Residual flow structure at a scour- hole in Bahia Blanca Estuary, Argentina

PIERINI J O, PERILLO GME, CARBONE M E, MARINI F.

JOURNAL OF COASTAL RESEARCH

Lugar: West Palm Beach USA; Año: 2005 p. 784 - 796

0749-0208

Two-dimensional data of mean and sediment flow collected at a natural confluence of channels in the inner Bahía Blanca Estuary are presented. We describe the two-dimensional flow field to assess the role of changes in bed morphology occurring during  transport-effective events on the structure of flow at a confluence; and to examine how the flow structure  varies with changes in the ratio of momentum flux. At each station, vertical profiles of velocity, salinity, temperature and suspended sediment were obtained during the whole tidal cycle and  residual fluxes were obtained. Lagrangean flux is higher than possible fresh water input, denoting an extra output of water which is the flux over the tidal flats along the southern and northern coasts of the channel.  The effect of the junction produces a distortion of the vertical profile flow and in the suspended sediment transport. The effect of the particular conformation of the system makes necessary for consider the lateral effect of the tidal flats in the circulation of the estuary which exerts a significant influence. When the residual fluxes where decomposed, the Lagrangean flux appears higher than possible freshwater input. Although river discharge was not determined, averages values do not coincide with the estimated values, denoting an extra output of water over the tidal flats along the southern and northern coasts of the channel. Due to the presence of the flats and the logistic difficulties measuring the circulation on top of them, becomes impossible to control  continuity in the system. Although the scour hole seems to be an important geomorphological feature at the cross-section, it did not produced major modifications in the circulation except when it was analyzed based on the momentum flux approach. In this analysis, it is clear that the sudden change in depth at the hole produces a relaxation in the momentum flux which becomes concentrated at both sides of the hole. This is particularly notable when the northern peak of momentum flux is associated with the tributary channel. The effect of the tidal flats in this type of estuaries, where water not necessarily returns with the same input pattern, must be considered further. The main difficulty is to be able to measure the circulation simultaneously with restricted instrumentation.