Study of the Water Circulation Patterns Using Landsat Imagery in San Blas Channel, Argentina

BEIGT, DEBORA; CUADRADO, DIANA G.; PICCOLO, M.CINTIA

Mendoza

Congreso; 18th International Sedimentary Congress; 2010

International Asociation of Sedimentology

San Blas channel (40º34´S, 62º11´W), located at the SW of Buenos Aires province (Argentina), is the southern portion of a large embayment (Bahía Anegada, 2371 km2). The channel is 3-km wide and 27-m deep with a SE-NW orientation. Winds and tides are the main forcing agents that influence the water circulation in the channel. Data recorded by a meteorological station installed at San Blas channel coast since December 2007 show prevailing N and NNE winds and stronger NNE (39.5 km/h) and S (38.8 km/h) winds. The channel is affected by a mixed tidal regime with a semidiurnal predominance. Recently performed ADCP velocity profiles showed maximum values of 200 cm s-1 during flood tide  and 180 cm s-1 during ebb tide (Cuadrado, pers. comm.).  This investigation deals with the application of satellite images to characterize some aspects of the water circulation in San Blas channel during a theoretical tidal cycle. Turbidity, a parameter detectable by Landsat reflective bands, was used as a natural tracer of water circulation patterns. Although the Landsat visible channels do not penetrate much in the turbid water, reducing the utility of turbidity patterns in dynamic studies to surface waters, the well- mixed San Blas channel, characterized by prevailing homothermal and homohaline vertical profiles and intense tidal currents, is considered an approppiate environment for the application of this approach. Six Landsat TM and ETM images acquired under clear-sky conditions during the warmer months of the year (November – March) and representing different tidal stages were selected from a pool of Landsat images provided by CONAE (National Commission on Space Activities). Geometric and radiometric corrections were performed on the visible and near-infrared bands. An atmospheric correction based on the COST method by Chavez (1996) was applied. Terrestrial areas were masked and an ISODATA unsupervised classification was performed. TM3 and ETM3 bands were chosen for this study. Turbidity patterns had close relation with some morphological features of the channel bed. Tidal deltas located at both ends of the channel were evidenced by turbidity plumes during ebb and flood tide, respectively. Sediment transport by  ebb currents (2 hours before low tide) was observed along the northeastern coast of the channel (Isla Gama and Banco Nordeste), characterized by fine-grained sediments (Cuadrado, pers. comm.). Sandy  sediments along the southwestern coast were also carried by ebb currents, apparently feeding the ebb-tidal delta. These banks were also reworked by flood currents. From the elongated shape of the turbidity plumes, general tidal circulation patterns were identified in San Blas channel: a) in the external area seaward of the channel mouth, the flood current direction is nearly S - N due to the tidal wave coming from the south, b) as they enter the channel, flood currents change direction and move along the SE - NW axis, c) as they reach the channel mouth, ebb currents flow in different directions into the open sea, showing a northward and southward transport of suspended sediment.