Numerical investigation of oscillations induced by submerged sliding masses within a harbor of constant slope. (February 2017)

• Record Type: Journal Article
• Authors: Shao, Dong; Feng, Xi; Feng, Weibing; Hong, Guangwen
• Is Part Of: Applied ocean research. Volume 63(2017:Oct.)
• Page Start: 49
• Page End: 64
• Publication Date: 2017-02
• Subjects: Harbor resonance -- Oscillations -- Boussinesq equations -- Wave generation -- Numerical experiments; Ocean engineering -- Periodicals; 620.416205
• DOI: 10.1016/j.apor.2017.01.002
• Languages: English
• ISSNs: 0141-1187

Abstract

Highlights: This manuscript involves the investigation of the behaviors and properties of harbor oscillations induced by submerged sliding masses on a constant slope which have not yet been reported in literature. Although the constant slope is an idealized geometry, it grabs the main feature of most of the harbors in reality. The submerged sliding masses may be resulted from numerous incidents within a harbor and can be regarded as one of the most frequent disturbances which may induce the harbor resonances. The characteristics of the sliding masses may influence the generated oscillations which manifest different behaviors compared with those generated by other type of disturbances (for example vertical seafloor movements). Involving at the same time the wave generation of the submerged sliding masses or landslides and the mechanism of harbor oscillations, this work can be seen as a combination of the researches in these two domains, which carries out the investigations numerically with Boussinesq-type equations in involving the possible origin of the harbor oscillation problem in practical account following the analytical solutions of the oscillations in a rectangular harbor of constant slope. The submerged sliding masses may induce obvious transverse oscillations which are trapped within the harbor. Oscillations induced by the slides of constant velocity are compared with those induced by the slides accelerated by gravity force while these are two representative types of the sliding movements. The effects of bottom friction are also considered. The augmentation of the velocity of the sliding masses leads to smaller amplitudes of the oscillation components, which corresponds to the physical understandings of waves generated by the underwater slides with a variable water depth. The movements with constant velocities may facilitate the development of some higher transverse modes together with the lower ones while those accelerated by gravity force are only in favor of the very basic modes. Similar to the resonances induced by vertical seafloor movements, the transverse oscillations generated by the sliding masses on a constant slope are also sensitive to the location of their initial position. Their trajectories also play an important role. The importance of the initial position of the sliding masses is manifested in their relative positions against the positions of the nodal lines and anti-nodal lines of the modes ( m, n ) in the cross-harbor direction. In the offshore direction, the components with higher mode number n may become predominant when the position of sliding departure approaches the mouth of the harbor. For longitudinal oscillations, they may be generated by relatively large sliding masses of the harbor width on a constant slope but cannot reach a steady or quasi-steady state when there is no forcing condition at the entrance. The patterns of several low-mode ones are investigated and found sensitive to the position of departure of the sliding masses and their moving velocities. As they are not steady and dissipate relatively rapidly, wavelet spectra are used to analyze their evolutions and possible comparisons are made with theoretical predictions. Abstract: Oscillations within a rectangular harbor of constant slope induced by submerged sliding masses are investigated numerically based on Boussinesq-type equations and results are used to reveal the characteristics of the generated oscillations. The numerical result of each transverse eigenfrequency is very close to the theoretical prediction and the spatial structure of each mode of the oscillations may also be well captured by the existing analytical solutions based on shallow water equations. The investigation shows that relatively small-scale sliding masses whose width is small compared with the harbor width may induce obvious transverse oscillations. The predominant transverse components are those with small mode numbers when the solid slides start moving from the backwall. In comparing the oscillations induced by the slides of constant velocity and those accelerated by gravity force with bottom friction, it is observed that the movements accelerated by gravity force may facilitate the development of very low transverse oscillation modes while those with constant velocity may also be in favor of the higher ones. The augmentation of the sliding velocity along the constant slope may shift the amplitudes of the oscillation components to smaller values, which corresponds to the physical understandings of the waves generated by underwater sliding masses or landslides. While the sliding masses may not act on an isolated point of the bottom but follow a certain trajectory along the harbor, the transverse oscillations induced by them are sensitive to their position of departure in both the cross-harbor direction and the offshore direction. Longitudinal oscillations may be induced by relatively large sliding masses of harbor width on a constant slope within the harbor. Although the longitudinal oscillations may not reach a steady state without forcing terms at the entrance of the harbor, some patterns of several low-mode ones occur and wavelet spectra are used to analyze their evolutions and comparisons are made with theoretical predictions. It is revealed that the longitudinal oscillations are also sensitive to the moving velocity and initial location of the sliding masses.