Caracterização morfossedimentar durante o ano de 2007 das praias estuarinas da ilha de Cotijuba (Baía do Marajó) no estado do Pará.

Abstract

The Cotijuba Island located at 90 km from the mouth of the Pará river is embedded in a tidal zone of the river, where the salinity is zero, and conductivity average is 65.70 μ S. The island belonging to the municipality of Belém is located 33 km of this, with 15 km2 of size and elongated shape in NE-SW direction. The island is supported by: (1) Barreiras sediments, outcroping in the form of exposed cliffs and platforms of abrasion, on the west side, (2) Pós Barreiras sediments, which are above and separated by a erosional surface (SÁ, 1969 apud SANTOS, 1996) and (3) Recent sediments (mangroves, marine terraces, bars and beaches). The study of sedimentmorph characterization of the morpho-sedimentary estuarine beaches of Saudade and Vai-quem-quer during the year 2007 (January, March and August), showed clear seasonal changes of these beaches. They were executed topographical profiles: nine on the beach of Saudade and twelve on the beach of Vai-quem-quer using a Total Station, in addition to the sediments collection. The beach of Saudade is 800 m length and is located on the southern island, with high topographic gradient ranging from tg d = 0,030 to tg d = 0,286. In the northern part of the island is the beach Vai-quem-quer with 1 km in length, which has a gradient topographic trivial compared to the beach of Saudade, ranging from tg d = 0069 to tg d = 0143. Have been applied on these beaches the morphological models of Guza & Inmam (1975), Wright & Short (1984) and Masselink & Short (1993) prepared for ocean beaches. Through the parameters used the beach showed state reflective of Saudade (b = 0.10 to 2.38) and longshore bar and runnel (b = 2.70 to 12.90), during the rainy season and only reflective (b = 0.05 to 1.84), during the dry season of Guza & Inmam (op. cit.). By the measure of Wright & Short (op. cit.) behaved the beach itself as refletiva (g = 1.08 and 0.86 in January and March, respectively, and in August g = 0.43) in the two periods. The beach of Vai-quem-quer presented by the measure of Guza & Inmam (op. cit.) states dissipativo (b = 20.05 to 31.28) and longshore bar and runnel (b = 9.21 to 18.23), in the rainy season. In the dry season, showed the states reflective (b = 1.61 to 2.46) and longshore bar and runnel (b = 2.63 to 3.44). For the parameter of Wright & Short (op. cit.) was classified as dissipative (g = 4.12) in January; longshore bar and runnel (g = 2.64) in March, and terrace of low tide (g = 2.38) in August. The parameter of RTR Masselink & Short (1993) stated that in January the beach of Saudade was influenced by waves and tides (RTR = 5.2) and in March by tides (RTR = 14.5). Already the beach of Vai-quem-quer either in January was influenced by waves (RTR = 2.15) and in March by waves and tides (RTR = 4.80). In the dry period, the two beaches were influenced by waves and tides (RTR = 10.5 and 3.16 for the beach of Saudade and Vai-quem-quer, respectively). The beaches are mostly covered by sand average (65%), moderately selected (59%), with kurtosis mesocúrtica (53%) and asymmetry negative (54%) on the beach of Saudade and approximately symmetrical (43%) on the beach of Vai-quem-quer. There was evidence of seasonality through profiles of acression and erosion, respectively, in dry and wet periods. On the beach of Saudade erosion occurred during the rainy season and acression, during the dry period. This beach has been divided into two sectors: North and South Sector. The South did not present major changes, however in the North Sector was developing a system of ridge and runnel and this has been intense variations with the changes of measures zones of intertidal and supratidal. On the beach of Vai-quem-quer either there was no notable changes during the rainy season. However, in the dry period was noticed acression in zones of upper intertidal and supratidal due to wind influence on the sediments that are at dried in this period. The characterization of the morphosedimentary estuarine beaches of Saudade and Vai-quem-quer is influenced by the Intertropical Convergence Zone (ITCZ), in the rainy season. It intensifies the winds in this period, which reached maximum speed of 7.5 m/s and preferential NE direction, and consequently, formed waves with higher energy (Hb = 1.3 m in January on the beach of Vai-quem-quer) and contributed to the beach erosional process. In addition to the high tides (Hm = 2.9 m in March) who achieved the most internal of these beaches.