Flow and Scour around Idealized Porous Engineered Log Jam Structures

Abstract

Engineered log jams (ELJs) are used widely in practice as a river restoration measure. One desired feature of an ELJ is its porous nature, which plays an important role in hydrodynamics and sediment response. Currently there are very limited studies of porous hydraulic structures such as ELJs, and ELJs often are treated as solid obstructions for the purpose of flow and scour predictions. In this study, laboratory experiments were conducted to examine flow and bathymetric response near porous versus nonporous ELJ structures emplaced in an experimental flume. Time-averaged velocity, turbulent fluctuations, and the initial and final bathymetry were measured for four cases of porous and nonporous structures. Alterations in the flow field near the porous structure resulted in reduced maximum scour depths and lower total transport of bed material. It was found that existing predictive equations for scour geometry fail to match experimental data for the porous structures due to the lack of consideration of porosity’s role in altering the flow conditions. Conceptual models of the flow zones and the stages of erosion were presented and discussed. A simple predictive model for scour depth around ELJs was developed.