دانلود رایگان مقاله مشاهدات و مدل سازی فرآیندهای مرتب سازی رسوب نزدیک ساحل

Abstract

The understanding of cross-shore sediment sorting is of primary importance for the design of sand nourishments and for assessing the suitability of the seabed to different ecological species. In this paper, sediment sorting processes were investigated by using a combination of physical and detailed numerical modelling. Data from large-scale wave flume experiments were used to validate a 2DV cross-shore Delft3D model. The model solves coupled short-wave averaged equations for flow, sediment transport, bed composition and bed level change. The infra-gravity wave motions were explicitly resolved. In order to investigate sorting processes, eight sediment fractions were used as well as a layered bed stratigraphy. The effects of different wave conditions (high energetic and more moderate energetic waves) on the morphodynamic profile development and sorting processes were investigated. The Delft3D model reproduced the profile development and bar position very accurately. Additionally, model predictions of sediment sorting across the profile fitted very well with the available observations. The numerical model simulations showed the importance of including short-wave grouping and infragravity wave effects in order to reproduce the cross-shore profile development, especially the breaker bar dynamics and sediment sorting processes. Infragravity waves contribute to larger sediment entrainment and more offshore bar development. Besides leading to a better prediction of the bed profile, infragravity waves also lead to a better prediction of the bed composition. Model results are in agreement with experimental data, showing its capabilities in functioning as a tool to predict sorting processes.

6. Conclusions

In this paper, cross-shore morphodynamics and sediment sorting processes for a barred beach were investigated based on a combination of large-scale wave flume experiments and detailed numerical modelling using the Delft3D modelling system. The Delft3D model was implemented in 2DV mode, also including infragravity (IG) wave effects using the instationary version of the roller module of Delft3D-FLOW. Furthermore, sediment sorting processes were included by modelling eight sediment fractions and by keeping track of the bed composition with a layered bed stratigraphy. This study focused on two test cases forced by different wave conditions (high-energetic wave conditions and moderate-energetic wave conditions, respectively Case 1 and Case 2). During Case 1 an offshore breaker bar developed, that remained approximately at the same location and with the same dimensions during the subsequent Case 2. The measurements clearly showed the effect of sediment sorting processes along the profile. During both cases, fining offshore of the breaker bar was observed, due to fine sediments staying in suspension longer than the coarser sand and settling more offshore. At the breaker bar and more onshore of the breaker bar, sediment became coarser. Coarser sediments are more difficult to keep in suspension and as such they settle earlier in the profile. Case 2, characterized by more moderate wave conditions, also showed coarsening higher in the profile, related to selective entrainment of the sediment bed.