Assessing the risk of infrastructure scour due to turbulence, using miniaturized instrumented particles

During extreme river-flow conditions induced by the continually worsening effects of climate change, the riverbed granular surface may get destabilized and can potentially be the cause of infrastructure failures [1]. Such conditions signify the start of the geomorphic change of the river's boundaries, affecting natural river habitat and the built infrastructure in its vicinity, especially near surface water bodies, costing billions of pounds per year globally. Given its importance, identifying the conditions leading to hydraulic infrastructure scour (i.e., scour around abutments and piers) has been a topic of intense focus for hydraulic researchers and engineering practitioners alike, especially over the last decades.

This research aims at studying the conditions leading to the start of hydraulic infrastructure scour by assessing the turbulent energy of flow structures leading to the destabilization of the bed surface around them. Specifically, a physical model of a cylindrical bridge pier is used in a flume to conduct lab experiments for various flow rates, aiming at probing the risk of critical failure of the riverbed surface. The experiments are conducted at a water recirculating laboratory flume with a cylindrical pier under four different flow rates. The experimental setup involves a flat fixed bed surface hydraulically roughened by spherical beads packed closely in a hexagonal arrangement, with a similarly roughened 3D-printed test section, on top of which an instrumented particle [2] can be positioned at distinct distances from the model pier. The risk of bed surface destabilization and scour initiation is assessed by the probability of entrainment of the instrumented particle for the combination of flow rates and distances downstream of the model cylinder [3]. The latter can be estimated as the rate of entrainment of the instrumented particle, monitored from the appropriate post-processing of the fused sensor data and validated from video observations (from a top and side camera). In this work, the 3-axis accelerometers and gyroscopes that offer records to help directly produce estimates of the probability of entrainment are embedded within an instrumented particle with an external diameter of 3.5cm.

These observations are further linked to the flow turbulence energy by aiming to establish correlations of the entrainment risk of the exposed instrumented particle to the probability of occurrence of turbulent eddies shed downstream the cylindrical model pier. Profiles of point flow turbulence measurements are obtained with acoustic Doppler velocimetry (ADV) at distinct distances downstream of the model pier. Flow energy and impulses are calculated from the probed flow velocity data at seven longitudinal distances.

 

References

[1] Michalis P, Xu Y, Valyrakis M (2020). Current practices and future directions of monitoring systems for the assessment of geomorphological conditions at bridge infrastructure. River Flow 2020. In: Proceedings of the 10th Conference on Fluvial Hydraulics, Delft, Netherlands, 7–10 July, pp.1–6. ISBN 9781003110958

[2] AlObaidi, K., Valyrakis, M. (2021). Explicit linking the probability of entrainment to the flow hydrodynamics, Earth Surface Processes and Landforms, DOI: 10.1002/esp.5188.

[3] Valyrakis, M., Diplas, P., Dancey C.L. (2011). Entrainment of coarse grains in turbulent flows: an extreme value theory approach, Water Resources Research, 47(9), W09512, pp.1-17, DOI:10.1029/2010WR010236.