In-situ study of the spatiotemporal variability of sediment erodibility in a microtidal estuary

Estuarine, Coastal and Shelf Science 232 (2020) 106530

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In-situ study of the spatiotemporal variability of sediment erodibility in a microtidal estuary Weihao Huang a, Heng Zhang a, b, c, Lei Zhu a, b, Lianghong Chen a, Guang Zhang a, Wenping Gong a, b, c, *, Jiahuan Liu a a b c

School of Marine Sciences, Sun Yat-sen University, Guangzhou, 510275, China Southern Marine Science and Engineering Guangdong Laboratory, Zhuhai, 519000, China Guangdong Provincial Key Laboratory of Marine Resources and Coastal Engineering, Sun Yat-sen University, Guangzhou, 510275, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Sediment erodibility Turbulent flux Resuspension Bottom shear stress Pearl River Estuary

Seabed erosion is a vital process for sediment transport in estuaries and coasts. In this study, we investigated the seabed erosion process and its spatiotemporal variability in the Pearl River Estuary, China, via in-situ obser­ vations and laboratory experiments. It is found that the seabed was more erodible in the channel than at the shoal in most cases due to the difference in sediment size. In a neap-spring-neap tidal cycle, a laminated seabed with different sediment ages was inferred to form due to deposition on flood tides and erosion on ebb tides. The seabed was more erodible during spring tides than neap tides because of the difference in sediment availability at the unconsolidated fresh layer. Furthermore, the erosion curve showed evident flood-ebb asymmetry, which was found to be related to the variations of sediment availability and bottom stress. Besides, erosion experiments with different sediment consolidations revealed that sediment erodibility decreased with the growth of consolidation. This study promotes our understanding on the spatiotemporal variability of sediment erodibility, and has sig­ nificant implications for simulation and prediction of estuarine sediment transport and morphological evolution.

1. Introduction The transport of sediment in estuaries and coasts plays a key role in pollutant dispersion, morphological evolution, and ecological balance (Brand et al., 2010). Because of the cohesive properties of fine sedi­ ments, contaminants are often transported as adsorbates attached to them (Geyer and Ralston, 2018). Moreover, the transport of cohesive sediment can cause siltation in navigation channels and harbors, affect the development or destruction of coasts (Mercier and Delhez, 2007). Thus, a detailed understanding of sediment transport is vital for pre­ diction of the evolution of bathymetry, water quality, and ecology in estuaries and coasts. Sediment transport is a complex process. It involves many processes including resuspension, horizontal advection, flocculation and defloc­ culation, deposition and consolidation (Mercier and Delhez, 2007; Li et al., 2018), among which resuspension is the dominant process at the boundary between the solid seabed and the liquid sea water, which is largely determined by sediment erodibility. However, it is difficult to quantify seabed erodibility due to complicated physical, geochemical,

and biological properties of the sediment (Grabowski et al., 2011). Most studies on the temporal and spatial variations of erodibility were conducted by using three successive generations of technology. The first generation was laboratory flumes. Movable seabeds formed by model or natural sediments were the most common choice to test the sediment erodibility (Shields, 1936; Kamphuis and Hall, 1983; Parchure and Mehta, 1985; Thomsen and Flach, 1997). However, in nature, sediments undergo cycles of disturbance and reconsolidation, so this method cannot reproduce the in-situ erodibility of the natural seabed. To overcome that shortcoming, the second generation of in-situ devices was pioneered by Young and Southard (1978). Since then, a wide range of in-situ devices, such as the annular flume, Gust microcosm, sedflume and carousel, have been developed (Gust and Morris, 1989; Houwing and Van Rijn, 1998; Tolhurst et al., 1999). Despite great progress made in recent decades, complicated devices are still required to collect un­ disturbed seabed samples. This is not practicable in some estuaries and coasts with relatively large water depth. For this reason, studies using the second generation devices focused mostly on mudflats or other shallow water zones (Austen et al., 1999; Brouwer et al., 2000; Xu et al.,

* Corresponding author. School of Marine Sciences, Sun Yat-sen University, Guangzhou, 510275, China. E-mail address: [email protected] (W. Gong). https://doi.org/10.1016/j.ecss.2019.106530 Received 8 March 2019; Received in revised form 27 November 2019; Accepted 4 December 2019 Available online 5 December 2019 0272-7714/© 2019 Elsevier Ltd. All rights reserved.

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2016). In addition, the time required for each individual erosion test (several hours) has limited the implementation of such experiments. To overcome these shortcomings, Brennan et al. (2002) proposed a method using bottom acoustic Doppler velocimetry (ADV) to estimate the ver­ tical suspended sediment concentration (SSC) flux to represent the erosion rate of unit area at the seabed. By estimating the vertical tur­ bulent flux while keeping the seabed undisturbed to represent the seabed erosion over a relatively short time scale, ADV offers much improved reliability. Sediment erodibility features great spatiotemporal variabilities. The horizontal variability of cohesive sediment erodibility in estuaries and coasts has been noted widely (Grabowski et al., 2011). It is found to be mostly correlated with bed morphology, sediment properties, and mi­ crobial community (Tolhurst et al., 2006; Xu et al., 2014, 2016). Vertically, the variability with depth in the seabed has also been a research focus (Grabowski et al., 2011). Studies indicate that the typical seabed structure can be roughly divided into three vertical parts (or layers) according to sediment properties (Parchure and Mehta, 1985; Amos et al., 2010; Wheatcroft et al., 2013): the most erodible fluffy layer lies on the surface and a consolidated layer that is less easily erodible lies at the bottom, separated by a transitional layer between them. However, some exceptions can happen. For instance, Geyer and Ralston (2018) noted an unusual movable bottom pool in the estuarine turbidity maximum (ETM) zone which was different from the typical vertical structure of the seabed. The movable bottom pool was much thicker and less erodible than the fluffy layer, but less consolidated and more erodible than the consolidated and transitional layers. Temporal variability has been found on various time scales (e.g., yearly, seasonally or diurnally) (Grabowski et al., 2011). Apart from the variations in physical processes, the variability is mostly related to biological effects such as variations in the production of extracellular polymeric substances (EPS) and diurnal or seasonal patterns of diatom migration (Paterson, 1989; Tolhurst et al., 2003, 2006; Friend et al., 2005). For instance, Amos et al. (2004) noted that the increase of critical shear stress in the summer compared to the winter was attributed to bio-stabilization by cyanobacteria and diatoms; and the asymmetry of erodibility during a tidal cycle was found to be the result of diurnal change in bio-stabilization (Brennan et al., 2002). In this study, the third-generation approach was used to investigate the spatiotemporal variations of the sediment erodibility in the Pearl River Estuary (PRE), China. We deployed a bottom-mounted tripod equipped with an ADV, an optical backscatter sensor (OBS), a conduc­ tivity, temperature, and depth (CTD) sensor, a pulse-coherent acoustic Doppler profiler (PC-ADP), and an acoustic wave and current (AWAC) meter to conduct in-situ measurements in the western part of the PRE. The bottom boundary layer (BBL) parameters, including near bottom velocity profile, high-frequency bottom velocity, bottom suspended sediment concentration, bottom salinity, seabed elevation, and wave parameters, were comprehensively measured. The field campaigns were conducted during neap and spring tides in the wet and dry seasons in 2015, respectively. The vertical SSC fluxes were estimated by the method proposed by Brennan et al. (2002) to assess the variability of seabed erodibility. In the meantime, grain sizes of the sediment samples collected at the observation sites were analyzed by Malven Mastersizer 2000. We also conducted experiments in the laboratory by using the Gust Erosion Microcosm System (GEMS) for the collected short sediment cores to test the erodibility. Our purpose in this study is to examine the spatiotemporal variability of the bottom sediment erodibility, such as those between channel and shoal, spring and neap tides, flood and ebb tides in a tidal cycle, in the western part of the PRE and to reveal the main factors affecting these variabilities. The paper is organized as follows. The background of the study area is introduced in Section 2. Section 3 describes the methodology and Section 4 shows the observation results in this study. The factors con­ trolling the spatiotemporal variability of erodibility are discussed in

Section 5, and a summary is given in Section 6. 2. Study area The PRE (Fig. 1a) is located in the Guangdong–Hong Kong–Macau Greater Bay Area, one of the most economically developed and densely populated regions in southern China (Ye et al., 2012). It is the largest estuary (Fig. 1b) of the three estuarine water bodies (Lingding Bay, Modaomen and Huangmaohai estuaries from east to west) in the Pearl River Delta area. It is a funnel-shaped estuary with an area of approxi­ mately 2100 km2, ranging from Humen in the north (~5 km wide) to Hong Kong and Macau in the south (~50 km wide) (Tan et al., 2019). Most of the PRE is shallow, with an average water depth of 4.8 m, ranging from 2 to 5 m upstream to about 20 m around the seaside entrance (Guo et al., 2019). A three-shoal and two-channel bathymetric pattern (Fig. 1b, including the West Shoal, Middle Shoal and East Shoal [< 5 m deep] and the West Channel and East Channel [> 5 m deep]) has been maintained for hundreds of years (Ouyang et al., 2017). Influenced by the subtropical monsoon climate, winds in the PRE are seasonally reversed over the coastal area. In summer (May to September), the wind is predominantly southerly or southeasterly, while in winter (December to February), the wind is relatively strong and northeasterly (Mo and Yan, 1986). The subtropical monsoon climate causes a mean annual temperature variation from 14 � C to 22 � C, and the mean annual precipitation ranges from 1200 to 2200 mm (Zhang et al., 2008). The wet season lasts from April to September and the dry season from October to March. About 80% of the annual rainfall occurs in the wet season (Liu et al., 2017). The uneven intra-annual distribution of the precipitation leads to higher runoff in the wet season than in the dry season. Approximately 3.13 � 1011 m3 of fresh water flows into the three estuaries annually, of which half is discharged into the PRE via four outlets (Humen, Jiaomen, Hongqili, and Hengmen) (Zhan et al., 2019). Tidal waves in the PRE are propagated from the Pacific Ocean. The tidal regime is characterized as semidiurnal, with daily inequalities in range and time. The principal semidiurnal tide, M2, is the most domi­ nant tidal constituent, followed by K1, O1, and S2 (Gong et al., 2012). The residual currents in the PRE exhibit a counterclockwise pattern with stronger ebb currents on the western side and stronger flood currents on the eastern side (Mao et al., 2004). With the input of large runoff, the tidal velocity asymmetry shows an ebb dominant characteristic (Gong et al., 2016). The mean SSC in the Pearl River is about 0.172 kg/m3, which is relatively low, and approximately 89 � 106 tons of sediment are trans­ ported annually, among which 46% is dumped into the PRE (Wu et al., 2017). The sediment in the PRE is mostly transported in the form of suspended sediment (Ouyang et al., 2017). More than half of the riverine sediment is trapped inside the PRE (Liu et al., 2009; Hu et al., 2011; Zhang et al., 2019). Multiple ETMs have been observed shifting with the variation of fresh water discharge (Wai et al., 2004). According to pre­ vious studies, the PRE experienced a natural deposition at a rate of centimeters per year before 1980, after that the morphological evolution was strongly affected by human activities (such as dredging and recla­ mations) (Wu et al., 2016). 3. Methods 3.1. Data collection Measurements were conducted at stations C1, S1, C2 and S2 (the observation times are listed in the figure caption of Fig. 1) located in a subchannel (Denglong Waterway) and a subshoal (Entrance Shoal) in the western part of the PRE (Fig. 1b) in 2015. The two stations in the channel (C1 and C2) were placed exactly at the same location but the two stations at the shoal were not (Fig. 1b). The two sites at the shoal were 4.9 km apart from each other. The distances between the stations in the channel and at the shoal were 4.43 km (C1 to S1) and 3.15 km (C2 2

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Estuarine, Coastal and Shelf Science 232 (2020) 106530

Fig. 1. Bathymetry of the PRE and locations of the observation sites: (a) overview map of the Pearl River Delta; (b) bathymetry of the PRE (red crosses represent observation sites), (c) the total daily freshwater discharge of the Pearl River during 2015 (the discharge during the observation periods in the wet and dry season are shadowed in the gray areas). Abbreviations: SCS: South China Sea, LB: Lingding Bay, M: Modaomen, H: Huangmaohai, 1: Yamen, 2: Hutiaomen, 3: Jitimen, 4: Modaomen, 5: Hengmen, 6: Hongqili, 7: Jiaomen, 8: Humen; C1 and C2 stand for observation sites in the channel in the wet season (neap tide: Jun. 25 - Jun. 26 and spring tide: Jul. 04 - Jul. 05 in 2015) and in the dry season (neap tide Dec. 18 - Dec. 19 and spring tide: Dec. 26 - Dec. 27 in 2015); S1 and S2 stand for observation sites at the shoal in the wet season (neap tide: Jun. 26 - Jun. 27 in 2015 and spring tide: Jul. 05 - Jul. 06 in 2015) and in the dry season (neap tide: Dec. 19 - Dec. 20 and spring tide: Dec. 27 - Dec. 28 in 2015). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

to S2) (Fig. 1b). The field measurements were conducted during neap and spring tides in the wet and dry seasons, respectively (Fig. 1c). For each station, the measurement lasted for more than 27 h, covering one to two tidal cycles. The winds were relatively mild with speeds less than 5 m/s during all the measurements. The total discharge of the Pearl River during the observation periods are shown in Fig. 1c. It indicates that the river inflow did not change considerably during our observation periods for the wet and dry seasons. This is attributed to the abnormal weather pattern induced by El Nino in 2015 (Chang et al., 2016). At each station, several parameters, including water depth (senor height included), current, wave height and period, salinity and tem­ perature, and turbidity, were measured with a bottom tripod (Fig. 2) equipped with ADV, PC-ADP, AWAC, OBS, and CTD. The configurations of the instruments are listed in Table 1. BBL velocity profiles ranging from 0 to 1.5 m above the seabed (mab) were measured by the downward-looking PC-ADP at a frequency of 1 Hz; the high-frequency current at 0.3 mab was measured by the ADV at a sampling frequency of 64 Hz every 10 min with each burst lasting for 5 min; turbidity and salinity at 0.3 mab were recorded by the OBS and CTD at frequencies of 6 Hz and 1 Hz, respectively; the wave parameters and water levels were recorded in acoustic surface tracking (AST) mode with a sampling frequency of 4 Hz by the AWAC every 15 min; the distance from the seabed to the ADV sampling volume was recorded by the ADV with an accuracy of 1 mm every 10 min, with the ADV mode being set in the bottom-tracking one. In addition to the bottom tripod measurements, water samples at surface, middle, and bottom layers were collected hourly and subjected to in-situ filtering and further analysis in the Sediment Dynamics Laboratory in Sun Yat-sen University.

Fig. 2. Bottom-deployed observation tripod system. The PC-ADP was oriented downward and functioned at a frequency of 1 Hz; the OBS, CTD, and ADV measured turbidity, salinity, and velocity at sampling rates of 6 Hz, 1 Hz, and 64 Hz, respectively; wave parameters and water level were recorded by the AWAC every 15 min; the distance from the ADV sampling volume to the bottom was recorded by the ADV.

3.2. Data processing 3.2.1. Preprocessing of ADV data Due to several interferential factors such as the instrument posture, existence of large particles of suspended matter or bubbles and the 3

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Estuarine, Coastal and Shelf Science 232 (2020) 106530

Table 1 Configurations of instruments.

where k and b are coefficients determined by linear regression analysis. The correlation coefficient (R2) ranged from 0.6 to 0.9, indicating a good linear relationship.

Title

Full title

Manufacturing company

Sampling frequency

Parameters

ADV

Acoustic Doppler velocimeter

Nortek Corporation

64 Hz

CTD

Conductivity, temperature, depth sensor Optical backscatter sensor Acoustic wave and current profiler

RBR Corporation

1 Hz

Accuracy: 0.5–0.1%; range: 1.0 m/s Accuracy: 0.01 psu

RBR Corporation

6 Hz

Accuracy: 0.1 NTU

Nortek Corporation

4 Hz in AST; 2 Hz in velocity

Pulse-coherent acoustic Doppler profiler

Sontek Corporation

1 Hz

AST accuracy: 1% or 1 cm; Velocity range: 1.5 m/s in horizon and 0.5 m/s in vertical Velocity accuracy: 1% or �0.5 cm/s Head frequency: 1 MHz; Velocity range: 1.15 m/s in horizon and 0.48 m/s in vertical; Velocity accuracy: 0.01 m/s; Cell size: 0.05 m; cell number: 28; blank distance: 0.4 m

OBS AWAC

PCADP

3.2.3. Estimation of the bottom shear stress (1) Current-induced shear stress The horizontal and vertical instantaneous velocities were separated into a time-mean flow and a turbulent deviation by Eq. (2). The time mean flow was obtained by averaging over every 5-min burst (Dyer, 1997). Because the turbulent part of the horizontal velocity consists of both wave orbital velocity and turbulent velocity, the wave orbital velocity might bias the bottom shear stress calculation (Bian et al., 2018). Owing to the fact that the orbital wave velocity is much smaller than the tur­ bulent velocity in the vertical direction, only the vertical turbulent ve­ locity was used to calculate the current-induced shear stress (Kim et al., 2000). The current-induced shear stress (τc, Pa) was estimated as follows: (2)

τc ¼ 0:9ρw’ 2

(3)

0

where ρ (kg/m3) is the density of the sea water, and w’ (m/s) is the turbulent velocity in the vertical direction. (2) Wave-induced shear stress The wave orbital velocity (Uw, m/s) associated with the significant wave height (H1/3, m) and the peak period (Tp, s) at the edge of the wave boundary layer was obtained as (Xiong et al., 2018): �pffiffi πH1=3 2 Uw ¼ (4) Tp sinhðkhÞ

Notes: AST stands for the acoustic surface tracking measurement.

Doppler effect, recorded signals are often contaminated by a large amount of noise and burr. To remove these disturbances from the raw data, several data processing steps were conducted as follows:

where k is the wave number and h is the water depth in meter. The parameters (H1/3, Tp and h) were measured by the AWAC every 15 min and obtained by post-processing the AWAC data using the software STORM (Pedersen et al., 2007). The wave-induced shear stress (τw, Pa) is estimated as (Xiong et al., 2018):

(1) The first step was to determine the posture of the ADV instru­ mentation. The posture (heading, pitch and roll) of the ADV was recorded during the observations. The pitch and roll measured by the tilt sensor represent the tilt angels which rotates around the x and y axes, and the heading measured by the compass shows the rotation angle which rotates around the z direction. Kim et al. (2000) concluded that a deflection of 5� would exert a non-negligible influence on the estimated Reynolds stress. Thus, data with a rotation of more than 5� in any direction (heading, pitch or roll) was discarded. (2) Secondly, the validity of the data was examined. According to the user’s manual of the ADV, the validity of the data can be judged by the signal correlation and the signal-to-noise ratio (SNR). Data with a signal correlation of less than 70% or an SNR of less than 5 was considered unreliable. If these unreliable data accounted for more than 10% in any burst, the entire burst was discarded.

1 2

τw ¼ ρw fw U 2w where fw is the wave friction factor, expressed as: 8 A > > 0:28; � 1:7 > < ks fw ¼ � � �0:194 � > A > > exp 5:213 A : 5:977 ; > 1:57 ks ks

(5)

(6)

where A ¼ Uw T=2π (m) is the semi-orbital excursion, and ks is the Nikuradse roughness, which is related to the sediment size (ks ¼ 2:5d50 , where d50 is the median grain size).

3.2.2. Estimation of SSC All the water samples were filtered in-situ by pre-weighted filter membrane (0.45 μm). The SSC of the water samples were then estimated by drying them in an electric dry oven in 100 � C for more than 5 h in the laboratory. Because no combustion was performed to remove organic matters, SSC reported in this study included both organic and inorganic (mineral) components. The calibration curve relating the OBS turbidity (T, NTU) to the SSC (c, mg/L) was obtained as follows: c ¼ kT þ b

w¼wþ w

3.2.4. Estimation of vertical sediment fluxes (1) Uncertainty of the observation height Because the observed SSC includes the SSC caused by both the resuspension and horizontal advection (which is not easy to separate from each other), we adopted the Brennan’s Method (Brennan et al., 2002). Only the vertical turbulent velocity and turbulent SSC were used (the mean component of the SSC and velocity are removed). Because the vertical SSC flux is much less influenced by horizontal advection in the

(1)

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Estuarine, Coastal and Shelf Science 232 (2020) 106530

bottom boundary layer (Brand et al., 2010), it is reliable to be used as the proxy for resuspension. To obtain the high-frequency fluctuation of the SSC, the acoustic backscattered signal (ABS) intensity recorded by ADV was converted into high-frequency SSC data. According to acoustic principles, the relationship between the SSC (c, mg/L) and the ABS in­ tensity (IADV, dB) can be expressed as follows (Brand et al., 2010; Scheu et al., 2015): lgðcÞ ¼ kIADV þ b

� � �ω dc ω h z KU� 1 � a �KUω � ¼ ca dz z h a KU�

c ¼c

c0 w0 ¼ εy

(7)

ω¼

(9)

z h

�

a h a

�B

and B ¼

KU* .

ω

The settling velocity of sediment

c’ w’ c

In the BBL near the seabed, z2 < z1 ≪ h, leading to hh

(18) can be simplified as: � �B c0 w0 z z1 γ¼ 0 0 2 � z2 c w z1

(17)

z2 z1

� 1. Then, Eq.

(19)

Eq. (19) indicates that, the vertical fluxes at different heights above the seabed near the bottom are transformable with each other. It is obvious that the flux is larger in the bottom layer and smaller in the upper layer in the water column. Theoretically, when the sampling volume of the ADV is close enough to the seabed, the eroded sediment of unit area from the seabed will be suspended into the water and the erosion rate of unit area will be recorded by the high-frequency ADV in the form of vertical SSC flux (Brennan et al., 2002; Brand et al., 2015):

(11)

where h is the water depth. Due to the inertia of sediment, the sediment diffusion coefficient (εy) is often larger than the turbulent-moment diffusion coefficient (εm) (Brush, 1962):

εy ¼ β εm

Ca ω

(16)

The values of A and B are related to the friction velocity (U*), water depth (h) and sediment settling velocity (ω). The ratio between vertical turbulent SSC fluxes of different heights (z2 < z1) above the seabed can be expressed as: � �B � �B c0 w0 z z1 h z2 γ¼ 0 0 2 ¼ (18) z2 h z1 c w z1

where U* is the bottom shear velocity, and can be expressed as U* ¼ pffiffiffiffiffiffiffi τ=ρ, where ρ is the density of the sea water; K is the dimensionless von Karman constant, with an empirical value of 0.4; z is the height above the seabed; z0 is the boundary roughness length. The vertical turbulent-momentum diffusion coefficient (εm) can be calculated as (Qian and Wan, 1983): h

(15)

can be obtained by the method proposed by Fugate and Friedrichs (2002):

As 0.3 mab maybe not close enough to the seabed and the data there may not represent the bottom resuspension, we tried to convert the data at 0.3 mab to even lower heights above the seabed. Because the BBL is a thin water layer close to the seabed, the current in it was often consid­ ered as a fully developed flow without stratification (e.g., Grant and Madsen, 1979; Kim et al., 2000; Ali and Lemckert, 2009; Brand et al., 2015). For the well-developed unstratified BBL, the velocity follows the logarithmic relationship (Qian and Wan, 1983; Trowbridge and Lentz, 2018). � � u 1 z ¼ ln (10) U* K z0

εm ¼ KU* z

dc dz

where A ¼

The vertical turbulent sediment flux (F) was then estimated by the eddy-correlation method for each 5-min burst (Brennan et al., 2002): F ¼ c’ w’

(14)

Inserting Eqs. (11) and (14) into Eq. (15), we get: � �B h z c0 w0 ¼ A z

(8)

c

�

The vertical turbulent SSC flux could be expressed as (Trowbridge and Lentz, 2018):

where k and b are coefficients determined by linear regression analysis. The correlation coefficients R2 between the logarithmic SSC and ABS intensity were more than 0.9, showing a good correlation. The turbulent fluctuation in SSC (c’) was estimated as below: 0

h z2

Fz ¼ E=γ ¼ Fz0 =γ

(20)

where Fz0 is the vertical turbulent flux at z0 just above the seabed, which is assumed to be the accurate erosion rate of unit area (E); Fz is the vertical turbulent flux at z above the seabed. In fact, there is not a distinct interface between the resuspended sediment and the sediment at the seabed. Also, the observation point cannot infinitely approach the seabed. In previous studies, the bottom SSCs and fluxes were mostly observed at a height ranging from 0.1 to 0.5 mab in situ (e.g., Brennan et al., 2002; Yuan et al., 2008; Brand et al., 2010). The observation height in this study was 0.3 mab, lying within the range of previous studies. For more accurately representing the erodibility of the seabed, the vertical SSC flux observed at 0.3 mab was transformed to 0.05 mab by Eq. (19).

(12)

where β is a ratio between them. Brush (1962) indicated that β decreased with the sediment grain size and approached 1.0 when the grain size was less than 0.19 mm. Also, Jobson and Sayre (1970) found that the value of β did not have significant effect on the vertical distribution of SSC and it was reasonable to be taken as 1.0. Besides, the Rouse distribution of SSC is typical in well-developed boundary layer derived from the vertical sediment diffusion equation (ignoring the horizontal advection of the SSC) (Rouse, 1937). It can be expressed as: �KUω � * c h z a ¼ (13) ca z h a

(2) Selection of erosion modes The erosion process can be expressed in different ways under different modes (Sanford and Halka, 1993; Sanford and Maa, 2001; Brand et al., 2015). Two typical erosion types (Type I and Type II) had been noted by Amos et al. (1997) and Sanford and Maa (2001). The Type I erosion happens when the erodible sediment is limited at the applied shear stress. If the available sediment at the seabed was not sufficient for the shear stress to erode, the erosion rate will decrease and approach 0 finally (e.g., it happens when the shear stress is persistently applied

where ca is the reference SSC at a reference height a in the BBL, ω is the settling velocity of the sediment. Therefore, the vertical SSC gradient can be derived from Eq. (13):

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Estuarine, Coastal and Shelf Science 232 (2020) 106530

and the erodible sediment is limited). The Type II erosion happens for an infinite erodible sediment (or the erodible sediment is continuously supplied), and the erosion rate stays stable. However, both the typical Type I or Type II erosion are not common in nature. The transitional types between them are frequently observed (Sanford and Maa, 2001). For the two typical erosion modes, the erosion rates can be expressed as: Mode 1 (Sanford and Halka, 1993):

Table 3 Estimated M, n, R2 and peak vertical SSC flux.

(21)

E ¼ M τn Mode 2 (Sanford and Maa, 2001): E ¼ Mðτ

(22)

τc Þm

in which E [g/(cm2/s)] is the erosion rate of unit area; M and n (m) are empirical constants controlled by sediment properties (Lavelle et al., 1984); τ (Pa) is the shear stress applied to the seabed. The difference between two modes is whether or not the critical shear stress τc is included. The choice of different modes depends on the timescale on which we are addressing. When we focused on the tidally averaged time scale (e. g., between neap and spring tides), the variation of critical shear stress in intratidal timescale was not considered. Considering that the critical shear stresses on different layers in the seabed are difficult to obtain, we adopted the empirical formula (Eq. (21), Mode 1) without considering the critical shear stress of the seabed explicitly (Sanford and Halka, 1993; Brand et al., 2015). Taking the logarithm of both sides of Eq. (21), it can be transformed into:

Peak ebb velocity (unit: m/s)

Neap-Wet Channel Neap-WetShoal SpringWetChannel SpringWetShoal Neap-DryChannel Neap-DryShoal SpringDryChannel SpringDryShoal

0.38

0.20

0.48

0.26

0.42

0.17

0.30

0.18

0.57

0.30

0.74

0.40

0.36

0.25

0.38

0.23

0.52

0.23

0.41

0.23

0.61

0.34

0.42

0.26

0.51

0.27

0.74

0.41

0.53

0.36

0.69

0.33

Neap-Wet Channel Neap-Wet-Shoal Spring-WetChannel Spring-Wet-Shoal Neap-Dry-Channel Neap-Dry-Shoal Spring-DryChannel Spring-Dry-Shoal

1.26 1.18 0.86

3.7 4.3 3.0

0.84 0.60 0.71

0.63 1.04 1.17

0.93 1.30 1.75 0.86

0.5 3.9 3.2 3.9

0.14 0.55 0.80 0.63

1.37 0.91 1.08 2.32

0.83

2.6

0.35

1.91

3

g/

Dsi

(25)

3.3. Sediment experiments in the laboratory Surficial sediments were collected by a short gravity corer during the observation periods. The collected sediment samples were used for grain size analysis and ‘consolidation-erosion’ experiments. 3.3.1. Sediment properties analysis All the sediment samples were analyzed for grain sizes by the Malven Mastersizer 2000 in the laboratory. This laser analyzer can measure grain sizes ranging from 0.02 to 2000 μm. The analyses were conducted following routine procedures as described in previous studies (Sperazza et al., 2004; Simms et al., 2006). Finally, the fractions of each sample were determined [sand (>64 μm), silt (4–64 μm) and clay (<4 μm)] (Xu et al., 2016). Besides, the water content and organic content were measured. The pre-weighted samples collected at the stations were dried in 100 � C in the electric oven for more than 5 h and burned in 550 � C for more than 8 h to remove the water and organic matters (Xu et al., 2016). The dried and burned samples were reweighted to estimate the water and organic contents in the sediment.

Table 2 The bottom peak and averaged velocities of flood and ebb in each observation period. Averaged flood velocity(unit: m/s)

Peak vertical SSC flux [10 (cm2s)]

where Dsi (mm) is the distance between the ADV sampling volume and the seabed for burst i, and Ds1 (mm) is the initial distance from the seabed in each cycle. As the seabed elevation in the channel stations were well recorded, they were used for the following study.

Using the least-squares method, M and n were obtained. The results of M and n are listed in Table 3. The calculated M and n herein were the empirical coefficients representing the erodibility of the sediment averaged in each observation period. However, when discussing the erodibility in smaller time scales, the variability of the critical shear stress should not be ignored, and we adopted Eq. (22) for Mode 2, in which the variability of the critical shear

Peak flood velocity (unit: m/s)

R2

B ¼ Ds1

(24)

Name

M

3.2.5. Estimation of seabed elevation The altimetry data of ADV was used to track the seabed change. The distances (Ds) from the sampling volume to the seabed were recorded by the ADV. Ds was then converted into the relative bed elevation (B) as:

The vertical SSC flux measured by the ADV at 0.3 mab was trans­ formed to at 0.05 mab to represent the erosion rate of seabed of unit area. Eq. (23) can be rearranged into: lgðc’ w’ Þz¼0:05m ¼ lgM þ nlgτ

n

stress was taken into consideration.

(23)

lgE ¼ lgM þ nlgτ

Name

Averaged ebb velocity(unit: m/s)

3.3.2. Erosion experiments by GEMS Sediments collected in the channel were used for the ‘consolidationerosion’ experiments to study the effect of consolidation. Sediment samples were mixed with water and stirred into mud fluid with an initial sediment concentration of 230 g/L. The initial mud fluid was left to rest for different times to form new beds (from 0 to 14 days) (Fig. 3a). The concentration of sediment indicated that the porosity of seabed was almost stable after 24 h’ consolidation (Fig. 3b). Then, the consolidated sediment with different ages (1, 2, 7 and 14 days) were used for the erosion experiments. Experiments were conducted by the GEMS (Gust and Muller, 1997; Xu et al., 2014). A picture of the GEMS system can be found in Xu et al. (2014). Sediment on the core surface can be eroded by applying a shear stress via a magnetically-controlled rotational head. The shear stress ranged from 0.01 to 0.6 Pa. As the shear stress increased, the sediment surface was eroded, and the sediment was resuspended into the cham­ ber. The mixed water was pumped through a turbidimeter and collected 6

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Fig. 3. Settling and consolidation processes of the mixed fluid mud: (a) diagram of settling and consolidation experiment, (b) averaged sediment concentration (blue line) and sediment thickness (red line), and (c) distribution of the grain sizes for sediment in the channel and at the shoal. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

in bottles for further analysis. Seven steps of shear stresses (0.01, 0.05, 0.10, 0.15, 0.30, 0.45 and 0.60 Pa) were applied and each step continued until the available sediment in the core was completely eroded. The calibration equation between the bottom stress and rotational speed is expressed in terms of the shear velocity which is defined as U* ¼ pffiffiffiffiffiffiffi τ=ρ (τ is the bottom shear stress and ρ is the water density). The calibration equations for the microcosm (10 cm above the core) are:

related to the properties of the sediment, τ (Pa) is the shear stress applied to the seabed, τco (Pa) is the critical shear stress when τ is first applied, τeo (Pa) is the excessive shear stress.

(26)

The tidal averaged water depth was more than 5.0 m at the stations in the channel and less than 5.0 m at the stations at the shoal (Fig. 4) (the 5 m isobath is often taken as the dividing line between channel and shoal in the PRE). The tidally averaged water depths were 5.36 m at C1, 3.9 m at S1, 6.25 m at C2, and 4.23 m at S2, respectively. According to the water depth data measured by the AWAC, OBS and CTD sensors, the maximum tide range was 2.62 m in the spring tide and 1.07 m in the neap tide in the wet season, and 2.51 m in the spring tide and 1.37 m in the neap tide in the dry season. Two rising and falling periods were observed each lunar day, showing irregular semidiurnal tide characteristics. From the available ADV data, the bottom flow direction in the channel was mostly parallel to the alignment of the deep channel and showed a characteristic of reciprocating flow, whereas the flow at the shoal was mostly rotational. The peak and averaged velocities of the flood and ebb tides are listed in Table 2. The peak flood velocity was lower than the peak ebb velocity in the spring tides, and vice versa in the neap tides (Table 2). One exception occurred in the neap tide during the wet season in the channel. The velocities recorded by the PC-ADP indicated that no extreme events (e.g., storms) happened during the observation periods, consistent with the results of the ADV. Estimated shear stress indicated that the amplitude of the waveinduced shear stress was much smaller than that of the currentinduced one (Fig. 4). Therefore, the bottom shear stress was domi­ nated by the current during the observation periods. In the following analysis, we will not present the bottom stress induced by waves and that by the combined action of waves and currents.

U* ¼ 10 5 n2 þ 0:011n þ 0:0956 28:31U 2* þ 170:2U*

Q¼

23:85

4. Results 4.1. Hydrodynamics

(27)

where U* is in unit of cm/s, n was the disk rotation speed in unit of rotations per minute, and Q is the pumping rate in mL/min. The turbidity of the pumped water was measured by the turbidimeter and recorded by the laptop. It was transformed into SSC (c, mg/L) by the calibrated relationship with high correlational coefficient (R2 ¼ 0.9995), c ¼ 0:0005T 2 þ 0:3905T þ 29:753

(28)

where c is the SSC of the pumped water from the chamber (mg/L) and T is the turbidity (NTU) measured by the turbidimeter. The erosion rate and the eroded mass of unit area were calculated as: E¼

cQ

(29)

π R2 Z

t1

m¼

Edt 0

(30)

where R is the inner radius of the chamber (9.1 cm). Plateaus of eroded mass at the end of each shear stress (Fig. 6c) was assumed to be the erodible mass at the applied shear stress (Sanford and Maa, 2001). The erosion rate at the shear stress (peak erosion corre­ sponding to the peak SSC) was estimated following Sanford and Maa (2001), E0 ¼ Mðτ

τc0 Þ ¼ M τe0

(31)

4.2. Sediment properties at the seabed

where E0 [g/(cm2s)] is the erosion rate at shear τ (Pa), M is the erosion coefficient, which is variable with time and depth in the seabed and

Results analyzed by the Malven Mastersize 2000 showed that the 7

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Fig. 4. Typical results of the water depth, significant wave height (Hs), peak wave period (Tp), current-induced shear stress (τc), wave-induced shear stress (τw) during the neap tide (a) in the channel, (b) at the shoal, and during the spring tide (c) in the channel, (d) at the shoal, in the dry season.

sediment in the channel and shoal were quite different. The grain size distribution in the channel showed a unimodal pattern (Fig. 3c), with a peak at 6 μm. On average, clay (<4 μm) and silt (4–64 μm) represent 27.76% and 72.24% in mass, respectively. Silt was the largest fraction and no sand was detected. While, sediment sampled at the shoal showed a bimodal pattern (Fig. 3c). Two peaks were located at 1125 μm in the sand fraction and at 6 μm in the silt fraction, respectively. On average, clay, silt and sand took percentages of 26.55%, 51.57% and 21.88% in mass, respectively. Silt was still the main component, but more sand appeared along with a decrease of the proportion of clay. The organic contents in the sediments were 15% and 13% for that in the channel and at the shoal, respectively. The corresponding water contents were 128% and 130%, with the bulk densities of 1.75 g/cm3 and 1.85 g/cm3, respectively.

4.3. Variation in bottom SSC and bed elevation In the wet season, the seabed elevation in the channel fluctuated slightly during the neap tide (Fig. 5a). When the neap tide turned into the spring tide, the seabed started to fluctuate periodically with an amplitude of 12 mm (Fig. 5b). In the dry season, the behaviors during the neap and spring tides were similar to that during the spring tide in the wet season, with the seabed fluctuating with an amplitude of 23 mm in the spring tide and an amplitude of 15 mm in the neap tide (Fig. 5c and d). The maximum erosion depth in each tidal cycle was related to the strength of the tidal current: the greater the peak ebb current (shear stress), the greater the maximum erosion. Because of the absence of observation data at the beginning of the tidal period in Fig. 5c (Neap-Dry-Channel), it is impossible to judge the trend of bed elevation in a ‘neap-spring’ cycle by directly estimating the net change of seabed in the tidal cycles. However, since there are two rises and two falls in a tidal cycle, the half tidal cycle with smaller tidal

Fig. 5. Time series of water depth (black lines), bottom velocity (red lines), bottom salinity (blue lines), bottom SSC (brown lines), and seabed elevation (magenta lines) in the channel during: (a) neap tide in the wet season, (b) spring tide in the wet season, (c) neap tide in the dry season, (d) spring tide in the dry season (Green lines in the figures represent the half tidal period selected to calculate the net change of bed elevation). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 8

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Fig. 6. Results of consolidation and erosion experiments conducted by the GEMS: (a) the observed SSC at applied seven-step shear stress (0.01, 0.05, 0.10, 0.15, 0.30, 0.45 and 0.60 Pa) for different ages (1, 2, 7 and 14 days); SSC at the first 5 steps is zoomed in (b); (c) the eroded mass at applied seven-step shear stress for different ages, and the eroded mass at the first 5 steps is zoomed in (d).

current speed during initial flood tides (Chen et al., 2017). A peak SSC appeared immediately after the application of each shear stress. The peak SSCs increased with the applied shear stress from 0.05 to 0.10 Pa (an exception was observed for the sediment with an age of 14 days, which was almost not erodible at 0.05–0.10 Pa). The peak SSC decreased at 0.15 Pa owing to the available sediment at that shear stress being exhausted. At the 0.3 Pa shear stress, the peak SSC increased again. The peak SSC increased sharply after the application of 0.45 and 0.6 Pa, indicating a significant erosion. The eroded mass at each shear stress was estimated by Eqs. (29) and (30). The estimated mass represents the total amount of eroded sediment at the applied shear stress. The eroded masses were consistent with the changes in SSC (Fig. 6c), with larger amount of masses corresponding to higher SSC and vice versa. The effect of consolidation ages on the erodibility of sediment was also noted by comparing the results with four different ages (1, 2, 7 and 14 days) (Fig. 6a, c). The changing trend of SSC in each experiment was similar, but the peak SSC varied. Both the mass eroded at 0.45 and 0.60 Pa decreased sharply with the increase of consolidation ages from 1 to 7 days and tended to be stable when the age is longer than 7 days (Fig. 6c). Experiment results indicated that the peak SSC (c0) and the eroded sediment mass decreased with the sediment age.

range in the neap tide and the half tidal cycle with larger tidal range in spring tide were selected to calculate the seabed evolution (green lines in Fig. 5). Net changes of seabed in the half-tidal period were calculated to be 0 mm (Neap-Wet-Channel, Fig. 5a), 7 mm (Spring-Wet-Channel, Fig. 5b), 0 mm (Neap-Dry-Channel, Fig. 5c) and 5 mm (Spring-DryChannel, Fig. 5d), respectively. Seabed in the neap tides was observed to be in geomorphic equilibrium without net erosion nor deposition. While, more seabed erosion was observed in the spring tides. The net change in seabed indicated that the strong hydrodynamics during spring tides facilitated seabed evolution. Fig. 5 shows that during flood tides, the bottom salinity and water depth increased, and vice versa. The changes in seabed elevation cor­ responded well with these changes. The bed was mostly under accretion during flood tides and under erosion during ebb tides in the channel. This is consistent with the tidal velocity asymmetry in the PRE. As shown in Fig. 5, the bottom SSC varied with the bottom shear stress for each tidal cycle. Specifically, the bottom SSC followed the changes in the bottom current shear stress remarkably in the channel for most of the time. The close link between SSC and the shear stress indi­ cated that the SSC was mostly resuspended from the seabed in the channel. Exceptions occurred sometimes. For instance, during the period shadowed in Fig. 5, the SSC reached a maximum before the peak of the shear stress, and decreased while the shear stress was still increasing, suggesting a limited supply of erodible sediment at the seabed (Sanford and Maa, 2001).

4.5. Vertical turbulent SSC flux The results of vertical SSC fluxes are listed in Table 3 and shown in Fig. 7. The parameters of M and n were obtained by fitting Eq. (24) to the observation data in a tidally averaged sense. The peak vertical SSC fluxes were mostly on an order of 10 3 g/(cm2s), with a larger magnitude in the spring tides than in the neap tides. The relationships between the vertical SSC flux and the bottom shear stress were in good accordance with Eq. (24) in the channel, with the R2 > 0.55 (Fig. 7a, c, e, g). However, the correlations at the shoal were much lower in the spring tides. The R2 was only 0.14 in the spring tide in the wet season and 0.35 in the spring tide in the dry season (Fig. 7d, h). For all other periods at the shoal, the R2 was above 0.6 (Fig. 7b, f), which also showed good consistence with Eq. (24). As shown in Table 3, n was greater than 1.0 during the neap tide and less than 1.0 during the spring tide, showing that the erodibility is more

4.4. Erodibility of the sediment For each erosion experiment in laboratory in this study, sufficient long time was taken until all the available sediment at the seabed was exhausted. Therefore, the erosion processes could be categorized as Type I (Fig. 6a). The initial pumping rate at 0.01 Pa was used to clear the ambient suspended sediment before the formal erosion experiment, and the data at 0.01 Pa was included for later analysis. A thin fluffy layer (with an initial critical shear stress around 0.05 Pa) was noted to exist at the surface. The first erosion process from 0.05 to 0.10 Pa could be taken as the organic ‘fluffy layer’ erosion (Fig. 6b). In this process, particles with low density at the surface was eroded. This occurs in nature at low 9

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Fig. 7. Relationships between the shear stress and vertical SSC flux: (a) neap tide in the wet season in the channel, (b) neap tide in the wet season at the shoal, (c) spring tide in the wet season in the channel, (d) spring tide in the wet season at the shoal, (e) neap tide in the dry season in the channel, (f) neap tide in the dry season at the shoal, (g) spring tide in the dry season in the channel, and (h) spring tide in the dry season at the shoal.

sensitive to the changes in excessive shear stress in the neap tide, with the reasons unknown. Fig. 8 shows that the sediment was more erodible in the spring tide than in the neap tide under shear stress between 0.01 and 1 Pa. In addition, our results suggested a spatial difference in erodibility between the channel and shoal (Fig. 8). As seen in the erosion curves (Figs. 7 and 8), the channel was more erodible than the shoal in most of the observation periods. The tidal variations of the vertical SSC flux are shown in Fig. 9. In the neap tide, the vertical SSC flux was well in accordance with the shear stress both in the channel and at the shoal (Fig. 9a and b). While in the spring tide, the situation became different. On the flood, the vertical SSC flux increased with the bottom shear stress. On the ebb, the vertical SSC flux increased firstly with the shear stress before reaching a peak, after then it stopped further increasing or even decreased even the shear

stress was still much high. The reason for such a behavior will be explored later. Unfortunately, the discharges during the observation periods in the wet season were not significantly different from that in the dry season (shadowed in Fig. 1c), and the seasonal variability of sediment erod­ ibility was not discerned in this study. 5. Discussion 5.1. Limitation of sediment supply Shown in Fig. 9c (also in Fig. 11a and b), the vertical SSC flux always existed in the period from hour 10 to 17. It seemed that a continuous erosion happened in the seabed. However, it was still a limited erosion and did not contradict the assumption of ‘limited sediment supply’. The amount of erodible sediment in the seabed is relevant to the applied shear stress on the seabed (Sanford and Maa, 2001; Sanford, 2008; Xu et al., 2014, 2016). Sanford and Maa (2001) believed they obey a power law relationship and suggested that the distinction be­ tween limited erosion (Type I) and unlimited erosion (Type II) was related to the time scale on which the shear stress is applied (i.e., it depends on whether erodible sediment at the applied shear stress was completely exhausted within the time scale). Generally, Type II erosion exists only in short erosion time scales. When the erosion time scale becomes longer, the erosion of seabed tends to change into the type of ‘limited erosion’, and eventually to be com­ plete Type I erosion for a long enough time scale (Sanford and Maa, 2001). In Fig. 9c, the period from hour 10 to 17 showed a limited erosion event. The former half (hour 10 to 14) was an erosion event (the seabed was continuously eroded), but in the latter half (after the peak of shear stress), erosion almost stopped (Fig. 9c). In the latter event, the change in bed elevation was close to 0. Though there were fluctuation in bed elevation, its amplitude was quite small (less than 2 mm), for which no erosion occurred in the seabed. The turbulent flux during the latter half was small though the bottom shear stress was still much high.

Fig. 8. Combination of the relationships between the shear stress and vertical SSC flux (neap tide in the channel, neap tide at the shoal, spring tide in the channel and spring tide at the shoal, in the dry season). 10

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Estuarine, Coastal and Shelf Science 232 (2020) 106530

Fig. 9. Results of the water depth, bed elevation, bottom shear stress and vertical SSC flux at 0.05 mab, during (a) the neap tide in the channel, (b) the neap tide at the shoal, (c) the spring tide in the channel, (d) the spring tide at the shoal, in the dry season.

Deposition and erosion occurred simultaneously (dm/dt ¼ E-D; dm/dt is the changing rate of sediment in seabed, E is the vertical turbulent flux, and D is the deposition flux of suspended sediment). In the latter half period, the vertical SSC and the deposition flux were in balance. The small amount of newly deposited sediment was resuspended into the water body in the form of turbulent flux, while the seabed stayed stable (D ¼ E, i.e., dm/dt ¼ 0). Another evidence was that the erosion gradually slowed down and the SSC decreased (Fig. 5d), showing a limited supply in the seabed. Thus, though the vertical SSC turbulent flux was still more than 0 in the latter half event, it was still a limited erosion.

of our research. 5.3. Formation of seabed structure Sanford and Maa (2001) described a typical vertical structure for well consolidated seabed. In the structure, critical shear stress was smaller in the upper layers and larger in the bottom layers (power law relationship). If there is not enough time for the seabed to consolidate to the equilibrium profile, the vertical profile would be different (Sanford, 2008). As shown in Section 4.3, the seabed underwent erosion in the ebb and deposition in the flood. For fully consolidated seabed, larger shear stress applied will erode more sediment (Sanford and Maa, 2001; San­ ford, 2008; Xu et al., 2014, 2016). During the ‘neap-spring’ cycle, the peak velocity (or shear stress) increased (Fig. 5). Thus, the eroded thickness of seabed increased from neap to spring tides (also discussed in Section 5.2). Conversely, during the ‘spring-neap’ cycle, the peak ve­ locity (or shear stress) decreased. For the newly deposited sediment, the critical shear stress in the seabed grows towards the equilibrium profile (vertical profile for fully consolidated seabed) (Sanford, 2008). Labo­ ratory results (Tsai and Lick, 1988; MacIntyre et al., 1990) indicated that newly deposited sediment needs more than 7 days to become fully consolidated. Limited by the short consolidation time (less than 7 days), the newly deposited sediment did not reach the equilibrium profile. However, the critical shear stress in the deeper layer is still larger than that in the upper layer of the newly deposited sediment. When the peak ebb velocity (or shear stress) decreased from the spring tide to the neap tide, the mass of erodible sediment would decrease. In Fig. 10, a conceptual model was proposed to illustrate the for­ mation of the seabed structure. The initial seabed was assumed to be well consolidated with typical three-layer structure (Parchure and Mehta, 1985; Amos et al., 2010; Sanford and Maa, 2001). The peak velocity (or bottom shear stress) is much larger in spring tide than neap tide (ebb dominated). Thus, a vertical two-layer structure, which is fresher at the surface and older at the bottom, can form when the tide changes from neap to spring (day 0–7 in Fig. 10). Conversely, when the tide transits from spring to neap (day 7–14 in Fig. 10), the decreased tidal strength leads to a decrease in the maximum erosion depth in the ebb tides. The upper part of the newly fresh layer is eroded, while the bottom part of the fresh layer remains to grow in age. Thus, a seabed laminated with different ages (fresher in the upper layer and older in the bottom layer) forms when the tides change from spring to neap (day 13 in Fig. 10). Similar closed cycles were found by Sanford (1992) in the northern

5.2. Factors controlling the spatiotemporal variability of erodibility in a tidally averaged timescale In this section, we focused on the difference between spring and neap tides to illustrate the temporal variability and the difference between channel and shoal for the spatial variability. We tried to identify the differences in physical properties that may affect the erodibility in the PRE. As shown in Fig. 8, the sediment was more erodible in the spring tide than in the neap tide under shear stress between 0.01 and 1 Pa. It was studied by Sanford and Maa (2001) through numerical ex­ periments that seabed experiencing frequent erosion and deposition is prevented from further consolidation, which makes it more erodible. The newly deposited fresh layer at the surface in our study site experi­ enced deposition on flood and erosion on ebb in each tidal cycle. As the spring tides have stronger ebb currents, more sediments are resuspended into the water column during ebbs, which are deposited at the seabed surface in the next flood tides. Thus, the fresh layer formed on flood in the spring tide was much thicker than in the neap tide, leading to a more erodible seabed (Fig. 8). In addition, our results (Fig. 8) indicated a spatial variability in erodibility between the channel and shoals. As seen in the erosion curves (Fig. 8), the channel was more erodible than the shoal in most of the observation periods. Previous studies indicated that the particle size distribution and consolidation were the dominant physical properties that affected the erodibility of the marine seabed (Dickhudt et al., 2009). Besides, Gerbersdorf et al. (2005) found that sand fraction has a negative effect on the erodibility of the seabed. In the PRE, the sediment in the channel is dominated by silt, while the shoals have more sand fractions. This difference could largely explain why the seabed in the channel is more erodible than that at the shoal. Other mechanisms such as differ­ ence in bioturbation and biostablization may play roles, but are beyond 11

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Estuarine, Coastal and Shelf Science 232 (2020) 106530

Louisiana Shelf (Xu et al., 2014). It is thus a ubiquitous phenomenon for seabed in marine environments. Since the erodibility decreases with the ages of sediment (Section 4.4), the seabed laminated with ages should be laminated with erodibility. 5.4. Erodibility asymmetry within a tidal cycle The flood-ebb asymmetry in erodibility was noticed to occur in our study site. A typical case in the channel during the spring tide of the dry season was chosen for illustration (shadowed in Fig. 11a and b). On the flood tide, the vertical SSC flux kept in pace with the shear stress (Fig. 11b). It became different on the ebb (Fig. 11b). The first peak on the ebb (the first red arrow in Fig. 11b) was similar to the peak on the flood (green arrow in Fig. 11b). During that time, the seabed had just expe­ rienced deposition at the former flood. The erosion occurred at the freshest sediment layer (sediment above the magenta line ‘—’) which was quite erodible. Three peaks (the first three red arrows) on the ebb occurred for the surficial fresh layer. The vertical SSC flux increased with the shear stress sharply. When seabed reached the elevation lower than that of the former cycle (denoted by the magenta line ‘—’ in Fig. 11a and b), the vertical SSC flux decreased sharply, because an older, less erodible seabed layer was encountered. The fourth peak in Fig. 11b was much smaller than the former three peaks though the applied shear stress was larger, indicating that the sediment in the consolidated layer was much less erodible than the newly deposited fresh layer. This was coincident with the results of GEMS that the erodibility decreased with the sediment ages. The asymmetry of erodibility in intratidal timescale in this study is closely related to the tidal velocity asymmetry in the PRE. It is expected that this asymmetry would happen but may display different patterns in other estuaries with different tidal asymmetries.

Fig. 10. Conceptual diagram about the non-dimensionalized bed elevation variation in a ‘neap-spring-neap’ tide cycle. Blue line stands for seabed eleva­ tion variation (the maximum erosion thickness in the spring tide was assumed to be non-dimensionalized as 1.0); black line in the neap-spring cycle is the interface of the newly deposited fresh layer and the well consolidated layer; and colored lines in the spring-neap cycle are interfaces between sediments of various ages (‘Nd’ stands for the age of the sediment since deposition). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Chesapeake Bay, but the averaged deposition rate was much faster than our study site. In the Chesapeake study, sediments were deposited and resuspended repeatedly, leading to periodic erosion and deposition cy­ cles. Thus, a laminated seabed formed. Laminated seabeds were also observed via X-radiography at the margin of the Adriatic Sea (Stevens et al., 2007), the York River estuary (Dickhudt et al., 2009), and the

6. Summary In this study, tripod observations were conducted in the western part

Fig. 11. Time series of (a) seabed elevation (red line) and water depth (black line) and (b) shear stress (magenta line) and vertical SSC flux (blue line) in the channel in the spring tide in the dry season. The data for analysis in (c) is shadowed with gray color in (a) and (b), the increasing/decreasing trend of the vertical SSC flux with shear stress is shown by the arrows in the flood and ebb. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 12

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of the PRE during the neap and spring tides in the wet and dry seasons of 2015. High-frequency velocity, bottom salinity, bottom SSC, wave pa­ rameters, and bed elevations were accurately recorded. The erodibility of the seabed was investigated via estimating the vertical SSC flux through the robust ADV measurements with high temporal resolution, combined with erosion experiments (GEMS) con­ ducted in the laboratory. The channel was found to be more erodible than the shoals, and this variation was related to the difference in sediment size distribution. The seabed was more erodible during the spring tides than the neap tides, mainly owing to the larger thickness of fresh sediment layer at the seabed surface in the spring tides. Erosion variability was also noticed between flood and ebb within a tidal cycle. The relationships between the vertical SSC flux and shear stress differed when the erosion occurred at different depth in the seabed in a tidal cycle. The processes of erosion/deposition and formation of seabed laminated with different ages were noted as important causes of the erodibility asymmetry. Since the sediment experiences erosion and deposition all the time, the spatiotemporal variability of the erodibility is much relevant to sediment transport. This study helps to deepen our understanding of the spatiotemporal variability of sediment erodibility of the PRE, and the results will have significant implications for simulation and prediction of sediment transport and morphological evolution of this important es­ tuary. The findings may also apply to other estuarine systems for process study on sediment transport in different timescales.

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Declaration of competing interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any kind in any product, service or company that could influence the position pre­ sented in the manuscript entitled “In-situ study of the spatiotemporal variability of sediment erodibility in a microtidal estuary”. CRediT authorship contribution statement Weihao Huang: Conceptualization, Methodology, Investigation, Resources, Data curation, Writing - original draft, Visualization. Heng Zhang: Conceptualization, Methodology, Formal analysis, Investiga­ tion, Resources, Writing - review & editing, Visualization. Lei Zhu: Formal analysis, Investigation, Writing - review & editing. Lianghong Chen: Formal analysis, Investigation, Writing - review & editing. Guang Zhang: Formal analysis, Investigation, Writing - review & editing. Wenping Gong: Conceptualization, Methodology, Formal analysis, Investigation, Resources, Data curation, Writing - review & editing, Visualization, Supervision. Jiahuan Liu: Resources, Investigation, Data curation. Acknowledgments This research is funded by National Natural Science Foundation of China [grant number 51761135021, 41576089, 41890851], and the Guangdong Provincial Water Conservancy Science and Technology Innovation Project (grant number 201719). We thank our graduate students from Sun Yat-sen, including Zhongyuan Lin, Rui Zhang, Wei­ cong Cheng, Shuaishuai Liu, Yuren Chen,and Jiawei Qiao, for their help in field work and sediment sample analysis in the laboratory. Three anonymous reviewers are greatly appreciated for their careful reviewing and constructive comments. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.ecss.2019.106530.

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