Stability of stones in the surf zone

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Abstract

Much research has been done on the stability of stones in breaking waves, but up to now, most of these studies were based on experiments with slopes varying from 1: 1 to 1:7. The stability of stones on mild slopes, slopes not steeper than 1: 10, has not yet been researched very extensively. Applications of mild slopes in practice are for example landfalls of oil pipelines and outfalls of sewage systems. The objective of this study is to improve the theoretical knowledge of the stability of stones on mild slopes in the surf zone by researching the flow in breaking waves. The stability relations for stones on mild slopes established so far, followed the trend of experimental results quite well in a qualitative way, but the difference in stability for spilling breakers and plunging breakers was predicted too small by the relations. Probably the main reason of this imperfection is the influence of the plunging jet in a plunging breaker. Therefore, the processes which take place in plunging breakers are studied. From a study by Basco (1985) it was concluded that processes in spilling and plunging breakers are similar, albeit that the vortex systems in plunging breakers are of a much larger scale. Experiments were carried out in the large wave flume of the Laboratory of Fluid Mechanics for a better understanding of the stability of stones on a slope subjected to wave attack. The model structure consisted of a 1: 10 impermeable slope, on which a layer of stones (Dnso =1.21 cm) was laid. Only regular waves were used, because these wave are more suitable for researching the flow in a particular wave. For three waves with different wave steepnesses, incipient motion of the stones was determined. SUbsequently, in the breaking regions of these waves, velocity measurements were carried out by means of LDV and video recordings. From the damage experiments it was concluded that maximum damage was located at about h/Ho =0.6 and that the direction of displacement of the stones depends on the breaker parameter. Furthermore, the stone displacement in upslope direction seemed to be caused by the plunging jets of the breaking waves. The velocities in the plunging jet were equivalent for the three different waves, which is in line with the fact that these waves cause incipient motion. The plunging jets of the breaking waves cause incipient motion of the stones. Up to now no theories were available for the stone stability in plunging jets. Therefore, an attempt was made to model the stone stability in a plunging jet. Two different models were considered, which both schematize the plunging jet on a stone as static forces on a single cubical stone. From the modelling it was concluded that the results deviate from experimental results. The missing of the turbulent fluctuations of the jet and to a less extent the dynamic characteristics of the stone stability were probably the main reasons of this deviation. Nevertheless, the modelling can improve the theoretical understanding of the stone stability in plunging jets. The numerical results of the stability relation by Izbash for uniform flow are close to the experimental results. Therefore, it seems that stone stability in a plunging jet is not as unfavourable as expected, compared to stone stability in uniform flow. The resulting stability equations for the stone stability in plunging breakers is in conformance with existing relations.