1 Introduction
Aquatic vegetation (AV) provides a wide range of ecosystem services. As a primary producer, AV supplies food for herbivorous animals and creates habitats and shelter areas for fish and shellfish (e.g., Green & Short, 2003 ; Waycott, Longstaff, & Mellors, 2005 ). In rivers, lakes, and costal zones, AV protects shorelines, inhibits erosion, and enhances local water quality (e.g., Barbier et al., 2011; Mitsch & Gosselink, 1986 ). AV can also provide significant carbon storage and support infauna diversity (e.g., Fourqurean et al., 2012 ; Irlandi & Peterson, 1991 ). Many of these ecosystem services arise as AV has the ability to alter local hydrodynamic conditions. For example, AV can reduce sediment resuspension by damping wave energy (e.g., Wang, Wang, & Wang, 2010; Ros et al., 2014 ; Luhar, Infantes, & Nepf, 2017 ), and thereby increase light penetration, creating a positive feedback for continued vegetation growth and a stable state with clear water (e.g., Carr, Dodorico, Mcglathery, & Wiberg, 2010 ;Scheffer & Carpenter, 2003 ; Scheffer, Carpenter, Foley, Folke, & Walker, 2001 ; van der Heide et al., 2007 ). Besides, by reducing sediment resuspension AV suppresses the release of nutrients associated with bed material and thus efficiently inhibits algal bloom (e.g., McGlathery, Sundback, & Anderson, 2007 ). Therefore, the changes in water motion associated with vegetation need to be investigated to fully understand the ecological function of AV.
In the last decades, flow resistance and structures with the presence of AV under pure-current conditions have been widely studied in the laboratory (e.g., Ghisalberti & Nepf, 2002, 2004, 2006; Jarvela, 2005; Okamoto & Nezu 2009; Tanino & Nepf, 2008a, b; Yang, Kerger, & Nepf, 2015; Zhang, Lai, & Jiang, 2016 ), in the field (e.g.,Cameron et al., 2013; Leonard & Croft, 2006; Leonard & Luther, 1995; Lightbody & Nepf, 2006; Neumeier & Amos, 2006; Zhang et al., 2020 ), and using analytical methods or numerical models (e.g.,Etminan, Ghisalberti, & Lowe, 2018; Neary, 2003; Nicolle & Eames, 2011; Pu, Shao, & Huang, 2014; Ricardo, Grigoriadis, & Ferreira, 2018 ). Nepf (2012) has reviewed the mean and turbulent flow structures in detail, and emphasized on the canopy- and stem-scale turbulence generated by AV and their effects on mass transport. Many previous studies have also investigated the interaction between water motion and AV under pure-waves, and most of them were mainly on the damping of waves by vegetation (e.g., Bradley & Houser, 2009; Lovstedt & Larson, 2010; Luhar, Infantes, & Nepf, 2017; Mendez & Losada, 2004 ), the mean and turbulent flow structures within canopy (e.g., Abdolahpour, Hambleton, & Ghisalberti, 2017; Lowe, Koseff, & Monismith, 2005; Luhar, Coutu, Infantes, Fox, & Nepf, 2010; Luhar, Infantes, Orfila, Terrados, & Nepf , 2013; Pujol, Serra, Colomer, & Casamitjana, 2013 ). For example, Lowe et al. (2005) studied the velocity attenuation within a model rigid canopy and a theoretical model was developed to predict the magnitude of in‐canopy wave orbital velocity under oscillatory flow. Luhar et al. (2010, 2013) investigated the flow structures within and above a model seagrass meadow and found that a mean current in the direction of wave propagation was generated within the meadow. Zhang et al. (2018) revealed the turbulent structures within submerged seagrass meadow forced by oscillatory flow, and noted that compared with bare bed the turbulence level within meadow was enhanced when the ratio of wave excursion to stem spacing Ew /S> 0.5.
In many natural settings (e.g., estuaries, shallow lakes connecting to rivers), AV is exposed to conditions with currents and waves coexisted, for which only a handful of studies have considered. Related studies have focused on the wave damping by AV with the presence of currents (e.g., Hu, Suzuki, Zitman, Uittewaal, & Stive , 2015; Lei & Nepf, 2019; Li & Yan, 2007; Losada, Maza, & Lara, 2016; Paul, Bouma, & Amos, 2012 ). For example, Paul et al. (2012)conducted flume experiments with flexible model vegetation and observed that the presence of current reduced wave dissipation by vegetation. Using real vegetation, Losada et al. (2016) found that wave damping was enhanced by current flowing in the opposite direction, but reduced by current in the same direction with wave propagation. However, to our knowledge, few studies concentrated on the flow structures with the presence of AV under combined current and wave conditions.
Field observations were conducted to study the flow structures influenced by AV in floodplains of Poyang Lake. Connected to Yangtze River, Poyang Lake has running water all the year around with stable flow directions from its upstream basin to Yangtze River. Combined with surface wind frequently occurring, the hydrodynamic environment of our study area is dominated by both currents and wind-driven waves. In natural world, the wave field under the direct effect of local wind is an interaction of large numbers of component with different wave periods, direction of propagations and phases, characterized as an erratic (irregular) pattern (e.g., Toffoli & Bitner-Gregersen, 2017 ). This is much more complicated than waves generated by the paddle wavemaker in most lab studies, for which the whole water mass was subjected to wave forcing and the waves generated were regular and linear. In this study, wave and turbulent velocity components were decomposed from the velocity time series by velocity spectrum and analyzed separately. The goals of present study are to investigate the influence of both emergent and submerged vegetations on the vertical distributions of time-averaged velocity, wave orbital velocity, and turbulent kinetic energy (TKE ) under combined current and wave conditions.