Effect of sea level rise and tidal current variation on the long-term evolution of offshore tidal sand ridges

Abstract

Tidal sand ridges are large-scale bedforms that occur in the offshore area of shelf seas. They evolve on a time scale of centuries due to tide-topography interactions while being further shaped by wind waves. During their evolution, ridges are also affected by changes in sea level, strength and direction of the tidal current. According to their present-day behavior, ridges are classified as ‘active’ (sand transport everywhere), ‘quasi-active’ (sand transport only on parts of the ridges) and ‘inactive’ (no sand transport anywhere). Using a nonlinear morphodynamic model, the present study extends earlier work by investigating the effect of sea level rise and changes in the amplitude and principal direction of the tidal current on the growth time and height of active tidal sand ridges. Besides, the role of sea level rise and tidal current variation in the presence of quasi-active/inactive ridges is explored. Two specific settings are considered, which are characteristic for the Dutch Banks in the North Sea and for sand ridges in the Celtic Sea. The time range considered here is less than 20,000 years, roughly the period from the Last Glacial Maximum to the present. Generally active tidal sand ridges occur if the tidal current amplitude is larger than 0.5 m/s. For these ridges, with increasing rates of sea level rise, their growth time becomes longer, and the root mean square height keeps on increasing. A smaller initial tidal current amplitude gives rise to a larger growth time, while changes in the principal current direction have a minor effect on the characteristics of the ridges. On the considered time scale, assuming a constant wave climate, quasi-active tidal sand 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 inactive on a time scale that depends inversely on the rate of sea level rise. Modeled ridges are compared with observed ridges. Similarities are found and quantitative differences are explained.