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Coastal morphology
Published in Dominic Reeve, Andrew Chadwick, Christopher Fleming, Coastal Engineering, 2018
Dominic Reeve, Andrew Chadwick, Christopher Fleming
One of the key mechanisms causing profile erosion is the offshore current near the seabed, commonly termed ‘undertow’. Outside the surf zone there is a shoreward steady flow near the bed, caused in part by the asymmetry in the (nonlinear) wave orbital velocities. At the transition zone waves transform from a non-breaking state to one resembling a turbulent base in which turbulence has become fully developed. The transformation does not occur instantaneously at the point of breaking but develops over a distance beyond the breaking point. Two ways of modelling this transition zone are described by Nairn et al. (1990). Wave-induced sediment transport formulae typically involve one or more moments of the wave orbital velocity at the seabed.
Modelling Procedures
Published in Vanesa Magar, Sediment Transport and Morphodynamics Modelling for Coasts and Shallow Environments, 2020
Mass conservation for bedload and suspended load is a critical issue, which has to be addressed in all sediment transport models. For example, a discrete-vortex, particle-tracking model developed during a project funded by the European Commission between 2002 and 2005 (contract no. EVK3-2001-00056), on Sand Transport and Morphology of Offshore Sand Mining Pits/Areas (the SANDPIT project), to analyse sediment dynamics above rippled beds, although validated against experimental data (Van der Werf et al. 2008), required extensive adjustments before the results could be confidently presented in a further work (Malarkey et al. 2015). In the research by Jacobsen and collaborators, they follow a similar development path, and once the mass conservation procedure has been validated, they develop the model further for morphodynamic studies. Using a wide range of the surf similarity parameter (Equation 3.25) allows them to consider spilling and plunging breaker cases. Dean’s parameter (Equation 3.27), on the other hand, is used to classify different breaker bar types. The cross-shore evolution of breaker bars is driven by different cross-shore coastal processes, including nonlinear wave effects, such as Stokes drift, wave asymmetry, and skewness, and wave-driven currents such as streaming and undertow. According to Jacobsen et al. (2014), the undertow is a crucial process in suspended sediment dynamics. Undertow is caused by the depth-varying shear stress, combined with an overall balance between a net shoreward volume flux above the wave crest and a seaward volume flux below the wave trough. In a more recent work, Roelvink & Stive (1989) showed that the wave asymmetry drives the breaker bar shoreward, whereas the undertow drives it seaward.
Surface Processes
Published in F.G.H. Blyth, M. H. de Freitas, A Geology for Engineers, 2017
F.G.H. Blyth, M. H. de Freitas
With an onshore wind blowing at right angles to the shore-line, water is heaped up against a coast; this is compensated by a return current away from the land, called the undertow, which may be concentrated into narrow channels. The undertow can transport finer sediments out to sea, and in storms a strong undertow can remove large quantities of beach.
Lagrangian two-phase flow modeling of scour in front of vertical breakwater
Published in Coastal Engineering Journal, 2020
Abbas Yeganeh-Bakhtiary, Hamid Houshangi, Soroush Abolfathi
Fluid–sediment interactions during the scour–deposition process are further studied by comparing the horizontal velocity of sediment particles and horizontal orbital velocity of fluid particles during a wave period at the halfway between first node and antinode from the vertical wall. Figure 13 presents the horizontal velocity variation against the nondimensional sediment particle layers, , where is the sediment particle diameter. The height range, between to , relates to the hyper-concentrated layer; to 13 is within the saltation layer, and values higher than depicts the suspended particle layer. Further analysis of numerical results show that after a short time from the start of the simulation (), the sediment particles accelerated their motion and reached to similar horizontal velocity as the fluid particles. At the horizontal velocity of sediment particles exceeded the fluid particles, and after a time-lag difference was clearly noticed. Figure 13 illustrates that in the upper part of the water column when the direction of fluid flow changes, the sediment particle acceleration increases due to bore and undertow motion. Hence, the interphase momentum is transferred remarkably from the fluid phase to the sediment phase at .
Simulating 2DH coastal morphodynamics with a Boussinesq-type model
Published in Coastal Engineering Journal, 2018
Georgios T. Klonaris, Constantine D. Memos, Nils K. Drønen, Rolf Deigaard
It is also noted that the convective terms in Equation (2) derive from the product of the depth-averaged concentration and the depth-integrated horizontal velocity, which is different from the depth-integrated product of the vertical profiles of the horizontal velocity and sediment concentration. This latter quasi-3D approach can more accurately predict the offshore-directed sediment transport produced by undertow currents (Fredsøe and Deigaard 1992). However, the present wave module relies on the depth-averaged Boussinesq equations and thus the analytical profile of the horizontal velocity is not available.
Hydrodynamical and morphological patterns of a sandy coast with a beach nourishment suffering from a storm surge
Published in Coastal Engineering Journal, 2022
Xuejian Han, Cuiping Kuang, Lei Zhu, Lixin Gong, Xin Cong
Storms play an important and sometimes even dominant role in the evolution of beach geomorphology. Jimenez et al. (2012) found that part of the beaches in Catalonia, Spain, had completely disappeared after 1950 due to storm action. The rise of the nearshore water level during a storm will enlarge the flooded area of the beach, which will further extend the range of the wave action on the beach (Nielsen 1988). The intensified waves in the storm lead to great differences in the relative proportion and action range of wave shallowing, wave breaking, and swash in the coastal area compared with the normal wave conditions (Rocha et al. 2013), which is the fundamental dynamic mechanism for the morphodynamic change of sandy coast under storm conditions (Anthony 2013). The sediment transport is driven by the cross-shore current (stokes drift and undertow) induced by shoreline-perpendicular incident waves and the longshore current induced by oblique waves. An observational study by Hill et al. (2004) on nine beaches in Saco Bay, Maine, USA, which experienced multiple storms, showed that the net sediment transport at the beach was controlled by the combined driving of longshore current and undertow. The cross-shore migration of the submerged sandbar is mainly controlled by the wave energy variation: the sandbar moves slowly onshore and may eventually merge into the backshore under the low-energy waves (Trowbridge and Young 1989), in which case the beach profile is known as the normal profile (Dean 1973); the strong waves, especially in a storm scour the beach berm even the dune and the eroded sediment is transported offshore as longshore sandbars (Van Enckevort and Ruessink 2003), in which case the beach profile is known as the storm profile. Surf beat plays an important role in coastal geomorphology as it will not break in the very shallow waters unlike short waves. As the measurements in Torrey Pines beach, California, shown by Guza and Thornton (1982), the surf beat in a storm will intensify with wind and waves propagating toward the shore, and dominate the water motion in the surf and swash zone. The monitoring study by Aagaard and Greenwood (1994) on Stanhope Lane beach in Canada shows that the suspended sediment transport in the surf zone during storms was mainly controlled by the enhanced surf beat. A storm impact scale was proposed by (Sallenger 2000) to evaluate the influence of storm hydrodynamics on the barrier islands, based on which four storm impact regimes are defined, i.e. swash, collision, overwash, and inundation regime.