Consequences of limited sediment supply for long-term evolution of offshore tidal sand waves, a 3D model perspective

Field data show that offshore tidal sand waves in areas where sediment supply is limited have different characteristics (shape and dimensions) compared with their counterparts in areas with sufficient sediment supply. So far, only the initial formation of tidal sand waves on a sediment-starved shelf has been studied with a 2DV model that ignores variations along the crests. In this study, a 3D non-linear morphodynamic model is used to investigate the effects of sediment availability on the long-term evolution of offshore tidal sand waves. Overall, the simulated sand waves have characteristics that resemble those of observed sand waves. The mature sand waves that develop in the case of limited sediment supply (i.e., thickness of erodible sediment layer is smaller than the height of sand waves) are more three-dimensional, i.e., having isolated and more irregular crestlines compared with those in the case of sufficient supply. With decreasing sediment supply, sand waves have larger spacings between successive crests, smaller heights and they migrate faster. These differences in the characteristics of the sand waves start to occur once the hard bed underneath the erodible sediment layer is exposed

Пошук витоків народного календаря

Рецензія на монографію: Мойсей А.А. Магія і мантика у народному календарі східнороманського населення Буковини. – Чернівці, 2008. – 320 с.: 16 іл

Background Topography Affects the Degree of Three‐Dimensionality of Tidal Sand Waves

Offshore tidal sand waves on the sandy bed of shallow continental shelf seas are more three-dimensional (3D) in some places than others, where 3D refers to a pattern that shows variations in three spatial directions. Such sand waves have crestlines that meander, split or merge. The degree of three-dimensionality seems to vary especially when large-scale bedforms, such as tidal sand banks, are present underneath the sand waves. Understanding this behavior is important for offshore activities, such as offshore windfarm construction or the maintenance of navigation channels. In this study, the degree of three-dimensionality of sand waves at five sites in the North Sea is quantified with a new measure. Results show that tidal sand waves on top of tidal sand banks are more two-dimensional (2D) than those on bank slopes or in open areas. These differences in sand wave pattern are supported by numerical simulations performed with a new long-term sand wave model. The primary cause of these differences is attributed to the deflection of tidal flow over a sand bank, which causes sand wave crests to be more aligned with the bank at its top than at its slopes. It is subsequently made plausible that the different patterns result from the competition between two known mechanisms. These mechanisms are nonlinear interactions between sand waves themselves (SW-SW interactions) and nonlinear interactions between sand banks and sand waves (SB-SW interactions). On bank tops, SB-SW interactions favor a 2D pattern, while SW-SW interactions, which elsewhere produce a 3D pattern, are less effective

Long-term morphodynamics of a coupled shelf-shoreline system forced by waves and tides, a model approach

Sand ridges, with length scales of several km, are prominent features of the seafloor landscape of many sandy continental shelves. Knowledge about the extent to which these ridges influence the large-scale (i.e., decadal and kilometer scales) morphodynamic evolution of the adjacent shoreline and vice versa (shelf-shoreline morphodynamic coupling) is limited. The present work aims at quantifying this coupling by using a coupled nonlinear shelf-shoreline model forced by tides and different wave conditions. Model results show that the presence of sand ridges on the shelf creates longshore non-uniform wave patterns, which act as a forcing template for the morphodynamic development of the shoreline. The shelf-shoreline coupling primarily works one way, meaning that the morphodynamic evolution of the shelf affects the evolution of the shoreline. When wave propagation is predominantly aligned with the long axis of the shelf ridges, the forced shoreline undulations are so prominent, that they affect the shelf morphology (significant two-way coupling). Moreover, for those waves, the longshore spacing of the ridges is strongly imprinted on the shoreline morphology. Weaker shoreline undulations develop for waves that propagate more across the ridges and the weakest for time-varying wave conditions with large variability in their angles of propagation. Model results compare fairly well with observations. Physical mechanisms underlying the different morphodynamic responses of the coupled shelf-shoreline system to different wave conditions are also given.Postprint (author's final draft