Wood Retention at Inclined Bar Screens: Effect of Wood Characteristics on Backwater Rise and Bedload Transport

In forested mountain catchment areas, both bedload and large wood (LW) can be transported during ordinary flows. Retention structures such as sediment traps or racks are built to mitigate potential hazards downstream. Up to now, the design of these retention structures focuses on either LW or bedload. In addition, the majority of LW retention racks tend to retain both LW and bedload, while bedload transport continuity during ordinary flows is an important aspect to be considered in the design. Therefore, a series of flume experiments was conducted to study the effect of LW accumulations at an inclined bar screen with a bottom clearance on backwater rise and bedload transport. The main focus was put on testing different LW characteristics such as LW size, density, fine material, and shape (branches and rootwads), as well as a sequenced flood. The results demonstrated that a few logs (wood volume of ≈ 7 m3 prototype scale with a model scale factor of 30) are sufficient to reduce the bedload transport capacity to below 75% compared to the condition without LW. Fine material and smaller wood sizes further reduced bedload transport and increased backwater rise. In contrast, LW density and LW shape had a negligible effect. The test focusing on a sequenced flood highlighted the need for maintenance measures to avoid self-flushing of the bed material. The results of this study further indicate that an inclined bar screen may need to be adapted by considering LW characteristics in the design of the bottom clearance to enable bedload continuity during ordinary flows.

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Hazards due to large wood accumulations: Local scour and backwater rise

E3S Web of Conferences ◽

10.1051/e3sconf/20184002003 ◽

2018 ◽

Vol 40 ◽

pp. 02003 ◽

Cited By ~ 2

Author(s):

Isabella Schalko ◽

Lukas Schmocker ◽

Volker Weitbrecht ◽

Robert M. Boes

Keyword(s):

Shear Stress ◽

Flood Hazard ◽

Local Scour ◽

Bridge Piers ◽

Bed Shear Stress ◽

Large Wood ◽

Flume Experiments ◽

Governing Parameter ◽

Bed Material ◽

Movable Bed

Large wood (LW) in rivers increases the flow variability and provides habitats for various species. During flood events, transported logs can accumulate at river infrastructures and increase the flood hazard. LW accumulations result in an upstream backwater rise and may increase local scour, for instance at bridge piers. Consequently, estimates of the resulting backwater rise and local scour are necessary to improve the flood hazard assessment. This study presents the findings of flume experiments with a movable bed on local scour and backwater rise due to LW accumulations. The approach flow conditions and the bed material were varied systematically for a specific LW accumulation volume. For all experiments, the initial condition for the bed material was defined as weak transport, since the bed shear stress was slightly below the critical bed shear stress for incipient motion. The inflow Froude number was identified as the governing parameter for backwater rise due to LW accumulations. The present study confirms the hypothesis that the resulting local scour reduces backwater rise. For the local scour, the unit discharge and the grain size diameter are the decisive parameters.

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Flume Experiments Evaluating the Efficacy of a Large Wood Trap Featuring Horizontal Rods

Water ◽

10.3390/w13131837 ◽

2021 ◽

Vol 13 (13) ◽

pp. 1837

Author(s):

Mayuko Furukawa ◽

Daizo Tsutsumi ◽

Hironori Muto ◽

Taro Uchida ◽

Takuro Suzuki ◽

...

Keyword(s):

Relative Length ◽

Large Wood ◽

Basic Information ◽

Trapping Rate ◽

Supply Rate ◽

Flume Experiments ◽

Water And Sediment ◽

Constant Rate

Large wood (LW) disasters, which often accompany sediment-related disasters, occur worldwide. To prevent and mitigate such disasters, we developed a unique LW trap featuring horizontal rods aligned with the flow. When LW enters the trap, it is scooped up by the rods and thus separated from water and sediment. We explored trapping efficacy using a flume of slope 0.087. Water circulated at a constant rate of 1.8 L/s, LW was added to the flow, and the trapping rates were measured. We focused on the relative wood length (Lw) with respect to the horizontal rod spacing (Sr), the number of LW units supplied, and the supply rate. A longer relative length (Lw/Sr) of LW was associated with a higher trapping rate. The trapping rate was also high when the LW number or supply rate was high. The critical Lw/Sr value was 1.5; below this value, LW was not trapped. This study yields the basic information needed to design traps featuring horizontal rods to mitigate LW-related disasters.

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Bedforms, bed material, and bedload transport in a salt-wedge estuary: Fraser River, British Columbia

Canadian Journal of Earth Sciences ◽

10.1139/e89-122 ◽

1989 ◽

Vol 26 (7) ◽

pp. 1440-1452 ◽

Cited By ~ 49

Author(s):

R. A. Kostaschuk ◽

M. A. Church ◽

J. L. Luternauer

Keyword(s):

British Columbia ◽

River Discharge ◽

Bedload Transport ◽

Fraser River ◽

Seasonal Fluctuations ◽

Salt Wedge ◽

Migration Rates ◽

Material Load ◽

Bed Material

The lower main channel of the Fraser River, British Columbia, is a sand-bed, salt-wedge estuary in which variations in velocity, discharge, and bedform characteristics are contolled by river discharge and the tides. Bed-material composition remains consistent over the discharge season and in the long term. Changes in bedform height and length follow but lag behind seasonal fluctuations in river discharge. Migration rates of bedforms respond more directly to river discharge and tidal fall than do height and length. Bedform characteristics were utilized to estimate bedload transport in the estuary, and a strong, direct, but very sensitive relationship was found between bed load and river discharge. Annual bedload transport in the estuary is estimated to be of the order of 0.35 Mt in 1986. Bedload transport in the estuary appears to be higher than in reaches upstream, possibly because of an increase in sediment movement along the bed to compensate for a reduction in suspended bed-material load produced by tidal slack water and the salt wedge.

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Controlled experiments and finite element simulations with the Swiss plate geophone bedload monitoring system: particle size identification and transport mode

10.5194/egusphere-egu21-2078 ◽

2021 ◽

Author(s):

Zheng Chen ◽

Siming He ◽

Tobias Nicollier ◽

Lorenz Ammann ◽

Alexandre Badoux ◽

...

Keyword(s):

Particle Size ◽

Grain Size ◽

Finite Element ◽

Quantitative Agreement ◽

Bedload Transport ◽

Material Structure ◽

Flume Experiments ◽

Fem Simulations ◽

Transport Mode ◽

Signal Response

The Swiss plate geophone (SPG) system is an indirect bedload transport monitoring device that records the acoustic signals generated by bedload particle impacts, with the goal to derive the bedload flux and grain size distribution. Particle drop experiments with quartz spheres in quiescent water in a flume setting were performed to investigate the dynamic signal response of the SPG system impacted by particle-like objects varying in size and impact location. Systematic flume experiments with natural bedload particles in flowing water were conducted to study the effects of impact angle and transport mode (saltating, rolling and sliding) on the SPG signals. For each impact caused by a single particle, the number of signal impulses, the amplitude, the positive area surrounded by the signal envelope, and the centroid frequency were extracted from the raw geophone monitoring data. The finite element method (FEM) was used to construct a virtual model of the SPG system and to determine the propagation characteristics of the numerical stress wave in the material structure. The experimental and numerical results showed a qualitative and partially quantitative agreement in the changes of the signal impulses, the amplitude, and the envelope area with increasing colliding sphere size. The centroid frequencies of the SPG vibrations showed qualitatively similar dependencies with increasing particle size as some field measurements for the coarser part of the investigated range of impact sizes. The effects of variable particle impact velocities and impact locations on the geophone plate were also investigated by drop experiments and compared to FEM simulations. In addition, the signal response for different bedload transport modes and varying impact angles were explored. In summary, the FEM simulations contribute to the understanding of the signal response of the SPG system and the findings in this study may eventually result in improving the bedload grain size classification and transport mode recognition.

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The Development of a 1-D Integrated Hydro-Mechanical Model Based on Flume Tests to Unravel Different Hydrological Triggering Processes of Debris Flows

Water ◽

10.3390/w10070950 ◽

2018 ◽

Vol 10 (7) ◽

pp. 950 ◽

Cited By ~ 5

Author(s):

Theo van Asch ◽

Bin Yu ◽

Wei Hu

Keyword(s):

Debris Flow ◽

Mechanical Model ◽

Debris Flows ◽

Overland Flow ◽

Slope Angle ◽

Great Influence ◽

Flume Experiments ◽

Triggering Mechanisms ◽

Bed Material ◽

Triggering Process

Many studies which try to analyze conditions for debris flow development ignore the type of initiation. Therefore, this paper deals with the following questions: What type of hydro-mechanical triggering mechanisms for debris flows can we distinguish in upstream channels of debris flow prone gullies? Which are the main parameters controlling the type and temporal sequence of these triggering processes, and what is their influence on the meteorological thresholds for debris flow initiation? A series of laboratory experiments were carried out in a flume 8 m long and with a width of 0.3 m to detect the conditions for different types of triggering mechanisms. The flume experiments show a sequence of hydrological processes triggering debris flows, namely erosion and transport by intensive overland flow and by infiltrating water causing failure of channel bed material. On the basis of these experiments, an integrated hydro-mechanical model was developed, which describes Hortonian and saturation overland flow, maximum sediment transport, through flow and failure of bed material. The model was calibrated and validated using process indicator values measured during the experiments in the flume. Virtual model simulations carried out in a schematic hypothetical source area of a catchment show that slope angle and hydraulic conductivity of the bed material determine the type and sequence of these triggering processes. It was also clearly demonstrated that the type of hydrological triggering process and the influencing geometrical and hydro-mechanical parameters may have a great influence on rainfall intensity-duration threshold curves for the start of debris flows.

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Flume experiments on the geomorphic effects of large wood in gravel-bed rivers

River Flow 2020 ◽

10.1201/b22619-225 ◽

2020 ◽

pp. 1609-1615

Author(s):

D. Ravazzolo ◽

G. Spreitzer ◽

H. Friedrich ◽

J. Tunnicliffe

Keyword(s):

Large Wood ◽

Flume Experiments ◽

Gravel Bed ◽

Gravel Bed Rivers

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Experimental observations and simulation of non-equilibrium bedload transport of graded bed material

Environmental Hydraulics, Two Volume Set ◽

10.1201/b10553-139 ◽

2010 ◽

pp. 829-834

Keyword(s):

Bedload Transport ◽

Bed Material ◽

Non Equilibrium

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An 1-D Integrated Hydro-Mechanical Model to Unravel Different Hydrological Triggering Processes of Debris Flows

10.20944/preprints201801.0032.v1 ◽

2018 ◽

Author(s):

Theo W.J. van Asch ◽

Bin Yu ◽

Wei Hu

Keyword(s):

Mechanical Model ◽

Debris Flows ◽

Overland Flow ◽

Model Simulation ◽

Slope Angle ◽

Great Influence ◽

Source Area ◽

Material Model ◽

Flume Experiments ◽

Bed Material

Many studies, which try to analyze the meteorological threshold conditions for debris flows ignore the type of initiation. This paper focuses on the differences in hydrological triggering processes of debris flows in channel beds of the source areas. The different triggering processes were studied in the laboratory and by model simulation on the field scale. The laboratory experiments were carried out in a flume, 8 m long and a width of 0.3 m. An integrated hydro-mechanical model was developed, describing Hortonian and Saturation overland flow, through flow, maximum sediment transport and failure of bed material. The model was tested on the processes observed in the flume. The flume experiments show a sequence of hydrological processes triggering debris flows, namely erosion and transport by intensive overland flow and by infiltrating water causing failure of channel bed material. Model simulations carried out on a schematic hypothetical source area of a catchment show that the type and sequence of these triggering processes are determined by slope angle and the hydraulic conductivity of the bed material. It was also clearly demonstrated that the type of initiation process and the geometrical and hydro-mechanical parameters may have a great influence on rainfall intensity-duration threshold curves, indicating the start of debris flows.

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The role of catastrophic floods in shaping the morphology of river beds in small mountain catchment areas

Forest Research Papers ◽

10.1515/frp-2017-0003 ◽

2017 ◽

Vol 78 (1) ◽

pp. 28-38

Author(s):

Paweł Franczak

Keyword(s):

Catchment Area ◽

Heavy Rainfall ◽

Flash Flood ◽

Mountain Streams ◽

River Bed ◽

Mountain Catchment ◽

Catchment Areas ◽

Forested Areas ◽

Transported Material

Abstract Mountain streams are subjected to the continuous reshaping of their river beds during floods, with the greatest changes occurring during extreme floods caused by sudden and heavy rainfall. River bed transformations during these flash floods are more severe in forested areas, where wooden logs carried by swollen streams are more likely to be deposited on the ground, which in turn leads to the greater accumulation of other transported material and debris. The study was conducted in the Rybny Potok catchment area (Babia Góra National Park). An extreme flash flood occurred on 15–16 May 2014 because of heavy rainfall, which, on 15 May amounted to 138 mm. The total amount of precipitation in the catchment area was 216.5 mm in three days. This resulted in sudden and full streams in spate, contributing to significant geomorphological transformations reaching all the way to the bottom of the river beds. During the flash flood, already established river beds and streams increased in size and many new river courses were formed.

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Effects of episodic sediment supply on experimental step-pool channel morphology, bedload transport and channel stability

10.5194/egusphere-egu21-10160 ◽

2021 ◽

Author(s):

Jiamei Wang ◽

Marwan A. Hassan ◽

Matteo Saletti ◽

Xingyu Chen ◽

Xudong Fu ◽

...

Keyword(s):

Grain Size ◽

Morphological Changes ◽

Mass Movement ◽

Channel Morphology ◽

Bedload Transport ◽

Sediment Supply ◽

Primary Control ◽

Channel Stability ◽

Flume Experiments ◽

Small Magnitude

Steep step-pool streams are often coupled to adjacent hillslope, directly receiving episodic sediment supply from mass movement processes such as landslides and debris flows. The response of step-pool channels to the variations in sediment supply remains largely unexplored. We conducted flume experiments with a poorly sorted grain-size distribution in an 8%-steep, 5-m long flume with variable width at the University of British Columbia, to study the effects of episodic sediment supply on channel evolution. After a conditioning phase with no feed, the channel was subjected to sediment pulses of different magnitude and frequency under constant flow discharge. High-resolution data of hydraulics, bedload transport, bed surface grain size, and channel morphology were collected every 10-20 minutes and an additional time at the end of each pulse.In response to sediment pulses, we recorded an increase in bedload transport rates, channel aggradation, bed surface fining, and continuous step formation and collapse. In between pulses, bedload rates dropped by several orders of magnitude, net erosion occurred, the bed surface gradually coarsened, and steps became more stable. The small-magnitude high-frequency pulses caused smaller but more frequent spikes in bedload transport, bed surface evolution, and thus step stability. Instead, the large-magnitude low-frequency pulses cause larger changes but provided a longer time for the channel to recover. This suggests that in step-pool channels pulse magnitude is a key control on channel rearrangement, while pulse frequency controls how fast and strong the recovery is.The frequency and stability of steps varied as a function of local channel width, showing that channel geometry is a primary control on step formation and stability even under episodic sediment supply conditions. Instead, the effect of sediment pulses is less important because the total number and average survival time of steps were similar among runs with different pulses. The critical Shields stress decreased following sediment pulses, then increased immediately after, and fluctuated until the next pulse. The variations in sediment supply caused cycles in bedload transport rate, surface and bedload texture, thus controlling the variability in the threshold for motion.Our results indicate that episodic sediment supply is a primary control on the evolution of step-pool channels, with sediment feed magnitude affecting mostly morphological changes, and sediment feed frequency controlling channel stability.

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