Numerical Modeling of Local Scour Around Cilindrical Piers Including Secondary Flows Effects

ABSTRACT: Local scour in cylindrical piers is a high complex phenomenon due to its three-dimensional behavior with highly turbulent processes acting in a bed that is variable in time and space. When interacting with the sediment in the background, these vorticity structures make it notably difficult the development of a mathematical and numerical model for predicting local scour in a precise way. The developed mathematical and numerical models that reach an acceptable degree of accuracy are computationally expensive and become impractical in the engineering exercise. The objective of this research is to provide a low-cost computational tool that allows calculating the local scour in cylindrical piers at a temporal and spatial scale of the river without compromising its precision. For this purpose, a mathematical model based on the equations of shallow waters is developed including the effects of secondary flows, which allows to introduce three-dimensional processes in a two-dimensional model. This mathematical model was implemented in Open Telemac software by modifying its source code in order to build the numerical model. To carry out the calibration and / or validation of the proposed model, a measurement campaign was carried out in the Roncador Bridge piers located on the Magdalena River at the height of the town of Magangue, Bolívar. The measurements consisted in obtaining velocity data, depth of the water sheet, bathymetries, solid flow in suspension and grain size of the bed. Measurements of velocity and depth of the water sheet were obtained with an Acoustic Doppler Current Profiler (ADCP). The bathymetries were performed with an echo sounder and a sonar type deeper. The solid flow in suspension was calculated from a sediment trap and contrasted with a hydrometric station near the study area. Finally, from the model proposed in this research, local scour maps are presented around the cylindrical piers for the simulation time of the Roncador bridge.

Keywords: Local Scour, Secondary Flow, Acoustic Doppler Current Profiler (ADCP), Open Telemac