Morfodinâmica da praia estuarina do Cajuúna, Soure, Marajó – Pará

Abstract

The Cajuúna beach is located in the municipality of Soure (0 ° 37 '41,971 "S - 48 ° 28' 56,980" W and 0 ° 38 '54,888 "S - 48 ° 29' 6.867" W), on the eastern shore of the Marajó island (Pará state). This beach is cut by tidal channels (one in the middle and two at the ends), and is influenced by meteorological parameters (wind, rain, thunderstorms), hydrological (water and solid discharge of Pará estuary) and hydrodynamic (waves, tidal currents). The aim of this work is to identify the morphodynamics of estuarine Cajuúna beach through seasonal analysis, may clarify questions about which morphological features are characteristic during the rainy and less rainy periods; which the textural changes of the sediments; which most forcing this change in the transport and distribution of sediments and with what intensity and preferred direction they are transported. There were two field campaigns: during the rainy periods of equinoctial tide (04/01/2014) and less rainy (11/23/2014). The beach stretches for 4 km, your lenght reaches about 100-489m, was applied 7 topographic profiles with equidistance of 300 m. The beach was divided into three sectors: profiles A and B - Sector 1, profiles, C, D and E - sector 2, F and G profiles - sector 3. The sediment samples were collected from: (1) timely measurement longitudinal transport in the surf zone (traps) and (2) over topographical profiles, adopting morphological zonation (Souza Filho et al., 2003). Still, waves and currents data acquired in the surf zone. The sediment samples were subjected to analysis and classified according to Folk and Ward's method (1957), Shepard diagrams (1954) and Pejrup (1988). Besides these were produced morphometric parameters: Sedimentary volume variation (Vv), declivity of praial face (β), width of the beach (Yb), coefficient of shoreline change (CVYb), Speed decanting of sedimentary particles (Ws ) beach morphodynamic state (Ω) Dean (1973) and the relative variation of the tide (RTR – Relative Tide Range) Davies and Hayes (1984), suggested by Masselink and Short (1993). The results show that the Cajuúna beach is composed of sediments ranging from fine sand (< 2 to 3 Φ) to very thin (< 3 to 4 Φ) with a good degree of selection (0.35 to 0.50 Φ) very well selected (<0.35 Φ) to moderately selected (from 0.50 to 1.00 Φ) in less rainy season. The asymmetry remained approximately symmetrical tending to positive values (0.10 to 0.30 Φ), very positive (from 0.30 to 1.00 Φ) and negative (-0.30 to -0.10 Φ) and kurtosis predominant in both periods was leptokurtic (1.11 to 1.50 Φ), hooting for very leptokurtic (1.50 to 3.00 Φ), mesokurtic (0.90 to 1.11 Φ) and platykurtic (0.67 to 0.90 Φ). This beach had a positive balance of the sediment volume during the rainy season (lower volume 134 m³ / m higher volume 955 m³ / m) to the less rainy (lower volume 74 m³ / m higher volume 1,567 m³ / m), and growing, with the exception in sector 2 which is the decrease in sediment volume (136 m³ / m in the rainy season to 74 m³ / m in less rainy). The Cajuúna beach remains wider in the rainy season; the coefficient of variation of shoreline was also higher in this period; however the sectors 1 and 3 were larger in the less rainy season (sheltered areas). This beach had low slope (<2.0 °) in both periods, related to the presence of fine sediments. Prevailed morphodynamic state (Ω) intermediate in both periods; Terrace Low Sea (TLS) in the rainy season and variation between Terrace Low Sea and Bank Cusp Beach (BCB) in less rainy season. The RTR classified as intermediate beach in less rainy and wet seasons, with wave-tide interaction (RTR < 7), occurring variation in the least rainy season, a reflective classification, dominated by wave with RTR < 3. In the surf zone, the fine sand fraction (< 3 to 4 Ф) was dominant in rainy periods (up to 74.27g) and less rainy (maximum 562.61g) during ebb tides in the sector 2; the silt remained high fraction (100g), even surpassing the dominant fraction in the less rainy season; and the clay had the lowest amount in both periods (minimum of 0.27g and maximum 23,48g). The incidence angle of the waves varies from NW (rainy season) to NE (less rainy season). The largest wave height was recorded during the ebb tide in both periods (1,074 m - rainy season and 2.94 m - less rain). The wind speed was more intense during the flood in the rainy season (8.4 m/s), and the ebb in less rainy season (10.3 m/s). The intensity of tidal currents was higher during the flood in rainy seasons (0.73 m/s) and less rain (0.35 m/s). Various features such as ridges and troughs, paleomangrove soils, dunes, sand spit and canals occur in the Cajuúna beach are formed and / or seasonally modified by weather forcings, hydrological and hydrodynamic. Understanding the morphodynamics of the Cajuúna beach is important, due to his strategic location above the beaches of the eastern part of the Marajó island, suffers influence of the waters of Pará mouth, and is part of an Environmental Protection Area Despite the implementation of a Coastal Management Plan, in some parts of the Northeast region of Pará ; to date, the Marajó Island does not have a methodology and set to the prognosis and monitoring of coastal erosion . Given these shortcomings , it is necessary to determine how the rising sea level is affecting the eastern margin of Marajó Island . This study makes contribution the Organization of the Amazon Cooperation Treaty Organization, the project " Adaptation to Sea Level Rise in the Amazon Delta " which aims to assess the impact of rising sea level caused by global climate change on island environments Marajó (Eastern and Northern margins).