Numerical simulation of suspended sediment transport induced by near-shore wave breaking.

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Maria-Angeliki Sfouni-Grigoriadou, Gerasimos Kolokythas, Athanassios Dimas

Monday 29 june 2015

16:30 - 16:45h at Amazon (level 1)

Themes: (T) Water engineering, (ST) River and coastal engineering

Parallel session: 3C. Coastal engineering

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The study of sediment transport in the coastal environment areas is essential for predicting significant morphological changes, such as erosion, which increase considerably the risk of failure of near-shore structures. In this paper the behavior of sediment suspension induced by wave breaking over a constant-slope beach, is investigated. The simulations are based on the coupled numerical solution of the flow equations (continuity and Navier-Stokes) with the transport equation for suspended sediment load. The Navier-Stokes equations subject to the fully nonlinear free-surface boundary conditions and appropriate bottom, inflow and outflow boundary conditions, while the bed concentration boundary condition for the suspended sediment, is defined by means of empirical formulas depending on the instantaneous bed shear stress. The settling velocity of sediment grains is also calculated by use of an empirical the formula proposed by Hallermeier (1981) depending on the grain diameter. The equations of fluid and sediment motion are properly transformed so that the computational domain becomes time-independent. A hybrid scheme is used for the spatial discretization with finite differences in the streamwise direction and a pseudospectral approximation with Chebyshev polynomials in the vertical direction. A fractional time-step scheme is used for the temporal discretization. In the present study, the cases of characteristic grain size normalized to the characteristic water depth, Ds/d = 10-4, 2_10-4 and 5_10-4, are investigated. During wave breaking and also in the surf zone, strong uplift of bed sediment is observed, while the maximum height of sediment suspension reaches almost the 80% of the local depth for the case of the smallest grain size. On the contrary, sediment suspension is restricted in the vicinity of the bed, for the case of the bigger grain size. The spatial distribution of the net discharge of suspended sediment clearly indicates the tendency of sediment to move to the offshore direction.