Guilherme C. Lessa

Oceanographic characteristics of Baía de Todos os Santos, Brazil

Based on a vast set of in situ data, a first comprehensive overview of the oceanographic characteristics of Ba ´ia de Todos os Santos (BTS) is provided. BTS is the second largest coastal bay in Brazil (maximum area of 1223 km 2 and average depth of 9.8 m), and is located in the northeast Brazil, in the vicinity of Salvador city. The circulation inside the bay is mostly tidally driven and does not vary significantly throughout the year. On the other hand, the wet (winter) and dry (summer) seasons does alter significantly the distribution of water properties inside the BTS. During summer, the waters inside the bay have oceanic characteristics, with Tropical Water (TW) penetrating along the whole region, except for the mouth of Rio Paraguacu. The water temperature inside the bay is higher than in the coastal zone, and variations can be up to 3 ◦ C, reaching a maximum of nearly 30 ◦ C. During winter, with the increase of freshwater inflow, salinity variations of about 4 are observed between the innermost stations inside BTS and the adjacent coastal region. Salinity values inside the bay can be as low as 32.3, inhibiting the penetration of TW into the BTS, which is totally occupied by a locally formed Coastal Water (CW). An evaluation of the flushing time is also provided and shows that during summer, a 60-fold increase can be observed compared to winter (38 days). While the circulation does not vary seasonally inside the bay, the associated inner shelf is characterized by two different scenarios. During summer, the upwelling favorable easterlies drive a southwestward flow, while during winter the more frequent occurrence of cold fronts (southerly winds) tend to reverse the circulation.

The tides and tidal circulation of Todos os Santos Bay, Northeast Brazil: a general characterization

The Todos os Santos Bay, located on northeastern Brazil, is the second largest coastal bay in the country, after Sao Marcos Bay. The three main drainage basins convey an average of 120 m3/s of freshwater towards the bay, the majority of it, however, held by a dam in Paraguacu River since 1985. The original average freshwater inflow was two orders of magnitude smaller than the estimated tidal discharge through the main bay entrance, and the oceanographic characteristics of the bay, as indicated by bay salinity measurements, are clearly marine. The tides are semi-diurnal, and are amplified up the bay by a factor of 1.5. Shallow water constituents become more important as the tide propagates along Paraguacu Channel and Paraguacu River, where they generate time asymmetries that change between spring and neap tides. Currents in the bay are mainly bi-directional, and are stronger during the ebbing tide in most of the bay. Offshore, relatively strong tidal currents appear to be felt in a radius of about 10 km, where they are superimposed on winds driven currents orientated to the southwest.

Evidence of a Mid-Holocene Sea Level Highstand from the Sedimentary Record of a Macrotidal Barrier and Paleoestuary System in Northwestern Australia

Abstract This study addresses the mid- to late Holocene stratigraphy and sea level history of a macrotidal barrier and paleoestuary system located along the relatively unstudied northwest coast of Australia. Thirty-nine shallow cores were obtained from three transects perpendicular to the barrier and paleoestuary axis. Seven sedimentary facies were identified on the basis of sediment texture, carbonate content, and foraminifera assemblages: slope, upper intertidal mud, upper intertidal sand flat, lower intertidal sand flat, barrier, estuarine beach, and flood tide delta. The sedimentary infill reveals a fining upward succession of marine sediments up to 6 m thick, mostly along a regressive sequence. All facies are of Holocene age and started to be laid down when sea level was approximately 3 m below present elevation. A radiocarbon date from the topmost sedimentary facies (upper intertidal mud) indicates that relative sea level was at least 1 m higher than today by 2720 years BP. At this time, the estuary was at the final stages of sedimentary infill, with tidal inundation reduced to a minimum. Further evidence of a higher relative sea level during the Holocene is in the form of estuarine beach deposits found at the back of the paleoestuary at an elevation above the present day beach/dune interface. Net nearshore transport in the area, driven by tidal current asymmetry, is northward, and it is proposed that this has significantly influenced the alongshore component of the barrier progradation and evolution of the barrier estuary.

A reevaluation of the late quaternary sedimentation in Todos os Santos Bay (BA), Brazil

Todos os Santos Bay is a large ( 1000 km2), structurally controlled tidal bay in northeast Brazil. Three main drainage basins debouch into the bay, providing a mean freshwater discharge of 200 m3/s (prior to 1985), or less than 1% of the spring tidal discharge through the bay mouth. Based on the result of several sedimentological studies performed in the 1970s, five surface sedimentary facies were identified inside the bay, namely i) transgressive siliciclastic marine sand facies; ii) transgressive bay sand-mud facies; iii) a transgressive carbonate marine sand facies; iv) regressive bay-mud facies, and v) regressive fluvial sand facies. The spatial distribution of these facies would follow, somewhat closely, the hydrodynamic-energy distribution inside the bay. Seismic profiles along the bay bottom indicate the existence of several paleochannels, 5-10 m deep, blanketed at least by three different sedimentary units. The topmost sedimentary unit, 5-20 m thick, appears to be associated with the regressive bay-mud facies, and assuming that it was laid down within the last 5000 years, sedimentation rates for the central and northeastern part of the bay would average at 2,4 mm/y.

ProcED: A MATLAB PACKAGE FOR PROCESSING ADCP ESTUARINE DATA

ProcED is a MATLAB based computational package designed to facilitate the handling of a large amount of data derived from mount vessel ADCPs that monitor tidal flows and discharge in estuarine cross-sections. The package routines were written to process water current information obtained with an RD Instruments Rio Grande ADCP and its WinRiver software - version 1.03. They are capable to eliminate bad ensembles, to extrapolate current velocities for the surface and bottom blanked areas and to smooth noisy cross-profiles, enhancing the visualization of the velocity field. By performing space and time correlation between cell velocities, ProcED allows for the computation of the residual velocity field, both in the entire cross-section and in the discrete vertical profiles. Results are provided in graphical and table formats, the latter also including a table with the main flow characteristics such flooding and ebbing time, tidal elevation and current asymmetry, tidal prism and computations errors. A comparison with computations performed by WinRiver shows that ProcED discharge calculations are in average 5.5% smaller. The apparent underestimation of ProcED computations is ascribed to interpolation of the cross-profiles and to its different bottom extrapolation method. In the bottom area there is a discharge overestimation by WinRiver.

High and Low Frequency Erosive and Constructive Cycles in Estuarine Beaches: an example from Garcez Point, Bahia / Brazil

Monitoring of the morphodynamic variations of the beaches associated with an estuary contiguous with Garcez Point, Bahia, Brazil, and the superposition of aerial photographs from the region, show the presence of distinctive erosive and constructive cycles of low and high frequencies. Between 1959 and 1989, one event of shoreline erosion and progradation was recognized on the oceanic beaches just outside the estuary. Inside the estuary, an erosion phase at the southern margin coincides with a constructive phase at the other side, and vice-versa. On the southern estuarine beach, low-frequency cycles of erosion and progradation are also perceived, but with the inverse trend when compared to the contiguous oceanic beach. During the beach monitoring period (February/1991 to July/1992), the oceanic beach showed retreat rates varying from 23.7 m/year, at the channel entrance, to 1.0 m/year, three kilometers away from it. During the same period, the estuarine beach advanced at a rate of 60.3 m/year. The long-term dynamics of the shoreline position in both sides of the estuarine entrance appears to be related to the position of the channel in the ebb-tidal delta.

Brazilian Estuaries: A Geomorphologic and Oceanographic Perspective

Estuaries are primarily sites where rivers meet the sea, where fresh water is mixed with sea water by the tidal flow and where salinity gradients are set up along a mixing zone. In this chapter, we consider that the estuary is an accommodation space landward of the coastline, where fluvial sediments are largely retained with no appreciable effect on the sedimentation processes of the open coast. Here we recognize 41 large estuaries along the Brazilian coast, ranging in size from 40 km2 to more than 10,000 km2, encompassing areas with different topography, climate, geology, tidal ranges, and alongshore drift. The majority of these estuaries are located along fault lines or within grabens, and although scattered, structural and stratigraphic evidences suggest that land subsidence has caused sea level to locally rise, which has been an important factor in the preservation of these coastal features. A larger number of estuaries exist in the macrotidal and humid north coastal sector where large-scale coastal subsidence occurs. The estuary sizes along the coast bear no relation with tidal ranges, but have a positive linear correlation with river discharge in the east-southeast and northeast coastal sectors. Based on the existing hydrographic data, it could be stated that the large Brazilian estuaries are hypersynchronous, ebb-dominated, and well mixed. In the deeper estuaries of the east-southeast sector, the longitudinal density gradient is an important driver for water circulation, causing vertically stratified residual flows that end up retaining suspended sediments within the estuary and promoting the accumulation of organic, muddy sediments.

The Impact of Different Forcing Agents on the Residual Circulation in a Tropical Estuary (Baía de Todos os Santos, Brazil)

ABSTRACT Santana, R.; Teixeira, C., and Lessa, G., 2018. The impact of different forcing agents on the residual circulation in a tropical estuary (Baía de Todos os Santos, Brazil). This study investigated the forcing of the residual circulation in a tropical estuary, Baía de Todos os Santos, Brazil (BTS). A numerical model (Regional Ocean Modeling System), forced with climatological means, was used to investigate the seasonal roles of the tide, wind, net heat, surface water fluxes, and river discharge on the residual circulation. The tide is the main driver of the circulation and its residual flow is structured at the bay mouth with a net-landward flow in the channel center and a net-seaward flow on the shoulders. The addition of wind drag affected the surface circulation, forcing W-bound (N-bound) currents in the spring–summer (fall–winter) months and generating a positive sea-level slope toward the continent. The addition of heat and surface water fluxes to the simulation established an incipient gravitational circulation, mainly because of the large (approximately 0.2°C/km) temperature gradients in the summer. The river discharge, added last, was second to the tide in driving the residual circulation in the BTS and established the gravitational circulation through most of the bay, including the bay mouth. The residual velocities attained magnitudes smaller than 0.13 ms−1 in the main channel and were seasonally controlled, with stronger currents during the summer when the river discharge peaked. The onset of the baroclinic forcing (heat and surface water fluxes plus river discharge) reduced the e-folding flushing time by a factor of five relative to the experiment with barotropic forcing (wind and tides) only.