Modos de Transporte de Sedimentos Finos no Estuário do Rio Itajaí, SC
Palavras-chave:Itajaí estuary, fine sediment transport, particulated suspended matter.
This paper presents an assessment about the modes of fine sediment transport in the Itajaí River estuary. ·The available information to approach a conceptual model is derived from three primary sources: (1) the river sediment delivery; (2) the distribution of salinity and particulate suspended mailer (MPS) along the estuary under influence of different river discharge conditions; and (3) near bed hydro-sedimentological processes. The river sediment delivery was achieved by daily monitoring the water and the suspended sediment discharge, since November 1998 at the Indaial lymnimetric station, which represents about 70 % of the drainage basin. The salinity and MPS distribution in the estuary were obtained through 47 weekly surveys along the estuary when vertical profiles of these variables were acquired every 1-1,5 km from the mouth to the upper limit of the salt intrusion. The surveys were carried out between November 1998 and November 1999, using an inflatable boat. The results allowed obtain the relationship between the salt wedge extension as function of the river discharge and the estuarine trapping efficiency. The near-bed hydro-sedimenlological processes were assessed through the deployment of an instrument tripod on the estuarine bed al the channel thalweg 4 km upstream from the mouth. The current speed, water level and MPS data were recorded hourly, 1 m above the bottom, from September to November 1999. Based on this data set, two main modes of fine sediment transport in the Itajaí River estuary were identified: a tidal driven and a fluvial driven mode. The tidal driven mode occurs when the river discharge is bellow 200 m3.s-1. Near bottom sediments arc in all erosion-deposition cycle following the semi-diurnal tides and the anomalistic cycle of spring and neap tides, with net sediment transport landwards. During such periods fine sediment is imported from the inner shelf. This mode occurs at time scales of weeks to months. The fluvial driven mode are fully developed when the river discharge exceeds 1,000 m3.s-1. accompanied by suspended sediment load as high as 10,000 ton.day-1. Fluvial advective process fully dominates the sediment transport resulting in zero trapping efficiency.