During flood events, transported large wood (LW) may accumulate at river infrastructures or is retained intentionally at retention structures. In both cases, the accumulation results in backwater rise upstream of the clogged cross section. Previous studies determined various governing parameters for the estimation of backwater rise, but the findings are still contradictory. In addition, the effect of organic fine material (e.g., branches or leaves) was so far neglected. Therefore, a series of flume experiments was conducted to clarify the influence of the governing parameters on backwater rise due to predefined LW accumulations. The hydraulic conditions, LW accumulation characteristics (compactness, accumulation length, LW diameter), and the organic fine material were varied systematically. Based on the results, backwater rise mainly depends on the approach flow Froude number, compactness of LW accumulations, and percentage of organic fine material. 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. Furthermore, the present study contributes to an enhanced process understanding of LW accumulations in rivers.

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. In this 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).

In this 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.

Wood reintroduction is now considered an important aspect of stream restoration, due to ecohydraulic benefits associated with wood presence. Channel‐spanning wood jams create an upstream backwater, increasing flow heterogeneity, sediment deposition, and ecological productivity, but also flood risk. Backwater rise prediction is necessary to evaluate flood hazards in hydraulic models, improve design of engineered logjam projects, and compare jam effects across river systems. We present experimental results demonstrating that a jam can be modeled as a porous obstruction generating momentum loss proportional to the number, size, and packing density of the logs and the jam length. Energy and momentumconstraints are combined to predict backwater rise from unit discharge and a dimensionless structural parameter. This novel approach allows description of preexisting jams with a common metric. The model was used to demonstrate how backwater length, pool size, and upstream sediment deposition depend on jam structure and channel slope.

Wood is an integral part of a river ecosystem and the number of restoration projects using log placements is increasing. Physical model tests were used to explore how the wood position and submergence level (discharge) affect wake structure, and hence the resulting habitat. We observed a von-Kármán vortex street (VS) for emergent logs placed at the channel center, while no VS formed for submerged logs, because the flow entering the wake from above the log (sweeping flow) inhibited VS formation. As a result, emergent logs placed at the channel center resulted in ten times higher turbulent kinetic energy compared to submerged logs. In addition, both spatial variation in time-mean velocity and turbulence level increased with increasing log length and decreasing submergence level. Submerged logs and logs placed at the channel side created a greater velocity deficit and a longer recirculation zone, both of which can increase the residence time in the wake and deposition of organic matter and nutrients. The results demonstrate that variation in log size and degree of submergence can be used as a tool to vary habitat suitability for different fish preferences. To maximize habitat diversity in rivers, we suggest a diverse large wood placement.

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.