Rauwoens, Pieter

Suzuki, Tomohiro

Troch, Peter

Van Oyen, Tomas

Zijlema, Marcel

English

Ghent University Academic Bibliography

A FINITE-AMPLITUDE MORPHODYNAMIC MODEL OF TIDALLY GENERATED BED FORMS BASED ON SWASHTomas Van Oyen,Department of Civil Engineering, Ghent University, tomas.vanoyen@ugent.beTomohiro Suziki, Flanders Hydraulics Research, tomohiro.suzuki@mow.vlaanderen.beMarcel Zijlema, Environmental Fluid Mechanics Section, Delft University of Technology, M.Zijlema@tudelft.nlPieter Rauwoens, Department of Civil Engineering, Ghent University, pieter.rauwoens@ugent.be Peter Troch, Department of Civil Engineering, Ghent University, peter.troch@ugent.beINTRODUCTION Tidal  sand  waves  are  prominent  bed  forms  occurring  in coastal  seas  characterized  by  non-cohesive  sediment deposits and relatively strong tidal currents (0.3 - 1.0 m/s). These bed features have a crest-to-crest distance between 100 and 500 m and are oriented approximately orthogonal to the main direction of the tidal current. Typical amplitudes are of the order of 1 to 10 m. Due to the ability of the bed forms to migrate, tidal sand waves pose serious hazards to many coastal structures such as pipe lines, monopiles and shipping lanes. Insight in the dynamics of these bed forms therefore  is  crucial  for  the long-term management  of  the coastal zone. Here, we describe the development of a new generic  numerical  morphodynamic  model  to  simulate  the finite amplitude  of tidal sand waves.  MODELSince  the  crests  of  tidal  sand  waves  are  almost perpendicular to the main direction of the tidal current, we adopt  a  two-dimensional  model  in  the  horizontal  and vertical directions, respectively x and z-axes; assuming that the crests of the waves are aligned with the x-axis. The tidal flow  field  is  simulated  using  a  modified  version  of  the numerical non-hydrostatic free surface flow solver SWASH (Zijlema et al., 2011). In particular, an additional forcing F(t) is added to the horizontal momentum balance, resembling the  gradient  in  the  tidal  potential.  For  details  on  the properties of the numerical solution procedure, we refer to (Zijlema et al., 2011). Once the flow field is evaluated, we compute the bed shear stress and determine the sediment transport  Q(t).  The  latter  is  calculated  by  means  of  the bedload  transport  formula  proposed  by  van  Rijn  (1991), accounting  also  for  slope  effects.  Considering  that  the changes in the bed during the tidal cycle are very small, the evolution of the bed can be described by with n the porosity, d the bed level and T the period of the tide. When the adjusted bed elevation is known, a new step in the morphodynamic loop is performed by reevaluating the flow field, bed shear stresses and corresponding sediment transport. Here, it worth to point out that, to speed up the calculations, this step is only taken when the changes in the bed elevation are larger than a percentage α of the depth. RESULTSFigure 1 (top panel) presents the evolution in time of a tidal sand  wave  considering  a  mean  depth  of  15  m  and  the maximum of the depth-averaged tidal velocity equal to 0.4 m/s. The mean grain size of the bed is set 0.3 mm. Initially a sand wave with a wavelength of 390 m is considered with an amplitude  of  0.15 m.  We considered a wavelength of 390 m based on a linear stability analysis of the bed using the model of Blondeaux and Vittori (2005).  Figure 1 shows that the sand wave growth slows down and eventually  stops  after  approximately  500  years  when the amplitude of the bed form is about 7.5 meter. DISCUSSION AND CONCLUSIONComparing  the  obtained  finite  amplitude  with  field observations, it appears that the model overestimates the final amplitude of the sand waves. Also, the growth of the bed forms is found to occur over much longer time scales than suggested by linear stability analysis. Taken together, we  conclude  that  further  work  is  still  needed  before  the morphodynamic  model  can  be  confidentially  used  as  a modelling tool. Figure 1 – Top: evolution in time of the depth at the crest of the sand wave.  Bottom:  profile  of  the  initially  considered sand wave.REFERENCESBlondeaux and Vittori (2005): Flow and sediment transport induced by tidal wave propagation. Part 2: the wavy bottom case. J. Geophys. Res. 110 van Rijn (1991): Sediment transport in combined currents and waves. In  Sand transport in rivers, estuaries and the sea. Euromech 262Zijlema, Stelling and Smit,  (2011):  Swash: An operational public domain code for simulating wave fields and rapidly varied flows in coastal waters. Coastal Engineering, vol. 58

A finite-amplitude morphodynamic model of tidally generated bed forms based on SWASH

https://biblio.ugent.be/publication/3144699/file/3144707.pdf

A finite-amplitude morphodynamic model of tidally generated bed forms based on SWASH

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