Drift seriously increases the destructive power of a flood event. Drift accumulations and blockages at river bridges are a widespread problem, possibly leading to their total destruction. Although drift is a major threat, limited knowledge is currently available on the likelihood of drift blocking. Drift either accumulates at a single pier, or it spans between two or more piers, or it gets blocked at the bridge deck. The main purpose of this experimental study is to analyze the drift-blocking probability at bridge decks depending on: (1) drift dimensions, (2) freeboard, (3) flow characteristics, and (4) bridge characteristics. Systematic model tests include the accumulation of both single logs and rootstocks. The test flow conditions represent a major flood event, where the freeboard tends to zero and the drift is able to touch the bridge deck. The results indicate significant effects of the freeboard, the approach flow Froude number, and the bridge characteristics on drift accumulation. They allow for an...

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 driftwood and woody debris during floods may lead to accumulations and blockages at river bridges or weirs, and can result in excessive scour or an increase in backwater that may lead to flooding of the nearby areas. Although driftwood-related problems occur predominantly in forested mountainous regions, driftwood may eventually reach densely populated lowland areas. Prevalent retention structures, such as nets or racks, across the river often fail for larger alpine rivers because of the excessive structural loading or the resulting backwater rise that may overtop the flood embankments. Therefore, this paper presents an overview of driftwood risk analysis and retention measures in large alpine rivers. Several methods on how to determine the potential driftwood volume for a given catchment area are summarized. The most reliable data are thereby obtained with a detailed investigation of the catchment characteristics. Observations during past flood events may also add valuable information for future driftwood management. Furthermore, methods on how to determine the likelihood of driftwood blockage at river crossings are presented in order to identify endangered structures prior to a flood. A driftwood transport diagram is introduced that indicates how much driftwood is expected at a certain location along the river and provides the basis for an optimum location for a driftwood retention structure. Finally, a novel driftwood retention structure is presented, where the driftwood is retained in a bypass channel located at the outer river bend. Because of the secondary currents that are induced by the river bend, the bed load remains in the primary channel, whereas the floating driftwood is directed into a bypass section. The bypass retention was tested in flume experiments and successfully implemented in a small-scale case study.

To prevent damages due to transported wooden debris during a flood, the debris is often retained in defined zones upstream of populated areas using a debris rack. Because thorough numerical modeling of the debris accumulation process is still challenging given the randomness of debris accumulation and the interaction between sediment and wooden debris, one main tool to study debris retention structures involves small-scale modeling. The main purpose of this experimental study is to analyze various aspects regarding the modeling of wooden debris and to provide information on reducing the laboratory effort. The focus is on the various factors influencing the accumulation process and the backwater rise and whether or not it is important to model them properly in small scale. The results indicate a significant effect of the approach-flow Froude number on the debris accumulation process, whereas the debris properties have only a minor effect on the resulting backwater rise so that effects of debris mix...

Due to increased flood discharges during the past years, overtopping of dikes and earthen embankments has caused large damages to the surrounding infrastructure and the environment. Despite the numerous dike breaches and the high damage potential, the dike-breach process is still only poorly understood. Therefore, hydraulic model tests were conducted to investigate the main features of the plane dike-breach due to overtopping. The temporal dike-breach progress was optically recorded allowing for a detailed analysis of the sediment and water surfaces. The results indicate a decisive effect of the sediment diameter on the breach process. A systematic variation of the dike dimension, the sediment diameter and the inflow discharge resulted in basic findings relative to the breach discharge and the dike degradation. The governing parameters of this dike-breach process include the dike height, the grain size and the critical flow depth.

Dike overtopping is an important issue in both scientific and practical aspects of sediment embankment erosion. This research is concerned with the validation of a one-dimensional numerical code applied to an unsteady two-phase flow over an initially trapezoidal-shaped sediment dike, including the transition from sub- to supercritical flows and the flow propagation over a steep slope. The approach is based on the shallow-water equations coupled with a sediment erosion code. The numerical results are compared with recent laboratory experiments, resulting in a reasonable prediction of the temporal free surface and bed evolution. The remaining model inaccuracies mainly originate from overflow initiation with extremely small flow depths, streamline curvature effects with the related underestimation of flow velocities close to the sediment surface, and the changing saturation in the dike body not accounted for in the model. Despite these deficiencies, the computational model will be helpful, particularly in flood risk assessment and management.