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Follett et al. (2020a, https://doi.org/10.1029/2020gl089346) developed an analytical model to predict backwater rise by log jams, using the size and packing density of logs and the jam length, as well as river slope and bed roughness. We show that the model formulas can be rewritten using the Froude number instead of river slope and roughness, thus improving their applicability in engineering practice. The equation terms and results of Follett et al. (2020a, https://doi.org/10.1029/2020gl089346) are found to be similar to those of the empirically derived formula by Schalko et al. (2018, https://doi.org/10.1061/(asce)hy.1943‐7900.0001501). However, some differences are identified, calling for further study. Most notably, these distinctions pertain to the effect of accumulation porosity, with additional minor differences in the exponent of the Froude number. Lastly, model implications for some broader applications are explored, showing a methodology to calculate the representative log size for log mixtures, and the expected effect of log orientation on backwater rise. Plain Language Summary Accumulations of wood in rivers (log jams) can block the flow and thereby cause water level rise. Follett et al. (2020a, https://doi.org/10.1029/2020gl089346) developed a theoretical model to predict how this water level rise depends on log jam properties and local river conditions. For the local river conditions, they used the river slope and bottom roughness. In this comment, we show that the Froude number can be used instead, with exactly the same result. The Froude number is a dimensionless number that depends directly on the local river conditions, making the adapted formula easier to apply in practice. The resulting formula shows good agreement with an earlier one based on experimental work by Schalko et al. (2018, https://doi.org/10.1061/(asce)hy.1943‐7900.0001501). Still, some differences were found that raise questions. Most notably, the formulas differ for the effect of accumulation porosity. This becomes especially clear when logs are packed closely together. Next, model implications for slightly different settings than those studied by Follett et al. (2020a, https://doi.org/10.1029/2020gl089346) were explored. This showed how to determine the average log size for a mixture of logs with different sizes, and how the expected water level rise changes with log orientation. Key Points Follett et al. (2020a, https://doi.org/10.1029/2020gl089346) predicted backwater rise by log jams using river slope and roughness. We show the Froude number can be used instead By using the Froude number, the link to the local river conditions becomes stronger, improving formula applicability in engineering practice The resulting formula is shown to be similar to earlier empirical work. But differences in jam porosity effects call for further study

Large wood is an important component of headwater streams in forested mountain regions and currently considered an important composition of debris flow due to the fact that the most serious disasters are associated with it. The interaction between large wood and debris flow processes can increase the disaster risk by creating an upstream backwater at slit-check dams, decreasing transport capacity, and increasing disaster risk. To better evaluate large wood debris flow hazards in disaster prevention and mitigation, the key influencing factors of backwater rise due to large wood accumulations at slit-check dams were investigated through a series of physical model experiments. We first summarized the characteristics of large wood accumulation and the backwater rise process, and then a characteristic wood volume was cited and extended to debris flow as the volume generating the primary backwater rise. It was additionally observed that sediment concentration and Froude number can affect the characteristics of large wood accumulation and the process of backwater rise. The relative backwater rise increases with the rising sediment concentration and initial Froude number, and the maximum relative backwater rise is 2.15 when sediment concentration is 0.55 and initial Froude number is 2.50. Finally, a revised equation was proposed for estimating the relative backwater rise that accounts for initial condition and large wood characteristic. This study may provide a scientific basis for better understanding and evaluating debris flow disasters containing large wood and the efficient design of the structural height of the slit-check dam to a certain extent.

Fallen trees enter the adjacent stream and are carried away downstream by the current. As the stream joins another one, the complex hydrodynamics near their confluence make the movement of wood hard to predict. These woods may accumulate near the confluence resulting in backwater and subsequent potential flooding. A laboratory study was conducted to investigate the movement and accumulation behavior of individual pieces of wood near the confluence. The characteristics of wood (i.e., the length, diameter, and density) and the hydraulic conditions (i.e., the discharge ratio and the release distance) were varied in this investigation. It was found that the wooden pieces released from the tributary got occasionally trapped in the flow separation zone of the confluence, whereupon they were mainly trapped by a clockwise vortex and continued to stay driven by a reverse cluster of currents within this zone. The accumulation probability of wood was mainly related to its length, the discharge ratio and the release distance. The effect of wood diameter and density within the tested parameters was negligible. The probability increased with an increase in the discharge ratio as well as a decrease in the release distance. The longer pieces had a higher probability of being trapped, whereas for those exceeding some critical value, the probability was nearly the same, or dropped sharply. A generalized model for wood accumulation near the confluence was developed for practical application. These findings carry significant implications for river management, particularly in preventing the risk of flooding caused by wood blockage. Key Points Conducting a laboratory study to investigate the transport, accumulation and trapping mechanism of wood near the confluence Evaluating the wood accumulation probability depending on different wood characteristics and confluence hydrodynamic conditions Wood released from the tributary may be trapped by the clockwise vortex and thus accumulate in the separation zone

Accumulation of instream large wood (i.e., fallen trees, trunks, branches, and roots) at bridges during floods may exacerbate flooding, scour and cause structural failure. Yet, explaining and predicting the likelihood of a bridge trapping wood remains challenging. Quantitative data regarding wood accumulation at bridges are scarce, and most equations proposed to estimate the accumulation probability were derived from laboratory experiments, and include variables such as flow velocity, Froude number, and approaching wood volume or size which are difficult to obtain. Other evaluations based on technical reports and information regarding wood removal have been proposed but are mostly qualitative. Until now, a data-driven approach combining multiple quantitative accessible variables at the river reach and catchment scales remains lacking. As a result, the controlling parameters explaining whether a bridge is prone to trap wood are still unclear. This work aims to fill this gap by analysing a database of 49 bridges across the United Kingdom (UK) classified as prone and not prone to wood accumulation. The database contained information regarding the geometry of the bridge (i.e., number of piers and pier shape) and we added parameters describing the upstream river channel morphology, the riparian landcover, and high-flow characteristics. We applied multivariate statistics and a machine learning approach to identify the variables that explained and predicted the predisposition of bridges to wood accumulation. Results showed that the number of bridge piers, the unit stream power, the pier shape, and the riparian forested area explained 87% of the total variability for the training dataset (0.87 training accuracy), and the selected model had a testing accuracy of 0.60 (60%). Although limited by the sample size, this study sheds light on the identification of bridges prone to wood accumulation and can inform bridge design and management to mitigate wood-related hazards.

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.

Many cities in the world experience significant air pollution from residential wood combustion. Such an advection–diffusion problem as applied to geographically distributed small-scale pollution sources presently does not have a satisfactory theoretical or modeling solution. For example, statistical models do not allow for pollution accumulation in local stagnation zones – a type of phenomena that is commonly observed over complex terrain. This study applies a Parallelized Atmospheric Large-eddy simulation Model (PALM) to investigate dynamical phenomena that control variability and pathways of the atmospheric pollution emitted by wood-burning household stoves. The model PALM runs at spatial resolution of 10 m in an urban-sized modeling domain of 29 km by 35 km with a real spatial distribution of the pollution source and with realistic surface boundary conditions that characterize a medium-sized urban area fragmented by water bodies and hills. Such complex geography is expected to favor local air quality hazards, which makes this study of general interest. The case study here is based on winter conditions in Bergen, Norway. We investigate the turbulent diffusion of a passive scalar associated with small-sized particles (PM2.5) emitted by household stoves. The study considers air pollution effects that could be observed under different policy scenarios of stove replacement; modern woodstoves emit significantly less PM2.5 than the older ones, but replacement of stoves is a costly and challenging process. We found significant accumulation of near-surface pollution in the local stagnation zones. The simulated concentrations were larger than the concentrations obtained only due to the local PM2.5 emission, thus indicating dominant transboundary contribution of pollutants for other districts. We demonstrate how the source of critical pollution can be attributed through model disaggregation of emission from specific districts. The study reveals a decisive role of local air circulations over complex terrain that makes high-resolution modeling indispensable for adequate management of the urban air quality. This modeling study has important policy-related implications. Uneven spatial distribution of the pollutants suggests prioritizing certain limited urban districts in policy scenarios. We show that focused efforts towards stove replacement in specific areas may have a dominant positive effect on the air quality in the whole municipality. The case study identifies urban districts where limited incentives would result in the strongest reduction of the population's exposure to PM2.5.

Large wood (LW) in debris flows during flooding can cause significant damage to check dams, bridges, and other major infrastructure in its flow path. When LW accumulates and forms logjams, large volumes of sediment can be deposited, resulting in increased backwater. The logjams can then suddenly collapse, with the deposited sediment and accumulated water increasing the magnitude of debris-flow peak discharge. To better understand the characteristics of debris flow discharge caused by LW accumulation and collapsed logjams, as well as the associated magnitude amplification effects, a series of experiments was carried out in a laboratory experimental channel. The main purpose of this study was to analyze the process of LW accumulation and collapse and to determine the magnitude amplification ratio of the debris flow peak discharge with LW. The results revealed three types of debris flow movement with LW: flow with no clogging or breakage, flow with clogging but without breakage, and flow with both clogging and breakage. The results indicated a significant effect on the magnitude amplification ratio caused by LW clogging and breakage. The maximum magnitude amplification ratio was 1.60 in the experiments, with a relative LW length and volume of 0.875 and 0.75, respectively, during the clogging and breakage process.

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.

The monitoring of acoustic emission (AE) has allowed tracing of the damage in wooden cultural objects exposed to variations in ambient relative humidity (RH). A year-long on-site AE monitoring of the Song Dynasty shipwreck confirmed the usefulness of the technique in tracing climate-induced damage in wood. New coupling material is tested to make it conform to the conservation rules which is non-corrosive to monitoring objects and a reversible operation. As sensitive parameter of wood damage caused by variations RH, the accumulated ringing counting tends to increase with the increase of daily fluctuation of RH (DFRH). In addition, the damage of wooden cultural objects during shrinkage is stronger than that during swelling. The relationship between the probability of AE activity and the daily DFRH is established and it is determined that the daily variation of RH for long-term protection of the Song Dynasty shipwreck should be controlled within 4%, and an early warning will be given if it exceeds 10%.