of ISWs,we should consider not only the changes of forces exerted by ISWs with depth and time in previous studies,but also the maximum values and their influencing factors.In this paper,neglecting higher modes,we obtain the theoretical expressions of forces on a pile exerted by ISWs including the unit force,total force,shear force and bending moment by using the KdV equation and Morison formula.Besides,we develop the expressions of their maximum values.The theoretical results are verified by the mooring data observed on the northern slope of SCS.We discuss the vertical and temporal changes of forces and obtain the monthly variations of the maximum values.The correlations between maximum values and amplitude,nonlinear phase speed and horizontal characteristic scale are analyzed.It is found that1)The ocean can be divided into two layers by the turning point of the horizontal velocity.The unit force and total force are westward in the upper layer and eastward in the lower layer.The shear force and bending moment are always negative.The unit force is largest on the ocean surface.The shear force and bending moment are largest at the turning layer and the seafloor,respectively.2)During the period of ISWs,the values of all forces first increase and then decrease with time.The total force integrating from bottom to surface reaches its maximum value when the trough arrives.The maximum values of other forces arise before the trough arrives.3)The maximal forces are larger in summer and autumn,while smaller in winter and spring.The advanced time is smaller in summer and autumn,while larger in winter and spring.4)The maximal total force integrating from bottom to surface is concerned with amplitude and nonlinear phase speed,while the maximum values of other forces are concerned with amplitude,nonlinear phase speed and horizontal characteristic scale.The maximum values have positive correlation with amplitude and nonlinear phase speed,while have negative correlation with horizontal characteristic scale.The advanced time has negative correlation with amplitude and nonlinear phase speed,while have positive correlation with horizontal characteristic scale.