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

Abstract

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.

Non-hydrostatic modeling of drag, inertia and porous effects in wave propagation over dense vegetation fields

“…Besides drag force, inertia force is the other component in the total force that acts on vegetation (Morison et al, 1950;Chen et al, 2018;Yao et al, 2018). Inertia force is commonly ignored in wave-vegetation modeling because the work done by the theoretical inertia force over one wave cycle is zero if linear wave theory is applied.…”

Laboratory data on wave propagation through vegetation with following and opposing currents

“…In submerged canopy cases (h=0.33m), u was measured at 9 locations: z/h=0.12, 0.18, 0.24, 0.30, 0.39, 0.5, 0.63, 0.79 and 0.94. The direct measurement method of CD in combined current-wave flows was first introduced in Hu et al, (2014) and it was further improved in Yao et al, (2018). Such method is proposed for both pure wave and combined wave-current flows.…”

“…Therefore, the period-averaged # can be derived based on the following equation: Before applying direct measurement to derive # , the force data and velocity data should be aligned (Figure 5d). Detailed procedure of alignment can be found in Yao et al, (2018). As drag force ( # ) is a function of velocity ( ) Eq.…”

“…Jadhav et al, 2013;Vuik et al, 2016;Garzon et al, 2019), laboratory experiments (e.g. Lara et al, 2016;Yao et al, 2018;He et al, 2019;Tinoco et al, 2020), theoretical and numerical models (e.g. Méndez and Losada, 2004;Losada et al, 2016;Hu et al, 2019;Suzuki et al, 2019).…”

Coastal Vulnerability and Mitigation Strategies: From Monitoring to Applied Research

“…Yao et al's [5] technical note provides a practical set-up to derive both time-varying and period-averaged vegetation drag coefficients following the direct measuring method. Standard force sensors are applied to compose four synchronized force-velocity measuring systems in the current experiment.…”