Near bed sediment transport in the Itajai-acu River estuary, southern Brazil.

The objective of this study is to assess the near bed sediment transport in the Itajai-acu River salt-wedge estuary, especially regarding two fortnight periods: the first being dominated by tidal currents, and the second being dominated by river flood currents. The data acquisition system consisted of an acoustical current meter moored in a tripod, with an optical backscatter turbiditymeter and a pressure sensor. The tripod was deployed at the channel thalweg 10 m deep and at 4 km from the estuarine mouth, throughout a 75-day period. Daily river discharge data were provided by the National Power Agency. During the period of data acquisition two discharge peaks greater than 1,000 m(3).s(-1) occurred within a seven-day interval, the first being in neap tide and the second in spring tide condition. During the high discharge events, the near bottom current speed peaks reached 1.0 m.s(-1), while during normal conditions they are usually lower than 0.7 m.s(-1), even during spring tides. Furthermore, the current during the river flood events was unidirectional towards the sea, lasting 49 and 35 hours, respectively. The tidal signal could still be observed, however, as the oscillation of the seaward current. Strong ebb current asymmetry followed such periods. The yield shear stress exceeded 2 Pa during the current peaks, and the bottom suspended sediment concentration increased from 0.02 to more than 0.25 kg.m(-3). The mean seaward sediment transport during the events was 303 kg.m(-2) and 509 kg.m(-2) per hour, respectively. Comparatively, during the previous fortnight period with low river discharge, the mean sediment transport was 32 kg.m(-2) per hour landward. These results show that critical sporadic high discharge events play an important role in the sediment dynamics in the Itajai-acu estuary, furnishing higher critical bottom shear stress than tidal currents as well as unidirectional seaward flow. During the tide-dominated period, erosion appears to take place only during the flood of spring tide, contributing to estuarine basin infilling. On the other hand, during river the flood-dominated period, erosion appears to take place as a function of river discharge only, promoting intense seaward sediment transport.