Combining hydroacoustics and large-scale particle image velocimetry : Flow dynamics at Hiitolanjoki River restoration site in Southeast Finland

Study region: Finland's largest dam removal project at Hiitolanjoki River restoration site provides a unique opportunity to assess the hydraulic impacts of river restoration. The study site includes 220-m-long restored whitewater rapids and adjacent slower flowing upstream and downstream reaches. Study focus: Based on an extensive multi-year measurement campaign we investigated flow dynamics and hydraulic forces using hydroacoustics and image velocimetry techniques. 40,000 + Acoustic Doppler Current Profiler (ADCP) measurements from August 2020 to May 2023 were used to assess flow velocities and sediment transport capacity upstream and downstream of the restoration site. Additionally, seasonally relevant surface flow velocity patterns (0.25 × 0.25 m) within the rapids were calculated using large-scale particle image velocimetry (LSPIV) from nadir Unoccupied Aerial Systems (UAS) video recordings. New hydrological insights for the region: By integrating LSPIV and hydroacoustic surveying, we captured seasonally relevant flow dynamics across the entire restoration site. The transition from a regulated to a more free-flowing river reach has resulted in a shift in the high-velocity core due to the reactivation of the bypass channel. The river restoration reconnects the upstream and downstream reach, alters sediment transport capacity and supports the development of novel physical habitats critical for ecological restoration. This study highlights the value of multi-method hydrodynamic assessments for diverse river environments and provides insights into methodological applicability, benefits, and potential limitations.