دانلود رایگان مقاله استحکام گیاهی و بیوماس به عنوان درایور برای نیروهای درگ موج شدید

Abstract

Moving water exerts drag forces on vegetation. The susceptibility of vegetation to bending and breakage determines its flow resistance, and chances of survival, under hydrodynamic loading. To evaluate the role of individual vegetation parameters in this water-vegetation interaction, we conducted drag force measurements under a wide range of wave loadings in a large wave flume. Artificial vegetation elements were used to manipulate stiffness, frontal area in still water and material volume as a proxy for biomass. The aim was to compare: (i) identical volume but different still frontal area, (ii) identical stiffness but different still frontal area, and (iii) identical still frontal area but different volume. Comparison of mimic arrangements showed that stiffness and the dynamic frontal area (i.e., frontal area resulting from bending which depends on stiffness and hydrodynamic forcing) determine drag forces. Only at low orbital-flow velocities did the still frontal area dominate the force-velocity relationship and it is hypothesised that no mimic bending took place under these conditions. Mimic arrangements with identical stiffness but different overall material volume and still frontal area showed that forces do not increase linearly with increasing material volume and it is proposed that short distances between mimics cause their interaction and result in additional drag forces. A model, based on effective leaf length and characteristic plant width developed for unidirectional flow, performed well for the force time series under both regular and irregular waves. However, its uncertainty increased with increasing interaction of neighbouring mimics.

5. Conclusions

In this study, we conducted direct force measurements on mimic arrangements representing vegetation elements of varying stiffness and material volume characteristics. All mimic arrangements were exposed to hydrodynamic forcing under regular and irregular waves, covering a wide range of conditions including high energy events. The results confirm that vegetation stiffness, rather than biomass, is the driving parameter behind the force-velocity relationship as it is stiffness that determines bending and hence effective leaf length under hydrodynamic forcing. Under low forcing, forces are distributed according to the still frontal area of the mimic arrangement; this may be due to the lack of bending under these conditions. While under increased orbital velocities, the combination of characteristic width and bending can lead to the same response for mimic arrangements with identical material volume but different still frontal area. Moreover, the observations of different mimic arrangements suggest that plants within a patch interact with each other in the cross-stream direction. If shoots grow close enough to each other, the turbulence at their edges will affect neighbouring plants and increases the drag force acting on them even if the plants are not in direct contact with each other. The force measurements were also modelled, applying the model based on effective leaf length by Luhar and Nepf (2011) to orbital velocities. Overall, the model performed very well and was able to reproduce force time series for regular as well as irregular waves. However, it did not reproduce the force increase due to the interaction of neighbouring mimics which led to small deviations between modelled and measured data. In order to incorporate these interactions in the model and allow for its application to more complex plant shapes, visual observations alongside force measurements are now required for different mimic configurations. Such work would further develop existing models, improve characterisation of vegetated foreshores and aid better design of soft engineering interventions on low-lying sedimentary shorelines.