Roughness and geometry effects of engineered log jams on 1-D flow characteristics

Graduation date: 2014The re-introduction of large woody debris (LWD) into streams and rivers for stream restoration purposes is rapidly growing. Engineered log jams (ELJs) are man-made structures intended to mimic natural LWD structures, designed and installed to protect stream banks from erosion while increasing habitat diversity. Several studies have evaluated the flow resistance of single cylinder wooden objects; however, limited information is available on complex ELJs. Design guidelines recommend using hydraulic models to evaluate the flooding impact of proposed ELJ designs and one-dimensional (1-D) hydraulic models are often used in the design of ELJs. However, while 1-D models have contributed some new knowledge, their application in the design of ELJs is still underdeveloped. For example, ELJs are often represented in practice as high ground or increased roughness in one-dimensional hydraulic models, but the accuracy and influence of adjusting channel geometry or roughness to represent ELJs has not been evaluated. This study thus evaluates the performance and characterizes the hydraulic impacts of different ELJ representations in a 1-D hydraulic model. The objective of this study is to investigate how representation of ELJs in a 1-D hydraulic model influences a) accuracy of WSE predictions, and b) 1-D flow characteristics of velocity, area, and hydraulic depth. The analysis is conducted for a case study of an ELJ at which channel geometry and hydraulic flow properties were measured and calibrated in a 1-D hydraulic model. We also present a sensitivity analysis of roughness and geometry at high flows. Calibration results demonstrate that geometry has a greater effect on model accuracy than hydraulic roughness for the evaluated ranges. Results of the sensitivity analysis indicate that increasing the roughness associated with an ELJ is simulated as a backwater effect, increasing WSEs upstream of the ELJ. Increasing ELJ roughness also causes a reduced velocity upstream of and at the ELJ. In contrast, the effect of adding ELJ geometry reduces the hydraulic depth and increases the velocity at the ELJ cross section. When combined, the effects of ELJ geometry and roughness combine to lower the WSE and raise velocities at the ELJ and raise WSE and lower velocities upstream of the ELJ, and geometry appears to dominate effects of adding roughness. Designers may choose to represent an ELJ as a modified geometry or increased roughness depending on whether upstream flood risk or localized scour adjacent to the ELJ is of concern