Detailed investigation of overwash on a gravel barrier

This paper uses results obtained from a prototype-scale experiment (Barrier Dynamics Experiment; BARDEX) undertaken in the Delta flume, the Netherlands, to investigate overwash hydraulics and morphodynamics of a prototype gravel barrier. Gravel barrier behaviour depends upon a number of factors, including sediment properties (porosity, permeability, grain-size) and wave climate. Since overwash processes are known to control short-term gravel barrier dynamics and long-term barrier migration, a detailed quantification of overwash flow properties and induced bed-changes is crucial. Overwash hydrodynamics of the prototype gravel barrier focused on the flow velocity, depth and discharge over the barrier crest, and the overwash flow progression across and the infiltration through the barrier. During the BARDEX experiment, overwash peak depth (0.77 m), velocity (5 m s−1 ) and discharge (max. 6 m3 m−1 ) were high, especially considering the relatively modest wave energy (significant wave height = 0.8 m). Conversely to schemes found in the literature, average flow depth did not linearly decrease across the barrier; rather, it was characterised by a sudden decrease at the crest, a milder decrease at the barrier top and then propagation as a shallow water lens over the backbarrier. The barrier morphological evolution was analysed over a series of 15-min experimental runs and at the timescale of individual overwash events. Overall, the morphological variation did not result from an accumulation of many small consistently erosive or accretionary events, but rather the mean bed elevation change per event was quite large (10 mm) and the overall morphology change occurred due to a small imbalance in the number of erosive and accretionary events at each location. Two relationships between overwash hydrodynamic variables were deduced from results: (1) between overwash flow depth and velocity a power-type relation was obtained; and (2) a linear relation was observed between overwash flow depth and maximum overwash intrusion distance across the barrier top (i.e. overwash intrusion). Findings from this study are useful to enhance the knowledge of overwash processes and also have practical applications. On the one hand, results shown here can be use for the validation of overwash predictive models, and additionally, the simple empirical relations deduced from the dataset can be used by coastal managers to estimate overwash intrusion distance, which in turn can assist in the location of areas under risk of overwash and breaching.N/

Measurements of air/water interfaces in plunging breaking waves

Of the energy dissipation that takes place in violent breaking waves, some is accounted for by work done against the buoyancy of bubbles, and some by work done in generating splashes that may rise to elevations far above the crest. In deep water the probability of encountering air in such conditions varies from almost zero at large submergences to almost unity at high elevations. Between these extremes in regular long-crested waves, there is a two dimensional continuum of time-dependent ensemble-averaged void fractions. A detailed knowledge of this distribution would represent a major contribution to a better understanding of the process of wave breaking, but largely for practical reasons, little of this information exists. Previous studies of air entrainment beneath breaking waves have made use of a variety of techniques including local conductivity probes (Cox & Shin, 2003; Hoque 2002), global conductivity probes (Lamarre & Melville, 1994), acoustic techniques and laser methods (Hwung & Jih, 1993). Many of these have shortcomings such as large measurement volumes, limited sensitivity at at least one end of the range of void fractions, or probes that are significantly intrusive. This paper describes detailed measurements made with an instrument that detects individual air/water interfaces over extremely small areas at high frequency. The results allowed us to compute time-dependent void fractions, not only in that part predominantly occupied by water, but in the region above, including (probably for the first time) the splash-up created by the plunging jet

Void fraction measurements and scale effects in breaking waves in freshwater and seawater

This paper follows from the work of Blenkinsopp and Chaplin (2007) and describes detailed measurements of the time-varying distribution of void fractions generated by breaking waves in freshwater, artificial seawater and natural seawater under laboratory conditions, along with flow visualisation of the entrainment process. The measurements were made with highly sensitive optical fibre phase detection probes and the results demonstrate that although an additional population of fine (d < 0.3 mm) bubbles existed in the seawater cases, the total volume and distribution of entrained air, and the spatial and temporal evolution of the bubble plumes were very similar in all three water types. The influence of water type may be relatively insignificant, but a numerical bubble tracking model shows that the effect of scale is an important consideration when modelling the post-entrainment evolution of breaker-entrained bubble plumes. Consequently the results suggest that while the use of freshwater in laboratory models of oceanic processes can be considered valid in most situations, the effect of scale may impact interpretation of the results

The effect of relative crest submergence on wave breaking over submerged slopes

Experiments were performed in a wave flume to measure the intensity, transmission and reflection of waves breaking over a submerged reef with an offshore gradient of 1:10. The results demonstrate that the relative water depth over the reef crest (hc/Ho) is a dominant factor affecting the breaking characteristics. In particular it is found that as the relative crest submergence is reduced, there is a considerable increase in the intensity of wave breaking over the reef that can be quantified through measurements of the air cavity enclosed beneath the plunging jet. It is also shown that there is a corresponding decrease in wave transmission and reflection as the submergence is reduced

Validity of small-scale physical models involving breaking waves

The presence of air bubbles entrained by the action of wave breaking can strongly influence a number of physical processes including wave impact forces on coastal structures, sediment transport and the rate of air-sea gas transfer. Due to their complex nature, laboratory facilities are commonly used to examine the influence of breaking waves on such processes and the large majority of these experiments are made in small-scale flumes filled with freshwater. In experiments involving non-breaking and hence non-aerating waves, scale effects are often unimportant. However very few authors have commented on the effect of scale on air entrainment by breaking waves, and the information available (Chanson et al., 2002; Lamarre, 1993) is contradictory. It is also noted that many researchers including Haines & Johnson (1995) and Chanson et al. (2002) have suggested that there are considerable differences in the total volume and size distribution of bubbles entrained in freshwater and seawater. It is clear therefore that in order to correctly interpret the results of laboratory tests involving breaking waves, the influence of scale on the entrainment and subsequent evolution of bubble plumes generated by breaking waves, as well as the effect of water type on the size, concentration and distribution of entrained bubbles must be understood

Bubble size measurements in breaking waves using optical fibre phase detection probes

Knowledge of the two-phase flow generated by breaking waves is desirable if we are to fully understand their effect on environmental processes such as air?sea gas exchange and production of sea-salt aerosol. However, due to the difficulties in making measurements in the high void fraction bubble plumes generated immediately after breaking, little detailed information is available. This paper describes laboratory measurements of the size of bubbles entrained in the dense plumes generated by wave breaking made using a pair of highly sensitive optical fiber phase detection probes. The results compare well with those of previous authors in the low void fraction parts of the flow, and go on to include data from within the highly aerated region present in the period shortly after breaking, close to the area of most active air entrainment. The data highlights the spatial and temporal evolution of the bubble sizes within breaker generated bubble plumes and demonstrates that some large air cavities with diameters of tens of millimeters are initially entrained. It is observed that the bubbles resident within the plume rapidly decrease in size with time and distance away from the point of primary entrainment as the large cavities initially entrained are broken up into smaller bubbles

Wave runup on composite beaches and dynamic cobble berm revetments

International audienceThe effects of climate change and sea level rise, combined with overpopulation are leading to ever-increasing stress on coastal regions throughout the world. As a result, there is increased interest in sustainable and adaptable methods of coastal protection. Dynamic cobble berm revetments consist of a gravel berm installed close to the high tide shoreline on a sand beach and are designed to mimic naturally occurring composite beaches (dissipative sandy beaches with a gravel berm around the high tide shoreline). Existing approaches to predict wave runup on sand or pure gravel beaches have very poor skill for composite beaches and this restricts the ability of coastal engineers to assess flood risks at existing sites or design new protection structures. This paper presents high-resolution measurements of wave runup from five field and large-scale laboratory experiments investigating composite beaches and dynamic cobble berm revetments. These data demonstrated that as the swash zone transitions from the fronting sand beach to the gravel berm, the short-wave component of significant swash height rapidly increases and can dominate over the infragravity component. When the berm toe is submerged at high tide, it was found that wave runup is strongly controlled by the water depth at the toe of the gravel berm. This is due to the decoupling of the significant wave height at the berm toe from the offshore wave conditions due to the dissipative nature of the fronting sand beach. This insight, combined with new methods to predict wave setup and infragravity wave dissipation on composite beaches is used to develop the first composite beach/dynamic revetment-specific methodologies for predicting wave runup