Abstract:
Usando metodologia de coleta de sedimentos com armadilhas portáteis (fundo e suspensão)
dentro da zona de surfe e metodologias de aquisição de imagens, topografia e sedimentologia conduzidas com o auxílio de filmadora, estação total e trabalhos em laboratório, respectivamente, determinou-se os parâmetros hidrodinâmicos, morfométricos e a deriva litorânea da região. Dados de
ondas em águas profundas foram obtidos a partir de modelo do CPTEC/INPE. Os trabalhos de campo foram realizados em 50 campanhas distribuídas pelo período de 1 ano. Do total, 42 foram dedicadas exclusivamente a dados hidrodinâmicos e morfométricos enquanto que as 8 restantes foram utilizadas também para coleta de sedimentos. Ao mesmo tempo foram obtidos dados sobre a direção e velocidade dos ventos e das correntes longitudinais. Todos os dados foram obtidos em condições
atmosféricas normais. Os trabalhos de campo foram conduzidos em 6 perfis transversais distribuídos entre as praias do Cassino e Mar Grosso (3 de cada lado da embocadura).
The growing coastal occupation by urbanization and the presence of engineering investments to attend these demands has showed the need of detailed studies of littoral drift with the objective to determine the total and liquid longshore sand transport rate since this phenomenon play an important role in several aspects related to coastline evolution and coastal engineering.
Using stream-traps for collect sediments (bedload and suspended load) into the surf zone according the methodology described by Kraus [11], topographic and hydrodynamic data respectively obtained by a total station and video images, allowed the morphodynamic characterization of the study area. Deep water wave data were obtained by the WAVEWATCH-III model from the Brazilian
National Space Research Institute (INPE) and compared with data acquired by a non directional wave rider at the same location (PNBoia Project). Wave characteristics at 15 m water depth and in the breaker zone were respectively obtained from Coli [5] and by visual observations. The field work was
done in 50 field trips distributed along one year. Among the 50 field data collection, 42 were dedicated exclusively to hydrodynamic and morphodynamic data, while 8 were also used for collect sediments.
All acquired data were obtained in normal atmospheric conditions.
The field work was done in 3 cross-shore profiles distributed along a stretch of 10 Km of coast south of the Patos Lagoon inlet encompassing Cassino and Querência beaches and 3 cross-shore profiles distributed along a stretch of 10 Km of coast north of Patos Lagoon inlet called Mar Grosso beach. It was also studied the participation of bedload and suspended load transport and there are strong evidences that bedload represents approximately 18% of total transport.
The measurements indicate a bi-directional flow of sediments, with a low net transport towards the northeast, like following: Profile-1(07/01/03) 1.92 m3/h NE-SW; Profile-2 (07/19/02) 2.60 m3/h NE-SW, (01/16/03) 0.23 m3/h NE-SW e (02/20/03) 25.1 m3/h SW-NE; Profile-3 (05/02/03) 4.24 m3/h SW-NE; Profile-4 (09/05/03) 43.33 m3/h SW-NE; Profile-5 (01/17/03) 2.54 m3/h NE-SW e Profile-6 (02/21/03) 31.74 m3/h SW-NE . The low values of total and net transport rate found in this area are consistent with the rate of sediment accumulation south of the inlet in the last 90 years.
The results suggest that the low longitudinal transport rates can be associated to both the small angle of wave approach (70% below 3°, with the remaining 30% inferior to 8°) and low wave height values at the breaker line (85% below 0.70 m and only 5% above 1 m). The low local hydrodynamics can be associated with the presence of jetties, as well as with the presence of fluid mud banks at the
nearshore area. These muddy areas can be responsible by the partial dumping of the incident wave energy and for the reduction of available sand to be transported towards the surf zone. Observations also suggest that the direction of the longshore current and the littoral drift is strongly dependent of the
longitudinal wind component along the coast.
