Fluvial hydrodynamics in contractions: Case study sector Guaimaro Magdalena River.

Predicting the morphological behavior of rivers is complex, due to the interaction of geomorphological and hydrosedimentological variables and processes. Geometric characteristics such as width, depth, slope, radius of curvature and sinuosity, among other variables, induce hydrodynamic conditions that affect the morphology associated with the same geometric variables. This dynamic is permanent and uncertain associated with the hydrological regime of the river. Therefore, the level of risk associated with the bank erosion is highly dependent on the hydrodynamic conditions of a sector, especially when contractions and radius of curvature are tight. This article presents an analysis of the evolution of the hydrodynamic conditions in a sector of the Magdalena River, called Guaimaro, which has a high morphological dynamics and is recently associated with an important section contraction change, from 1,000 m to 250 m wide, together with a radius of curvature sufficient to cause secondary currents that increase the velocities, the shear stress magnitudes, the depth on the outer bank, and the risk of bank erosion. The hydrodynamic analysis in this sector was carried out for four (4) years with different morphological conditions: 2009, 2013, 2014 and 2018. Each of these scenarios were characterized and modeled with Delft-3D to determine their differences associated with the geometric conditions. The numerical model was set, calibrated and validated with field measurements such as bathymetries, velocities and flow rates measured with ADCP, sediment transport data and satellite images. The results showed that at low flow rates (3,000 m³/s) the average velocity increased by 40% at the contraction. For the average flow rate (7,000 m³/s) the average velocity increased by 50%. However, at high flow rates (12000m³ / s) the energy gradient is much higher causing a 100% increase of the average velocity in the contraction. The results of the shear stress are similar. The analysis was conducted with different geometry scenarios over the years of analysis, with which were possible to determine a correlation between geometry variables and hydraulic variables caused by the combination of width contraction and curvature radius.