Hydrodynamic and morphologic processes associated with wood placements:
Numerous rivers have been confined and are eco-morphologically impaired, resulting in an increased demand for river restoration projects. Wood placements are a common and inexpensive measure for river restoration. To plan and evaluate river restoration projects including wood, it is important to understand the interactions between flow, sediment, and wood. Using physical modeling, this project aims to quantify flow and morphological structures associated with different wood placements.
This project was funded by the Swiss National Science Foundation and conducted at MIT with Heidi Nepf with Ellen Wohl from CSU as an external advisor.
Results:
Schalko I. , Wohl W., Nepf H. (2021). Flow and wake characteristics associated with large wood to inform river restoration. Scientific Reports
Interaction of wood and sediment at inclined racks:
Large wood (LW) transport can highly increase during floods, leading to accumulations at river infrastructures. To mitigate the potential flood hazard, racks are a common method to retain LW upstream of endangered settlements or infrastructures. The majority of LW retention racks consists of vertical bars and, therefore, disrupt sediment transport. It can be hypothesized that inclined racks reduce backwater rise and sediment disruption, as wood will block the upper part of the rack, thereby increasing the open flow cross-section below the accumulation. Flume experiments were conducted under clear water conditions to analyze backwater rise and local scour as a function of (1) rack inclination, (2) hydraulic inflow condition, (3) uniform bed material, and (4) LW volume. In addition, first experiments were performed under sediment transport conditions to study the effect of sediment transport capacity on local scour at the rack and its subsequent effect on backwater rise.
This project was conducted at the Laboratory of Hydraulics, Hydrology and Glaciology at ETH Zurich.
Risk reduction measures of large wood accumulations at bridges:
Bridges with and without piers are prone to large wood (LW) accumulations during floods, possibly resulting in an upstream backwater rise, local scour, or destabilization of the structure. To reduce the flood hazard, measures are required that decrease the accumulation probability p of LW. This paper presents a literature review on existing measures to reduce p at bridges. In addition, a series of flume experiments was conducted to examine structural measures at bridge piers regarding their accumulation risk reduction effect. The objective was to test the efficiency of (1) LW fins and (2) bottom sills including various configurations. The resulting p was then compared to the setup without measures. The tested configurations of a LW fin did not decrease p. Bottom sills, in contrast, are a promising measure to reduce p for a defined range of boundary conditions. The installed sills lead to enhanced turbulence and increased surface waves. The best results to reduce p were obtained with two consecutive sills, leading to an average reduction of p by 30%. In contrast to retaining LW with retention structures, LW can be safely guided downstream, thereby preserving its relevant ecological role in rivers. However, the efficiency of bottom sills strongly depends on the approach flow and the sediment transport conditions.
This project was conducted at the Laboratory of Hydraulics, Hydrology and Glaciology at ETH Zurich in the frame of my doctorate.
Laboratory study on wood accumulation probability at bridge piers:
River infrastructures like bridges are prone to accumulations of transported large wood (LW) during floods. To contribute to an improved risk evaluation, the prediction of LW accumulation probability (AP) is crucial. Previous studies on LW AP focused mainly on the influence of a bridge deck. In the present study, flume experiments were conducted to investigate LW AP at bridge piers with special emphasis on (1) approach flow conditions, (2) bridge pier characteristics with different pier roughness, shape, diameter, and pier number, (3) LW characteristics, involving various log lengths, log diameter, log density, LW with and without branches, and uncongested versus (semi-) congested LW transport, and (4) channel bed, i.e. scour. Based on the experiments, AP is mainly a function of approach flow velocity and log length. The results were combined in a design equation to predict AP for risk assessment in engineering application.
This project was conducted at the Laboratory of Hydraulics, Hydrology and Glaciology at ETH Zurich in the frame of my doctorate.
Backwater rise and local scour due to wood accumulation:
Transported large wood (LW) in rivers may lead to accumulations at natural or artificial obstructions. The hydraulic and geomorphic conditions change due to these accumulations. Backwater rise as well as scour can evolve in the vicinity of such an accumulation. This study was performed for 3 experimental setups, in 2 different flumes and with 3 model scale factors, resulting in ~900 model tests. This project was conducted at the Laboratory of Hydraulics, Hydrology and Glaciology at ETH Zurich in the frame of my doctorate.
Predefined accumulation – backwater rise: A specific wood accumulation was placed between two rack rows to study the resulting backwater rise. Based on the results, backwater rise mainly depends on the approach flow Froude number, compactness of LW accumulations, and percentage of organic fine material (branches and leaves). The findings of this study resulted in a design equation to calculate backwater rise due to LW accumulations, contributing to the estimation of the hazard potential assessment of river infrastructures for flood events.
Natural accumulation – backwater rise: The previous setup was adapted to study a natural wood accumulation by removing one rack row and adding wood continuously to the flow upstream of the rack. The experiments were performed with a solid and movable bed. The results were published in two companion papers in Water Resources Research. In the first companion paper, results of hydraulic model tests are presented on backwater rise due to spanwise LW accumulations in combination with a movable bed. The findings are summarized in design equations and allow the estimation of (1) characteristic LW volume generating the primary backwater rise, (2) effect of LW accumulation shape and bed material on resulting backwater rise, and (3) effect of LW volume on backwater rise. Compared to a fixed bed, movable bed reduces backwater rise as the open cross‐section area and thus discharge capacity increase. This work improves the understanding and predictability of the formation and impact of spanwise LW accumulations at natural or artificial obstructions (e.g., LW retention racks).
Natural accumulation – local scour: In the second companion paper, hydraulic model tests were conducted to analyze local scour due to natural spanwise large wood (LW) accumulations. Spanwise accumulations were modeled using a vertical barrier, similar to a LW retention rack in prototype. The flume experiments were conducted according to Froude similitude in a scale of 1:30 for various approach flow conditions (subcritical and supercritical flow) and different uniform bed material (2.7–13.1‐mm model dimensions). The findings allow the estimation of local scour depth due to spanwise LW accumulations as a function of unit discharge, sediment diameter, and wood volume. Higher unit discharge, finer bed material, and increasing wood volume lead to an increased scour depth. The scour length can be estimated based on the scour depth and a geometrical scaling factor. The longitudinal shape of the cross‐sectional scour depth can be described with a Gaussian normal distribution. Based on the results of both scour depth and length, the design of LW retention structures can be significantly improved. At the same time, the results demonstrate that LW accumulations strongly affect the geomorphic conditions and may consequently create more heterogeneous morphological structures.
Isabella Schalko 