Scour and deposition patterns in complex flow around stream restoration structures in a meandering stream channel

Instream structures are often employed in stream restoration projects to minimize erosion on the outside of a meander bend where shear stresses are highest, but guidelines for installation are often based on subjective criteria or professional experience. As part of a multiphase study to develop comprehensive quantitative design guidelines for instream structures, a series of experiments were conducted in the sand-bed meandering stream channel in the Outdoor StreamLab (OSL) at the St. Anthony Falls Laboratory (SAFL). Following an experiment with a single rock vane, three arrays of three evenly spaced structures (rock vanes, J-hooks, and bendway weirs) were installed in a single meander bend. To improve fundamental understanding of the interaction of the complex flow field around these structures with the sediment bed in a field-scale meandering stream, high resolution channel topography data were obtained for the entire meander bend at bankfull flow conditions (280 LPS) with and without structure arrays. Three-dimensional flow velocity and turbulence was measured using acoustic Doppler velocimetry for each scenario in nine cross sections located before, after, between, and over the structure installation locations. Velocity point spacing was decreased close to boundaries (bed, bank, or structure). The velocity data confirmed that the velocity core moved away from the outside of the meander bend in the presence of structures; however, increased local shear stresses around the structures increased scour which threatened structure stability. For each structure array, individual structures introduced different velocity patterns including visible recirculation zones and turbulent structures depending on the structure type and where in the meander bend the structure was placed. The results from these experiments will inform stream restoration structure design in a meandering stream.