Oscar Orfeo

Monitoring Suspended Sediment Dynamics Using MBES

This technical note describes use of a multibeam echosounder (MBES) to quantify the dynamics of suspended sediment in a large open channel. A methodology is detailed that uses the backscatter magnitude from the MBES water-column data to adjust the magnitude of sonar returns for the various sonar settings, spatially and temporally average the data to account for the random nature of acoustic backscatter from the suspended sediment, and calibrate the processed data with direct samples. A case study of flow at the confluence of the Rio Parana and Rio Paraguay, Argentina, where there is a distinct turbidity difference along the mixing interface of the two flows, is used to demonstrate the unique capabilities of MBES to quantify sediment concentrations and dynamics within the water column.

The role of discharge variability in determining alluvial stratigraphy

We illustrate the potential for using physics-based modeling to link alluvial stratigraphy to large river morphology and dynamics. Model simulations, validated using ground penetrating radar data from the Rio Parana, Argentina, demonstrate a strong relationship between bar-scale set thickness and channel depth, which applies across a wide range of river patterns and bar types. We show that hydrologic regime, indexed by discharge variability and flood duration, exerts a first-order influence on morphodynamics and hence bar set thickness, and that planform morphology alone may be a misleading variable for interpreting deposits. Indeed, our results illustrate that rivers evolving under contrasting hydrologic regimes may have very similar morphology, yet be characterized by marked differences in stratigraphy. This realization represents an important limitation on the application of established theory that links river topography to alluvial deposits, and highlights the need to obtain field evidence of discharge variability when developing paleoenvironmental reconstructions. Model simulations demonstrate the potential for deriving such evidence using metrics of paleocurrent variance.

Sedimentological characteristics of small rivers with loessic headwaters in the Chaco, South America

Some sedimentological characteristics of #uvial environments located in the northeastern region of the Chaco}Pampa plain were analysed in order to determine the possible in#uence of the loessic headwaters of local basins. Concentrations of suspended sediments, as well as grain size of the river beds, were compared. Unexpectedly, it was found that the suspended load has an inverse relationship with the hydrometric levels, with the exception of highly saline streams (conductivity values between 4300 and 11,000 l Sc m~1). The mean concentration of suspended sediments was 63.1 and 206.3 mg l~1 in #ood and low water periods, respectively. The grain size of bed sediments was relatively homogeneous along the whole hydrological cycle. Sandy}silty bottoms predominate in channels, followed by silty}sandy and clayey}silty bottom types. Sediment transport mainly occurs in accelerating #ow suspension, overcoming the critical erosion velocity. During #ood periods, some parameters exceeded by 70% the values recorded during the low water phase. ( 2000 Elsevier Science Ltd and INQUA. All rights reserved.