EFFECTS OF SPACING BETWEEN ENGINEERED LOGS JAMS ON FLOW, SCOUR, AND DEPOSITIONAL PATTERNS

Large Woody Debris (LWD), a term that refers to natural accumulations of tree boles, roots, and branches, assembles in natural streams and rivers and affects the bed morphology. The presence of LWD contributes to the development of scour pools, bars, and floodplains; as such, these fluvial formations provide structures necessary to maintain healthy riverine ecosystems. Recently, many stream restoration projects have used structures that are similar to LWD, called engineered log jams (ELJ) similar to LWD to stabilize banks and create ecological habitats. Many engineering firms have installed ELJ’s into streams; however, in many cases, trial and error is the dominating method of stream restoration (Shields et al 2004). There are few details known of the complex interplay between the components of ELJ structures, fluid dynamics, and the evolution of bed morphology. In this presentation, we focus on one particular aspect of an ELJ design, the spacing between individual log components on the erosion and deposition of the surrounding bed. Normally, when a large log is placed on to a stream or river bed, a scour pool develops upstream of the structure while a bar is deposited downstream of the structure. In this research, a flume scaled experiment using sand sized sediments demonstrated that the size of the scour pools and the bars were dependent on the spacing between two parallel rooted logs. With increased distance between two logs, the maximum scour pool depth of the structure exponentially decayed to that of the control test (a single rooted log). At the same time, the pool area and eroded volume created by the structure exponentially decayed to double the corresponding measures of scour in the control test. The values converged after the spacing between the logs was twice the diameter of the root wad. However, the exponential trends were not observed in the bar deposition results, possibly due to the finite width of the flume. Last, zones of recirculation and flow constriction were observed in particle image velocimetry (PIV) measurements of the surface velocities, which we associate with the increased scour per log when the logs were in close proximity. We conclude with a discussion that similar tests can provide quantitative data and relationships that can be adapted to improve ELJ designs and other structures containing LWD.