Multi-scale modeling of the hydrodynamics, sediment transport, and morphodynamics in the nearshore
Date
2021
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Publisher
University of Delaware
Abstract
Understanding the dynamics in coastal regions and forecasting the beach response to different wave events has always been demanding in coastal zones. Here, a multi-scale modeling approach was taken to study the hydrodynamics and the underlying sediment transport and morphodynamics for a variety of bed sediment configurations under different wave conditions. First, using a laboratory-scale model (scale of meters) the detailed fluid-sediment interactions were simulated followed by regional-scale (hundreds of meters) simulation of waves and underlying sediment transport in the nearshore.
Using a two-phase Eulerian-Lagrangian CFD-DEM model we first validated the model for oscillatory sheet flow sediment transport and simulated a variety of sediment bed configurations under a variety of near-bed flow fields representing a variety of wave conditions in the nearshore. Accordingly, sediment beds of well-sorted and mixed sediment ranging from 0.21 mm (medium sand) to 0.97 mm (coarse sand) were simulated under oscillatory wave motions. It was found that sediment size gradation influences the wave induced sediment transport by the inverse vertical sorting of bed sediment particles (coarsening upward). As a result, the transport of bed sediment is dominated by the coarse fraction (exposure effect) while the fine fraction is shielded by the coarse fraction (armoring effect). Model results show that due to the co-existing exposure and armoring effect, the wave-induced onshore sediment transport rate can be enhanced or reduced depending on the bed sediment configuration and wave condition. Model results also showed that sediment size gradation can significantly influence the phase-lags in sediment response to the flow field as well as generation of plug flow under high flow acceleration. It was observed that for the mixed sand consisting of 70% medium fraction (d50 = 0.21 mm) the sediment transport under high-energy waves is remarkably different than that of well-sorted medium sand. Model results corresponding to the mixed sand showed significantly weaker phase-lag effects in sediment response to the flow field. Furthermore, plug flow is generated for the mixed sand attributed to the coarse particles facilitated to be picked up due to the fine particles underneath acting as a relatively soft foundation.
Using the regional-scale XBeach-Surfbeat model, hydrodynamics and morphodynamics during sandbar migration events at Duck, NC, USA were simulated under high-energy and low-energy incident waves. It was found that the model calculations of intense offshore-directed flow referred to as undertow is highly dependent on the wave roller energy formulation. Accordingly, the model skill was improved by implementing a roller energy formulation based on local wave characteristics. It was also found that the model calibration of sediment transport is dependent on the parameterization used for the nonlinear wave shapes referred to as skewness and asymmetry. Generally, during high energy waves the parameterization of skewness and asymmetry were more skillful compared to that during low-energy waves. Accordingly, the calibration coefficients of sediment transport for the offshore sandbar migration under high-energy waves were more similar to those previously obtained for beach erosion under storm events, compared to the calibration coefficients obtained for onshore sandbar migration under low-energy waves.
Future work will focus on phase resolving simulation of hydrodynamics and morphodynamics, resolving more scales of the flow field and sediment transport which will reduce the model skill dependency to the calibration coefficients. The future work will also incorporate some environmental applications of the CFD-DEM model by simulating olivine particles in the nearshore which are aimed at capturing CO2 in the shallow beach region and transfer that to the deep ocean.
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Keywords
Multi-scale modeling, Hydrodynamics, Sediment transport, Morphodynamics