Luis Parente Maia

Wind velocity and sand transport on a barchan dune

We present measurements of wind velocity and sand flux performed on the windward side of a large barchan dune in Jericoacoara, northeastern Brazil. From the measured profile, we calculate the air shear stress using an analytical approximation and treat the problem of flow separation by an heuristic model. We find that the results from this approach agree well with our field data. Moreover, using the calculated shear velocity, we predict the sand flux according to well-known equilibrium relations and with a phenomenological continuum saltation model that includes saturation transients and thus allows for nonequilibrium conditions. Based on the field data and theoretical predicted results, we indicate the principal differences between saturated and nonsaturated sand flux models. Finally, we show that the measured dune moves with invariant shape and predict its velocity from our data and calculations.

Measurements and numerical simulations of the degree of activity and vegetation cover on parabolic dunes in north-eastern Brazil

In this work we present measurements of vegetation cover over parabolic dunes with different degrees of activation along the north-eastern Brazilian coast. We extend the local values of the vegetation cover density to the whole dune by correlating measurements with the relative brightness index C of high resolution QuickBird panchromatic satellite images of the dune field. We then introduce the vegetation data into a continuous model for vegetated dunes, coupling sand erosion and vegetation growth, and perform simulations of the evolution of the morphology and vegetation cover of parabolic dunes. Finally, from the comparison of both, the measurements and the simulation results, we show that the model is able to predict the dune shape and the vegetation distribution of real parabolic dunes as result of the evolution of a blow-out.

Measuring Aeolian Saltation: A Comparison of Sensors

Abstract We report the results of field experiments designed to compare four types of aeolian saltation sensors: the Safire; the Wenglor® Particle Counter; the Miniphone; and the Buzzer Disc. Sets of sensors were deployed in tight spatial arrays and sampled at rates as fast as 20 kHz. In two of the three trials, the data from the sensors are compared to data obtained from sand traps. The Miniphone and the Buzzer Disc, based on microphone and piezoelectric technologies, respectively, produced grain impact counts comparable to those derived from the trap data. The Safire and the Wenglor® Particle Counter produce count rates that were an order of magnitude too slow. Safires undercount because of their large momentum threshold and because its signal is saturated at relatively slow transport rates. We conclude that the Miniphone and the Buzzer Disc are appropriate for deployment as grain counters because their small size allows them to be installed in closely-spaced sets.

Accelerated Dune Migration and Aeolian Transport During El Niño Events along the NE Brazilian Coast

Abstract Dune migration response to regional inter-annual climate variability in Ceará, Northeastern Brazil was investigated. Dunes along the study area are mainly barchans and sand-sheets and they migrate at an averaged rate of 17 m/yr although this value depends on dune dimensions, the larger the dune the lower the migration. This is explained by the existence of a representative regional aeolian transport rate that is almost constant along the region inducing different dune response depending on dune size, which is a common feature of barchans fields. Estimated yearly transport rates from dune migration were compared to the values obtained by using the calibrated aeolian transport formula when fed by regional wind climate data. The results obtained differ by about 20% (100 m3/m/yr) from actual aeolian transport measurements (80 m3/m/yr) from dune evolution, which can be considered as a good agreement between both approaches. We show a relationship between dune displacement and strength of the dry season in the region. With larger displacements occurring in drier years, coincident with a more northern position of the Intertropical Convergence Zone (ITCZ), which occurred during strong El Niño periods. This relationship serves to link the dune migration in Northeastern Brazil with El Niño events. As El Niño Southern Oscillation (ENSO) supposes a major climatic perturbation, its effects will be transferred to all the dynamical processes controlled in a direct or indirect manner by the regional climate. These findings should be incorporated into Integrated Coastal Zone Management (ICZM) strategies for the region.

Profile measurement and simulation of a transverse dune field in the Lençóis Maranhenses

In this work, we report measurements of the height profile of transverse dunes in the coastal dune field known as “Lencois Maranhenses”, northeastern Brazil. Our measurements show that transverse dunes with approximately the same height present a variable brink position relative to the crest, in contrast to the case of barchan dunes. Based on our field data, we present a relation for the dune spacing as a function of the crest–brink distances of transverse dunes. Furthermore, we compare the measurements with simulations of transverse dunes obtained from a two-dimensional dune model, where a phenomenological definition is introduced for the length of the separation streamlines on the lee side of closely spaced transverse dunes. We find that our model reproduces transverse dune fields with similar interdune distances and dune aspect ratios as measured in the field.

Dynamics of sediment storage and release on aeolian dune slip faces: A field study in Jericoacoara, Brazil

Sediment transport on the lee sides of aeolian dunes involves a combination of grain-fall deposition on the upper portion of the slip face until a critical angle is exceeded, transport of a portion of those sediments down the slip face by grain flows and, finally, deposition at an angle of repose. We measured the mean critical and repose angles and the rate of slip-face avalanching using terrestrial laser scanning (TLS) on two barchans of different size in Jericoacoara, Brazil. Wind speeds and sand fluxes were measured simultaneously at the dune crests. We found that the mean critical and repose angles decrease with increasing wind speed. We attribute this to turbulent shear stresses, the magnitude of which are predicted using 3D Large-Eddy Simulation (LES) modeling, that episodically act down the slip face (i.e. in the direction of gravity) to trigger grain flows at lower angles than with gravity stresses alone. The rate of avalanching is a maximum in the morning at our study site and coincides with the maximum rate of increase in wind speed and not with the maximum rate of sediment supply to the slip face. We developed and tested a new predictive model for the rate of avalanching that includes both sediment flux delivered to the slip face and the derivative of the critical angle with time. Our data also suggest that the mean critical angle varies inversely with slip-face height. These results have important implications for aeolian dune evolution, interpretations of aeolian stratigraphy, and granular mechanics.

The Beaches of Ceará

The coastal zone of Ceara State, in Northeastern Brazil, extends for 573 km in a SE- NW direction. It is predominantly composed of sandy sediments of Upper Tertiary-Quaternary age with several generations of Pleistocene transgressive dunes, together with beaches, estuarine plains and localized occurrences of cliffs. The Precambrian rocks also occur on some beaches. The climate is semi-arid tropical and the rivers only flow into the sea during the rainy season. The dunes, beaches and estuary margins have experienced serious problems derived from the loss of sand material that have been taken to be used in coastal engineering, edifications and natural environmental degradation processes concerned to the sediment budget. In this context, this chapter focuses the description of the morphodynamics characteristics of the beaches of Ceara State, highlighting the factors controlling spatial and temporal processes, as well as discussing the impacts, potential uses and limitations of these areas. The results indicate that today this coastal erosion, either natural or induced by man, is perceived as the most significant threat to maintaining services in areas that depend upon tourism, traditional economic activities, housing and other pertinent uses of the beaches. It has been pointed out that the natural and human impacts represent the main and outstanding challenges for dwellers and the coastal managers who need to find out the new ways of living and occupation, includes redesigning the processes of house constructions and keeping up with minimizing the impacts. Then, the main actions that deal with the use and abuse of this littoral are discussed in this chapter. The environmental problems have to be strictly controlled, particularly stabilization of the mobile dune and preventing development of cliffs and headlands.

Impact of Drainage Basin Changes on Suspended Matter and Particulate Copper and Zinc Discharges to the Ocean from the Jaguaribe River in the Semiarid NE Brazilian Coast

ABSTRACT da Silva Dias, F.J.; Valente Marins, R., and Parente Maia, L., 2013. Impact of drainage basin changes on suspended matter and particulate copper and zinc discharges to the ocean from the Jaguaribe River in the semiarid NE Brazilian coast. This study aims to understand the fluvial contribution to the estuary and thence to the ocean and the behavior of the suspended particulate matter (SPM) and particulate Cu and Zn during spring tide cycles and during the dry and rainy seasons in 2005, 2006, and 2008, at the Jaguaribe River Estuary, NE Brazil. The distribution of metals concentrations in SPM during dry and rainy seasons suggests a lithogenic origin of aluminum (Al), iron (Fe), and copper (Cu), as a result of erosion and leaching of soils in the drainage basin due to anthropogenic drivers, such as urbanization, shrimp farming, and agriculture. Anthropogenic drivers also affect Zn flows associated with SPM during the dry season. The highest discharges of SPM and particulate metals occurred during rainy periods due higher freshwater volumes observed in the estuarine channel. The results strongly suggest that the high variability of discharges typical of semiarid drainage basins can be underestimated with the use of secondary data, showing the necessity of obtaining data in situ.

First Brazilian Symposium on Dunes Systems and International Symposium on Coastal Dunes: A Report of Meeting

The First Brazilian Symposium on Dunes Systems and International Symposium on Coastal Dunes (Management and Sustainable Development) were held on the Marina Park Hotel in Fortaleza, CE, Brazil, from August 24, 2005, to August 28, 2005. 1 Luis Parente Maia and coworkers at the Institute of Marine Sciences at Federal University of Ceara (LABOMAR/UFC) organized the meeting. Cosponsors provided assistance in many ways that helped to make the meeting a success. Travel grants were made available to plenary speakers from overseas. With nearly 100 delegates in attendance, this was the largest single grouping on coastal dune research in Brazil. Because the main theme of the meeting focused on coastal sand dunes management and sustainable development, a range of research was presented by researchers with professional backgrounds in physics, geography, geology, biology, and various social and human sciences. The format of the meeting included oral presentations, poster sessions, and a 3-day field excursion along the coast in Ceara (Jericoacoara Dune Field). All of the professional talks were presented in single plenary sessions with no concurrent sessions. This arrangement was effective because it provided participants with different professional training to appreciate new or different points of view. Half of the talks were delivered in English and half in Portuguese, both with simultaneous translation. The talks were of high quality and were accompanied by ample time for questions and discussion.

Using Wind Direction and Shoreline Morphology to Model Sand Dune Mobilization

ABSTRACT Carvalho, A.M.C.; Ellis, J.T.; Lamothe, M., and Maia, L.P., 2016. Using wind direction and shoreline morphology to model sand dune mobilization. This paper aims to establish a relationship between wind direction, shoreline position, and aeolian transport directions. Ceará State in NE Brazil, which mainly comprises barchans, barchanoids, parabolic dunes, and large flattened composed dunes was used to formulate and test a dune transport model that can be applied to north and east facing beaches. The headland-bay beaches of Ceará progressively increase in angle from E to W. This is accompanied by a corresponding change in the near-beach dune migration direction. A significant change in the wind pattern for the northern portion of Ceará was found, which resulted in a shift in dune migration from E-W to ENE-WSW as the shoreline orientation changed from NW-SE to W-E. The different aeolian transport conditions along the stretches of the headland-bay beach gradually shifted from transport parallel to the shoreline to unimpeded aeolian transport that moves sediments inland. Beach feeding occurs from headland bypass as dunes occupy the area landward of the headland. A zone of composite wind transport directions, winds carrying sand directly from the beach and those transporting sediment landward of the headland, was identified. This variation of wind transport conditions is confirmed by the dune morphology and influenced by the relationship between shoreline position and wind angle approach. The proposed model uses the sine of the angle between wind direction and shoreline orientation to predict aeolian transport and determines the volume of sediment transported by the wind. This model identifies stretches of shoreline under different aeolian transport directions and exhibits compatibility with measured dune migration.