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Wave-driven dynamics of Shoreward propagating acretionary waves in the nearshore

Weerd, Lianne van der (2012) Wave-driven dynamics of Shoreward propagating acretionary waves in the nearshore.

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Abstract:A new phenomenon in the evolution of nearshore topography is a small-scale natural mode of shoreface nourishments observed by Wijnberg and Holman (2007). It is referred to as Shoreward Propagating Accretionary Waves (SPAWs) and is a bar-like feature shed of from the nearshore bar. It is observed to transit the through between bar and shore as an intact form. This study identified which nearshore processes control the shoreward propagation of a SPAW phenomenon after it has been initiated. The wave-driven flow field and related initial sediment transport patterns were simulated with a three-dimensional Delft3D model with a high spatial and temporal resolution. Based on statistics of SPAWs observed near Duck (North Carolina, USA) (Wijnberg and Holman, 2007) a schematized bathymetry was defined and typical wave conditions were selected (Hs=0.56 m and Tp=8.2 s). Additional to this base case, the influence on initial sedimentation and erosion patterns was assessed of different water levels, SPAW size and location, and nearshore bar geometry. Results showed that under typically prevailing wave conditions the process of wave transformation (i.e. increasing wave skewness and asymmetry) over the SPAW is important to generate onshore sediment transports over the feature. Near-bed transport processes in the direction of wave propagation due to wave asymmetry were dominant in all cases. These processes consisted of (i) bed load transport due to waves and currents, and (ii) suspended load due to wave asymmetry. Furthermore, the process of local wave breaking (i.e. energy dissipation), generates a horizontal circulation current around the SPAW. Since near-bed transport is dominant for our cases with a low wave height, it was shown that the generated circulation pattern did hardly influence sediment transport patterns over the SPAW. The onshore transports over the SPAW result in a shoreward displacement of the SPAW, consistent with SPAW observations in nature. This pattern persisted for different water levels, different SPAW sizes and location, and different nearshore bathymetry.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:52 mechanical engineering
Programme:Industrial Design Engineering MSc (66955)
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