Research on bilateral reverse migration of one-group seabed sand waves in a small shallow shelf sea

Abstract

Bilateral reverse migration of one-group seabed sand waves is found recently in many small sea areas and is a potential threat to the safety of many near seabed engineering structures, such as pipelines and cables. The mechanism cannot clearly be explained by existing theories. This paper focuses on the bilateral reverse migration of one-group seabed sand waves around a sand ridge in a small shallow shelf sea of the South China Seas, and presents a developed numerical model consisting of three-dimensional wave–current coupling module and sand transport module to simulate this complex phenomenon and perform relative analysis on the characteristics of the current and bilateral reverse migration formation mechanism. Taking the ocean environment characteristics of the surrounding sea area into account and with sufficient grid resolution, this model successfully depicts the process of sand wave bilateral reverse migration. Bottom friction velocity is taken as the inspecting physical quantity and divided into tidal, residual and short-term fluctuation components through harmonic analysis; with a method based on bedload transport calculation, the contributions of the above three parts are determined. It is found that long-term sand wave migration trend is primarily controlled by both tidal constituents and residual velocity. Under nonlinear pure tidal impact, the sand waves migrate southwards; the residual current is northwards and will cancel the tidal impact. When the magnitude of the residual velocity is large enough, i.e., in the western part of the sand wave area, sand waves migrate northwards; when the residual velocity is small, i.e., in the eastern part, sand waves migrate southwards and the bilateral reverse migration pattern is formed. The residual velocity distribution relates closely to the surrounding topography. This paper discloses one kind of mechanisms for bilateral reverse migration of one-group seabed sand waves, and the model and analysis framework can also be extended and applied to other similar situations to prompt deeper and more comprehensive understanding about sand wave migration.