Study of the water circulation patterns using Landsat imagery in San Blas channel, Argentina

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

Mendoza

Congreso; 18th International Sedimentological Congress; 2010

IAS International Association of Sedimentologists

San Blas channel (40º34´S, 62º11´W), located at the SW of Buenos Aires province (Argentina), is the southernportion of a large embayment (Bahía Anegada, 2371 km2). The channel is 3-km wide and 27-m deep with aSE-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 showprevailing N and NNE winds and stronger NNE (39.5 km/h) and S (38.8 km/h) winds. The channel is affected bya mixed tidal regime with a semidiurnal predominance. Recently performed ADCP velocity profiles showedmaximum values of 2.0 m/s during flood tide and 1.8 m/s during ebb tide (Cuadrado, pers. comm.). This investigationdeals with the application of satellite images to characterize the water circulation in San Blas channelduring a theoretical tidal cycle. Turbidity, a parameter detectable by Landsat reflective bands, was used as a naturaltracer of water circulation patterns. Although the Landsat visible channels do not penetrate much in the tur -bid water, reducing the utility of turbidity patterns in dynamic studies to surface waters, the well- mixed SanBlas channel, characterized by prevailing homothermal and homohaline vertical profiles and intense tidal currents,is considered an appropriate environment for the application of this approach. Six Landsat TM and ETMimages acquired under clear-sky conditions during the warmer months of the year (November – March) and rep -resenting different tidal stages were selected from a pool of Landsat images provided by CONAE (NationalCommission on Space Activities). Geometric and radiometric corrections were performed on the visible andnear-infrared bands. An atmospheric correction based on the COST method was applied. Terrestrial areas weremasked and an ISODATA unsupervised classification was performed. TM3 and ETM3 bands were chosen forthis study. Turbidity patterns have close relation with some morphological features of the channel bed. Tidaldeltas 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 ofthe channel (Isla Gama and Banco Nordeste) which is characterized by fine-grained sediments (Cuadrado, pers.comm.). Sandy sediments along the southwestern coast were also carried by ebb currents, apparently feeding theebb-tidal delta. These banks were also reworked by flood currents. From the elongated shape of the turbidityplumes, general tidal circulation patterns were identified in San Blas channel: a) in the external area seawardfrom the channel mouth, the flood current direction is nearly S to N due to the tidal wave coming from the south.This fact generates the re-transport of sediment from the banks closed to the shore, b) in the inner channel, floodcurrents cause sediment transport mainly on the south coast, c) in contrast, ebb currents trigger turbidity mostlyover the northern flank, and d) as they reach the channel mouth, ebb currents flow in a wide spread direction intothe open sea, showing a northward and southward transport of suspended sediment.