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

Do tidal sand waves always regenerate after dredging?

Tidal sand waves are rhythmic bedforms found on sandy continental shelves that pose a threat to offshore activities. While emphasis is placed on studying their natural morphodynamic evolution, little is known about if and how fast sand waves recover after dredging. This work presents an analysis of multibeam echosounder data collected at three former sand extraction sites on the Belgian continental shelf. At one of the sites, sand waves seemed to reappear approximately 5 years after dredging had stopped, which did not happen at the other two sites during the measurement period (5 and 9 years). The lack of recovery in those sites is likely the result of larger depths and smaller local sediment availability compared with the site where recovery occurred. Furthermore, these data reveal that in the latter site sand wave recovery was established mainly through local sediment redistribution. • Tidal sand waves are isolated from bathymetric data of the Belgian continental shelf. • At only one of the three sites, sand waves seemed to regenerate after dredging. • Possible explanations are differences in water depth and local sediment availability. • The regenerating tidal sand waves do so as a result of local redistribution of sand

Formation and long-term evolution of shoreface-connected sand ridges : Modeling the effects of sand extraction and sea level rise

This thesis focuses on shoreface-connected sand ridges, which are large-scale bedforms observed on many storm-dominated inner shelves. A new nonlinear finite-difference model (MORFO56) is used to study effects of sand extraction and sea level rise on the dynamics of these ridges. MORFO56 uses depth-averaged shallow water equations for currents and includes sediment transport and bed updating. The model setting resembles the Long Island inner shelf. In Chapter 2, a series of short-term runs with MORFO56 are conducted to compare initial growth and migration rates of sfcr versus the longshore wavenumber with those computed with earlier models, which are based on stability analysis. It turns out that MORFO56 yields similar initial growth and migration curves to those from earlier models. Furthermore, MORFO56 is able to simulate finite amplitude sfcr for more realistic bottom slopes than earlier models. If subharmonic modes (modes with wavelengths larger than that of the initially fastest growing mode) are included, ridges merge such that one ridge eventually remains in the system. In Chapter 3, the response of sfcr to extraction of sand is studied. After extraction, ridges partially restore on decadal time scales. However, the original sand volume of the ridge is not recovered. Most sand that accomplishes the pit infill originates from upstream areas, as well as from the areas surrounding the pit. Depending on the pit location, additional sand sources contribute: if the pit is located close to the downstream trough, the pit gains sand by reduction of sand transport from the ridge to this trough. If the pit is located close to the adjacent outer shelf, the recovery of the ridge is stronger due to an import of sand from that area. In Chapter 4, the response of sfcr to sea level rise is investigated. A rising sea level increases the height of sfcr and it decreases their migration until they eventually drown. In contrast, in absence of sea level rise, the model simulates sfcr with constant heights and migration rates. In case of including subharmonic modes, sea level rise reduces the merging of ridges, such that multiple ridges occur in the end state, thereby yielding better agreement with observations. In Chapter 5, the dynamics of sand ridges is investigated in a setting that is characterized by coastal retreat and shelf steepening due to sea level rise. New ridges appear in the shallow area of the inner shelf, which remain active in time. Old ridges that were already formed in the antecedent area become less active with the rising sea level. If migration of offshore parts of the ridges vanishes, these parts change orientation to become more shore-parallel compared with the active onshore parts. In case of small landward inner shelf depths and a decreasing rate of sea level rise, the active onshore parts migrate too fast, thereby causing the drowned offshore parts to detach and to become a field of shoreface-detached ridges. The characteristics of the modeled shore-oblique shoreface-connected and more parallel shoreface-detached ridges are in agreement with those of observed sand ridges

Formation and long-term evolution of shoreface-connected sand ridges : Modeling the effects of sand extraction and sea level rise

This thesis focuses on shoreface-connected sand ridges, which are large-scale bedforms observed on many storm-dominated inner shelves. A new nonlinear finite-difference model (MORFO56) is used to study effects of sand extraction and sea level rise on the dynamics of these ridges. MORFO56 uses depth-averaged shallow water equations for currents and includes sediment transport and bed updating. The model setting resembles the Long Island inner shelf. In Chapter 2, a series of short-term runs with MORFO56 are conducted to compare initial growth and migration rates of sfcr versus the longshore wavenumber with those computed with earlier models, which are based on stability analysis. It turns out that MORFO56 yields similar initial growth and migration curves to those from earlier models. Furthermore, MORFO56 is able to simulate finite amplitude sfcr for more realistic bottom slopes than earlier models. If subharmonic modes (modes with wavelengths larger than that of the initially fastest growing mode) are included, ridges merge such that one ridge eventually remains in the system. In Chapter 3, the response of sfcr to extraction of sand is studied. After extraction, ridges partially restore on decadal time scales. However, the original sand volume of the ridge is not recovered. Most sand that accomplishes the pit infill originates from upstream areas, as well as from the areas surrounding the pit. Depending on the pit location, additional sand sources contribute: if the pit is located close to the downstream trough, the pit gains sand by reduction of sand transport from the ridge to this trough. If the pit is located close to the adjacent outer shelf, the recovery of the ridge is stronger due to an import of sand from that area. In Chapter 4, the response of sfcr to sea level rise is investigated. A rising sea level increases the height of sfcr and it decreases their migration until they eventually drown. In contrast, in absence of sea level rise, the model simulates sfcr with constant heights and migration rates. In case of including subharmonic modes, sea level rise reduces the merging of ridges, such that multiple ridges occur in the end state, thereby yielding better agreement with observations. In Chapter 5, the dynamics of sand ridges is investigated in a setting that is characterized by coastal retreat and shelf steepening due to sea level rise. New ridges appear in the shallow area of the inner shelf, which remain active in time. Old ridges that were already formed in the antecedent area become less active with the rising sea level. If migration of offshore parts of the ridges vanishes, these parts change orientation to become more shore-parallel compared with the active onshore parts. In case of small landward inner shelf depths and a decreasing rate of sea level rise, the active onshore parts migrate too fast, thereby causing the drowned offshore parts to detach and to become a field of shoreface-detached ridges. The characteristics of the modeled shore-oblique shoreface-connected and more parallel shoreface-detached ridges are in agreement with those of observed sand ridges

DataRepository_ImpactSecondaryBasins

Historical bathymetry Western Scheldt Estuary, Delft3D model output data <br

Modelling the impact of a time-varying wave angle on the nonlinear evolution of sand bars in the surf zone

Sandy beaches are often characterized by the presence of sand bars, whose characteristics (growth, migration speed, etc.) strongly depend on offshore wave conditions, such as wave height and angle of wave incidence. This study addresses the impact of a sinusoidally time-varying wave angle of incidence with different time-means on the saturation height, migration speed and longshore spacing of sand bars. Model results show that shore-transverse sand bars (so-called TBR bars) eventually develop under a time-varying wave angle. Depending on the time-mean, amplitude and period of the varying angle of wave incidence, the mean heights and mean migration speeds of the bars can be larger or smaller than their corresponding values in the case of time-invariant angles. Bars might not even form when the wave angle varies around a too large oblique mean value, whereas bars exist in the case of a time-invariant wave angle. The oscillations in both bar height and migration speed are large if the period of the time-varying wave angle is close to the adjustment timescale of the system and if large differences in the local growth and migration rates of the bars occur during one oscillation period. The oscillations in bar height are a combination of harmonics with the principal period and half the period of the time-varying wave angle, whereas those of migration speed contain only the principal period. Bars that are subject to time-varying wave angles have larger longshore crest-to-crest spacings than those which form under fixed wave angles. Physical explanations for these findings are given