The Influence of Wave Nonlinearity on Cross-Shore Sediment Transport in Coastal Zone: Experimental Investigations

Abstract

On the basis of field experiment data, the main features of influence of non-linear wave transformation scenarios on cross-shore sediment transport in coastal zones were investigated. The bottom deformations due to the non-linear wave transformation follow the specific scenario. The increase in the second non-linear harmonic amplitude leads to the erosion of the underwater slope at the distances corresponding to this process, with the subsequent accumulation of sandy material closer to the shore at distances where the amplitude decreases during the backward energy transfer to the first harmonic. This can be explained by the change in the phase shift between harmonics during non-linear wave transformation. The second harmonic maximum provides the point near which the bottom deformations occur in different directions. Scenarios of non-linear wave transformation in which backward energy transfer from the second non-linear harmonic to the first is close to the shoreline will contribute to the transport and accumulation of the sediment on the coast. These scenarios are more characteristic of “small waves”. The scenario without a periodical exchange of wave energy between non-linear harmonics (with an increase in the second harmonic only) that is characteristic of large storm waves and plunging breaking waves will lead to the erosion of the underwater bottom profile.

Wave Energy Dissipation of Spilling and Plunging Breaking Waves in Spectral Models

“…The scheme of the experiment and positions of wave breaking are shown in Figure 1. A more detailed description of the wave regime on that day is given in [1,20,23,24]. For the analysis, we selected chronograms of wave transformation between gauges 9 and 12 (spilling breaking) and 1 and 4 (plunging breaking).…”

“…The process of wave breaking is characterized by significant losses of their energy. Changes in the steepness of waves and their asymmetry, occurring both due to linear and nonlinear processes and due to breaking, lead to a change in the higher statistical moments of wave motion, which determine the magnitude and the direction of sediment flow in the coastal zone [1].…”

Nonlinear Wave Transformation in Coastal Zone: Free and Bound Waves

“…During the experiment at the Shkorpilovtsi study site of the Institute of Oceanology of the Bulgarian Academy of Sciences, 15 wire wave gauges were evenly spatially located along the research pier (240 m) and installed at a depth range of 4.5−0.5 m. The free surface elevations at 15 points were synchronously measured at a sampling frequency of 5 Hz. The length of wave records was from 20 min to 1 h. A more detailed description of this field experiment and experimental results concerning irregular wave transformations can be found in [5,8,[15][16][17].…”

“…In this case beach erosion will occur because there will be no wave component of sediment transport to the shore and the undertow directed to the sea will prevail. It was shown that the spatially periodic exchange of energy between the first and second harmonics contributes to the formation and movement of underwater sand bars [4,5]. In addition, the periodic exchange of energy between harmonics leads to the arising and disappearance of secondary waves that significantly change the mean wave period [6,7].…”

The Method for Evaluating Cross-Shore Migration of Sand Bar under the Influence of Nonlinear Waves Transformation

“…During storms, high waves break on the top of bars and cause strong offshore currents ("undertow") that transport the sediment toward the sea, resulting in sand bar migration from the coast and beach erosion [4]. During the attenuation of the storm and decreasing in the heights and periods of the waves, as well as at moderate energy wave conditions, the sediment transport due to wave nonlinearity leads to a gradual migration of sand deposits, accompanied by accumulative processes of beach restoration (for example, [4,5]). In addition to movement on the time scale of individual storms, it has been noted by many researchers that sand bars migrate quasi-periodically depending on the seasonal wave climate during the year [6,7].…”