Carmen Medeiros

Influence of the Amazon and Para Rivers on Water Composition and Phytoplankton Biomass on the Adjacent Shelf

Abstract In the context of the REVIZEE programme, an extensive sampling cruise was carried out in the northern section of the Brazilian Exclusive Economic Zone on board O.V. Antares (Operation North III). Several parameters were surveyed along the Amazon continental shelf during this cruise, such as horizontal and vertical distributions of temperature, salinity, dissolved oxygen, pH, dissolved inorganic nutrients, and phytoplankton biomass. Water with salinity below 33 psu and high silicate content was observed throughout the shelf, indicating the influence of the Amazon River plume up to a distance of 147 km from the coast; pH always remained alkaline. Water temperature was relatively homogeneous at the surface, with an average of 28°C. A permanent thermocline was detected in oceanic regions. Coastal waters were eutrophic, well oxygenated (maximum of 6.54 mL O2 L−1), and presented high chlorophyll a (maximum of 9.8 mg chlorophyll a m−3) and nutrient concentrations (up to 9.54 μmol L−1 NO3−, 0.89 μmol L−1 PO42−, and 108.59 μmol L−1 Si044−). Oceanic waters adjacent to the Amazon shelf presented high transparency (Secchi depth up to 35 m) and chlorophyll a (mean: 0.1 mg m−3) and low nutrient concentrations (maximum of 1.58 μmol L−1 NO3−, 0.18 μmol L−1 PO42−, and 5.92 μmol L−1 Si044−). Cluster analysis performed with all data showed a clear separation between coastal and oceanic waters at the surface layer and at 1% light penetration layers. This separation into two groups of stations clearly represents areas with different degrees of influence from the discharges of the Amazon Basin. The Amazon River is very important to the nutrient budgets of the shelf.

Tidal characteristics of the Strait of Magellan

The Strait of Magellan is a 500 km long navigational waterway connecting the South Atlantic and South Pacific oceans. On the Atlantic side, the Strait is characterized by semidiurnal macrotides with mean and spring tide ranges of 7.1 and 9.0 m, respectively. On the Pacific side, tides are mixed, mainly semidiurnal, with mean and spring tide ranges of 1.1 and 1.2 m, respectively. Wide tidal flats and basin geometry tend to amplify the Atlantic shelf tides in the eastern extreme of the Strait. Tidal choking occurs in a series of narrows, causing stepwise decreases in tidal range from east to west. The mean tide range is initially amplified to 8.4 m, then reduced to 4.0 m, and subsequently to 1.2 m within 150 km of the Atlantic Ocean. Peak tidal currents exceed 3 m s−1 in the narrow constrictions and may reach 4.5 m s−1. The tidal form number is 0.12 in the eastern Strait, but exceeds 0.25 less than 125 km from the Atlantic. Bahi´a Gente Grande, an embayment located 165 km into the Strait from the Atlantic, experiences a tidal regime that shifts from semidiurnal to third-diurnal to quarter-diurnal during each month. This regime results from reduction of semidiurnal constituent amplitudes, existence of appreciable diurnal constituent amplitudes, and a four-fold amplification of the M6 amplitude, which in combination give rise to the double water phenomenon.

Remote sensing and numerical modeling of suspended sediment in Laguna de terminos, Campeche, Mexico

Abstract It is necessary to understand the complex physical processes at work in coastal lagoons in order to manage them effectively. Improved methods of data collection and analysis must be found to provide synoptic, timely hydrodynamic information because of the sheer size of some lagoons and the difficulty of acquiring in situ data (particularly in the tropics). This paper summarizes research to model salinity and suspended sediment distributions in Laguna de Terminos, Mexico, using 1) a coupled hydrodynamic and dispersion model and 2) analysis of two Landsat Thematic Mapper images collected on 25 November 1984 and 24 April 1987. Atmospherically corrected chromaticity data derived from Thematic Mapper data were significantly correlated with modeled total suspended sediment concentrations for the two dates. Comparison between numerically modeled and remotely sensed suspended sediment maps at 1.5× 1.5 km resolution yielded a covariation map useful for identifying areas of discrepancy between the remotely sensed data and model output.

Current vanes for measuring tidal currents in estuaries

Abstract Current vanes used as submerged drags are simple, reliable and inexpensive instruments for measuring instantaneous currents in shallow, tidal estuaries. They are superior to current crosses and well-suited for environmental surveys of estuarine current distributions. The vanes have been designed for current measurements in the range 0.1–1.6 m s −1 and have an average standard error of the mean of 0.04–0.06 m s −1 , depending on the choice of current speed range. The drag coefficient for the vane is calculated to vary between 1.39 and 1.48.

Longitudinal Salt and Sediment Fluxes in a Tropical Estuary: Itamaracá, Brazil

Abstract Along-channel velocity, salinity, and total suspended sediment concentration profiles were measured during 15 complete neap and spring tidal cycles in the dry and rainy seasons in four cross-sections in a tropical estuary, Itamaracá, Pernambuco, Brazil. The net longitudinal fluxes of salt and sediments were decomposed into advective, tidal, and cross-sectional shear transports. The advective transport of both salt and suspended sediments were for most parts export-directed and greater than the combined river discharge, suggesting a non-steady state. The dispersive tidal transport was decomposed into four separate mechanisms of which tidal pumping transport and tidal wave transport (similar to transport by Stokes’ drift) are the main transport mechanisms. The cross-sectional shear transport was negligibly small, and thus, vertical nor lateral effects are significant with respect to longitudinal dispersion in the case of Itamaracá.

Energy Balance and Mixing Timescales in a Stirring Tropical Estuary, Itamaracá, Brazil

Abstract Stratification and mixing dynamics in estuaries are controlled by different physical processes, which determine estuarine hydrodynamics and the transport of plankton, nutrients, and organic compounds. In this paper, we present an investigation of features that control the dynamics of the Itamaracá estuary, Pernambuco (northeastern Brazil) through quantitative and comparative analyses between stratification processes (surface heating, rainfall precipitation, and differential advection of the longitudinal density gradient resulting from the vertical velocity structure) and mixing processes (stirring of bottom tidal stress, stirring of surface wind stress, and surface evaporation). Thermodynamic and cinematic field data were obtained during rainy and dry seasons. Results indicate that mixing intensity in the water column was about one and a half to twice as high as the potential capacity of external forcing in promoting vertical stratification. The bottom tidal stirring was the most important cause of vertical mixing. A theoretical analysis showed that, during the dry season, the stratification timescale (30 min) was greater than the turbulence decay timescale (9–10 min) and also greater than the slack water timescale. During the rainy season, the stratification timescale (7 min) had the same order of magnitude as the turbulence decay timescale (9–10 min). The Itamaracá estuary is a vertically well-mixed/weakly-stratified system during both periods (dry and rainy), except during rainy slack water periods, when stratification can be expected. These theoretical results are in agreement with previous field data and recent works involving numerical simulations at the Santa Cruz Channel.

Application of a probabilistic sediment transport model to guide beach nourishment efforts

ABSTRACT Rollnic, M. and Medeiros, C., 2013. Application of a Probabilistic Sediment Transport Model to Guide Beach Nourishment Efforts This study focused on establishing the probability of different sediment grain size be transported by coastal currents, as a prognostic tool to guide nourishment efforts at a coastal section near Recife, NE-Brazil (UTM 9105000-9089969N; 0297000-0289227E). The SisBahia hydrodynamic module FIST3D was applied linked to the Lagrangian-Probabilistic Transport Module over a 27km-long coastal band from 0 to 17m isobaths. The bathymetry and sedimentary coverage was characterized in a 200m grid. Input and calibration data were originated from a 3-years field work comprising 40 stations and considering seasonal variability. Data base included instantaneous current measurements, current, tide and wave time-series and CTD profiles. A finite-element grid (876 quadratic elements; 3703 nodes) was generated and twenty one depth levels defined through the water column. Simulations were performed with a 50s time-integration intervals, resulting an average 4.57 Courant number. Dry and rainy season scenarios under neap and spring tides were simulated with good agreement between simulated and in situ data. Simulation for each scenario was conducted over a 10-days period, and the probability of the different currents intensity near the bottom relative to its ability to move the various sediments fractions were mapped. Simulation indicated a high potential for sediment transport in the area under more severe erosion. Here current intensification was verified at both seasons and during neap and spring tides. Fine sand have a 90% probability of being transported by local currents, medium and coarse sands a probability of 80% and very coarse sands a probability of 60%.

Equilibrium Beach Profile in the Presence of Beachrocks

ABSTRACT Rollnic, M. and Medeiros, C., 2016. Equilibrium Beach Profile in the Presence of Beachrocks In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 452–456. Coconut Creek (Florida), ISSN 0749-0208. Equilibrium Beach Profile (EBP) models are used to estimate sandy beach profiles, they assume the existence of a shoreface shape that represents an equilibrium condition between wave climate and sediments, making it possible to identify erosion/accretion trends. EBP models suit beaches with near homogeneous grain size sediments. However, in subtropical and tropical areas, beachrocks are often found underlying the beach face, such that EBP models may not be suitable as shadowing areas may favour settling of variable grain size sediments. Our goal was to evaluate the Bruun-Dean EBP model for a beach along which beachrocks were at times absent and at times present as onshore or submerged reefs. Eight 1km-long bathymetric profiles were performed from the surf zone to the 10m isobath and sediment samples obtained at 50, 100, 200, 500 and 1000 m from the surf zone. EBP was computed for distinct coefficient (m) values and evaluated as a function of resulting summed square error. A closure depth of 3.97 m was computed using wave characteristics were derived from 1 year radar records. Along the study area the 3.97 m isobath is positioned 140 to 400m offshore, disregarding the quotas of the beachrocks. At the control areas where beachrocks were absent, m=0.67 yielded the best fit. At areas where beachrocks were present, profiles tended to be steeper and m=0.77 results in a better adjustment near the shore. For the more seaward portion of the profiles, the incorporation of a parameter related to the reef high could improve fitting.

An Analytic Approach to Model the Tidal Circulation in a Double-inlet Estuary

ABSTRACT Araujo, M.; Medeiros, C. and Endres, J-P., 2016. An Analytic Approach to Model the Tidal Circulation in a Double-inlet Estuary. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 223-227. Coconut Creek (Florida), ISSN 0749-0208. Estuaries comprise broad spectra of systems whose morphology often rule theirs hydrodynamics. The Itamaracá estuarine system (NE-Brazil) is formed by the Santa Cruz Channel (SCC), connecting to the Atlantic Ocean through two inlets. Water level and in depth current measurements were used to evaluate an analytical approach for representing its tidal circulation. Depth-averaged currents were analytically predicted with 11% error (δ1 = 0.11). Currents measurements for five different sampling stations and depths were compared to model responses for various values of eddy viscosity (ν) and bottom friction parameter (r). The best-fit quadratic error δ2 = 0.155 was obtained with ν = 6.3×10−3m2s−1 and r = 6.5×10−3ms−1. Model improvements, considering bottom friction and eddy diffusivity formulations, indicated a boundary layer depth of 0.10H (H=channel depth), and a large (6.1×10−3m) mean roughness length of the sea-bed to couple with the intricate roots system of red mangroves along the SCC. Simulations were also used to test Taylors (1954) scale analysis, yielding c = 0.080 as best value (c = ν/u*H constant; u* =bottom friction velocity) and a mean eddy viscosity of 5.8×10−3 m2s−1). The low sensibility of momentum distribution to changes in eddy viscosity verified suggests that stronger viscosity dumping may be compensated by higher bottom shear stress. This simple analytical approach could also be used to predict spatial and temporal distribution of pollutants and other materials at SSC and at similar systems as advection of those components could easily be simulated combining modelled currents with measurements of theirs concentrations.