Rip currents, morphodynamic simulations

Rip currents are narrow and intense, offshore directed flows in the nearshore zone. A rip current is able to transport a considerable amount of water and sediment seaward and may dominate the morphological exchange between surfzone and the offshore. Rip currents are often located nearby lateral boundaries that obstruct the longshore current, but they can appear as well on long straight beaches, Rip currents can arise with a regular spacing along the beach. In case of a barred beach profile the return flow is concentrated in eroded channels across the bar. Rip currents are not a permanent phenomenon. Under a combination of favourable hydrodynamic conditions they may be generated. Within half a day rips can develop more pronounced. The rip channel may migrate along the open beach and vanish after having persisted for some time, up to several months. The objective of this study is to construct a morphological development associated with an initial rip current. It is investigated in what way the simulations can be valdidated by the ARGUS video system. This system generates images of the nearshore zone on an hourly basis. A rip current is a wave induced flow. Transforming waves exert a force on the fluid mass in which they propagate. In order to balance this force, a pressure gradient or a velocity will arise, This leads to variations in the local mean water depth and currents in the nearshore zone. At the same time the generated current influences the waves. A current in the nearshore zone opposing the waves, like a rip current yield an increased wave height and a reduced wave length so that the wave is steepened, even up to breaking. For successful simulations of an evolving rip current, it is required to know plausible mechanisms that are involved in rip current generation. In literature several theories can be found, explaining the generation of rip currents. Most of them assume differences in wave set-up as mechanism that drives the current. This difference in wave set-up can originate from either hydrodynamic interactions or morphological irregularities. The simulations in this study are executed in DELFT2D-MOR, a numerical model developed at Delft Hyrdaulics. It comprehends modules for simulating hydrodynamics and morphodynamics. In order to obtain a realistic and steady state wave-current field, iterations between wave- and current computations are required. The most direct feedback between the hydrodynamic modules along with a short period of the flow simulations give the best results. A barred profile is used as input for the simulations, On a uniform beach, without any longshore perturbation, the simulations show weak, but clearly distinguishable circulation cells. Simulations starting with a small initial rip channel show stronger currents, Increasing the width of the inner trough increases rip velocities. Increasing of the wave height at the offshore boundary results in increase of the rip current velocity. But, beyond a certain value of the applied wave height, the velocity decreases because of rise of the set-up at the rip head. Waves approaching under a relatively large angle ( > 5 dgr to shore normal) fade the effect of the initial rip channel away. Small angles of wave incidence stimulate the development of a rip. On the one hand because the longshore current is much stronger than the current only generated by differences in set-up. On the other hand because, the longshore current drops sediment from the nearshore zone not directly at the offshore end of the rip channel but more downstream. In the initial rip, the breaking of the waves due to the depth at the bar face, coincides with the maximum current velocity. This results in an increase in the dissipation intensity more than a shift of the dissipation peak down stream. So the wave dissipation, as observed on ARGUS images from rips in Noordwijk, is strongly related to the development of sill at the rip head. Morphodynamic simulations show the deposition of a sill at the downstream end of the rip and a slight growth of the rip channel. The morphological development of the rip, resulting from simulations is strongly dependent on the course of the morphodynamic computation. Frequent feedback shows a splitting of the current and sideward discharge. Longer morphological time steps between calling of the hydrodynamic modules show that the rip current is excavating through the sill. Addition of wave mass flux formulations to the model does not modify the flow pattern of the initial rip current significantly. Differences resulting from taking wave mass fluxes into account, arise from other computed shear stresses, because these are based on a different defined velocity. Computations with wave mass flux cause an increase of the sediment transport rates. This effect is not confined to the rip so the whole beach profile strongly changes.Hydraulic EngineeringCivil Engineering and Geoscience