Hydrodynamic and sedimentary characteristics of a small tidal channel in the Ria de Aveiro

Abstract

Owing to the decline of traditional salt production, a large number of saltpans in the Ria de Aveiro, Portugal, are currently abandoned and deteriorating. A study was undertaken to characterize the hydrodynamic and sediment transport conditions, in order to understand their role in the deterioration of the saltpan walls and to assist in the development of rehabilitation strategies. In a small tidal channel, samples of surface sediment were collected both in a saltpan wall and in the channel axis. Field measurements of water level, velocity, salinity, water temperature and suspended sediment concentration were made over two tidal cycles. It has been found that the sediment from the saltpan wall consists mainly of silt and clay material (about 80%), with median grain-size of the order of 0.018 mm. The sediment collected in the bottom of the channel is much coarser (median grain-size of 0.3-0.5 mm). The water levels indicate that tidal propagation between the inlet and the study area is not appreciably affected in terms of amplitude. Velocities showed maximum values of 0.40 m/s in the channel axis and 0.20 m/s near the bottom of the saltpan wall. Salinities (31.3-34.4‰) and water temperatures (17.1-27.9o C in July) reflected the strong marine influence and the shallow depth of the lagoon. The variation of suspended sediment concentrations (3.9-78.5 mg/l) was in agreement with previous studies. These findings, particularly regarding the nature of the sediment and the local tidal range, suggest that, in the absence of maintenance, the saltpan walls are significantly vulnerable to wave- and tide-induced deterioration.