Kinematic characteristics of blunt nosed chevrons in movable bed channels

Rivers are a crucial component of freshwater ecosystems. Nevertheless, they are increasingly threatened by anthropogenic activities, resulting in environmental degradation, and affecting the sediment transport capacity and channel stability. Active restoration of such ecosystems can be achieved by implementing low-head eco-friendly structures, e.g., cross vanes, J-hooks, W-weirs [1], block ramps [2] and, more recently investigated, wood bundles [3], and chevrons [4], which modify the sediment load and create low-velocity localized scour zones. In particular, blunt nosed chevrons are U-shaped structures facing upstream used to divert the flow and to promote scour processes in order to allow navigability in rivers. In addition, it has been found that such structures are also effective in enhancing the habitat for fish species, promoting biodiversity [5]. In this regard, different species of macroinvertebrates have been found in proximity of the rock body of chevron structures, providing nourishment for aquatic species. Despite the availability of guidelines for the design of blunt nosed chevrons for a long time ([6], [7]), information concerning the effects of such structures on the surrounding morphology is fragmented. Recently, [5] investigated the effects of blunt nosed chevrons on the equilibrium scour characteristics in a straight channel, while [4] extended the analysis to the temporal scour evolution for both straight and curved channels, as well as the equilibrium morphology in curved channels. Despite the advancements in predicting the equilibrium morphology and its temporal evolution in proximity of blunt nosed chevrons, the characterization of the flow kinematics in proximity of such structures has not been fully assessed yet, especially in proximity of the generated scours. Therefore, the present study focuses on the velocity fields occurring around blunt nosed chevrons at selected vertical positions below the water surface, with particular focus at the maximum scour location. To this end, a set of experimental tests were performed under clear water conditions for a range of hydraulic (i.e., water discharges between 0.008 and 0.016 m3/s; tailwater depths between 0.06 and 0.12 m) and geometrical configurations of blunt nosed chevrons inserted in a straight channel (i.e., different combinations of structure height, number of structures and spacing for series combination, position along the curved channel). Punctual velocity measurements at selected locations were performed by means of an Acoustic Doppler Velocimeter (ADV). Preliminary data analysis showed that flow velocities steeply decrease along the vertical depth at the location of maximum scour created downstream of the chevron structure, allowing favorable nursing conditions for aquatic species. Finally, Figure 1 shows an exemplary configuration of two chevrons in series arrangement, under dynamic (left) and static (right) conditions.