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

Authors: BENNETT, SEAN - University Of Buffalo; GHANEEIZAD, S - University Of Buffalo; GALLISDORFER, MICHAEL - University Of Buffalo; CAI, DONGHUA - University Of Buffalo; ATKINSON, JOSEPH - University Of Buffalo; SIMON, ANDREW - Cardno Entrix; Langendoen, Eddy

Submitted to: Geomorphology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/21/2015
Publication Date: 11/1/2015
Publication URL: http://handle.nal.usda.gov/10113/63326
Citation: Bennett, S.J., Ghaneeizad, S.M., Gallisdorfer, M.S., Cai, D., Atkinson, J.F., Simon, A., Langendoen, E.J. 2015. Flow, turbulence, and drag associated with engineered log jams in a fixed-bed experimental channel. Geomorphology. 248:172–184.

Interpretive Summary: The design of stable stream bank protection measures have generally relied on materials such as rock and concrete. While such measures ensure protecting adjacent floodplain uses, they typically do not improve in-stream, aquatic habitat. Therefore, stream restoration and river engineering projects are increasingly employing engineered log jams to provide both bank protection and new habitat. However, design guidelines that characterize the forces acting on the whole engineered log jam as well as its individual members are lacking. Scientists of the USDA, ARS, Watershed Physical Processes Research Unit (Oxford, MS) in collaboration with researchers of the University of Buffalo and professionals from industry (Cardno, Inc), have conducted laboratory experiments to assess the introduction of engineered log jams along the Big Sioux River, SD. The experimental results provided further insight into the design criteria necessary for the effective deployment of engineered log jams in rivers. First, engineered log jams provided significant bank protection via flow deceleration. Second, the measured drag coefficients can be used to quantify the balance of forces acting on engineered log jams in field conditions as well as aid in the assessment of engineered log jams in river corridors using numerical models. Finally, the experimental results can be used to inform and assess the design of engineered log jams for use in river restoration and bank stabilization projects.

Technical 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.