New laboratory data are presented on the influence of long waves on sediment transport in the surf zone. Due to the very significant difficulties in isolating the morphodynamic processes induced by long waves in field conditions, the laboratory study was designed practically to measure the net sediment transport rates, and gradients in sediment transport, arising from the interaction between long waves and short waves in the surf zone. The bathymetric evolution of model sand beaches, with dB50B = 0.2 mm, was observed under monochromatic short waves, long-wave short-wave combinations (free long waves), and bichromatic wave groups (forced long waves). The beach profile change and net cross-shore transport rates, Q(x), were extracted and compared for conditions with and without long waves. The experiments include a range of wave conditions, e.g. high-energy, moderate-energy, low-energy waves, and the beaches evolve to form accretionary, erosive, and intermediate beach states. Hydrodynamic measurements were made to identify the influence of long waves on short waves and to determine the correlation between surf zone bars and standing long waves. A shallow water wave model was modified for this application to surf zone morphodynamics and compared to both hydrodynamics and measured sediment transport. This data clearly demonstrate that free large-amplitude long waves influence surf zone morphodynamics not only under accretive conditions, by promoting onshore sediment transport, but also under erosive conditions, by decreasing offshore transport. For the dominant berm-bar feature, the strong surf beat induces offshore transport in the inner surf zone and onshore transport around the outer surf zone and throughout the shoaling zone. In contrast, forced (bound) long waves and wave groups correlated with bichromatic short wave groups play a pronounced role under erosive conditions, increasing offshore sediment transport across the whole beach profile. For accretionary conditions, only a very narrowbanded wave group promotes onshore sediment transport across the whole beach profile, while broader banded wave groups again promote offshore transport. The modified numerical model of Li et al. (2002) provides good predictions of the standing long wave pattern for the long-wave short-wave combinations, but generally poor agreement for the bichromatic wave groups. Similarly, this model performs poorly in terms of predicting the net sediment transport for all waves, even after optimising the sediment transport coefficients. This is because the model cannot predict the correct hydrodynamics around the breakpoint position and does not correctly represent net sediment transport mechanics. Overall, the model does not correctly predict the trends in beach profile evolution induced by the long waves and wave groups. Further, there is little evidence that the long wave nodal structure plays a dominant role. The influence of the free long waves and wave groups is consistent with the concept of the Gourlay parameter, H/wBsBT, as a dominant parameter controlling net erosion or accretion. Free long waves tend to reduce H/wBsBT, promoting accretion, while wave groups tend to increase H/wBsBT, promoting erosion.
| Identifer | oai:union.ndltd.org:ADTP/279372 |
| Creators | Panut Manoonvoravong |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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