Validation of large-scale particle image velocimetry to acquire free-surface flow fields in vegetated rivers

The reliability of large-scale particle image velocimetry (LSPIV) methodology to measure a 2D surface velocity field in a vegetated lowland stream is evaluated. To this end, measurements of the free-surface flow field obtained with LSPIV are compared with measurements with an electromagnetic current meter (ECM) close to the surface at four different locations. The measurements were performed monthly, allowing the evaluation of the LSPIV measurements in relation to different vegetated conditions. The difference observed between the mean velocities measured with ECM and LSPIV remains low in winter, whereas an increase is observed in summer. Inappropriate particle seeding density and unsteadiness of the flow are the main sources of LSPIV reliability reduction. Nonetheless, the seasonal average frequency of reliable LSPIV measurements is 97%, 95% and 78% in winter, spring and summer, respectively. The results illustrate that LSPIV is an inexpensive methodology, which provides high-resolution and reliable data to study the flow-field distribution in vegetated rivers, provided some considerations are taken into account to deal with the added complexity of the vegetation presence and the field conditions.