Impact of Lateral Gap on Flow Distribution, Backwater Rise, and Turbulence Generated by a Logjam

AbstractLogjams may form at natural obstructions and are also used as nature‐based solutions for river restoration and natural flood management. Previous research has described backwater rise due to logjams that span the full channel cross‐section and logjams with a gap between the lower edge of the logjam and the bed. Logjams that fill the channel depth, but not its width, leaving a lateral gap between the logjam and the channel bank, are also common natural formations and the focus of this study. The flow distribution between the logjam and the lateral gap, backwater rise, and wake turbulence are key factors in determining the ecologic and flood risk impact of a logjam. Specifically, relative to a channel‐spanning logjam, the introduction of a lateral gap can reduce backwater rise and increase the potential for trapping particles, such as nutrients or microplastics, within the wake region, but may also promote erosion in the gap. The choice of logjam and gap widths can be used to maximize flow and habitat diversity in rivers, while reducing erosion risk. We present experimental results demonstrating that the flow distribution between the logjam and the lateral gap can be predicted by assuming equal resistance through the logjam and gap sections. Further, we show that backwater rise can be determined from the predicted discharge through the logjam using a momentum balance developed for channel‐spanning logjams. Finally, turbulence generated within the jam was observed directly downstream of the logjam, and, for the densities considered, increased with jam density.</jats:p

Effect of water withdrawal on the appearance and sound level of waterfalls

For rivers or streams with waterfalls, water withdrawal for multiple uses can lead to potential conflicts of interest. The question arises to what extent water withdrawal impairs the appearance and acoustics of waterfalls. A data set was analyzed of 15 waterfalls of different morphological types in Switzerland, Austria, and Norway. It comprises discharge measurements as well as photo imaging and sound level measurements. The variations of waterfall appearance and sound level decreased after reaching a threshold discharge value for all investigated morphological waterfall types. For low discharges, visible and acoustics changes were more significant compared to higher discharges. Acoustics changes were less pronounced than visual changes. The acoustics and visual extent of a water withdrawal can be calculated with a novel assessment equation using a normalized discharge. Based on an impact intensity, which is a function of the extent of the water withdrawal and the non-monetary waterfall value, the selected residual discharge can be evaluated. In addition, a software tool was developed to enable a sensitivity analysis of the required residual discharge to maintain a certain waterfall appearance.ISSN:0043-1397ISSN:1944-797

Wood blockage and sediment transport at inclined bar screens

Semi-permeable sediment traps are designed to retain sediment during large floods and enable sediment transport continuity during ordinary floods by combining two barriers with a guiding channel. During floods, not only sediment but also large wood (LW) can be transported, that may block the inclined bar screen (mechanical barrier) and inhibit sediment transport. The design of the inclined bar screen has not been tested with LW. We conducted a first set of simplified flume experiments to study the effect of (1) initial conditions, (2) bar screen design, and (3) LW volume on sediment transport. First results demonstrate that a LW volume blocking 20% of the flow cross-section is sufficient to reduce sediment transport by 50%. Hence, LW volume is a relevant parameter for the design of inclined bar screens. We provide future research ideas to enable the local separation of LW and sediment.ISSN:0022-1686ISSN:1814-207

AVETH follow-up survey on salary and duties of ETH doctoral students

The present survey was conducted as a follow-up of the AVETH survey in 2014. Based on 1′052 completed answers this report summarizes the current employment situation and the corresponding opinion of doctoral students at ETH Zurich about their salary rate and additional duties

Flow redistribution and backwater rise due to brush accumulation upstream of logjams with a lower gap

&amp;lt;p&amp;gt;Engineered logjams with a gap at the bed are used in engineering practice to provide natural flood management and ecological benefits while preserving river connectivity at base flow. In addition, logjams with a gap at the bed form naturally in small streams with river width less than log length. The accumulation of wood pieces acts as a porous obstruction, and the distribution of flow through and beneath a jam with a lower gap satisfies a two-box, momentum-based model constrained by drag generated in the jam, momentum loss in flow through the lower gap, and net pressure force. Accumulation of brush and fine material upstream of logjams occurs naturally as small wood pieces and leaves are transported to the river channel. However, the impact of accumulated upstream material on logjam-generated increase in backwater rise presents a potential concern for long term maintenance of engineered logjam projects. We present recent results demonstrating that initial accumulation of wood pieces upstream of a jam with a lower gap has little impact on backwater rise, but backwater rise increased during a simulated flood cycle as wood pieces blocked the lower gap. The effect of varying brush size and shape and impact on flow redistribution between the jam and gap is examined.&amp;lt;/p&amp;gt;</jats:p

Field Study on Wood Accumulation at a Bridge Pier

Transported large wood (LW) in rivers may block at river infrastructures such as bridge piers and pose an additional flood hazard. An improved process understanding of LW accumulations at bridge piers is essential for a flood risk assessment. Therefore, we conducted a field study at the River Glatt in Zurich (Switzerland) to analyze the LW accumulation process of single logs at a circular bridge pier and to evaluate the results of previous flume experiments with respect to potential scale effects. The field test demonstrated that the LW accumulation process can be described by an impact, rotation, and separation phase. The LW accumulation was described by combining two simplified equilibria of acting forces and moments, which are mainly a function of the pier diameter, pier roughness, and flow properties. We applied the resulting analytic criterion to the field data and demonstrated that the criterion can explain the behavior of 82% of the logs. In general, the field observations confirmed previous results on the LW accumulation probability in the laboratory, which supports the applicability of laboratory studies to investigate LW–structure interactions.ISSN:2073-444