Hydrodynamic Pattern and Erosive Process at Tidal Channel Junctions

SILVIA SUSANA GINSBERG; SALVADOR ALIOTTA; MARTA E. MINOR SALVATIERRA

Viña del Mar

Congreso; GEOSUR; 2013

Universidad Andres Bello

Flow division at bifurcations or the flow confluence at junctions may assert a strong control over the morphological evolution of tidal channels, as the flow brings the material needed for progradation or take out the sediment to the channels for degradation. As a consequence, tidal processes play a prominent role in the morphological evolution of the channels, which may also have an impact in the estuary on the response to permanent changes in the channels and ultimately on their evolutionary structures. Some studies on channel confluences or bifurcations show that channel bottom morphology has an important role in the flow structure and sedimentary dynamics. The morphological development in the channel junctions at the Bahia Blanca estuarine system (Argentina) is driven by tidal currents, i.e. ebb flow as well as flood flow. Thus, during the flood phase, the junction side of both branches forms a bifurcation and during the ebb phase, the junction zone forms a confluence, such that bifurcations or confluences exert a crucial control over the nature of the processes that occur in this area. Accordingly, an important morphological feature at junction channels, which is due to erosive processes, is the formation of different depressions as result of changes in hydraulic conditions. This process is characterised by the removal of bed material as a result of flow acceleration, secondary currents and/or vortices that occur in this zone. Geophysical equipment (multibeam echosounder, 3.5 kHz profiler and side scan sonar) and an acoustic Doppler current profiler (ADCP) mounted on a vessel to more accurately define the velocity and direction of flow were used to provide new information on morphology, sub-bottom and the flow field at tidal channel junctions of the Bahia Blanca estuary (south of Buenos Aires province). Our focus of attention was centered on a confluence zone between the Tres Brazas Channel and Tierra Firme Channel (Fig. 1). In this sector, a depression developed that has an elliptical shape, with a maximum depth of 30 m relative to the surrounding seafloor and extends from the confluence of the channels to the principal channel. The average depth in the Tres Brazas Channel is 10 m, whereas that in Tierra Firme Channel is 9 m. A notable asymmetry is observed for the hole with the deepest section of the depression occurring in the inner sector. Thereby, towards the inner area of the Tres Brazas and Tierra Firme Channels, a marked 14° inclination topographic step can be observed. The average slopes corresponding to the northern and southern flanks of the depression vary between 8° and 4°, respectively, whereas the bottom, which is located towards the mouth of the Tres Brazas Channel, has a 1.5° slope. The base of the depression is irregular with outcrops of compacted material (Plio-Pleistocene) that constitute terrace-like structures of up to 0.5 m. The northern slope of the hole is composed of small scarps with a marked lateral continuity, which, towards the inner area of both channels, generates a V-shaped configuration at the bottom. On the contrary, on the southern side there is a uniform sedimentary veneer which leaves stepped terraces partially exposed, in certain sectors. The acoustic information on the marine bottom and sub-bottom reveals the presence of present-day depositional and erosive features as well as mechanisms of formation and evolution of the large depression that characterises the juncture of both channels in the study area. On the northern flank of the hole and towards the inner area of both channels, erosion therefore predominates, manifesting itself by outcrops of old sedimentary strata. The hole sector with the lowest slope, towards the principal channel, is affected by the deposition of sandy material. This process is manifested by a prograding seismic configuration, characterized by oblique clinoforms which evidence a progressive lateral development of slightly sloping depositional surfaces. The deep hole affects the hydrographic system and has a significant impact on local current patterns. According to Bernoulli dynamics, when a homogeneous current crosses a depression, the current velocities decrease with the increase of the water depth in the hollow. The topographic restoration of tidal currents over a hole, under a homogeneous water column, generates spatial variations of vorticity. Field observations have shown that during the ebb tide, when the flow is directed outward of the channel, the channel junction area may be compared to fluvial confluences. In this phase, turbulent flows were observed at the downstream confluence zone, where the eddy formation induces an erosive effect on the bed and therefore provokes the formation of the hole with its increasing depth and width. However, turbulence conditions were not observed outside of the deep hole. The ebb dominance occurs mainly through the center and southern channel. Depositional processes therefore occur on the side of the hole developed downstream on the gently sloping side. In contrast, during the flood current, the pathways of flow into the Tres Brazas Channel are through the northern bank. Its behaviour is analogous to fluvial meanders as it produces erosion of the outer part of the bend, i.e. erosion of the concave bank occurs in the northern sector of the Tres Brazas Channel and the northern flank of the hole. The flow division at the bifurcation has the potential to produce a significative erosive feature at the mouth of each channel and it is manifested by the outcrops of old sedimentary strata. In this sector, during the division of flows which are forced to enter into channels of shallower depths turbulence was not observed.