Narrow Results

The South Pacific Island nation of Tuvalu, which is composed of coral gravel and sand, is vulnerable to storm waves and sea level rise, resulting in beach erosion. Beach nourishment with self-produced coral gravel and sand was implemented in Tuvalu as the first trial of a user- and eco-friendly type of coastal conservation measure in Pacific Island countries. In order to examine the applicability of this type of coastal conservation measure, continuous monitoring has been carried out for one year to check the change in shoreline and beach profile. Beach monitoring for large-scale reclamation project, which was executed at the neighboring coast in almost the same period, was also conducted to compare the change of beach in the two different projects. The results show that the executed gravel beach nourishment can maintain stability under seasonal and extreme condition of wave actions.

This paper presents a discussion on the role of impulsive pressures induced by plunging breakers acting on gravel beaches. In previous works, this process has been hypothesized to play a major role in the observed sediment transport on these beaches. Thus, a diagnostic investigation is carried out here in order to elucidate, in more detail, the anatomy of the impulsive signature in the pressure signal. For this purpose, we employ an integrated approach, which comprises full scale laboratory measurements and their comparison against model results from a well-validated phase/depth resolving numerical model based on the Reynolds-Averaged Navier-Stokes (RANS) equations. Agreement between observations and model predictions allows us to investigate the contribution of each of the acceleration terms in the momentum balance under the wave-impact event by means of the numerical model. The discussion is presented in order to understand how impulsive pressures are generated under plunging waves and their role in sediment mobilisation. Consistent with prior studies, numerical results show that under plunging breakers the local acceleration (du/dt) alone cannot be used as a proxy for pressure gradients. In addition, the importance of the term wdu/dz of total acceleration is recognised for first time. Results from this study suggest that for different types of breaking a different characterisation of the pressure gradient may be sought.

There is a general lack of knowledge of combined cross-shore and longshore changes in gravel barrier beach morphology due to storms. A cluster analysis followed by a multidimensional scaling ordination revealed that the storms affecting a gravel barrier in the southwest of England (Slapton Sands) fall into two very distinct groups: easterlies with mainly wind waves, and southerlies dominated by swell. The morphological response of the barrier to these two typical storm conditions was found to be opposite. Easterly storms induce upper beach erosion and lower beach accretion, while southerly storms cause upper beach accretion and lower beach erosion. In addition, the net alongshore sediment transport for southerlies is northward, while for easterlies it is southward. Video-derived shorelines over three years indicate that the barrier has rotated significantly, as demonstrated by widening of the northern end, and narrowing of the middle section. It is argued that the rotation is caused by a recent change in dominance of one storm type over the other.

In this study, we investigated the characteristics of wave breaking on a gravel beach, considering the effects of the groundwater table. Wave breaking in 835 hydraulic model tests is studied. The experimental results of breaking index including the breaking wave height and breaking water depth in a gravel beach are compared with the existing breaking wave formula for computing breaker height and depth. The experimental results indicate that breaking wave heights and water depths are smaller than those in a sandy beach. Further, the higher the groundwater table, the higher breaking wave height and water depth.