Laboratory Studies on the Effect of Deflectors on Changes in Sediments Flow

River regulation often leads to uniform conditions within the river channel, alters sediment dynamics, and contributes to the degradation of aquatic habitats. Deflectors are increasingly used as habitat-forming elements in river restoration projects. However, the interaction between hydraulic conditions, sediment inflow, deflector location, and surface roughness and their effects on the spatial extent of sediment removal remain insufficiently studied. Laboratory experiments were conducted in a flow channel using three geometrically identical deflectors arranged according to the configuration observed in the Flinta River in western Poland. The studies were conducted for the following combinations: three discharges (Q = 0.40, 0.64, and 1.70 dm3 s−1), three water depths (h = 0.03, 0.06, and 0.09 m), three cumulative surrogate-sediment masses (Rum = 0.5, 1.0, and 1.5 kg), three dimensionless longitudinal positions (ξ = 0.21, 0.61, and 1.00), and two deflector roughness specifications—smooth or rough. The two-dimensional extent of the sediment-free zone was quantified based on aerial photographs using the normalized surface index A∗. Dimensionless water depth and dimensionless discharge were the dominant factors χ > Q∗ > R∗ ≫ ξ, and their interaction Q∗×χ constituted the strongest two-way effect, whereas sediment mass had a significant but secondary influence. Surface roughness did not independently affect the mean A∗ value but altered the spatial characteristics: longitudinal position was non-significant for smooth deflectors but became significant for rough deflectors, particularly through interactions between depth and location and between depth, location, and surface. These findings indicate that deflector roughness should not be specified as an isolated design parameter but should be selected jointly with the expected flow-depth regime and the longitudinal placement of successive structures. In practical terms, the results can support the preliminary design and positioning of habitat-forming deflectors intended to create or maintain spatially differentiated sediment-cleared zones in regulated channels, thereby contributing to more evidence-based and sustainable river restoration.