Cross-Shore Redistribution of Nourished Sand near a Breaker Bar: Fid-Bau Portal

Cross-Shore Redistribution of Nourished Sand near a Breaker Bar

Jacobsen, Niels Gjøl / Fredsoe, Jørgen

Abstract

This paper focuses on the optimal location for dumping nourished sand on a barred coastline. This is done by investigating the short-term behavior of the cross-shore redistribution of nourished sediment on a breaker-bar profile in a two-dimensional vertical plane. This is achieved by the use of a complete numerical description of the surf-zone processes with respect to both hydrodynamics and sediment transport. The numerical model is based on the finite-volume approach with a free surface-tracking method, also known as the volume of fluid (VOF), and the sediment transport is calculated applying the Engelund and Fredsøe deterministic concept. The methodology is as follows: a Dean/Bruun equilibrium profile is exposed to regular waves (chosen as $H = 1.3 m$ and $T = 4.8 s$ ). These waves will form bars on the Dean/Bruun equilibrium profile, and the wave impact is continued until a quasi-steady behavior of the bars has been obtained, i.e., bars that have stopped growing in height and only migrate slowly offshore. Next, the profile is nourished. The resulting continued morphologic developments are compared with the development of the unnourished (reference) profile. Also, the effect of a net shoreward current over the profile is consider as a means of imposing the net recirculation resulting from longshore nonuniformities or the impact of wind-induced shear stresses. The impact of wave height, irregularity in incident waves, and the size of the sediment grains on the cross-shore distribution of suspended sediment transport rates are described. In addition, all profiles are seen to lose sediment seaward when no net shoreward current is enforced over the outermost breaker bar. Alternative strategies for nourishment are investigated, and it is seen that the shape on the nourishment seaward of the outermost bar can have both destructive and protective impacts on the original shoreward-located cross-shore profile.