Wind impacts on suspended sediment transport in the largest freshwater lake of China

Poyang Lake, the largest freshwater lake in China, is distinguished by complicated suspended sediment (SS) dynamics. Apart from lake currents, wind is an important form of natural disturbance in driving SS transport. Combining ﬁ eld data, laboratory experiments, and numerical simulations, we gained valuable insight into wind impacts on SS dynamics in Poyang Lake. (1) Lake current patterns exert great in ﬂ uence on the level of wind impacts. Due to reduced sediment carrying capacity, SS under weak current suffers from stronger wind in ﬂ uence than under strong currents. (2) Wind speed determines the degree of wind impact, not only affecting horizontal SS transport, but also regulating vertical dynamics. Winds exceeding critical intensity can enhance horizontal transport through both surface drift and Stokes drift at different water depths, triggering sediment suspension to feed the loads in overlying water. (3) Wind impact is in ﬂ uenced by lake morphology. The broad water surface in the central lake permits formation of continuous waves, leading to the largest SS ﬂ uctuation, from (cid:1) 10.05 mg·L (cid:1) 1 to þ 20.17 mg·L (cid:1) 1 , while average variation in the south and north part of the lake is only (cid:1) 6.59 mg·L (cid:1) 1 to þ 10.36 mg·L (cid:1) 1 . (4) SS in four reserves are characterized by notable wind impact, while in the other two reserves SS show no obvious departure from values without wind.


INTRODUCTION
Suspended sediment (SS) is an important member of the water environment system (Westall ; Horowitz ).
The transport of SS has a wide variety of consequences in natural and artificial water bodies (Edmonds & Slingerland ; Constantine et al. ). First, SS contributes to the morphodynamic changes of water boundaries and bottom topographies, which may exert negative effects on flood control, irrigation, power generation, shipping, aquaculture, and water landscape. The over-supply of sediment may also result in aggradation that cover the habitats of aquatic animals and cause a decrease in the function of stream installations. A sediment under-supply causes the degradation of the river bed, which can endanger stream installations such as banks and bridge piers by undermining their foundations (Poff et al. ). Moreover, SS always acts as a carrier of pollutants and alters the chemical and biological properties of the aquatic environment (Hossain et al. ). It participates in the vertical transport of pollutants between overlying water and surface deposited sediment, as well as the horizontal migration; for example, heavy metal adsorbed onto SS in the upstream will aggravate the pollution load in estuaries (Yin et al. ). Sediment suspension-induced nutrient release may accelerate harmful cyanobacterial blooms in a shallow freshwater lake (Qiu et al. ).
The dynamics of SS in water are related to the combination of many factors, such as particle characteristics, water current, and plant resistance. Current intensity is Among the main studies of wind's influence on SS transport are those of Sheng & Lick (), Thomas & Takhar (), and Constantin (), who identified that wind could play a major role in sediment transport in various forms. Below a critical wind speed, SS in the upper water volume are apt to flow toward the downwind axis, while under the stronger wind speed, the generated turbulence can not only enhance the lateral SS transport but also be capable of resulting in spontaneous resuspension events which may increase the total SS load in overlying water. Furthermore, especially in the case of strong and long duration winds, besides the surface transport, SS distributed in deeper layers can still be driven by the wind-generated stokes drift which interacts with, and often contributes to, lateral SS transport (Stokes ). These researchers made efforts to explore the wind's impacts on sediment transport; however, most of the documented results remained at the level of qualitative description, and limited work was directed to probing the difficulties in quantitative determination. As wind generally affects a relatively limited area and is infrequent at a specific site, its impacts may not be as significant in lakes or rivers compared to more regular current forces. However, SS transport during different wind events may also have critical impacts on ecosystem functioning by modifying sediment delivery to nearby marshes (Perez et al. ; Draut et al. ), nutrient transport, and other biogeochemical fluxes both within, and out of, the system (Day et al. ; Carlin et al. ). Thus, a better understanding of wind impacts on SS transport is essential for a better understanding of many eco-environment processes.
Poyang Lake is the largest freshwater lake in China, as well as the most typical river-connected lake in the world's top 50 freshwater lakes ( Figure 1). Unlike isolated lakes such as Lake Taihu, China and Lake Tana, Ethiopia, Poyang Lake gathers water runoff from five upstream rivers and then feeds the Yangtze River at Hukou through a northern channel (Mekete et al. ; Wang et al. ).
SS in the lake is distinguished by a notable spatial-temporal distribution. In addition, Poyang Lake is one of the world's six largest wetlands in the Ramsar Convention List recognized by the Global Natural Fund. It is an ecological treasury with global significance (Ji et al. ; Zhang et al. ; Han et al. ). It provides the world's largest overwintering area for more than 95% of the world's white cranes, and provides important places for finless porpoises for feeding, nurture, and play. Both of these two species were identified as 'Critically Endangered A3b þ 4b' on the red list of International Union for Conservation of Nature (IUCN) (Su et al. ; Zhao et al. ). The complicated process of SS in Poyang Lake may change the distribution of varied pollutants including nutrients, heavy metals, organic contaminants, and nano-pollutants, which may exert direct or indirect health risks on the sensitive species.
A better study on SS dynamics can yield better insight into pollutant migration. Currently, some researchers are paying attention to SS transport in Poyang Lake, but most of them have focused on the runoff-induced dynamic conditions (Cui et al. ; Gao et al. ; Zhang et al. ).
Little information is available concerning the wind impacts on SS transport in the lake. As Poyang Lake is located in the monsoon region, wind is an important factor influencing SS transport. In the present work, we place the emphasis on wind impacts on SS transport in Poyang Lake. The objectives were to: (1) investigate the suspension mechanism associated with varied grain-size sediments, triggered by different shear stresses from the surface deposited sediment; (2) develop and validate an improved SS transport model that incorporates the contribution of surface transport and Stokes drift induced by different wind events; (3) use numerical experiments to quantitatively reveal the spatial SS distribution under the combination of varied currents, gravity-pattern, jacking-pattern, and backflow-pattern, and different wind events, light wind (0.3 m·s À1 -1.6 m·s À1 ), gentle wind (3.4 m·s À1 -5.5 m·s À1 ), moderate wind (5.5 m·s À1 -8.0 m·s À1 ), hard wind (8.0 m·s À1 -10.8 m·s À1 ), and strong wind (10.8 m·s À1 -13.9 m·s À1 ); (4) evaluate the influences of wind events on SS dynamics in the lake, especially the regions related to the critically endangered white cranes and finless porpoises.

Study area
Poyang Lake, with an area of 3,583 km 2 and a volume of 27.6 km 3 , on average, is located on the south bank of the Yangtze River in Jiangxi Province, China (Wu et al. ).
It is a typical river-connected lake. When the water level at Hukou station increased from its lowest 5.9 m (observed on February 6, 1963) to its highest 22.59 m (observed on July 31, 1998), the mean water depth increased from 4.5 m to 6.8 m. The lake, which hosts millions of birds (over 300 The lake receives water from five rivers (Raohe, Xinjiang, Fuhe, Ganjiang, and Xiuhe) and drains into the Yangtze River through a narrow outlet to the north (Feng et al. ). Due to the river-lake interaction, the area and volume of Poyang Lake vary considerably throughout the year (Liu & Rossiter ; Volpe et al. ). It expands to a large water surface during the wet season, but shrinks to little more than a river during the dry season. Poyang Lake is characterized by marked intra-and inter-annual variations of suspended sediment load (Wang et al. , ). According to the measured data from 1956 to 2015, the mean concentration of SS load in Poyang Lake is approximately 0.120 kg·m À3 , with the peak and valley values being 0.185 kg·m À3 and 0.036 kg·m À3 , respectively year could reach 9.16 × 10 6 t, 2.12 × 10 6 t, 1.43 × 10 6 t, 0.99 × 10 6 t, and 0.80 × 10 6 t, respectively (Xiong ). Investigation of the wind-induced impacts on SS transport is of great significance to explore water quality variation, bio-geochemical cycling, and eco-environment evolution in Poyang Lake.

Data acquisition and processing
Data for the simulation experiments were collected from various sources. The boundary data for the five upstream rivers, including water quantity and suspended sediment,  (Wang et al. ). Prior to the experiment, sawdust was selected as a tracer indicator to calibrate the device. According to the in situ flow velocity in Poyang Lake, the currents in the flume were set at six grades including 0 m·s À1 , 0.1 m·s À1 , 0.2 m·s À1 , 0.3 m·s À1 , 0.5 m·s À1 , and 0.7 m·s À1 .
After the change of rotational speeds, 30 minutes of waiting time was introduced to form a stable flow. The sediment was stirred and spread evenly at the bottom of the flume, and after 1 day's deposition, the overlying water was slowly poured into the test depth 24 cm to start the experiment.
Three groups of disturbance experiments were arranged separately for fine-, medium-, and coarse-silt. SS concentration was determined by gravimetric method during the experiment.
Generally, the incipient motion of sediment can be divided into the following three levels, individual movement, ounce movement, and universal movement (Xiao et al. ). To place the emphasis on wind impacts, individual movement with the suspension rate of 1% was fixed to determine the sediment starting criterion in the present work. Moreover, the starting velocity can be used as an index to reflect sediment incipient motion, but it is not convenient in popularization and application (Yang & Wang ; Pang et al. ). Thus, the depth-averaged velocity in the flume was transferred into bed shear stress by the following equations which are written as follows: where τ e is the critical sediment starting shear stress, ρ is water density, u Ã is the friction velocity, u c is the section- where h is water depth, t is time, u and v are the depth-averaged velocity components in x and y directions, g is the acceleration of gravity, δ is the difference between water surface elevation and the average elevation. ε is the eddy viscosity coefficient, f is the Coriolis force parameter, S i is the suspended sediment concentration associated with the i-class sediment (i ¼ 1, 2, 3, respectively, representing fine- where τ wx and τ wy are, respectively, the wind stress velocity components in x and y directions, τ bx and τ by are the friction force components of lake bottom. ρ is the water density, ω is the earth's rotation angular velocity, M is the scouring coefficient, ϖ is the deposition velocity of sediment, τ is the shear stress at the deposit sediment surface, τ d is the critical deposition shear stress, τ e is the critical starting shear stress. When τ ! τ e , the deposited sediment starts to suspend, and the bottom bed is scoured. When τ τ e , the suspended sediment began to settle, and sedimentation was exerted on the bottom bed. τ d is usually a bit less than τ e , and here, to simplify calculation, the two shear stresses were approximately recognized as the same. It was considered that the flow velocity at the balanced status (non-deposition and non-eroding) was not a range (from the critical deposition velocity to the critical starting velocity) but a point (the critical starting velocity). φ is the latitude of research area. Pu ): where ρ a is air density, taken as 1.02 kg·m À3 ; U x and U y are, respectively, the wind speed components in x and y directions 10 m from water surface; α is the wave stress coefficient, which is related to the wave-particle amplitude; c is the Chezy coefficient; n is the Manning coefficient; τ c is the shear stress of lake bed; τ w is the shear stress of lake bed under wind force; θ is the argument. λ is the correction coefficient of wind on physical SS transportation, which can be calculated by Equation (5 where, u w is the wind speed; u sd is the wind-induced surface where, ρ a is the density of air; η is the wind drag coefficient; η ¼ (0:75 þ 0:067U 10 ) × 10 À3 where, U 10 is the wind speed at 10 m elevation.

Numerical simulation schemes
Water currents in Poyang Lake are strongly influenced by the combined impacts of the five upstream rivers and the Yangtze River downstream. Given the temporal gap between the flood peaks of the upstream and downstream rivers, currents in Poyang Lake can be divided into the following three types.
(1) Gravity-pattern current, the primary current type. Water Keeping all other factors the same, the comparison calculation schemes were arranged synchronously with the winddriven parameter closed.

Model calibration
The model was calibrated and validated against the field investigated sediment data at ten field investigated points

Wind's contribution to bottom shear stress
The laboratory results revealed that SS release to the overlying water was proportional to bed shear stress and the suspension is closely related to particle diameters. SS loads in the overlying water subject to weak shear stress (<0.05 N·m À2 ) were low and just a few fine-silts were observed to preferentially suspend. After the disturbance was intensified above a critical level, the bed sediment was intensely affected and significant sediment suspension was detected. When the shear stress was approaching 0.10 N·m À2 , the bottom sediment was observed to increasingly suspend, with the stripes becoming evidently tortuous. A fully developed suspended sediment regime was gradually established. As the bottom shear stress was increased to 0.26 N·m À2 the tortuous level of the stripes was significant, with the ends of the stripes gradually joining together and, swinging like a broom, which caused intense sediment suspension. SS content of the overlying water was markedly increased to 63.48%, 48.54%, and 36.41%, respectively, in the tests for fine-silt, medium-silt, and coarse-silt. Subject to the same disturbance intensity, SS contents in the overlying water of the three group tests also exhibited an evident variation with the grain size. In the present work, the suspension rate of 10% was adopted as the starting standard. Hence, the fine-, medium-, and coarse-silt were characterized by the critical incipient shear stresses of 0.011 N·m À2 , 0.017 N·m À2 , and 0.024 N·m À2 , respectively (Figure 4(a)).
How wind triggers the deposited sediment to overlying water is related to the combination of water depth, wind intensity, and wind fetch (WF). Based on the lake morphology, the WFs of 10 km, 20 km, and 30 km were adopted to explore wind's contribution to the bottom shear stress of different water depths (Figure 4(b)-4(d)). It was detected that wind-generated shear stress negatively However, after the wind intensity rose to 7 m·s À1 , the disturbance contribution could reach 0.019 N·m À2 , which was capable of individually motivating the suspension of fine-silt and medium-silt. When the wind intensity went up to 8 m·s À1 , the contributed shear stress was enhanced to 0.063 N·m À2 , which markedly exceeded the critical starting shear stresses of fine-, medium-, and coarse-silt. WF interacts with wave height, wave length, and wave period, and thus influences the wind-driven SS dynamics. In general, if wind intensity were kept the same, wind fetch is an important factor affecting the wave height, wavelength, and wave period, which directly determines the influence of the wind on the suspension of sediments. The longer wind fetch always results in a stronger shear stress, and therefore a stronger regulation in suspended sediment distribution.

Wind impact evaluation
SS in Poyang Lake is closely related to external water volume, imported sediment concentration, hydrodynamic conditions, lake topography, and wind events. Generally, water current is the dominant factor governing the dynamics of SS. As different current patterns are characterized by Under the gravity-pattern, water flows from the south to north. SE wind in the south lake accelerated the spreading of SS from the river inlet area to the central lake, i.e., in the case of G-S, wind of 3.61 m·s À1 from SE in the south lake motivated the SS from Gan-, Fu, Xin rivers to extend northwards, which enhanced the SS in NGR to 78.9 mg·L À1 , increased by 18.5% than that without wind. However, the strong NW wind of 10.82 m·s À1 in the north resisted the current-driven transport from south to north, which could be evidently observed in the region south to Duchang. This resistance was generated by the combination of surface drift and Stokes drift. Under the situation of G-H, winds in the north, central, and south lake were, respectively, 9.01 m·s À1 (NE), 1.94 m·s À1 (NW), and 3.89 m·s À1 (SE).
The easterly wind in north and south lake resulted in a narrowed SS diffusion zone towards the west. Hence, the SS loads in PNNR and NNNR were increased to 95 mg·L À1 and 83.1 mg·L À1 , with wind contribution, respectively, being 9.10 mg·L À1 and 8.44 mg·L À1 . In the case of G-M, influenced by 2.5 m·s À1 (NE) in the central lake, SS diffusion intensity from the middle branch of Gan River to the central lake was weakened. SS concentration in the inlet area was decreased by 6.13 mg·L À1 than the value without wind. In the south lake, the NW wind of 2.67 m·s À1 exhibited an inhibitory effect on SS transport from the south branch of Gan, Fu, and Xin rivers. SE wind in the north was stronger than winds in the center and south, but due to the limited wind fetch, the wind's contribution to YFPR was just 2.68 mg·L À1 . In G-G mode, the south and north lake were influenced by NW winds of 2.22 m·s À1 and 3.89 m·s À1 , which stemmed the SS transport driven by current. SS in PNNR was decreased to 68.7 mg·L À1 . The wind direction in the central lake was consistent with water flow, whereas the weak intensity did not show any evident promotion in SS spreading northward.
Wind intensity under the G-L situation was lower, with the north and south lake influenced by SE winds of 1.11 m·s À1 and 1.39 m·s À1 , and the central lake by NW wind of 1.35 m·s À1 . The wind's impacts on SS dynamics were concentrated in the west lakeshore between the south branch and middle branch of Gan River. However, this contribution was not remarkable, and the fluctuation of SS in PNNR was only 0.076 mg·L À1 .
During the jacking-pattern, water in the lake cannot smoothly outflow to the Yangtze River. The reduced flow disturbance weakens the sediment carrying capacity, which makes stronger the impacts of wind on SS dynamics.
Besides, during the period when jacking-pattern current happens, the wind directions in different lake regions tend to be more consistent. Apart from scheme J-L, SE wind prevails in the south and north lake, and in the central lake wind shifts to the SW. In the case of J-L, the north and central lake were dominated by 1.11 m·s À1 (NE) and 1.67 m·s À1  NGR are characterized by more remarkable wind impacts, and the means of 10.3%, 12.7%, 13.2%, and 9.3% were enhanced by wind, respectively. Nevertheless, the influencing weights in different reserves had close relationships with current patterns, wind speeds, and wind directions.
For example, when the prevalent south wind in the central lake repressed sediments from the Yangtze River, the regulation of wind in YFPR was distinguished at the highest level. The degrees of wind impacts in PNNR, NNNR, and NGR were enhanced with rising wind speeds, especially in the events of high and strong winds. When wind acts as a hindrance to SS transport, its resistance ability is closely related to current intensity, i.e., in the central lake, the flow currents are reduced by the board water surface, and the mean resistance rate in PNNR, NNNR, and NGR could reach 6.2%. However, in YFPR where the flow current is stronger, the mean resistance rate was just 2.3%.
These results could yield insight into wind impacts on SS dynamics in Poyang Lake. However, there are several uncertainties that may need further investigation. First, in the present work, the interactions of sediments with some ecological processes were not considered, i.e., plant blocking, biological consumption, and bioaccumulation.
Simplification of these interactions indeed set up a barrier to simulate the field actual SS transport, but it will not affect the general trend of the dynamics in the whole lake.
Second, a lack of monitored data meant the model suffered from not accurately incorporating the sediment input of some small boundary rivers, which was recognized as a constant for the present work. Third, the model we established here was a depth-averaged one, and the vertical SS transport was simplified by the parameters of deposition and suspension.

CONCLUSIONS
Combining field data, laboratory experiment, and numerical simulation, we shed light on the wind impacts on SS dynamic in Poyang Lake. Wind impacts increase with rising wind speeds. They not only affect the horizontal SS transport, but also regulate the vertical dynamics. When