Residual fluxes of suspended sediment in a tidally dominated tropical estuary

Continental Shelf Research 70 (2013) 27–35

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Continental Shelf Research journal homepage: www.elsevier.com/locate/csr

Research papers

Residual fluxes of suspended sediment in a tidally dominated tropical estuary Carlos Augusto França Schettini a,n, Marçal Duarte Pereira b, Eduardo Siegle c, Luiz Bruner de Miranda c, Mário P. Silva d a

Departamento de Oceanografia—Universidade Federal de Pernambuco (DOcean/UFPE), Av. Prof. Moraes Rego 1235, Recife CE 50670-901, Brazil Programa de Pós-Graduação em Geociências—Universidade Federal do Rio Grande do Sul (PPGGeo-UFRGS), Av. Bento Gonçalves 9500, Porto Alegre RS 91509–900, Brazil c Instituto Oceanográfico—Universidade de São Paulo (IO-USP), Praça do Oceanográfico 191, São Paulo SP 05508-120, Brazil d Universidade do Rio Grande do Norte (UFRN), Campus Universitário CxP 1524, Natal RN 59072–970, Brazil b

art ic l e i nf o

a b s t r a c t

Available online 20 March 2013

This paper assesses the fine sediment fluxes in the Caravelas estuarine system (Bahia, Brazil, 17o45'S and 039o12'W). The estuary reaches the ocean at the shore across from the Abrolhos Bank, the largest tropical reef habitat in the South Atlantic. The Caravelas estuarine system is composed of several meandering channels, which are connected to the ocean by a double inlet system. These two openings – the Caravelas and Nova Viçosa estuaries – are connected by a narrow, 30 km long channel. The Caravelas estuary does not receive significant continental input, while the Nova Viçosa estuary receives the contribution of the Peruíbe River, which drains an area of approximately 5000 km2. To understand the fine sediment dynamics and net transport, observations of tides, currents, salinity and suspended sediment concentration (SSC) were recorded in 13-h tidal surveys (spring and neap tide) and with 20-day long CTDs/ADCP moorings at the Caravelas estuary and in the interconnection channel. The SSC dynamic in the Caravelas estuary is primarily driven by advection, with SSC originating in the inlet and inner shelf area. Residual water and sediment transport are up-estuary in the Caravelas estuary and toward the Caravelas estuary in the interconnection channel. The residual transport showed pronounced synodical modulation and was stronger during spring tide. The Caravelas estuary function as a trap for inner shelf materials and fine sediments delivered by the Peruípe River at Nova Viçosa. & 2013 Elsevier Ltd. All rights reserved.

Keywords: Fine sediment transport Tidal currents ADCP Residual currents

1. Introduction Despite extensive research on the topic, fine sediment dynamics in estuaries are still unpredictable, and understanding the processes of a particular system relies heavily on the physical environment and direct observations. Although some general trends can be observed (Postma, 1967; Uncles et al., 2002; Prandle, 2009), the complexity of the matter stems from both the intrinsically complex nature of estuarine hydrodynamics (e.g., Officer, 1976; Dyer, 1997; Miranda et al., 2002; Valle-Levinson, 2010) and the even more complex nature of fine sediment behavior in coastal waters (e.g., Dyer, 1986; Winterwerp and van Kesteren, 2004). Understanding fine sediment dynamics is a basis for further comprehending sediment fluxes throughout estuaries, which is of fundamental importance to functioning and coastal evolution (French et al., 2008) and is increasingly relevant in this period of rapid global change (Syvitski, 2008; Syvitski and Kettner, 2011). n

Corresponding author. Tel.: þ55 85 3366 7021; fax: þ 55 85 3366 7002. E-mail addresses: [email protected], [email protected] (C.A.F. Schettini), [email protected] (M. Duarte Pereira), [email protected] (E. Siegle), [email protected] (L.B. de Miranda), [email protected] (M.P. Silva). 0278-4343/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.csr.2013.03.006

Determining robust values for fine sediment flux is a complex task and can be achieved by direct or indirect means. Indirectly assessing morphology evolution in terms of sedimentation rate is possible (e.g., Patchineelam et al., 1999); therefore, time scales must normally span decades or longer. The result is a long-term trend that overcomes any shorter-term transfer mechanisms that may be relevant, such as seasonal fluvial regimes and episodic events. Direct measurements of currents and suspended sediment concentration (SSC) can provide effective instantaneous transport estimates, which may provide straightforward values of sediment flux when carried out for sufficient periods of time. A major problem, however, lies in defining ‘sufficient’ or ‘minimum’ periods for monitoring. The optimum length of time would be ‘ad infinitum,’ which, in this case, is feasible for a relatively small number of estuaries (e.g., Williams et al., 2010). Most estuaries are surveyed using a one or two tidalcycle approach (e.g., Schettini et al., 2006) that may provide insight about what is occurring in the tidal driven process. Although, the residual flux can be a fraction of ebb/flood transport and may be easily masked and/or biased in the dataset. The creation of optical (OBS, Downing et al., 1981) and acoustic (ABS, Gartner, 2004) backscatter sensors allows for new insights into the way we observe and monitor SSC (Schettini et al., 2010). Previously

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based on discrete water sampling and gravimetric analysis, the number of samples was limited either by logistics and the presence of an operator or by an expensive and complicated auto-sampling device. Water sampling is still necessary for calibration procedures. Therefore, the use of OBS and ABS devices allow for the acquisition of reliable data for long periods at relatively low costs. Long-term SSC time series may provide a unique, robust way to interpret processes controlling sediment regimes and allow for the verification of

hypotheses based on quantitative determinations of residual flux (Schoellhamer, 2002; French et al., 2008). The present work assesses fine sediment flux in the Caravelas estuarine system (Bahia, Brazil, Fig. 1) using the framework of the major ProAbrolhos Project – Productivity, Sustainability and Utilization of the Abrolhos Bank Ecosystem. The Abrolhos Bank includes an important Brazilian national park that surrounds the largest coral reef habitat in the South Atlantic, which is also a

Fig. 1. Location of the Caravelas estuary, in relation to South America and the Bahia State (A). Panel (B) presents the drainage basin of the Itanhém and Peruípe Rivers (thin black lines) and the Caravelas estuary (bold red lines). Panel (C) presents the entire Caravelas estuarine system and the nearby reefs offshore. Panel (D) presents detail of the Caravelas estuary, with the locations of surveying stations and mooring sites #A and #B. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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humpback whale breeding area (see http://www.icmbio.gov.br/ parna_abrolhos/). ProAbrolhos addresses meso-scale oceanic circulation and small-scale estuarine processes, which the present study examines. Coral reefs are very sensitive to environmental conditions and can be impacted by land-based activities affecting near-shore fine sediment patterns (Edinger et al., 1998; Wolanski et al., 1990, 2003). Some of the Abrolhos Bank's coral reefs are only approximately 15 km offshore, across from the Caravelas estuary (Fig. 1C). The estuary-shelf fine sediment exchange is of great importance to the reef's health, as has been reported for other reef habitats (e.g., Golbuu et al., 2003; Victor et al., 2004). One question addressed in the ProAbrolhos Project regarding the estuarine processes is the Caravelas estuarine system's role in the region's fine sediment dynamics. To gather field data to answer this question, two intensive field campaigns were conducted during both dry winter (August 2007) and wet summer (January 2008) conditions. Field campaigns involved 13-h synoptic tidal surveys at several stations, moored instrumentation and spatial surveys. This study focused on the moored instrumentation data and investigated the role of the Caravelas estuary as either a source or trap of fine sediments in the region using OBS and ABS techniques to monitor SSC. 1.1. Study site The Caravelas estuarine system (~171 45' S/ 391 15' W; Fig. 1) encompasses low-land area with several islands and meandering channels and nearly pristine mangrove forests that evolve into sand ridges and coastal plains forming a tombolo (Andrade and Dominguez, 2002). Cassumba Island is the largest island and separates the two main estuarine entrances: Caravelas to the north (Caravelas estuary) and Nova Viçosa to the South (Nova Viçosa estuary). The Caravelas estuary also contains two inlets: the wider (~900 m) and shallower (~8 m) Barra Velha inlet and the narrower (~350 m) and deeper (~25 m) Tomba inlet. The Nova Viçosa estuary contains a single inlet, which is ~650 m wide and ~6 m deep. The interconnection channels between the entrances are 30 m wide in the narrowest reaches, and nearly midway, there is a wider mudflat area that is considered the nodal point for tidal propagation. The inner shelf and the estuarine intertidal areas are mainly muddy (Falcão and Ayres Neto, 2010; Sousa et al., 2012). The surrounding coastline presents a narrow beach with medium to coarse sand and the Tomba inlet has a wide sandy ebb tidal delta. There is no information available about sediment characteristics in the upper estuarine channels, but the lower estuarine channels are composed by fine sands with its deeper parts covered by medium to coarse silts. Moving offshore through the inlets sediments become coarser being composed by medium to coarse sands (Barroso, 2009). The regional climate is tropical wet with well defined wet and dry seasons, from October to April and May to September, respectively, with an average precipitation rate of 1,600 mm yr-1 (Chaves, 1999). The total drainage area encompasses 5200 km2, with primary land use for cattle breeding and eucalyptus forests and sparse areas of pristine rain forest. The freshwater inflow to the Caravelas estuary consists of a few small streams with a total area of 600 km2, while the Nova Viçosa estuary receives the inflow of the Peruípe River with a drainage basin of 4600 km2 (Fig. 1B). The fresh water inflow follows precipitation climatology, and the mean long-term discharge of the Peruípe River ranges from 15 to 30 m3 s−1 for the wet and dry seasons, respectively, relative to a catchment area of 2840 km2. There are no records for river discharge for the Caravelas estuary, but given regional patterns, it can be roughly estimated at approximately 4 m3 s−1 (Pereira et al., 2010). At nearly 20 km north of the Caravelas estuary inlet

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indent lies the Itanhén River estuary with a drainage area of approximately 5200 km2 and a river discharge of the same magnitude as that of the Peruípe River. The major fluvial systems nearby the study site include the Jequitinhonha River at nearly 215 km to the north and the Doce River at 220 km to the south, with drainages of 70,000 and 83,000 km2, respectively. Regional tides are pure semi-diurnal, with form number Nf ¼[(K1 þ O1)/(M2 þ S2)]¼0.12, ranging from 1.1 to 3.0 during neap and spring tide periods, respectively (Lessa and Cirano, 2006). The wind regime is influenced by the South Atlantic Anticyclone, which results in dominant trade winds from the northeast in the region. The near-shore currents are primarily driven by a wind regime that is predominantly southward, with a tidal regime playing a lesser role (Knoppers et al., 1999; Leipe et al., 1999; Texeira et al., 2013). The Caravelas estuary dynamics are primarily driven by tides with well mixed salinity vertical structures and pronounced ebbdominance currents (Schettini and Miranda, 2010). The estuarine dynamics do not change much between dry and wet periods, which is primarily due to the relatively negligible fresh water inflow when compared to the tidal prism (Pereira et al., 2010). Schettini and Miranda (2010) found negative (seawards) water budgets incompatible with the fresh water inflow, raising the hypothesis that there is a significant exchange through the straits that connect the Caravelas and Peruípe estuaries. The Peruípe estuary presents a partially mixed salinity vertical structure, which is attributed to the inflow of the Peruípe River (Miranda et al., 2013). The SSC presents an inverse relationship with the Caravelas estuary's salinity in which higher SSC values are observed closest to the inlets, indicating a marine source instead of a fluvial source, although Travassos et al. (2006) found the estuary to be a source of dissolved inorganic nutrients. Schettini and Miranda (2010) found negative suspended sediment budget in a 13-h spring tide survey, while in four 13-h surveys (2 at spring tide and 2 at neap tide), Pereira et al. (2010) found a negative budget in one (spring tide) and a positive budget in the remaining three. The SSC was of the order of 10's and 100's mg l−1 during neap and spring tide conditions, respectively, with maximum values in the order of 500 mg l−1 (Pereira et al., 2010).

2. Materials and methods Two intensive field campaigns were carried out during a dry winter (August 2007) and a wet summer (January 2008). Each campaign lasted nearly 20 days and covered a full synodical spring/neap tidal period. During the campaigns, physical data were recorded in both 13-h tidal surveys and longitudinal surveys and with moored instrumentation. Meteorological data of wind speed and direction (at 0:00, 12:00 and 18:00) and rain (daily values) were obtained for the Caravelas meteorological station from the National Institute of Meteorology (INMET). River discharge data (daily values) were obtained from the Brazilian National Water Agency (ANA) for the Peruípe River (#55510000) and Itanhén River (#55490000). Tidal surveys were carried out at spring and neap tide at the following five sites: the Caravelas and Nova Viçosa inlets, the interconnection channel and the Peruípe and Itanhém estuaries (the latter is not shown in Fig. 1). Currents, salinity, temperature and SSC were acquired from anchored stations every half-hour. Longitudinal surveys were carried out at spring and neap tide along the Caravelas estuary and along its two main branches, the Macacos and Cupido Rivers. Vertical profiles of salinity, temperature and SSC were sampled at every 1 km along these profiles. Moored instrumentation was deployed in the Caravelas estuary

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(both campaigns) and in the interconnection strait (only during the January 2008 campaign), and water level, currents, salinity, temperature and SSC were monitored. Assessment of SSC residual flux was based primarily on records from the moored acoustic Doppler current profilers (ADCP) in which SSC was derived from the ABS signal (e.g., Gartner, 2004). During the first campaign, the moored instrumentation was under-weighted, and the bottom frame turned during the intense spring tide currents; thus, only the dataset acquired during the second campaign was used in the present assessment. It is important to note that Pereira et al. (2010) showed that hydrodynamics and water properties do not change much between wet and dry seasons. The mooring stations were located at nearly 5 km (#A) and 11 km (#B) upstream from the Boca do Tomba inlet and at the depth of 13 and 7 m, respectively, at the channel thalweg (Fig. 1D). Station #B is the only passage to the interconnection straits. At station #A, an ADCP model Aquadopp Profiler of 1000 kHz by Nortek with pressure transducer was set up to record every 20', with 0.5 m of vertical resolution (cell size) and 120 s of averaging. At station #B, an ADCP model Argonaut XR of 1500 kHz by Sontek with pressure transducer was set up to record every 20', with 1 m of vertical resolution (cell size) and 120 s of averaging. The nonmeasured near surface layer was of 1 m at both stations, and the near bed layer was of 1.1 and 1.8 m at stations #A and #B, respectively. The water column coverage was of about 85 and 60% at stations #A and #B, respectively. Salinity and temperature were recorded at the bottom in both stations with conductivity-temperature (CT) loggers by JFE-Advantech, and at the surface only in station #A with a CTD by Saiv A/S. The deployments lasted nearly 15 days, from January 15 to 29, 2008. Current velocity was reduced to the estuary longitudinal component. We adopted the convention of assigning the upestuary direction (@ #A) and the Nova Viçosa (@ #B) direction as positive and assigned all other directions as negative. To convert ABS to SSC, it was necessary to take synoptic measurements of both variables for a wide range of SSC. SSC was indirectly measured using OBS probes, which also required calibration. The field OBS data were recorded during the 13-h tidal surveys at stations #A and #B on January 16 (neap tide) and on January 23 (spring tide). Vertical profiles of salinity, temperature and turbidity were recorded every half-hour from an anchored boat. At station #A, a CTD probe by JFE Advantech with an embedded optical backscatter (OBS) was utilized, furnishing turbidity in NTU. At station #B, a direct reading OBS probe by SeaPoint was utilized, providing turbidity in VDC.

To calibrate the OBS probes, a highly concentrated bulk SSC solution was prepared. On a pier near to station #A, a 200 l tank was used to store water during the spring tide period when the concentration was higher. After 12 h, the water was siphoned out, and the bottom material was collected. This procedure was repeated several times. The bulk solution was periodically agitated between collection and calibration procedures in the laboratory to avoid changes in its material properties (e.g., Eh reduction of settled mud). In the laboratory, starting with clear salt water, SSC was step-added, and OBS readings and water samples were taken between the additions. The SSC was determined by gravimetric analysis. A volume of sample was filtered through a dry-weighted membrane filter of 0.45 μm of pore size. After drying at 50 oC for nearly 24 h, the filters were weighted again. The SSC was calculated by the ratio of retained mass to filtered volume. The calibration curves were as follows: SSCðOBSNTU Þ ¼ −1:05 þ1:03 OBSNTU

: r 2 ¼ 0:99

SSCðOBSV DC Þ ¼ −0:89 þ 450:85 OBSVDC

: r 2 ¼ 0:98

ð1Þ

Using ABS from ADCP to infer SSC has become a powerful tool in assessing the dynamics of fine sediments in coastal areas (Gartner, 2004; Schettini and Zaleski, 2006; Zaleski and Schettini, 2006; French et al., 2008; Schettini et al., 2010), where once rigorously calibrated, a main advantage is its excellent water column coverage synoptic with currents. To do so, the acoustic ADCP reading in ‘counts’ must be converted to acoustic intensity in decibels (dB) and normalized with respect to the attenuation effects with the distance from the transducers and the geometrical aperture of the beams. This result can be achieved with the sonar equation (Deines, 1999; Gartner, 2004): ABS ¼ K C ðE−Er Þ þ 20 log10 ðRÞ þ 2αw ðRÞ þ TS

ð2Þ

where E is the echo intensity (counts), which must be subtracted from the instrument background noise Er and converted to dB by the scale factor KC. The second and third terms account for the two-way pulse transmission loss; the former is due to spreading, and the latter is due to absorption. R is the distance from the instrument head, given by Z/cos μ, with Z being the distance from the transducer and μ the angle to the vertical. μW is the water absorption coefficient. TS is the target strength given by the particle's size and concentration. This term can be neglected in 1200 kHz ADCP, unless the particles are very small (e.g., o 10 μm; Gartner, 2004). We do not account for this term since we used ADCPs with 1000 and 1500 kHz, and expect typical coastal waters particles size, of the order of 10's and 100's of micrometers (Eisma, 1986).

Fig. 2. Calibration curve of optical backscatter (OBS) as a function of acoustic backscatter (ABS) recorded by the ADCP, (A) with OBS from the ctd probe with reading in NTU and (B) with OBS probe with direct readings in VDC.

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Once the ABS was determined, the conversion model to OBS was achieved by examining the relationship between OBS and ABS for each station. An intermediate step involved synchronizing the data from the moored ADCPs and the anchored stations. The vertical adjustment was conducted by taking the water surface as a reference level, thus, the ADCP reading levels were corrected by tidal variation. The time adjustment was performed by linear interpolation of the OBS readings (dt¼30') to fit on the ADCP readings (dt¼20'). The results are shown in Fig. 2. The calibrations curves are included below: OBSNTU ðABS1000 Þ ¼ expð−7:608 þ 0:1989 ABS1000 Þ OBSNTU ðABS1500 Þ ¼ expð−9:301 þ 0:093 ABS1500 Þ

: r 2 ¼ 0:78 : r 2 ¼ 0:81

ð3Þ

The calibration curve for station #B (Sontek Argonaut) during spring tide is particularly poor. This behavior may be due to the lack of high turbidity peaks as recorded at station #A and partially due to the discrete manual readings of the volt meter. A sensitive analysis was performed in order to verify the robustness of the calibration curve to provide SSC values. Two ancillary curves were fitted passing through the center of mass of sampling points displaced at 75% from the OBS and ABS mean values. The SSC calculated from the main and ancillary curves were 30.1, 24.6 (−21%) and 38.6 (þ 25%) mg l−1, respectively. Considering the expected SSC variability in coastal environments, from 10 s to 103 s of mg l−1 (e.g., Winterwerp van Kesteren, 2004), and also that the non linear nature of the calibration could produce fast raising values, the obtained values from all curves are in the same scale of 10's of milligram per litre, and thus, the calibration can be assumed as being a good proxy. The accumulative water displacement Lt was calculated by the cumulative sum of the instantaneous water parcel displacements l, as Lt ¼ ∑ti ¼ 1 li . The instantaneous water parcel displacements were calculated integrating the depth averaged longitudinal velocity to R Δt the sampling interval time of 20 min, as lt ¼ udt, where the over bar denotes the depth averaging operation. The cumulative suspended sediment transport Q t was calculated in the same way, substituting u by the suspended sediment transport rate qt ¼ u: c, and Q t ¼ ∑ti ¼ 1 qi . For the station #B the Q t was calculated also using the SSC obtained from the ancillary curves of the sensitive analysis. The results were very similar and corroborate the consistency of the calibration.

3. Results The meteorological and hydrological conditions for January 2008 are representative of the austral summer on the southern Bahia coast. The wind was predominantly from the northeast. Only at the end of the month the wind direction rotated to the eastsoutheast for a few days (Fig. 3A). The mean wind intensity was approximately 4 ms−1, with higher values of between 7 and 9 ms−1 on days 15 through 20. Due to the inland position of the meteorological station, these values are somewhat underestimated in comparison to offshore winds. The precipitation rate was 118 mm, which was slightly higher than the Caravelas meteorological station's ten-year average of 105 785 mm. A major rainfall event of 45 mm occurred on January 10. Strong rain showers of 20/25 mm also occurred on January 24 and 25 (Fig. 3B). Fig. 3C shows the discharge of the Peruípe and Itanhén Rivers. The mean discharges were 22.8 and 25.2 m3 s−1, respectively, which are close to the minimum values expected for the rainy season (December–February; Pereira et al., 2010). The Caravelas estuary showed a well mixed salinity vertical structure along the estuary for both spring and neap tides. The maximum vertical difference of salinity was on the order of one salinity unity at neap tide but was normally lower. The

Fig. 3. (A) Wind speed (m s−1) and (B) direction (degree), (C) daily precipitation rate (rain) at Caravelas meteorological station, and (D) river discharge at the Itanhén and Peruípe Rivers in January 2008.

longitudinal distribution of depth-averaged salinity and SSC are shown in Fig. 4. The salinity was slightly lower during neap tide, although the longitudinal distribution was similar to that recorded during the spring tide along the Caravelas estuary and Macacos River. Close to the inlet, the salinity was approximately 35, regularly decreasing landwards to 19 and 23 at neap and spring tide, respectively. The salinity in the Cupido River (the shorter lines in Fig. 4) was very similar to that found in the Macacos River during spring tide and was lower during neap tide. The longitudinal distribution of SSC was homogeneous during neap tide, when most values were approximately 5 mg l−1 with sparse peaks higher than 10 mg l−1. During spring tide, the SSC distribution presented a negative gradient up-estuary. A maximum value of 108 mg l−1 was recorded at 7 km from the mouth, decreasing to 10 mg l−1 in the upper estuary. During neap tide, there was no relationship between SSC and salinity, while during spring tide, a consistent direct and exponential correlation was observed (Fig. 4C). The time series of water level, depth-averaged currents, salinity (depth-averaged for #A and at bottom for #B), temperature (idem as salinity) and depth-averaged SSC are shown in Fig. 5. Water elevation, referred to the mean water level over the sampling period, was nearly identical at both stations. The minimum tidal range at neap tide was 1.14 m on January 16, and the maximum tidal range at spring tide was 2.96 m on January 23. The tidal signal was symmetrical, and the semi-diurnal inequalities reached 0.4 for the low waters of spring tide (Fig. 5A). The currents showed a tide-related pattern following the synodical tidal pattern and ebb dominance. The currents at station #A ranged between −0.41 and 0.30 m s−1 at neap tide (January 16) and between −1.03 and 0.79 m s−1 at spring tide (January 23); at station #B, they ranged between −0.28 and 0.25 m s−1 at neap tide (January 16) and between −0.89 and 0.61 m s−1 at spring tide (January 23) (Fig. 5B). The salinity showed tidal semi-diurnal variance. The maximum vertical differential salinity was nearly homogeneous at station #A, with a maximum vertical salinity difference of 1.3 between surface and bottom on January 16. The tidal variation of salinity at station #A was 2 units (33–35) at neap tide and 3 units (32.5–35.5) at spring tide. At station #B, the tidal variation was also 2 units (31–33) at neap tide and 4 units (31–35) at spring tide. After the

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Fig. 5. Time series of water level (A) and depth-averaged current velocity (B), salinity (C), temperature (D) and depth-averaged SSC (D) at stations #A (blue) and #B (red). The arrows in panel E indicate the SSC peaks discussed in the text. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 4. Longitudinal distribution of depth-averaged salinity (A) and SSC (B, mg L−1) at neap tide (January 18, 2008, continuous green line) and spring tide (January 25, 2008, dashed blue line). The longer lines represent the Caravelas and Macacos r. branch; the shorter lines represent the Cupido r. branch. (C) The relationship between salinity and SSC (mg L−1) at neap (blue circles) and spring (green squares) tides. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

spring tide peak (January 23), the salinity showed a general trend of decreasing, which was slightly more intense at station #B (Fig. 5C). The temperature showed a clear tidal signal only during the spring tide. At the beginning of the campaign, the temperature at both stations was 28.7 1C, which showed an increasing trend until day 22 and then decreased 1 oC until the end of the campaign. The temperature tidal variation was higher at station #A (Fig. 5D). The SSC presented noisier signals at semi-diurnal frequencies and a markedly synodical modulation (Fig. 5E). The lowest values were 15–20 mg l−1 at both stations, recorded on January 17 and 18. SSC increased as tidal range increased, reaching up to 160 mg l−1 and 60 mg l−1 at stations #A and #B, respectively. The tidal variability of the SSC was much higher at station #A than at station #B, which was most notably observed between January 20 and 24, during the increasing tidal range phase of the synodical

cycle when several SSC peaks were recorded. During the period of decreasing tidal range, some peaks still occurred, though they were less intense. All SSC peaks occurred during floods and never during ebbs. Accumulative water displacement (Lt ) and accumulative suspended sediment transport (Q t ) are shown in Fig. 6. The low frequency signals are represented by bold lines in Fig. 6B and C. Positive displacement and transport means up-estuary for station #A and toward the Nova Viçosa estuary for station #B. The Lt at station #A was up-estuary from the beginning of the campaign until January 22, being nearly constant during the peak of tidal height and again up-estuary until the end of the campaign. At station #B, the Lt was nearly constant until January 22, becoming negative until day 28 and again constant until the end of the campaign. The Q t was constant at both stations until January 19, after which it became positive at station #A and negative at station #B. After January 25, the Q t remains constant at station #B, and at station #A, it remained positive until the end of the campaign. Constant values of Lt and Q t means null residual displacement or transport. The net cumulative transport at the end of the monitored period was 5.5 and −1.6 t m−1 at stations #A and #B, respectively. Taking into account the cross sections width of 650 and 200 m, we have net transport of the order of 3600 and −320 t, respectively. However, in the absence of information about the cross section transport variability, these are speculative values, although is possible to ascribe that the expected transport at the cross section of station #A is much larger than that at station #B.

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4. Discussion 4.1. Fine sediment dynamics The fine sediment dynamics in the Caravelas estuary can be hypothesized as a tidally driven system in response to the tidal dominance of hydrodynamics. The river contribution is practically null, and the estuary shows a well mixed salinity structure

Fig. 6. Time series of tidal range (A), accumulative water displacement (B) and residual suspended sediment transport (C). Blue: station #A; red: station #B. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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(Schettini and Miranda, 2010; Pereira et al., 2010). Thus, mixing and vertical diffusion during flood and ebb would likely be the same (e.g., Simpson et al., 1990). It is also expected that SSC will respond directly to the tidal currents in a continuous cycle of erosion, transport and deposition. In this case, the relationship between near-bed SSC and depth-averaged SSC can be a good proxy to assess the role of such processes (Nichols, 1984). Based on the entire dataset, the correlation between current velocity and SCC were r2 ¼0.61 and r2 ¼ 0.51 for stations #A and #B, respectively. These values are quite low, suggesting other processes in addition to tidal driven vertical flux. Additionally, the resuspension/deposition processes do not explain the higher concentrations during floods in spite of weaker currents than those recorded during ebbs, nor do they explain the peaks of SSC observed only during floods and during the increasing tidal height phase of the synodical cycle. The higher values of SSC during floods and the SSC peaks suggest that the inlet area and/or adjacent shelf are the primary sources of the suspended sediments, which advect up-estuary. All SSC peaks occurred at the end of flood periods and only at station #A. The hypothesis of advection is supported by the tidal excursion and morphology of the Boca do Tomba inlet. Station #A is nearly 5 km landwards from the Boca do Tomba inlet. The tidal excursion, calculated by integrating the mean velocity during the flood phase, ranged from 5 to 15 km at neap and spring tide, respectively. These values may be overestimated, as Schettini and Miranda (2010) have suggested that the spring tidal excursion is approximately 10 km, based on longitudinal distribution of salinity in low and high water. During neap tide, the water volume that occupied the inlet and near-shore area did not reach the monitored point during the flood, which gradually occurs as the tidal range increases. The Boca do Tomba is a recent feature of the estuary and was formed by regression of the coastline due to erosion. Originally there was only a narrow creek, which began functioning as a second inlet once the shore retreated. Intense erosion occurred, deepening and widening it to its present morphology (25 m deep and 350 m wide). The currents in the Boca do Tomba inlet are higher than those observed in the main inlet (Barra Velha—Old Inlet), although the volume exchanges are approximately the same (Schettini and Miranda, 2010). The inlet now constricts tidal flow, with higher shear and, consequently, higher erosion rates being produced. Additionally, the inner shelf bed material consists primarily of fine-grained sediment constantly resuspended by

Fig. 7. Current velocity vs. suspended sediment concentration diagrams for #A (A) and #B (B). Bold lines indicate periods of neap (January 16) and spring (January 23) tides.

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wave action, which means that this material is also potentially advected into the estuary during flood. The SSC peaks were not observed at station #B because it is out of range of the tidal excursion of the inlet water volume. The SSC peaks' occurrence only during the phase of increasing tidal height can be explained in terms of erosion threshold. Bed strength increases with bed depth, and as the tidal currents become more intense, deeper bed layers are eroded until reaching their maximum at the maximum tidal height (Dyer, 1986; Mehta et al., 1989). Fig. 7 shows the current velocity vs. SSC diagrams for the entire period for both stations #A and #B, emphasizing the periods of January 16 and 23 when the extreme conditions of neap and spring tide, respectively, occurred. The track in environments where the tidal resuspension–deposition processes dominate resembles a butterfly-like shape (Nichols, 1984), where resuspension phases are identified by the direct proportionality between current velocity and SSC, and deposition phases occur when this relationship is the reverse. This pattern can be observed during neap tide (January 16). Therefore, the shape of the track changes with increasing tidal height. At spring tide (January 23), the ebbto-flood phase presents small variations in SSC. During the flood phase, there is a steep increase in SSC when the velocity is nearly steady. During the flood-to-ebb phase, the SSC decreases until the peak of ebb currents is reached, which is unrelated to the current magnitude, showing a lag effect from the flood conditions (Postma, 1967; Dyer, 1995). The poor relationship during the ebb-to-flood phase denotes the advection dominance on the SSC at the monitored sites. The steep increase at the end of the flood suggests allochthonous effects. 4.2. Net transport The Caravelas estuarine system still contains pristine mangrove forests, indicating that it continues to function as a depositional environment, with the mangroves acting as trapping areas for fine grained sediment (Kuehl et al., 1996; Furukawa and Wolanski, 2004; Victor et al., 2004; Wolanski and Ridd, 1986). It was initially hypothesized that the local drainage inflow was the main source, following the basic model for sediment dynamics in estuaries (e.g., Dyer, 1995). Schettini and Miranda (2010) showed a direct relationship between salinity and SSC at spring tide (and the inverse with fresh water), suggesting that higher SSCs toward the estuary mouth may be explained by local sediment reworking or by advection from the adjacent shelf as a source of suspended sediments. Our findings corroborated this observation and also showed that there is no relationship during neap tide when the longitudinal SSC was uniformly low and high salinity values were observed in the estuary upper reaches. These findings point to a synodical control of fine sediment dynamics and transport. Taking into account the reduced drainage basin that debouches directly into the Caravelas estuary, these results also highlight that the continental inflow is negligible (except following an episodic extreme hydrological event). Therefore, the origin of the fine sediments found in the Caravelas estuary is either the inner shelf rework and transport up-estuary or the Peruípe River via the interconnection channel with the Nova Viçosa estuary. Schettini and Miranda (2010) also found net seaward water transport incompatible with the direct inflow, which was explained by a residual circulation from Nova Viçosa estuary to Caravelas estuary through the interconnection channel. As shown in Fig. 6, this process was well noted at Station #B. There is consistent residual water displacement and suspended sediment transport towards the Caravelas estuary that coincides with the period of spring tide. Fig. 6 also shows similar behavior recorded at the Caravelas estuary at Station #A with net importation of water

and suspended sediments. The results from Station #B are robust because the cross-section is only 170 m wide with simple topography (deeper central thalweg and shallower flanks), while the cross-section at #A is 870 m wide with more irregular topography (deeper thalweg at southern flank and wide shallows at the northern flank). The width of the section will surely play an important role in lateral currents patterns. Due to differential advection through the cross-section, the deeper portion will present net transport up-estuary, and the shallower portion will present down-estuary net transport (Chant, 2010). Thus, the water excess from the net up-estuary flow at the thalweg at the Caravelas cross-section and the net inflow from the interconnection channel must be balanced with a residual outflow through the shallower part of the section at #A. This result is surely expected for water based on mass balance but not for suspended sediment transport, as its behavior is non-conservative. In spite of the fact that net inflow at #A can be understood as a cross-section effect, net inflow at #B cannot. Guyondet and Koutitonsky (2007) found residual fluxes of water in interconnected systems induced by morphological differences between entrances, which is likely the case for the Caravelas estuary. Other possibilities for this result could be the fresh water input from the Peruípe River or even subtidal currents at the inner shelf responding to wind forcing (e.g., Lessa and Cirano, 2006; Texeira et al., 2013). The former was nearly constant at approximately 25 m3 s−1 during all recorded periods; the latter is improbable because the wind pattern showed changes only at the end of the recorded period, and the resulting currents would have a lag with the wind change. Because changes in the trend of water displacement and accumulative suspended sediment transport were more marked during the transition between the neap and spring tides, they appear to be tidal-related processes. The residual water and sediment transport in the interconnection channel towards Caravelas can be explained in terms of differential tidal progression along the estuarine basins. The Nova Viçosa channel resembles a funnel shape starting from its inlet. The Caravelas channel, however, presents a nearly constant width until the upstream interconnection channel junction. After this point, the main channels bifurcate into several smaller branches, all with funnel shapes. In funnel-shaped estuaries, when the lateral convergence effects overwhelm the friction, tidal amplification can occur landwards (e.g., hypersynchronous estuaries), whereas when there are no significant changes in width and depth, tidal height can be nearly constant landwards (e.g., synchronous estuaries; Nichols and Biggs, 1985). Tidal amplification along the Nova Viçosa estuary can produce a barotropic pressure gradient towards Caravelas estuary, maximized during spring tides.

5. Conclusions The main findings of the present assessment on fine sediment transport in the Caravelas estuarine system can be summarized as follows: (1) The suspended sediment dynamic is primarily driven by advection and not by tidal related resuspension/deposition processes as hypothesized; (2) Up-estuary residual transport at the Caravelas estuary exists and is modulated by the synodical cycle, which is null at neap tide and maximum at spring tides; (3) Residual transport from the Nova-Viçosa estuary to the Caravelas estuary exists and is also modulated by the synodical cycle, with maximum values at spring tide; The Caravelas estuary functions as a trap for fine sediment, either from the inner shelf or from the Nova Viçosa estuary.

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