Characterization of bottom sediment resuspension events observed in a micro-tidal bay

. In this contribution we investigate the origin of the variability in near-bottom turbidity observations in the Alfacs Bay (NW Mediterranean Sea). This bay is characterized by a micro-tidal environment and a relevant seiching activity which may lead to flow velocities of more than 50 cm•s-1. A set of current meters and optical sensors mounted near the sea bottom were used to acquire synchronous hydrodynamic and optical information of the water column. The time-series observations 15 showed an evident relation between seiche activity and sediment resuspension events. The observations of turbidity peaks are consistent with the node/anti-node location for the fundamental and first resonance periods of the bay. The implementation of a coupled wave-current numerical model shows a strong spatial variability of the potential resuspension locations. Strong wind events are also a mechanism responsible of the resuspension of fine sediment within the bay. This is confirmed using retrieval of suspended sediment concentration from Sentinel-2 data. We suggest that the sequence of resuspension events plays a 20 relevant role in Suspended Sediment Concentration, in such a way that previous sediment resuspension events may influence the increase of suspended sediment in subsequents events. The suspended sediment events likely affect the ecological status of the bay and the sedimentary process at long-term period.

Line 292-293: don't use the same word through the entire text.Moreover, two "contribution" have different meaning.I suggest to replace the first one with "study" Done.

Done
Line 297: "must take into account" -> "should include" Replaced Line 300: The bay geometry characteristics cannot suggest.Please rewrite this sentence.
Line 305: "This may be consistent with …" Please rewrite this sentence.
Magenta is not very clear in the contour plots.I suggest to use a color with better contrast.
After a reviewer suggestion we include magenta because this color is not included in the color bar.
"Isobaths (in grey) are plotted each 3 m" -> Isobaths are plotted in grey solid lines in 3 m interval from ? m to ? m. Modified.
"the plot scale is transformed in log10" -> "the bottom stresses are plotted in log10 scale" Corrected 1 Introduction Suspended sediment in the water column and subsequent deposition plays a critical role in coastal ecosystems and for managing in coastal environments and management.High levels of suspended sediment concentration in the water column haves relevant substantial implications forin aquatic ecosystems and natural habitats (Ellis et al., 2002) in particular during large exposure periods ( (Newcombe and Macdonald, 1991).Also, SsedimentFurthermore, sediment supplied from rivers potentially transports loads significant amounts of organic matter, pollutants and heavy metals that may be deposited at the in the vicinity sea bottom or even transported offshore (Palanques et al., 2017).The sediment dynamics is relevant in coastal bays and estuaries due to the large amount of sediment delivered by the freshwater and the potential fine sediment trapping zones.In addition, sediment resuspension can results in a large contributionlargely contributes significantly to the total nutrient load (Sondergaard et al., 1992) and prevents the sunlight penetration (Mehta, 1989).Besides, Tthe analysis and prevention of fine sedimentation within basins and channels access is object of investigation also plays also a role for the purposes ofin the port engineering context inengineering, in order to examine and monitor the siltation processes (e.g.(Ghosh et al., 2001;van Maren et al., 2015)).Finally, the growth of harmful species, such as dinoflagellate cysts, may be related to significant local resuspension through the mixing of the upper layers, resulting into more homogenous cyst profiles in the sediment (Giannakourou et al., 2005).
In coastal areas, the transport sediment transport is related towith the hydrodynamic conditions.ForIn large long time scales, advection processes redistribute and determine the final depositional pattern as a function of the sediment and water current variables (Bever et al., 2009;Ogston et al., 2000;Bever et al., 2009).Hydrodynamics processes driven by wind --waves (Grifoll et al., 2013;Carlin et al., 2016;Grifoll et al., 2013), tides (Fan et al., 2004;Garel et al., 2009), winds ( Sherwood et al., 1994;Hofmann et al., 20111;Sherwood et al., 1994), surface seiches (Jordi et al., 2008) or internal-seiches (Shteinman et al., 1997) promote the resuspension, advection and settling of fine sediment, and conditioned by the continental sediment sources.Subsequent resuspension effects due to natural causes also contributes toat the reworking and final deposition of the sediment load (Guillén et al., 2006;Grifoll et al., 2014a;Guillén et al., 2006).MoreoverIn this sense, anthropogenic activities such as, fishing trawling, ship propellers and in general waves generated by vessels, may bring additional energy in the water system influencing the resuspension, transport and final sediment deposition, in particular in shallow waters (e.g.(e.g., Garel et al., 2009;Hofmann et al., 2011).
This study focuses on Alfacs Bay (NW Mediterranean Sea; Southern part of the Ebro Delta) which isis a micro-tidal estuary.
The entire bay area is and intensively exploited by commercial activities, including area with tourism, fishing and aquaculture, activities and hence, the being an ecosystem has a of highrelevantsignificant economic importance importancein the region.It has beenIn the past, the bay has been extensively investigated extensively in the past in terms of its hydrodynamics response ( Solé et al., 2009;Llebot et al., 2014;Cerralbo et al., 2015aCerralbo et al., , 2016Cerralbo et al., , 2018;;Llebot et al., 2014;Solé et al., 2009), tidal wave propagation (Cerralbo et al., 2014), biochemical processes (Llebot et al., 2010(Llebot et al., , 2011) ) and optical water properties (Ramírez-Pérez et al., 2017).The estuary receives freshwater discharge mainly from the rice fields of the Ebro river.Thus, Alfacs Bay is an intensively exploited area with tourism, fishing and aquaculture activities being an ecosystem of relevant economic importance in the region.Several episodes of algal blooms have been reported to be and were (linked to with the increased of nutrient concentrationss, possiblyand perhaps triggered by resuspension mechanisms.Moreover, ) andthe presence of harmful bacteriaum was found in bivalves with negative effects on aquaculturee have been reported (Loureiro et al., 2009;Roque et al., 2009).
With the purpose toThe goal of this study is to improve the knowledge in fine sediment dynamics in coastal bays and, the goal of this investigation is to provide a physical interpretation of insights ofon the controlling factors of the sediment resuspension events observed within the Alfacs a micro-tidal bay (Alfacs Bay; NW Mediterranean Sea).Using WithUsing sea-level heights, water currents and wind speed measurements we investigated the driving mechanisms forthat resuspension ofd fine bottom sediment within the bay.ThenSubsequently, the spatial and temporal interpretation of the resuspension mechanisms were linked with the hydrodynamic processes s is and analyszed, through the implementation of a coupled wave-current coupled numerical model.The contribution aims to provide explanation ofto explain resuspension mechanisms ; the knowledge of these mechanismwhich may have an a positiveevident benefit for humanmanagement activities activities management mentioned previously (e.g.harmful species resuspension or algal blooms with negative effects on aquaculture activities).
The water circulation in Alfacs Bay has been widelyextensively analyszed in previous contributions studies, using observational data sets and numerical results models (Camp and Delgado, 1987;Cerralbo et al., 2014Cerralbo et al., , 2015a;;Llebot et al., 2014).However, fine sediment dynamics and its their resuspension mechanisms haves not been examined yet.Synchronous optical measurements, jointly with velocity and sea-level measurements, have entailed facilitated an opportunity toa good chance to investigate the resuspension mechanisms in Alfacs Bay.Considering the area is a micro-tidal estuary, wind or windwaves are candidate mechanisms for dispersal of fine sediment.This area is an example of micro-tidal estuary, thus being the wind or wind-waves candidates mechanisms of fine sediment dispersal.

Study Area
Alfacs Bay, located in theat south of the Ebro delta, is formed by the prograding southern spit.The semi-enclosed bay is about approximately 16 km long and 4 km wideth.The average depth is 4 m and the maximum depth is about 6.5 m in the middle of the bay (Figure 1).A central channel of 2.5 km in length, and 6.5 m depthdeep, connects the is connecting bay towith theThe connection with the open seaocean.is 2.5 km, with a central channel of 6.5m and Sshallow edges of around 1-2 m canare be found on both sides.To the north, tThe bay is surrounded by rice fields.to the north, which From April to December, these fields spill around 10 m 3 •s -1 of freshwater loaded with nutrients into the bay.during 9-10 months per year (April-December) These nutrient rich waters are distributed among severalin several channels, to the eastern side of the delta, and aclose byto a sandy beach closing it on the east side.The seabed in the central part of the bay is composed ofby very fine sediments (typically 65-65% silt, 30-35% clay and aapproximately..round 5% sand) with increasing the sandy content towards the edges of the bay (Guillén and Palanques, 1997;Satta et al., 2013).The bottom sediment of Alfacs Bay is composed of mud, with a significant content of clay , and sand (Palacín et al., 1991).It was discoveredThey found that the muddy sediment extended toby the central part of the bay, whereasand the sand content of sand increased near theto spit which that separates the bay from the open seaocean.The same was found and also inalongfor the southern shallow edge.
The bay is categoriszed has been defined as a salt-wedge estuary (Camp and Delgado, 1987) with almost stable stratification all year.The highest tidal ranges during spring tides reachis around 0.2 m, and the hydrodynamic fluctuations are controlled by the wind modulated by the seiche activity duringin a short periods (Cerralbo et al., 2015a).Both winds and salinity gradients due to freshwater discharge dominate tThe water circulation in the low-frequency band. is dominated by both winds and salinity gradients due to freshwater discharge (Solé et al., 2009;Cerralbo et al., 2018).The most iIntense regional winds coming from in the area are from the north and and northwestern directions together with the orographic effects , establishing aresult in wind jets in the Ebro River valley due to the orographic effects in the Ebro River valley (Grifoll et al., 2015(Grifoll et al., , 2016)).This offshore wind is characterized by noticeable spatial variability due to the surrounding topography (Cerralbo et al., 2015b).
The water column within the bay used to be stratified due to the freshwater discharge, but well-mixed conditions are common during winter as a consequence of the hydrodynamic response to strong wind forcing (Llebot et al., 2014) and occasionally to seiches (Cerralbo et al., 2015a).During the summer, the contribution of the temperature at the stratification may be also be substantial (Cerralbo et al., 2015a).

Measurements campaigns
The bulk of the observational data correspond atwas collected during two months a 2-month field campaign from July to mid-September 2013.The data set consisted of wWater currents from were measured with two 2MHz Acoustic Doppler Current meter Profilers (ADCPs) moored in the mouth (Fig. 1 -A1) and inner bay (Fig. 1 -A2) (Fig. 1) and configured to record 10 min averaged data from 10 registers per minute and with 25 cm vertical cells.Both devices were equipped with an Optical Backscatter Sensor (Campbell Scientific OBS-3), a bottom pressure meter and a temperature sensor.The instruments , and they were mounted at on the sea bottom inat 6.5 m depth, while the sensors were 0.25 m above the sea bed.The signals from the OBS instruments signal iswere transformed to the Nephelometric Turbidity Units (NTU) withusing device calibration repportreport.In the past a Besides, the study area used to present a linear relation between optical signal and suspended sediment concentration has been observed in the study area (Guillén et al., 2000).The distance of the ADCPs and OBS sensor were 0.25 m above the sea bed.The ADCP has a 20 cm of blanking zone.Additional sea level data were obtained through a sea level gauge mounted in Sant Carles de la Ràpita harbor (Fig. 1) and bottom pressure systems from the ADCPs.Atmospheric data (wind, atmospheric pressure, solar radiation and humidity) were obtained from a land station (M-Sc) located in Sant Carles de la Ràpita, (M-Sc) mounted 10 m above the ground.

Current and wave model implementation
We use the coupled version of SWAN-ROMS models included in the COAWST system in order to simulate the hydrodynamics within the bay.The COAWST system (Warner et al., 2010) consists of several state-of-the-art numerical models that include ROMS (Regional Ocean Modeling System) for ocean and coastal circulation and SWAN (Simulating Waves Nearshore) for surface wind-wave simulation.SWAN is a third-generation numerical wave model that computes random, short-crested waves in coastal regions with shallow water and ambient currents (Booij et al., 1999).It is based on the wave action balance with sources and sinks and incorporates the state-of-the-art formulations of the processes of wave generation, dissipation and wavewave interactions.ROMS is a three-dimensional circulation model which solves the primitive variables on a sigma-level in the vertical and horizontal regular grid.Numerical aspects of ROMS are described in detail in (Shchepetkin and McWilliams, 2005).In COAWST system, the wave model provides hydrodynamic parameters (i.e., significant wave height, average wave periods, wave propagation direction, near-bottom orbital velocity and wave energy dissipation rate) to the water circulation model.The ocean model provides water depth, sea surface elevation, and current velocity to the wave model.The variables exchange is made "on-line" during the simulation processes, via Model Coupling Toolkit (Jacob et al., 2005), where a multiprocesses MPI protocol is used to distribute the computations among several nodes.The COAWST also include different formulations to parametrize the wave-current bottom boundary layer and the wave effect on currents (Warner et al., 2008;Kumar et al., 2012).
The implementation of the COAWST system in Alfacs Bay consistsed of a regular grid of 186 x 101 points with a spatial resolution of 100 m (in both x and y) and 12 sigma levels in the vertical direction.Details of the implementation and the skill assessment of the ROMS model in Alfacs Bay is provided by in (Cerralbo et al. , (2015a).The same regular grid is used by the SWAN model.A two-year water circulation simulation (2012-2013) was performed in order to obtain realistic threedimensional temperature and salinity fields.The barotropic time step for ROMS is set towas 30 s, and in for ROMS and SWAN the wave field is solved in a time interval of the wave field each 3600 s.The interval time between exchange of variables of ROMS and SWAN was established in 3600 s.For both simulations, water motion at the open boundary was forced bywith depth-averaged velocities and sea level measurementsd at A1 (interval data of 600 sec).The freshwater inputs are distributed on 8 points simulating the main rice channels with a total dischargeflow of 10m 3 s -1 (see (Cerralbo et al., 2015a)).
The bottom boundary layer was parameterizsed using the combined wave-current (Styles and Glenn, 2000) adopted in ROMS and SWAN coupling in (Warner et al., 2008).The input parameters for the model are the velocity components near the bottom and wave characteristics near the bottom (wave period, wave direction and the wave orbital direction).For each computational step, an initial assessment of bed roughness length is estimated as a function of the grain size, ripples and sediment transport.
ThenConsequnetly, the pure current bottom stress (τ c ) and pure wave bottom stress (τ w ) bottom stress are computed as: (1) where z is the vertical coordinate, z 0 total bottom roughness length, u and v are the water currents componentsspeed, u b is the orbital velocity, κ is the von Karman's constant, and f w is the Madsen wave-friction factor.Then, tThe maximum bottom stress under wave-current conditions is computed as (Soulsby, 1997): The wave effects on from currents are considered using vortex-force formalism, which is included in COAWST.This approach allows to considers the effect of fromthe gravity waves on the mean flow, and was tested in different experimental and real configurations by (Kumar et al., (2012).

Observations
In order to investigate the suspended sediments events within Alfacs Bay we used a sub-set of the total observations recorded atin A2: from 2 nd August to 8 th August 2013.This is because the sub-set data selected include the main hydrodynamic conditions susceptible to increase the near-bottom turbidity.Figure 2 shows the time-series recorded atin A2 in terms of NTU from the OBS, measured sea level height measured (additionally sea-level height measured atin A1 is also shown), bottom current speed in m•s-1 atin A1 and wind speed and direction measured atin M-Sc (see Figure 1).The sea level height reference was obtained by subtracting the mean value of the pressure meter time-series provided by the ADCP.
The wind characterizationTwo typical wind conditions are considered (Figure 2.a and 2.b) include two of the most typical situations in the region: sea breeze and the NW winds (Cerralbo et al., 2015a).The sea breeze is associated to an increase of wind speed during the central hours of the day (approximately from 11:00 hr to 18:00 hr with a wind direction within the range of approximately 30º to 180º approximately).From a daily point of view, this seems evident during the 1 st to 6 th of August.A different pattern is observed during the wind speed peak of (7 th -8 th of August) where 330º wind directions were measured.This corresponds to an offshore wind typical from for the region (NW winds called "Mestral").
DuringThe the analysis periodd of analysis, also include a seiche event was also captured during the 3 rd ofon the 3rd of August.
This seiche event was previously characterized described hydro-dynamically in Cerralbo et al., (2015a)  The near-bottom turbidity shows a fluctuating behaviour with values ranging values from almost zero to higher overthan 10 NTU (Figure 2.ed).In this sense, Tthree differentiated events with high turbidity are observed.These events are E1 (covering from 08:00 of 3 rd of August to 10:00 of 5 th of August), E2 (03:00 to 12:00 6 th of August) and E3 (between 08:00 7 th August and 15:00 8 th of August).The maximum turbidity is measured during the E1 (maximum turbidity 41.1 NTU).This event lasts for a longer time in comparison to E2 (with a maximum turbidity 4.6 NTU) and E3 (maximum turbidity 12.1 NTU).

Skill assessment near the sea bottom
The performance of the water circulation model used in this contribution study was examined in terms of sea-level, water currents and temperature/salinity evolution in previous worksresearch (Cerralbo et al., 2014).However, in this work we pay attention to the near-bottom velocities because due to its relevant role in the sediment resuspension and transport dynamics.
Thus, the skill assessment of the near-bottom velocities atin A1 and A2 is analyszed using a Taylor diagram (Taylor, 2001).
This diagram characterizes the similarity between numerical model and observations using their correlation, the Rroot-Mmean-Ssquare Ddifference (RMSD) and the amplitude of their variations (represented by their standard deviations).The skill of the model skill improves as the points triangles get are closer to the observation reference point in the diagram which means the full agreement between the model and the observations (Figure 4).In general, the model results showed a good agreement with the observations in the prevalent along-shelf direction, with correlations larger than 0.5 and RMSD below 1.In addition, the water current fluctuations are well represented in the model because the normalized standard deviation is closer to 1 in both measuring points.

Modelled bottom stress
The bottom stress is obtained from the coupled numerical model implemented in the Alfacs Bay.The Figures 5 and 6 show different snapshots of the modelling results in order to examine the bottom stress pattern for twoboth components (i.e.waveinduced and current-induced bottom stresses).These snapshots results corresponds to different episodes identified from the previous observational analysis.The plot scale of the bottom stress is transformed in log10 for clarity.to induce resuspension).For this event, the wave field during the sea-breeze is shown in Figure 7.This figure shows howIt reveals that the maximum significant wave height (equal to 0.3 m) occurs near the northern and southern shallow edge consistent with the maximum wave-induced bottom stress.
The bottom-stress pattern during the episode E2 (Figure 6.left) is similar to the second stage of the episode E1.Both wave and current bottom stress (08:00 7 th of August) tends to be small in at A2 in comparison to the seiche event.Only substantial bottom stress are is observed in the shallow edges of the bay due to the wave action originated by the sea-breeze.
During the episode E3 (NW wind, Figure 6.right), the combined bottom stress (23:00 8 th of August) is dominated by both wave and current action.The southern part of the bay shows that the maximum wave induced bottom stress is consistent with the wave climate (Figure 7).Also, the current induced bottom stress presents non negligiblenon-negligible values within the bay.
Focusing in on A2, both mechanisms contribute in a similar manner (wave and current bottom stress is 0.09 and 0.06 Pa respectively) in the combined bottom stress.

Discussion
The synchronous time-series of the meteo-oceanographic variables and turbidity shown in Figure 2, jointly with the bottom stress modelled provides a good opportunity to characterize the turbidity peaks measured atin A2.During the first stage of the episode E1, the bottom current speed responds at to the node-antinode pattern with velocities that raise increase 0.4 m•s-1 atin A2.Apparently, Tthis increase of the bottom velocity caused bottom sediment resuspension and a turbidity peak (Figure 2).
Even that if an increase of wind speed occurs (peaks that raise 8 m•s-1), the oscillating pattern of the current (see Figure 3), strongly polariszed strongly, following the along-shore direction with 1-hr periods.This, suggest an increase of turbidity due to the seiche instead of wind driven current.The analysis of modelled bottom stress modeled during E1 (Figure 5) also suggested that the seiche is the main mechanism for turbidity increase atin A2, during the first stage of event E1.Resuspension mechanisms in the water environments caused by seiches are is mentionedsuggested in observational investigations (Niedda and Greppi, 2007;Chung et al., 2009;Jordi et al., 2011;Niedda and Greppi, 2007).
However, these studies did not explain the high spatial variability of the importance of the seiche-induced sediment resuspension mechanism, which are implied by the modelled current-induced bottom stress.However, the numerical results of the current-induced bottom stress shown in Figure 5(left) suggest a high spatial variability of the seiche-induced resuspension not examined in the mentioned contributions.It means observational results about turbidity variability may differ significantly in function of the location of the node/anti-node and its consequent maximum and minimum velocities.
The turbidity still shows large values after the seiche was already dissipated and the bottom current decreased during the second stage of the E1 event.Typical sea-breeze wind conditions were observed (gentle variation of wind direction from 30º to 180º) with a noticeable increase of the wind speed during 4 th of August, unrelated with to the measured bottom current bottom intensity speed.Llebot et al. (2014) and (Cerralbo et al., (2015a) stated that water current profiles due to winds observed in Alfacs Bay does not imply a barotropic shape in the water columnvelocityies profiles, suggesting near the bottom a different behaviourbehaviour near the bottom, compared to thethan surface, related to wind set-up phenomena.In consequence, the local resuspension due to wind-breeze seems unlikely at this location of the bay.It seems more feasible that high turbidity measured atin A2 during E1 (second stage) are is associated to advection of fine sediment resuspended previously by seiche or by sea-breeze activity in the shallow edges of the bay, with a subsequent transport towards the middle of the bay.This last mechanism would also explain also the turbidity peak measured during the 5 th of August at 00:00; after the fine sediment settling occurred within the bay.The sediment advection within the bay is difficult to confirm according to our data set, but Alfacs bathymetry shows a characteristic shallow edge near the coastline (water depths below 2 m; see Figure 1).In these shallow edges the bottom stress arises increases by 0.8 Pa, suggesting a potential sediment resuspension.Theis shallow edge may be a source of fine sediment under energetic wind conditions in case of fine sediment availability.In consequence, the advection of resuspended sediment highlighthighlights the relevance of the water current patterns within the bay for turbidity measurements.
The eEpisode E2 is associatedattributed to at sea-breeze mechanism.This event is qualitatively less important in terms of turbidity measured atin A2.The comparison of the sea-breeze event during 4 th of August and 6 th of August (both have similar wind and bottom current speed but different turbidity values) seems to indicate the relevance of the previous events and the subsequent advection of fine sediment, following the mechanism way explained previously.Similar to the second stage of E1, the bottom stress is low (below 0.02 Pa) in the central basin of the bay, the bottom stress isare small (below 0.02 Pa); so indicating thatthe local resuspension is unlikely.In consequence, the turbidity measured atin A2 is probably due to advection processes of suspended sediment from the shallowest areas (combined bottom stress more than 0.8 Pa) into the central basin.
Finally, episode E3 corresponds to a strong NW wind event with wind speedsspeedsintensities in excess of that raise 12 m•s-1.The bottom current speed does not show significantly higher values during this episode, in comparison to calm periods.
However, in opposite contrast to the sea breeze, the sea waves generated by the NW wind conditions may have a relevant role in the are relevant to the resuspension mechanisms due to an increase of the wave induced bottom stress (Figure 6(right)).
UnhopefullyUnfortunately, the set-up of the ADCP did not allowed us to record the oscillatory pattern derived from the orbital velocities generated by waves and the relative importance of each resuspension mechanism (i.e.wind or waves) is difficult to be quantifyiedfy.
E2 and E3 are examples of two mechanisms that may produce local peaks inincrease the turbidity: wind-driven current and wind-waves.In Alfacs Bay, the role of these mechanisms in sediment resuspension is less clear in comparison to seiches because they are in a function of wind speed without a clear correlation between wind module and the turbidity observed.The resuspension of fine sediment due to wind and wind-waves in shallow environments have been reported in the literature (Luettich et al., 1990;Ogston et al., 2000;Guillén et al., 2006;Bever et al., 2011;Grifoll et al., 2014b;Guillén et al., 2006;Hawley et al., 2014;López et al., 2017;Luettich et al., 1990;Martyanov and Ryabchenko, 2016;López et al., 2017;Ogston et al., 2000).Some of these this works highlight the complexity of the sediment processes due to the temporal and spatial variability of the importance of resuspension mechanisms and the presence of available material to be resuspended.Apparently, this is the case of our observations, because similar wind conditions doesdo not imply the same turbidity measurements.A good example is the sea-breeze wind events during 4 th , 5 th and 6 th of August in which different turbidity values are were observed.As we mentioned in the previous section, advective fluxes and the sequence of events may have a relevant role in the observed water turbidity.In this sense, mMany authors have reported an apparentevident influence on advective fluxes correlated with suspended sediment concentration after an initial deposition of fine sediment ( Sherwood et al., 1994;Ogston et al., 2000;Guillén et al., 2006;Harris et al., 2008;Bever et al., 2009;Grifoll et al., 2014b;Guillén et al., 2006;Harris et al., 2008;Ogston et al., 2000;Sherwood et al., 1994).This means that on longer time scales, advection of sediment by currents may redistribute sediment and determine final deposition patterns (Wright and Nittrouer, 1995).This may be the mechanism responsible of high turbidity observed under relatively low hydrodynamic conditions.For instance, the fact that during the seabreeze event of 2 nd August does not appears cause high turbidity, in opposite in contrast to the event onto 5 th of August (second stage of E1 event), which may response indicate thatat this mechanism where an energetic event (i.e.seiche) may could mobilizse sediment, which is that after is resuspended easily in subsequent events.The lack of proportionality of the resuspension related to hydrodynamics is also found in extended data time-series where divergences are associated mainly at to sediment availability in the bottom, among other factors (e.g. in Wiberg et al. (1994) or (López et al., (2017) ; Wiberg et al., 1994)).
In the case of Alfacs Bay, more extended observations may clarify the relation between wind intensity, wind-waves, seiches and the amount of suspended sediment and fluxes taking into account the sequence of energetic events.
The sediment distribution in Alfacs Bay (high percentage of silt and clay in the central basin and sand prevalence in the southern, eastern and western shore) is consistent with the modeling results shown in this studycontribution, where larger bottom stresses were obtained in the lateral shallow edges due to the contribution of the wave induced bottom stress in shallow areas.However, the deposition mechanism may beis a complex process, composed from including an initial settling and a subsequent dispersal, in a similar pattern to as described in (Wright and Nittrouer, 1995).Further sediment transport simulations, including those considering sediment classes and erosion and settling effects, would help to investigate the sediment settling dynamics and its final deposition.These processes must take into accountshould include the cohesive nature of the fine sediment or others phenomenaphenomena's, such as armoringarmouring or bioturbation, that may modify the physical properties of the sediment layers (van Ledden et al., 2004;Amoudry and Souza, 2011;van Ledden et al., 2004).
The characteristics of the bay, such as geometry characteristics (for instance the relative narrow and shallow entrance,) favoursuggest the trapping effect of fine sediments, fed either delivered by the freshwater outflow or the exchangelink between the open sea and the inner bay.The trapping effect of the bay may entailed the presence of a thin surface bottom layer of fine sediment easily involved subject toin resuspension.This behaviour is typical from shallow and sheltered environments such as lagoons or lakes.According to (Luettich et al. (, 1990) and (Hofmann et al., ( 2011), the regular resuspension events in sheltered and shallow water bodies prevent the sedimentprevents sediment consolidation and the formation of a cohesive sediment layer.This may could explain be consistent with the high turbidity values observed in the Alfacs Bay under relatively weak conditions, such as sea-breeze events, as which would likely not occuropposite to be expected if the sediment was cohesive.
The image with unprecedent resolution obtained by the The Sentinel-2 satellites provide imagery whichshould allowwhich allow for further identification of to identify scenarios with resuspension linked to hydrodynamic forcing.Figure 8 shows the Total Suspended Matter (TSM in mgr•l-1) for the Alfacs Bay in two differentiate scenarios: NW wind and Calm conditions.
Without access to local calibration data, a generalized approach for TSM retrieval has been applied.Through SNAP (v.6.0.0) the Level 1C Sentinel-2 MSI data was converted to geophysical values (suspended sediment concentration) using the most recent version of the water quality processor 'C2RCC' (v.1.0).The C2RCC processor was run using default values.Following processing in SNAP the data was post-processed (tiles merged, and data noise corrected) and the TSM maps created.NW wind conditions increase the TSM substantially the TSM in the southeastern shallow edges.This would be a source of a subsequent advection of fine sediment towards the central bay as it was stated in the previous paragraphs.In oppositecontrast, the values of TSM decrease significantly during calm conditions.
Also, the proximity of the Ebro river mouth (15 km at north) may increase the suspended sediment within the bay under particular specific circumstances.River discharge is the main driver of the Ebro River plume, followed by wind and regional oceanic circulation that tends to be southward (Fernández-Nóvoa et al., 2015;Mestres et al., 2003).Analysis of the turbid plume by remote sensing products indicate that more than 70% of the plume extension was located south of the river mouth, influenced by the regional oceanic circulation (Fernández-Nóvoa et al., 2015).Others external sediment sources may be associated with freshwater discharge from channels, overwash in the bar, flash flood from small creeks or aeolian transport.
The complete study of the suspended sediment dynamics will provide objective information to address the problem of degrading water quality within the bay and how to make use of natural mechanisms to limit undesired concentrations of nutrients or pollutants.This applies in particular to harmful algae blooms prone to occur in the area under present and future conditions.

Conclusions
The observational set and the wave-current numerical results obtained for Alfacs Bay have permitted allowed for a thorough investigation ofto investigate the resuspension mechanisms of fine sediment.The results indicate evidence of a clear mechanism of resuspension induced by eventual seiche events, which according to the bottom stress patterns may have a relevant spatial variability within the bay.The wind and wind-wave mechanisms also are also responsible of for fine sediment resuspension during energetic wind events, especially in shallower areas of the bay.The relevance of the sequence of events in turbidity is highlighted, taking into account the effect of advective sediment fluxes within the bay (from the lateral shallow edges to the middle of the bay).The trapping effect of the bay may entail the presence of a thin surface layer of fine sediment, easily involved in resuspension neglecting the expected cohesive effects.However, these points deserve further analysis with extended data sets and sediment transport modeling.The exchange of fine sediment within the bay and the open sea is also evidentseems also evident according to remote sensing images.However, these points deserve further analysis with extended data sets and sediment transport modeling.As a region of with high-anthropogenic pressure, this research may contribute to

Figure 3
Figure 3 caption: (a)…; (b) as (a), but for the cross-shore direction.Modified."showed" -> "shown" Corrected revealing an characteristic oscillation of 1 hour periods in sea-level and currents.This oscillation is characterized by a node (approximately located at A2) where the velocities are maximum, and an anti-node (approximately located atitn A1) where the amplitude of sea level oscillation is maximum (see sea-level height atin A1 in comparison to A2 in Figure2.c).The homogeneous vertical profile in velocities measured atin A2 is shown in Figure3, where the along-shore direction reveal with velocitiyes peaks of in the order of 0.5 m•s-1 in the water column, at the along-shore direction (i.e.following the axis of the bay).The near-bottom water current speed atin A2 (Figure2.d)show fluctuations with peaks over 0.1 m•s-1, except foring the mentioned seiche event where peaks arising 0.4 m•s-1.
During the case E1 (3 rd of August 2013; 10:00 hr) the combined bottom stresses are is mainly due to the current bottom stresscaused by currents(Figure 5.left).Maximum values of 0.15 Pa for the combined bottom stress are obtained atin the center of the bay, and the mouth.This episode corresponds to a seiche event and the spatial variability of the bottom stress is consistent with the spatial pattern of the node/antinode position.It means that the maximum combined bottom stress (associated atwith maximum water currents) corresponds to the node position (minimum sea-level amplitude).In oppositecontrast, the minimum bottom stress corresponds to the antinode position (maximum sea-level amplitude).The position A2 is located near to the node, where the water currents are maximum during the seiche event (0.08 Pa for combined bottom stress).It is worth to mention the node/antinode pattern of the current-induced bottom stress, which presumably would indicate a large spatial variability on of the resuspension process within the bay.After the seiche activity (second stage of E1), where the wind speed increases due to the sea-breeze) and, the current-induced bottom stress (5 th of August 2013; 08:00) decreases significantly in particular in the center of the bay (Figure5.right).The bottom stress distribution shows how the maximum values are obtained near the shoreline (2.2 Pa) due to the contribution of the wave-induced bottom stress.AtIn A2, the combined bottom stress is equal to 0.03 Pa (value presumably far too smalllittle

Figure 1 :Figure 2 : 535 Figure 3 :
Figure 1: a: Regional lLocation of the Ebro River Delta in a regional context; b: Location of the Alfacs Bay in the Ebro River Delta;.c: Overview map of the Alfacs Bay.The location of Triangle shows tthe meteorological station is marked with a triangle (M-ASc) and a. wWhite cross marks the location of thefor Sant Carles de la Ràpita tide gauge (M-Sc).The Greay circles shows

Figure 4 :Figure 5 :
Figure 4: Taylor diagram comparing the error metrics between the observations and model results for the near-bottom currents.A1 and A2 corresponds to the ADCP locations shown in Figure 1.

Figure 6 :
Figure 6: Current-induced bottom stress ( ), wave-induced bottom stress ( ) and combined wave-current bottom stress ( ).in the Alfacs Bay Current-induced bottom stress (\tau_c), wave-induced bottom stress (\tau_w) and combined wave-current bottom stress (\tau_c + \tau_w).Distribution of the current, wave and combined wave-current bottom stresses log10(Pa) in the Alfacs Bay during the first stage of the episodes E2 (left) and E3 (right).The A2 station is shown in magenta.Isobaths (in grey) are plotted in 555

Figure 7 :
Figure 7: Snapshot of the wWave field for the episode E2 (sea-breeze; left) and E3 (NW wind; right).Color mapThe colours represents the significant wave heights in meters and black arrows the direction of propagation.Note that the value ranges of the significant wave height arecolourbars are different.

Figure 8 :
Figure 8: Total Suspended Matter (TSM in mgr•l-per 1iter) obtained from Sentinel-2 imagery for the Alfacs Bay in for two 565