Assessing different imaging velocimetry techniques to measure shallow runoff velocities during rain events using an urban drainage physical model

Juan Naves,Juan T García,Jose Anta,Jerónimo Puertas

doi:10.5194/hess-25-885-2021

Abstract

Abstract. Although surface velocities are key in the calibration of physically based urban drainage models, the shallow water depths developed during non-extreme precipitation and the potential risks during flood events limit the availability of this type of data in urban catchments. In this context, imaging velocimetry techniques are being investigated as suitable non-intrusive methods to estimate runoff velocities, when the possible influence of rain has yet to be analyzed. This study carried out a comparative assessment of different seeded and unseeded imaging velocimetry techniques based on large-scale particle image velocimetry (LSPIV) and bubble image velocimetry (BIV) through six realistic but laboratory-controlled experiments, in which the runoff generated by three different rain intensities was recorded. First, the use of naturally generated bubbles and water shadows and glares as tracers allows unseeded techniques to measure extremely shallow flows. However, these techniques are more affected by raindrop impacts, which even lead to erroneous velocities in the case of high rain intensities. At the same time, better results were obtained for high intensities and in complex flows with techniques that use artificial particles. Finally, the study highlights the potential of these imaging techniques for measuring surface velocities in real field applications as well as the importance of considering rain properties to interpret and assess the results obtained. The robustness of the techniques for real-life applications yet remains to be proven by means of further studies in non-controlled environments.

Highlights

Since the last years of the 19th century, urban drainage systems have fulfilled a fundamental mission that has enabled us to guarantee the hygienic-sanitary conditions and growth of denser cities (Butler et al, 2018; Brown et al, 2009)
The differences obtained in the velocity fields resulting from analyzing the experimental videos by four image velocimetry techniques (LSPIV, LSPIVb, LSPIVu and bubble image velocimetry (BIV)) are presented and discussed
This analysis focused on the intermediate rain intensity experiments

Summary

Introduction

Since the last years of the 19th century, urban drainage systems have fulfilled a fundamental mission that has enabled us to guarantee the hygienic-sanitary conditions and growth of denser cities (Butler et al, 2018; Brown et al, 2009). The increased flood risk is a consequence of these factors (Chen et al, 2015) that must be accurately assessed (Apel et al, 2004; Martinez-Gomariz et al, 2016) This continuous development of impervious areas leads to a significant increase in the load and peak concentrations of pollutants, which are accumulated on urban catchments surfaces and can be washed off and transported by runoff into drainage systems and eventually to aquatic media (Lee and Bank, 2000; Anta et al, 2006; Zafra et al, 2017; Muthusamy et al, 2018). This process depends on multiple factors (Hatt et al, 2004; García et al, 2017) and requires a clear understanding of the surface drainage in urban areas, from the hydrodynamic point of view

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