Suspended Sediment Concentration Gradients in the Complex Bathymetric Features of a Macro-Tidal Estuary

Suspended sediment dynamics were studied in the Upper Gulf of California (UGC), a macro-tidal estuary in a transformation process due to the cease of freshwater and sediments from the Colorado River. This area is of particular interest because of its high productivity and is where the endemic protected species “vaquita marina” and “totoaba” live. Strong currents of about 1 ms^-1, a tidal range of 8 m and a complex bathymetry consisting of ridges and troughs are the main features of UGC. Although the bathymetric features have been recognized to be related to sediment resuspension, the present work is the first focused in the study of the sediment dynamics and the complex bed morphology. A field campaign was carried out during July 2008 to take different measurements in a ridge and a trough supported by two moorings with a suite of instruments to investigate the relationships between hydrodynamics and sediment resuspension. Results showed slightly lower velocity current in the through, but maxima at both sides were as expected coinciding with the middle tidal phase level (between high and low water).

Nevertheless, bed shear stresses presented a particular behavior. Maxima were significantly higher in the trough than in the crest. Suspended sediments almost reflected this characteristic with higher concentration also in the trough than in the ridge. An event of high winds bringing higher waves was the only recorded with significant sediment concentrations in the ridge similar to those in the trough. These characteristics seem to be related to differential grain sizes and hydrodynamics at both sites and therefore, different critical bed stress for erosion. Recent measurements of the particle size distribution of suspended sediments in the study area showed that flocculation might also play a role in sediment dynamics.