Flow, turbulence, and drag associated with engineered log jams in a fixed-bed experimental channel

Abstract

Engineered log jams (ELJs) have become attractive alternatives for river restoration and bank stabilization programs. Yet the effects of ELJs on turbulent flow and the fluid forces acting on the ELJs are not well known, and such information could inform design criteria. In this study, a fixed-bed physical model was constructed to assess the introduction of ELJs along the Big Sioux River, SD. Two ELJ types were examined, referred to as ELJ-1 and ELJ-2. Both types were deflector jams, where ELJ-1 was rectangular and ELJ-2 was triangular, and oriented with one side attached to the channel bank. They were deployed either as single structures or in groups of two or three on the same side of the channel and at different separation distances. Results show that (1) time–mean and turbulent velocities and bed shear stresses were measurably altered near the ELJ, but spatially averaged flow just upstream and downstream of the structure was unaffected; (2) streamwise drag forces measured for the ELJs were significantly larger than the transverse forces, and the derived drag coefficients for the single structures were 2.72±0.19 for ELJ-1 and 1.60±0.37 for ELJ-2; and (3) the presence of an upstream structure created a near-bank wake region that extended a distance of more than 30 flow depths downstream, which greatly reduced drag forces and drag coefficients observed for the downstream structure by as much as 80%. These observations are further evidence of the efficacy of ELJs in providing near-structure scour pool development and bank protection downstream, and they can be used to inform and assess the design of ELJs for use in river restoration and bank stabilization projects.