The influence of sea state on formation speed of alongshore variability in surf zone sand bars |
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| Institution: | 1. Faculty of Civil Engineering and Geosciences, Section of Hydraulic Engineering, Delft University of Technology, Delft, The Netherlands;2. UNESCO-IHE Institute for Water Education, Delft, The Netherlands;3. Harbour, Coastal and Offshore Engineering, Deltares, Delft, The Netherlands;1. Department of Civil and Environmental Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-744, Republic of Korea;2. Department of Civil and Environmental Engineering & Integrated Research Institute of Construction and Environmental Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-744, Republic of Korea;1. Department of Water Resources Engineering, Lund University, Box 118, 22100 Lund, Sweden;2. Department of Civil Engineering, Eduardo Mondlane University, C.P. 257, Maputo, Mozambique;1. MARUM, Center for Marine Environmental Sciences, Universität Bremen, Germany;2. Department of Earth and Ocean Sciences, University of Waikato, New Zealand;1. Perpignan University, Centre Européen de Formation et de Recherche sur les Environnements Méditerranéens (CEFREM), UMR5110, 52 Avenue Paul Alduy, 66860 Perpignan Cedex 9, France;2. Aix-Marseille University & Institut Universitaire de France, Centre Européen de Recherche et d''Enseignement des Géosciences de l''Environnement (CEREGE), UM 34, Europôle Méditerranéen de l''Arbois, BP 80, 13545 Aix-en-Provence Cedex 4, France |
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| Abstract: | The formation time of alongshore morphological variability in surf zone sand bars has long been known to differ from one beach to the other and from one post-storm period to another. Here we investigate whether the type of sea state, i.e. distant swell waves or locally generated short period wind sea, affects the formation time of the emerging alongshore topographic variability.A numerical modeling approach is used to examine the emergence of alongshore variability under different shore-normal wave forcing. A research version of Delft3D, operating on the time-scale of wave groups, is applied to a schematised bathymetry with a single bar. The model is then used to investigate several wave scenarios, examining the impact of peak period, frequency spread and directional spread on the formation time of alongshore variability.Results show that an increase in wave period has a large effect, changing the formation time up to O (250%) in case the wave period is changed from a representative value for the Dutch coast (Tp ~ 5–6 s) to an Australian South East coast value (Tp ~ 10–12 s). In contrast, modifications in the directional and frequency spread of the wave field result only in a minor change in the formation time.Examination of hydrodynamics and potential sediment transport shows that the variations in formation time are primarily related to changes in the magnitude of the time-averaged flow conditions. Variations in the magnitude of very low frequency (f < 0.004 Hz) or infragravity (0.004 < f < 0.04 Hz) surf zone flow velocities do not affect the mean sediment transport capacity. Consequently the formation speed of patterns is primarily governed by positive feedback between mean flow and morphology, and low frequency flow fluctuations are of minor importance.These findings indicate that the development of alongshore topographic variability may be faster at swell dominated open coasts, primarily due to the occurrence of longer period swell. Also, at a given site, the arrival of a long wave period swell after a storm can accelerate the emergence of variability. |
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