Author(s): Simone Speltoni; Volker Weitbrecht; Robert M. Boes; Isabella Schalko

Linked Author(s): Volker Weitbrecht, Isabella Schalko

Keywords: Flow velocity fields; Macro-roughness elements; River restoration; Wood

Abstract: Instream -structures like boulder placements and engineered logjams are widely implemented in river restoration projects. These macro-roughness elements (MREs) influence flow dynamics by creating areas of reduced and elevated velocity, affecting riverbed morphology and, in case of deadwood elements, offering cover and carbon storage which fosters food webs. Materials like boulders and wood can be used as stand- alone or in combination, to create multiple designs that vary in width, length , and porosity. This diversity in design, however, makes it difficult to predict the resulting flow patterns. To address this challenge, physical experiments were conducted using scaled MRE models in a laboratory flume with a fixed bed, representing boulder formations, engineered logjams, and vegetated rootwads. Each MRE type was tested under identical hydraulic conditions, spanning a range of Froude numbers and relative submergence levels. Additional tests explored the influence of relative width and length for selected configurations. Water depth and flow velocity fields were measured using ultrasonic distance sensors and acoustic Doppler velocimetry, respectively, on a defined grid upstream and downstream of each structure. Results show that internal voids within the MREs generate jet-like accelerations and localized peaks in turbulent kinetic energy. Partial -spanning, emergent MREs produced higher flow variation than channel-spanning and submerged structures. Both design characteristics and hydraulic conditions strongly influenced flow acceleration in the lateral gap between the flume wall and the MRE, as well as the extent of the wake region.

DOI: https://doi.org/10.64697/iahr.proc.ise2026.abs.256

Year: 2026