Drag forces on vegetation due to waves and currents

Vegetation in coastal areas plays an important role in its environment. In addition, vegetation can also be utilized for coastal protection. Incoming wave energy could be effectively attenuated by the upstanding vegetation plants. The reduced wave energy results in stabilized seabed and harmonious environment in coastal zones. Nowadays, an increasing number of projects have been carried out to apply vegetation as a soft measure for coastal protection. Wave energy dissipation by vegetation is primarily induced by the work done by drag force acting on the vegetation. A drag coefficient (CD) is introduced to characterize the flow resistance from the plant stems. Knowledge of CD is of great importance for understanding and predicting the wave dissipation process. In previous studies, relations between CD and Reynolds number (Re) have been proposed in pure current or pure wave. In addition, relations between CD and the Keulegan-Carpenter number (abbreviated as KC and KC=Uw*Tw/d, Tw is the wave period and d is the plant stem diameter) have also been suggested. In Keulegan-Carptenter number, the wave period Tw is also considered. Since waves are oscillatory flow, it would be preferable to use the KC number to describe the behaviour of CD in wave-present conditions. However, contradictory conclusions are found in the literature on the CD-KC relation in pure wave condition. Monotonous decreasing of CD with KC has been reported for multiple vegetation mimics. On the contrary, the rise-and-fall variation trend has been observed in pure wave, but only for single cylinder. It is noted that the transition point (from rise to fall) occurs when KC value is small, which is often left out in the experiment with multiple vegetation mimics in previous studies. Hence, it is necessary to investigate the variation trend of CD-KC for multiple vegetation mimics in pure wave with a wider KC range. Moreover, background tidal currents may also play a role in the wave dissipation process. It is often the case that when the tide penetrates the coastal wetlands during flooding phase, waves propagate in the same direction as the tidal currents. The underlying current may affect the behaviour of oscillatory wave flow during the energy-damping process and the CD-KC relation. Yet, the CD-KC relation in combined current-wave flow has not been reported in previous studies. In order to fill the knowledge gap in the CD-KC relation, an experimental approach was adopted by using the laboratory flume to replicate such complicated hydrodynamics. The flume is 40m long and 0.8m wide, with a patch of rigid wooden cylinders as vegetation mimics installed over the entire channel width over a 6m long test section. Pure wave can be generated by the wave generator. The underlying current can be made by using a water circulation system in the flume. After the generation of underlying current, waves could be generated afterwards and propagate together with the underlying current in the flume. The velocity was measured using EMS within the vegetation patch. Direct measurement data of the force on individual rods within the array were collected by attchaching the rods to the force sensors and embedding them in the false bottom of the flume. Three densities of the vegetation mimics were investigated for two water depths in this study. The results of the experiments reveal a rise-and-fall variation trend of CD-KC for multiple vegetation mimics in pure wave. The rise-part occurs when KC is small, around KC=3 to 10 and this phenomenon could be phycisally explained based on the changes in vortexes shedding directions. In this range of KC, the vortexes motions would change its propagation direction from lateral to oblique and longitudally parallel with the incoming flow. It is the changes in vortexes directions that lead to the increase of flow resistance experienced by the cylinder. Beyond this range of KC, the vortexes motion would keep moving longitudinally parallel and behave much the same way as in steady current. Natuarally, similar to the behaviour found in steady current conditions, the values of CD would decrease gradually and converge to 1. In the combined current-wave flow conditions, it is necessary to make a distinction between oscillatory-dominated flow and unidirectional-dominated flow. For small underlying current applied in this study, the flow could be regarded as oscillatory-dominated. It is found that the similar rise-and-fall pattern of CD-KC relation occurs in this kind of combined flow. And the transition point of KC locates at around KC=10, which is also the case found in pure wave conditions. But the peak value of CD would decrease a little bit. However, for larger underlying current conditions, the combined flow is similar to pure current. Consequently, the peak values of CD would collapse. Thus, the values of CD obtained in these conditions are stable and close to 1. The experimental results suggest the vegetation density as well as the water depths has limited effects on the values of CD and its variation trend with KC. It is recommended to carry further investigation concerning the influence on CD caused by vegetation density (N) and relative vegetation height (?) in future studies. The product of this thesis is a general description and explaination for the variation trend of CD-KC in pure wave and combined current-wave flow conditions. Physically, more insights have been gained about the evolution of vortex shedding in different flow conditions, say from pure wave to combined current-wave flow conditions. Moreover, both the calibration approach (used by Mendez and Losada, 2004, etc.) and direct measurement method have been utilised for data processing. The direct measurement method is recommended to apply in all the complicated flow conditions. As to the calibration approach, it should not be applied to obtain CD values in combined current-wave flow.Hydraulic EngineeringHydraulic EngineeringCivil Engineering and Geoscience