SHORELINE SAND WAVES AND BEACH NOURISHMENTS

N. van den Berg, A. Falqués, F. Ribas

Abstract


The effects of the feedback between the changing coastal morphology and the wavefield on the generation and propagation of large scale (O(1-10 km)) shoreline sand waves is examined with a quasi-2D morphodynamic model. Traditional shoreline change models do not include this feedback and are only able to describe diffusion of shoreline sand waves and furthermore they are unable to describe migration. It is found with the present model that if there is a dominant littoral drift, the feedback causes downdrift migration of coastline features no matter if they grow or decay. Consistently with previous studies, simulations show that a rectilinear coastline becomes unstable and sand waves tend to grow spontaneously from random perturbations, if the wave incidence angle is larger then about 42oc) at the depth of closure (high angle wave instability). The initial wavelengths at which the sand waves develop are 2-3 km and this is similar to previous linear stability analysis. The implications of high angle wave instability for beach nourishments are investigated. The nourished shoreline retreats initially due to cross-shore transport because the nourished profile is steeper than the equilibrium profile. When a dominant littoral drift is present, the nourishment also migrates downdrift. If the wave angle at the depth of closure is below θc the alongshore transport contributes to the diffusion of the nourishment. However, if the angle is above θc (constant high angle wave conditions) the diffusion is reversed and the nourishment can trigger the formation of a shoreline sand wave train. Numerical experiments changing the proportion of ‘high angle waves’ and ‘low angle waves’ in the wave climate show that relatively small proportions of low angle waves slow down the growth of sand waves. These simulations with more realistic wave climates show shoreline sand waves that migrate downdrift maintaining more or less the same amplitude for years.

Keywords


alongshore sediment transport; beach morphology; beach nourishment; coastline instability; high angle waves; shoreline sand waves

References


Ashton A., A.B. Murray and O. Arnault, 2001. Formation of coastline features by large-scale instabilities induced by high-angle waves.) ature, 414, 296-300.

Ashton A. and A.B. Murray, 2006a. High-angle wave instability and emergent shoreline shapes: 1. Modeling of sand waves, flying spits, and capes. J.Geophys.Res, 111, F04011, doi:10.1029/2005JF000422. http://dx.doi.org/10.1029/2005JF000422

Ashton A. and A.B. Murray, 2006b. High-angle wave instability and emergent shoreline shapes: 2. Wave climate analysis and comparisons to nature. J.Geophys.Res, 111, F04012, doi:10.1029/2005JF000423.http://dx.doi.org/10.1029/2005JF000423

Bruun, P., 1954. Migrating sand waves or sand humps, with special reference to investigations carried out on the Danish North Sea Coast. Proc. 5th Int. Conf. Coastal Eng. ASCE, New York, 269– 295.

Davidson-Arnott, R.G.D. and A. van Heyningen, 2003. Migration and sedimentology of longshore sandwaves, Long Point, Lake Erie, Canada. Sedimentology 50, 1123–1137. http://dx.doi.org/10.1046/j.1365-3091.2003.00597.x

Dean, R. G., 2002. Beach nourishment. Theory and practice. World Scientific, Singapore

Falqués A. and D. Calvete, 2005. Large scale dynamics of sandy coastlines. Diffusivity and instability. J. Geophys. Res., 110, C03007,doi:10.1029/2004JC002587. http://dx.doi.org/10.1029/2004JC002587

Falqués A., N. van den Berg and D. Calvete, 2008. The role of cross-shore profile dynamics on shoreline instability due to high angle waves. Proc. 32nd Int. Conf. Coastal Eng. World Scientific, Singapore.

Grove, R.S., C.J. Sonu and D.H. Dykstra, 1987. Fate of a massive sediment injection on a smooth shoreline at san onofre, california. In: Coastal Sediments 1987. Am. Soc. of Civ. Eng., pp. 531–538.

Inman, D.L., 1987. Accretion and erosion waves on beaches. Shore and Beach 55 (3/4), 61–66.

Komar, P.D., 1998. Beach Processes and Sedimentation, 2nd Edition. Prentice Hall, Englewood Cliffs, N.J.

Larson M. and N.C. Kraus, 1991. Mathematical modeling of the fate of beach fill. Coastal Eng., 16, 83-114.http://dx.doi.org/10.1016/0378-3839(91)90054-K

List, J.H. and A.D. Ashton, 2007. A circulation modeling approach for evaluating the conditions for shoreline instabilities. Coastal Sediments 2007. ASCE Conf. Proc. 239, 25.

Ozasa, H. and A.H. Brampton, 1980. Mathematical modelling of beaches backed by seawalls. Coastal Eng. 4, 47–63. http://dx.doi.org/10.1016/0378-3839(80)90005-8

Ruessink, B.G. and M.C.J.L. Jeuken, 2002. Dunefoot dynamics along the dutch coast. Earth Surface Processes and Landforms 27, 1043–1056. http://dx.doi.org/10.1002/esp.391

Stive M.J.F., S.G.J. Aarninkhof, L. Hamm, H. Hanson, M. Larson, K.M. Wijnberg, R.J. Nicholls and M. Capobianco, 2002. Variability of shore and shoreline evolution. Coastal Engineering, 47, 211-235, DOI: 10.1016/S0378-3839(02)00126-6.http://dx.doi.org/10.1016/S0378-3839(02)00126-6

Thevenot M.M and N.C. Kraus, 1995. Longshore sandwaves at Southampton Beach, New York: observations and numerical simulation of their movement. Mar. Geology, 126, 249-269.http://dx.doi.org/10.1016/0025-3227(95)00081-9

van den Berg, N., A. Falqués and F. Ribas, 2010. Long-term evolution of nourished beaches under high angle wave conditions. J. Marine Systems, in press

Verhagen, H.J., 1989. Sand waves along the Dutch coast. Coastal Eng., 13, 129–147.http://dx.doi.org/10.1016/0378-3839(89)90020-3


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