Modeling the dynamics of offshore tidal sand ridges

Abstract

Tidal sand ridges are large-scale bedforms with horizontal dimensions of several kilometers and heights of tens of meters. They occur in the offshore area of shelf seas that have a wide range of water depths (10-200 m). Based on their present-day behavior, ridges are classified as `active' (sand transport everywhere), `quasi-active' (sand transport in parts of the ridge area) and `moribund' (sand transport nowhere). They evolve on a timescale of centuries due to tide-topography interactions and they are shaped by wind waves. During their evolution, ridges are also affected by changes in sea level, strength and direction of the tidal current. A literature survey resulted in three research questions on the dynamics of thsee ridges. The first one concerns the effect of different formulations of bed shear stress and sand transport on the growth characteristics of these bedforms (growth rate and wavelength). The second one is on the sensitivity of the characteristics of finite-height ridges (their shape and growth time) to either 1D or 2D configurations (topography varies in one/two horizontal dimensions). The third one concerns the effect of changes in the characteristics of the sea level and the tidal current on the long-term evolution of these ridges. An idealized nonlinear morphodynamic model was developed and used for the above questions. Main findings are listed below. Regarding the initial formation of tidal sand ridges, the formulations for bed shear stress and slope-induced sand transport are not critical. For these ridges, under rectilinear tidal currents, increasing the critical velocity for sand erosion decreases the growth rate and the wavelength of the preferred bedform significantly. Besides tidal sand ridges, long bed waves are also found if the critical velocity for sand erosion is slightly smaller than the tidal current amplitude. For the nonlinear evolution of tidal sand ridges, neglecting changes in the sea level and tidal conditions, in the case of a 1D configuration, the root mean square height hrms of ridges first grows exponentially and hereafter saturates. In the end, ridges in static equilibrium are obtained, i.e., hrms remains constant. In contrast, when the configuration is 2D, ridges are found with spatially meandering crests that oscillate in time. Initially the bedforms are composed of a finite number of bottom modes. The occurrence of the meanders relates to the presence of bottom modes with crests normal to those of the initially preferred bedform. If changes in the sea level and tidal conditions are accounted for, with increasing rates of sea level rise, for active tidal sand ridges, their growth time becomes longer, and hrms keeps on increasing. On the considered time scale (10000 years), assuming a constant wave climate, quasi-active ridges occur mainly as a result of a decreasing tidal current amplitude such that the effective velocity (in the sense of stirring sand) becomes smaller than the critical velocity for sand erosion. The ridges further become moribund on a time scale that depends inversely on the rate of sea level rise.