Physical-scale model designs for engineered log jams in rivers

• We review the basis for physical-scale models of rivers in the presence of engineered log jams.
• We construct fixed- and movable-bed laboratory models of the Big Sioux River, SD, with scaled engineered log jams.
• Results show that the ELJs had a measurable effect on localized flow and caused significant channel adjustment.

Stream restoration and river engineering projects are employing engineered log jams (ELJs) increasingly for stabilization and in-stream improvements. To advance the design of these structures and to evaluate their morphodynamic effects on corridors, the basis for physical-scale models of rivers with ELJs is presented and discussed. The prototype selected is the Big Sioux River, SD, chosen because ELJs will be used to mitigate excessive bank erosion. The underlying theory of physical-scale modeling and all primary and secondary scaling ratios are presented for two boundary conditions, a fixed- and movable-bed, given the experimental constraints of the intended facility. The scaling ratios for the movable-bed model sediment are relaxed, allowing for the use of typical experimental flows, facilities, and materials. Proposed ELJ designs are based on proven field installations, and these structures also are scaled to natural timber dimensions to be used in the prototype. Preliminary results from these physical models show that (1) ELJs greatly decelerate flow near the structure and accelerate flow in the main portion of the channel, yet spatially averaged flow velocity and depth remain unchanged, (2) derived drag coefficients for the ELJs based on force measurements vary from 0.3 to 0.7 depending on the scaling velocity employed, and (3) while significant localized erosion and deposition occurred in the vicinity of the ELJ, these effects extended well downstream of the structure and across the entire channel. Although physical experimentation using wood is not uncommon, the use of physical scaling theory appears to be employed infrequently, which potentially could limit the applicability of the results obtained. It is envisioned that the procedures outlined here would become more widely used in experimental research of rivers and in river restoration design.