Sand bed morphodynamics under water waves and vegetated conditions

morphodynamics;vegetation;water waves;bathymetry;ripples;bars;bedforms;wave tank

Landry, Blake J.

关键词: morphodynamics;    vegetation;    water waves;    bathymetry;    ripples;    bars;    bedforms;    wave tank;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/24055/Landry_Blake.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

The littoral zone plays an integral role in the success of today's thriving society, providing a stable food supply as well as points of trade/transport for goods around the world.Key environmental processes in this region such as sediment transport, wave attenuation, and boundary layer development are directly governed by the presence of bathymetric features, which include large-scale sand bars upon which smaller-scale sand ripples are superimposed, as well as the presence of submarine vegetation.As a result, understanding the complex interaction among these features is imperative to determining coastline morphological changes and protecting coastal zones and society as a whole due to our strong dependence on the region.The experimental large-scale laboratory work presented herein provides new insight into the complex sediment dynamics within this region.Specifically, this study explores the impact of spatial variability in the wave envelope on bed evolution, sand ripple geometric characteristics, and migration velocities as well as effects that vegetation imparts on the sediment dynamics.Key results of the study show that it is of the utmost importance to fully resolve the wave conditions along the entire domain of a facility to understand local morphodynamics.The study reports that mild wave reflections of 20% can generate 55% variability in the small-scale bed form geometries.Analysis of high-resolution temporal and spatial bathymetry measurements shows that ripple velocities under partially progressive waves are related to the local near-bed Lagrangian mass transport velocities within the bottom boundary layer.Additional laboratory experiments with near-deeply submerged vegetated canopies (current work has a ratio of mean still water depth to plant height, Hp/hp = 7.9) using idealized vegetation (6.35-mm diameter rigid wooden cylinders) beneath standing water waves provide evidence that significant modifications in bathymetry can result without vegetation directly attenuating the surface waves. While the introduction of vegetation decreases the bar growth rate, the final equilibrium bar height can be increased due to localization of flow velocities within the intra-canopy (i.e., enhanced streaming).In vegetated conditions with high lateral density (i.e., one plant diameter on-center), bar crests formed near wave antinodes rather than under wave nodes, which is indicative of a change in the dominant mode of sediment transport from bed load to suspended load.Ultimately, the study demonstrates that bottom roughness can be controlled with the help of vegetation to provide a sustainable means of altering sediment transport, adding valuable insight to aid in coastal erosion mitigation efforts.