Lukas Schmocker

Probability of Drift Blockage at Bridge Decks

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 estimation of the blocking probability and therefore can be used as a risk assessment tool to identify endangered bridges prior to a flood event. The model tests demonstrate further the randomness of the blocking process, resulting occasionally in a wide scatter of data.

1D numerical evaluation of dike erosion due to overtopping

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.

Driftwood: Risk Analysis and Engineering Measures

AbstractTransported 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...

Plane dike-breach due to overtopping: effects of sediment, dike height and discharge

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.

Effect of Weir Face Angles on Circular-Crested Weir Flow

The standard circular-crested weir is often found in engineering applications and is used as a discharge measurement device or as an overflow structure. This research determines the discharge coefficients for ten circular-crested weir configurations with various combinations of up- and downstream angles. Two different weir heights and four different overflow depths are considered for each weir shape. For free overflow, the discharge coefficient is determined experimentally by using the total head of the approach flow. The results indicate that the upstream weir face angle has only a small effect on the discharge coefficient. In contrast, increasing the downstream weir face angle increases the discharge coefficient notably. A new formula for the discharge coefficient is presented, including both the up- and downstream weir face angles. Further, the hydraulic performance of the circular-crested weir, the resulting discharge reduction from tailwater submergence, and transition flow are discussed.

Scale Modeling of Wooden Debris Accumulation at a Debris Rack

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 mixture are negligible.

Overtopping dike-breach: effect of grain size distribution

ABSTRACT A series of plane dike overtopping laboratory tests is reported, in which the effect of sediment mixture on the breach process is investigated. Three different sediment mixtures were tested, in addition to the reference dike consisting of uniform non-cohesive sediment. No sealing elements or surface protection were applied. The adopted constant reservoir inflow led to a falling reservoir scenario. The experiments demonstrate that the grain size distribution has a small effect on the overall breach process within the given test range, as the hydraulic conditions during the dike breach are much above the sediment entrainment condition. No increased erosion resistance was observed for the sediment mixtures. Therefore, the mean sediment diameter adequately describes the non-cohesive material characteristics and general dike-breach features in laboratory scale can be investigated using uniform material. This finding is significant both for laboratory experimentation and for prototype dike construction.

Rethinking Pumped Storage Hydropower in the European Alps

The European Alps are well positioned to contribute significantly to the energy transition. In addition to sites with above-average potential for wind and solar power, the “water towers” of Europe provide flexible, low-carbon power generation as well as energy storage. In the future, hydropower systems are expected to become more than mere electricity generators, serving a key role as flexible complements to intermittent power generators and as providers of large-scale seasonal and daily energy storage. Energy transition on national and European scales can be facilitated by expanding the capacity of pumped storage hydropower (PSHP) plants. Yet the extension of hydropower production, in particular PSHP, remains controversial, primarily due to environmental concerns. Focusing on 2 Alpine countries, Austria and Switzerland, this paper provides a system view of hydropower production and energy storage in the Alps. It discusses advantages and drawbacks of various assessment tools and identifies gaps and needs for the integrated assessment of PSHP plants. It concludes that instruments that evaluate the impacts and sustainability of PSHP projects need to be developed, elaborated, and applied in a participatory manner, in order to promote public dialogue, increase social acceptance, and, ideally, encourage energy consumers to become advocates of a sustainable energy future.

Application of a Videometric Measurement System to Investigate Spatial Dike Breach

Breach of a dike can lead to extensive flooding of nearby areas along with both monetary and human losses. Especially, dike breaches due to overtopping have occurred more frequently during the last years, as most dikes suffer from the lack of maintenance and revision. An accurate prediction of the dike failure process is therefore essential for developing effective emergency action plans or establishing adequate safety measures. Of particular interest is information on the breach profiles during a breach to estimate the outflowing discharge. Spatial dike breach tests due to overtopping were conducted at a laboratory channel of the Laboratory of Hydraulics, Hydrology and Glaciology (VAW), ETH Zurich. To record the temporal dike breach profiles, a three-dimensional (3D)-videometric recording system was applied, developed by AICON 3D Systems Ltd. This system allows for a continuous, non-intrusive recording of the sediment surface through the water surface during the dike breach process. The system is presented herein and first results regarding the experimental setup are discussed.

Measuring suspended sediments in periglacial reservoirs using water samples, laser in-situ scattering and transmissometry and acoustic Doppler current profiler

ABSTRACT Climate change will impact the water and sediment conveyance into periglacial reservoirs. It is therefore important to understand and forecast future reservoir sedimentation processes with regard to climate change. In the present project, particle size distribution (PSD) and suspended sediment concentrations (SSC) were measured in three reservoirs in the Swiss Alps whose catchment areas are covered by glaciers by at least 40%. The threefold combination of water sample analysis, laser in-situ scattering and transmissometry (LISST) and acoustic Doppler current profiler (ADCP) was applied and the results were compared to each other. The combination of the three measuring techniques was proven suitable for assessing PSD and SSC in periglacial reservoirs. Water sample analysis and LISST records showed that most of the suspended sediments in the reservoir are in the range of clay and silt. SSC was relatively low in the order of 100 mg/l. An increase in both PSD (e.g. median diameter d50) and SSC with increasing reservoir depth could be observed in deep reservoirs. Flow velocities and Signal-to-Noise ratios (SNR) were measured with ADCP. SNR values allowed to study the mixing of inflowing river water and the evolution and decay of turbidity currents. There is evidence of dominant homopycnal flows, whereas stratified flow was restricted to the regions close to the inflow. Flocculation, influence of mica, and organic content could be neglected. This paper presents detailed information about PSD and SSC gained with water sample analysis and LISST measurements. Furthermore, flow field measurements and tracking of mixing by means of ADCP will be illustrated. Finally, application experiences and limitations will be discussed.