The influence of wave groups and wave-swash interactions on sediment transport and bed evolution in the swash zone

Abstract

Large scale laboratory measurements of sediment dynamics in the swash zone are presented. Two bichromatic wave group conditions were generated, having the same energy content but different wave group period (T g = 15.0 s and 27.7 s). For the shortest wave group, due to bore focussing, the shoreline fluctuates predominantly at the T g time scale, showing a large runup and the presence of wave-swash interactions with strong momentum exchange. In contrast, for the longer wave groups, the swash excursion is dominated by the individual waves. The uprush generally promotes onshore sediment advection with consequent erosion at the rundown location but accretion close to the runup. On the contrary, the backwash promotes seaward sediment advection and accretion at the rundown location. The presence of repeated wave-swash interactions modifies these patterns slightly. A wave overrunning a previous uprush promotes a reduction in onshore sediment advection while weak wave-backwash interactions reduce seaward advection. Consequently, the measured sediment dynamics shows stronger intra-swash cross-shore sediment advection for the swash events produced by the short wave groups. Measurements of the sheet flow layer near the shoreline show that for the shortest wave groups the vertical structure of the concentration

Beach Profile Evolution towards Equilibrium from Varying Initial Morphologies

“…Overall, the volume of the sediment deposited onshore of x > 4 m is larger than the eroded volume resulting in overall onshore directed sediment transport in this area (see Figures 6a and 9a,b). In case 2, which started from the plane profile, no beach features are present and a clear separation (at x ≈ 3.5 m) between an area of sediment deposition (x > 3.5 m) and erosion (x < 3.5 m) can be observed towards the evolution of a concave swash profile, which is supported by previous studies [18,29,44]. The net transport of sediment is negative resulting in a net export of sediment from the swash to the inner surf zone (see, for instance, van der Zanden et al [29]).…”

“…It can be noted that the observed limits of the breaking zone do not always perfectly coincide with the separation between on-and offshore sediment transport which might be related to limitations in the visually observed breaking locations as well as the driving processes acting slightly beyond the observed breaking limits. (3) Positive sediment transport in the most onshore part of the swash zone (i.e., x > 2 m) resulting in filling of a previously formed runnel and/or building of a berm [15,18,43]. This berm is smaller and located further onshore than the swash berm of the initial profile (at x ≈ 6-7 m).…”

“…The smaller mean velocity magnitudes at the initial shoreline in case 1 are less likely to promote erosion; in fact, Figure 6a suggests onshore sediment transport in this area. Berm formation and onshore sediment transport in the swash zone have been linked to the advection of sediment from the inner surf and/or lower swash zone, even under conditions with negative mean near-bed velocities [17,18].…”

“…A large and wide berm is generally associated with low energy beaches [9]. The presence of a berm has been linked to berm overwash and sediment accumulation on the berm crest [15] and to horizontal sediment advection and the consequent cross-shore distribution of sediment transport [16][17][18].…”

Experimental Study on Uniform and Mixed Bed-Load Sediment Transport under Unsteady Flow

“…These strong flows cause significant erosion and deposition in affected coastal areas, and results in severe economic losses [3]. The unsteady flow is characterized as a process of acceleration or deceleration and a strong unsteadiness effects on sediment particles during a short time [4][5][6][7], which affects the threshold motion of sediment and bed-load transport. Besides, sediment transported in natural coastal zone and rivers is usually heterogeneous in particle sizes.…”