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 inâcanopy
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.