On bar growth and decay during interannual net offshore migration

Abstract Multiple bar systems often show a cyclic net offshore directed migration with return periods on the order of years. Generally, a bar is generated near the shoreline, grows in height and width, while migrating offshore before finally decaying at the seaward limit of the surf zone. Based on a three-year hindcast of a bar cycle at Noordwijk (Netherlands) and on additional synthetic runs using a wave-averaged cross-shore process model, this study identifies the dominant mechanisms that govern the bar amplitude growth and decay during net interannual offshore migration. The bar amplitude response is particularly sensitive to the water depth above the bar crest, h Xb , and the angle of wave incidence, θ . These variables largely control the amount of waves breaking on the bar and the strength and cross-shore distribution of the associated longshore current. The longshore current has its maximum landward of the bar crest, inducing additional stirring of sediment on the landward bar slope and trough. The enhanced sediment concentration in the trough region shifts the cross-shore transports peak landward of the bar crest forcing bar amplitude growth during offshore migration. For increased h Xb -values wave breaking becomes less frequent, consequently reducing the influence of the longshore current on sediment stirring. Therefore, the resulting dominance of the cross-shore current results in a sediment transport peak at the bar crest causing bar amplitude decay. All four types of bar response (viz. all combinations of onshore/offshore migration and bar amplitude growth/decay) can occur for a single wave height and wave period combination, depending on h Xb and θ . Additional hindcast runs in which the wave direction was assumed time-invariant confirmed that h Xb and θ largely control the transient bar amplitude response.

[1]  John A. Nelder,et al.  A Simplex Method for Function Minimization , 1965, Comput. J..

[2]  J. H. D. Vroeg.  Schematisering brandingsruggen met behulp van jaarlijkse kustmetingen , 1987 .

[3]  Michael Selwyn Longuet-Higgins,et al.  Mass transport in water waves , 1953, Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences.

[4]  J. A. Battjes,et al.  A laboratory study of longshore currents over barred and non-barred beaches [Coastal Eng., 30 (1997) 1–22] , 1997 .

[5]  B. G. Ruessink,et al.  Morphodynamic response of nearshore bars to a shoreface nourishment , 2005 .

[6]  Richard Soulsby,et al.  Evaluating the performance of morphological models , 2004 .

[7]  Steve Elgar,et al.  Modeling the alongshore current on barred beaches , 2001 .

[8]  B. G. Ruessink,et al.  Video observations of nearshore bar behaviour. Part 1: alongshore uniform variability , 2003 .

[9]  B. G. Ruessink,et al.  Dunefoot dynamics along the Dutch coast , 2002 .

[10]  Michele Capobianco,et al.  Shoreline evolution of the Holland coast on a decadal scale , 1999 .

[11]  J. A. Roelvink,et al.  Field Validation and Application of a Coastal Profile Model , 1996 .

[12]  J. A. Battjes,et al.  ENERGY LOSS AND SET-UP DUE TO BREAKING OF RANDOM WAVES , 1978 .

[13]  B. G. Ruessink,et al.  Numerical predictability experiments of cross‐shore sandbar migration , 2008 .

[14]  J. Ribberink.  Bed-load transport for steady flows and unsteady oscillatory flows , 1998 .

[15]  B. G. Ruessink,et al.  Models and scales for cross‐shore sandbar migration , 2010 .

[16]  Shunqi Pan,et al.  The predictability of cross-shore bed evolution of sandy beaches at the time scale of storms and seasons using process-based Profile models , 2003 .

[17]  E. Thornton,et al.  Morphodynamic modeling of an embayed beach under wave group forcing , 2004 .

[18]  Kathelijne Mariken Wijnberg,et al.  Extracting decadal morphological behaviour from high-resolution, long-term bathymetric surveys along the Holland coast using eigenfunction analysis , 1995 .

[19]  John D. Fenton,et al.  A Fourier approximation method for steady water waves , 1981, Journal of Fluid Mechanics.

[20]  Kathelijne Mariken Wijnberg,et al.  Intersite comparison of interannual nearshore bar behavior , 2003 .

[21]  Howard N. Southgate,et al.  Transition Zone Width and Implications for Modeling Surfzone Hydrodynamics , 1991 .

[22]  Donald G. Bailey,et al.  An Inter-Site Comparison of Net Offshore Bar Migration Characteristics and Environmental Conditions , 1999 .

[23]  Edward B. Thornton,et al.  Bar/trough generation on a natural beach , 1996 .

[24]  R. Soulsby.  Dynamics of marine sands , 1997 .

[25]  I. A. Svendsen.  Wave Heights and Set-up in a Surf Zone , 1983 .

[26]  H. N. Southgate,et al.  Calibration and verification of a parametric wave model on barred beaches , 2003 .

[27]  Jurjen A. Battjes,et al.  A laboratory study of longshore currents over barred and non-barred beaches , 1997 .

[28]  E. Thornton,et al.  Vertical flow structure during Sandy Duck: observations and modeling , 2004 .

[29]  L. Rijn.  Principles of sediment transport in rivers, estuaries and coastal seas , 1993 .

[30]  H. G. Wind,et al.  Cross-shore mean flow in the surf zone , 1986 .

[31]  B. G. Ruessink,et al.  Process-based modelling of a shoreface nourishment , 2004 .

[32]  Yoshiaki Kuriyama,et al.  Medium-term bar behavior and associated sediment transport at Hasaki, Japan , 2002 .

[33]  D. Walstra,et al.  Seasonal-scale nearshore morphological evolution: Field observations and numerical modeling , 2009 .

[34]  Richard Essery,et al.  Scaling and parametrization of clear-sky solar radiation over complex topography , 2007 .

[35]  Pierre Etchevers,et al.  Parameter sensitivity in simulations of snowmelt , 2004 .

[36]  O. Phillips.  The dynamics of the upper ocean , 1966 .

[37]  Ad Reniers,et al.  Modeling cross-shore sandbar behavior on the timescale of weeks , 2007 .

[38]  Dirk-Jan R. Walstra,et al.  Nourishing the shoreface: observations and hindcasting of the Egmond case, The Netherlands , 2004 .

[39]  Giovanni Coco,et al.  Observations of nearshore crescentic sandbars , 2004 .

[40]  Jorge Guillén,et al.  Morphodynamic response of a two-barred beach to a shoreface nourishment , 2008 .

[41]  Stig E. Sand.  Long Wave Problems in Laboratory Models , 1982 .

[42]  J. A. Roelvink,et al.  Bar-generating cross-shore flow mechanisms on a beach Barre provoquant des mecanismes d' ecoulement perpendiculaires a la plage , 1989 .

[43]  B. G. Ruessink,et al.  The behaviour of a multiple bar system in the nearshore zone of Terschelling, the Netherlands: 1965–1993 , 1994 .

[44]  R. Deigaard.  A note on the three-dimensional shear stress distribution in a surf zone , 1993 .