**Peng Yao 1,2,3, Hui Chen 4, Bensheng Huang 4, Chao Tan 4, Zhan Hu 1,3,\*, Lei Ren 3,5,\* and Qingshu Yang 3,5**


Received: 24 May 2018; Accepted: 6 July 2018; Published: 9 July 2018

**Abstract:** Coastal vegetation is effective in dissipating incident wave energy during storm conditions, which offers valuable protection to coastal communities. Determining vegetation drag coefficient (*CD*) 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 *CD* 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 *CD* can be obtained using this method. The derived *CD* can be used to accurately reproduce the measured maximum total acting force on vegetation (*R*<sup>2</sup> = 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.

**Keywords:** drag coefficients; oscillatory flows; force sensors; synchronization; automatic alignment
