Nils Drønen

A morphological stability analysis for a long straight barred coast

A morphological stability analysis is carried out for a long straight coast with a longshore bar. The situation with oblique wave incidence and a wave-driven longshore current is considered. The flow and sediment transport are described by a numerical modelling system. The models comprise: (i) a wave model with depth refraction, shoaling and wave breaking, (ii) a depth integrated model for wave driven currents and (iii) a sediment transport model for the bed load transport and the suspended load transport in combined waves and current. The direction of the sediment transport is taken to be parallel to the depth integrated mean current velocity, neglecting the effects of a bed slope and secondary currents. An instability is found to develop around the bar crest. The instability is periodic in the alongshore direction, and tends to form rip channels and to steepen the offshore face of the bar between the rip channels. The alongshore wave length of the most unstable perturbation is determined for different combinations of the wave conditions and the geometry of the profile.

An experimental study of rip channel flow

A laboratory study of the flow over a bar with a single rip channel has been performed. First, the well-known pattern of a bar circulation cell with a strong offshore-directed current out through the rip channel and a weaker onshore-directed return flow over the bar is documented. Then measurements of the three-dimensional structure of the flow in the area where the rip channel, the bar and the trough meet and well inside the rip channel are presented. These measurements reveal that 3D effects play an important role, and that a depth-integrated viewpoint may not always be sufficient for predicting the flow in the near bed region. Particle-tracking experiments illustrate the near bed flow pattern over the entire area. These demonstrate how the overall trajectory pattern changes as a function of the distance of wave breaking from the bar crest: For some conditions, the rip current is fed from the trough and for other conditions it is fed directly from the bar. Both the 3D measurements and the trajectory tests show the existence of a weaker onshore-directed near-bed drift in the area where the rip current ceases. Finally, in a series of sensitivity tests, measurements of the rip current intensity for different wave climate and water level conditions reveal a strong correlation between the rip current intensity and the wave height (both normalized).

LONG TERM MODELING OF SHORELINE RESPONSE TO COASTAL STRUCTURES

A new model concept for shoreline evolution is applied to the case of an offshore breakwater. The model is verified against bathymetrical surveys performed 4 years after the breakwater construction, and used to predict the consequences of an alternative design. The model concept is based on the combination of a one-line type of schematization of the nearshore morphology with a 2DH area modeling approach for waves, currents and sediment transport. This concept makes a model capable of taking into account: 1) variation in the offshore bathymetry and 2) the effect of coastal structures on waves and currents, and hence on sediment transport and shoreline response. The model results show very good quantitative and qualitative agreement with the observed morphology in the case of the original breakwater layout, and the results from the alternative breakwater layout demonstrate the model’s potential as a tool for shoreline assessment studies.

Morphological development of coasts at very oblique wave incidence

(19/01/2019) Morphological development of coasts at very oblique wave incidence This study focuses on one distinct feature to be found on coasts exposed to a very oblique wave incidence, namely an accumulating spit. That is a spit where no retreat of the shoreline is going on along the spit. This requires a monotonically decreasing sediment transport capacity from the updrift stretch of coast, along the spit as the coast curves away from the approaching waves towards the tip of the spit. A one-line model for the coastline development predicts that accumulating spits can exist on coasts exposed to waves approaching at angles larger than 45 degree only. It is suggested that such a spit grows without changing its shape i.e. an equilibrium form emerge if the coast is exposed to a constant wave climate. During experiments conducted in a wave tank where a uniform stretch of coast was exposed to waves approaching at a very oblique angle an accumulating spit was formed at the down-drift end of the coast. The spits approached equilibrium forms when constant wave climates were applied. The sediment transport around the spit has been investigated by twodimensional models. The characteristic length scale for the equilibrium form depends linearly on the width of the surf zone for constant wave incidence at the updrift stretch of coast. The factor of proportionality is in reasonable agreement with the experimentally found.