Applying a New Force-Velocity Synchronizing Algorithm to Derive Drag Coefficients of Rigid Vegetation in Oscillatory Flows

Coastal vegetation is effective in dissipating incident wave energy during storm conditions, which offers valuable protection to coastal communities. Determining vegetation drag coefficient (C-D) is of great importance to the quantification of vegetation-induced wave dissipation. Recently, a direct measuring approach has been developed to derive vegetation drag coefficient more accurately compared to the conventional calibration approach. However, as this approach requires perfectly in-phase force and velocity signals, there are two difficulties associated with it. The first difficulty is the availability of a suitable force sensor to compose synchronized force-velocity measuring systems. The second difficulty is related to realigning the obtained timeseries of force and velocity data. This technical note develops a new synchronized force velocity measuring system by using standard force sensors and an acoustic doppler velocimeter (ADV). This system is applied together with an automatic realignment algorithm to ensure in-phase data for C-D deviation. The algorithm reduces the phase shift between force-velocity signals from ca. 0.26 s to 0.003 s. Both time-varying and period-averaged C-D can be obtained using this method. The derived C-D can be used to accurately reproduce the measured maximum total acting force on vegetation (R-2 = 0.759), which shows the reliability of the automatic alignment algorithm. The newly-developed synchronized force velocity measuring system and alignment algorithm are expected to be useful in future experiments on vegetation wave interactions with various hydrodynamic and vegetation settings.