Flume Investigation Research Articles

Multi-scale flume investigation of the influence of cylindrical baffles on the mobility of landslide debris

Debris flows travel downslope at high speed and often cause damage to the infrastructure of societies around the world. With increasing extreme rainfall events and urbanization in mountainous regions, effective structural countermeasures are in increasing demand. Over recent years, engineers have proposed the installation of an array of cylindrical columns, called baffles, to reduce the velocity of debris flows in catchments. However, existing design methods are highly empirical in nature, so it is unclear whether they are adequate or over designed, and appropriate specifications and arrangement of cylindrical baffles have still not been suggested. Moreover, previous experimental studies have predominantly modeled debris flows as dry granular flows at a laboratory scale. In this study, to investigate the effect of cylindrical baffles on the dynamic characteristics of debris flow, a series of small-scale flume tests was conducted using a flume equipped with devices to measure the flow interaction between baffles and the dynamic loads of debris flow. In addition, to investigate the scale effect of debris flows and cylindrical baffles on flow characteristics, large-scale tests were also performed according to different numbers of rows of baffles for similar baffle configurations confirmed by small-scale tests. Using the small- and large-scale test results, this study analyzed the energy dissipation and dynamic impact characteristics according to the height and number of rows of baffles. The analysis results showed that the use of baffles increased the energy dissipation of debris flows, and an additional row of baffles produced greater effects on the energy dissipation in the debris flows. Based on the test results, the average dynamic pressure coefficient for cylindrical baffles was 0.31.

Flume Investigation Into Mechanisms Responsible for Particle Sorting in Gravel‐Bed Meandering Channels

AbstractMeandering gravel‐bed rivers tend to exhibit bed surface sorting patterns with coarse particles located in pools and fine particles on bar tops. The mechanism by which these patterns emerge has been explored in sand‐bed reaches; however, for gravel‐bed meandering channels it remains poorly understood. Here we present results from a flume experiment in which bed morphology, velocity, sediment sorting patterns, and bed load transport were intensively documented. The experimental channel is 1.35 m wide, 15.2 m long, and its centerline follows a sine‐generated curve with a crossing angle of 20°. Water and sediment input were held constant throughout the experiment and measurements were collected under quasi‐equilibrium conditions. Boundary shear stress calculated from near‐bed velocity measurements indicates that in a channel with mild sinuosity, deposition of fine particles on bars is a result of divergent shear stress at the inside bend of the channel, downstream of the apex. Boundary shear stress in the upstream half of the pool was below critical for coarse particles (>8 mm), leading to an armored pool. Inward directed selective transport was responsible for winnowing of fine particles in the pool. Fine and coarse sediment followed similar trajectories through the meander bend, which contrasts earlier studies of sand‐bedded meanders where the loci of fine and coarse particles cross paths. This suggests a different sorting mechanism for gravel bends. This experiment shows that a complex interaction of quasi‐equilibrium bed topography, selective sediment transport, and secondary currents are responsible for the sorting patterns seen in gravel‐bed, meandering channels.

Flume investigation of cylindrical baffles on landslide debris energy dissipation

Flume investigation of cylindrical baffles on landslide debris energy dissipation

Small-Scale Flume Investigation of the Performance of Step-Baffle Drainage Channels in Mitigating Debris Flows

Drainage channels are widely used for discharging debris flows into deposition basins or rivers. However, the current drainage channel designs for guiding rapid debris flows downstream do not account for the variations of the gullies’ gradient and debris flow energy. In this study, we evaluated the performance of different step-baffle geometries (square, triangle, and trapezoid) in regulating debris flows. Specifically, their effects on the flow patterns, sediment transport, energy dissipation, and impact pressure are investigated using flume experiments. Results here showed that the square baffles promote highly turbulent flows which in turn result in the highest lift height relative to the triangular and trapezoidal baffles. Maximum sediment interception and highest energy dissipation are obtained using the trapezoidal baffle, whereas the triangular baffle exhibits minimal solid interception and the lowest energy dissipation. Trapezoidal baffles generally experience the greatest impact forces relative to both square and triangular baffles. However, when only the first baffle in the channel is considered, it is the square baffles that experience the largest impact forces. The present work improves the understanding of the effectiveness of step-baffle drainage channels in mitigating debris flows.

Shear layer over floodplain vegetation with a view on bending and streamlining effects

Shrubby and woody vegetation growing on floodplains profoundly influences hydrodynamic and transport processes in riverine systems. Existing hydrodynamic research is mostly focused on conditions with aquatic plants and rigid model vegetation. To appreciate the different hydrodynamic impacts of submerged floodplain and riverbank vegetation, a novel flume investigation was carried out. We simulated conditions found in riparian environments in terms of vegetation density, plant structure and flexibility, and presence of a grassy understory. Four experimental cases were defined so that vegetation exhibited different degrees of bending and streamlining. Extensive set of velocity measurements allowed reliable description of the double averaged flow. Vegetation morphology, with the flexibility-induced streamlining and dynamic motion controlled the magnitude and distribution of the vegetative drag, shaping the shear penetration within the canopy. The flows were highly heterogeneous, thus calling for spatially averaged approaches for the flow field investigation. The relative importance of dispersive momentum fluxes was high in the canopy bottom region where both Reynolds and dispersive stresses were small. The contribution of dispersive fluxes to momentum transport decreased with increasing reconfiguration. The results revealed the shear layers over floodplain vegetation to be dynamically similar to other environmental flows over porous obstructions. However, the velocity-dependent vegetative drag and deflected height introduced additional complexity in the flow simulation. Altogether our findings implied that accurate description of vegetated floodplain flows can be achieved only when plant morphology and flexibility are appropriately described in drag models.Article highlightsA novel experimental setup with flexible woody plants and grasses was used to model the hydrodynamics of vegetated floodplains.Plant morphology and flexibility controlled the vegetative drag, affecting key shear layer features, including the shear penetration.The spatially heterogeneous flows had higher dispersive stresses at the canopy bottom, where the total fluid stress was small.

The impacts of bioturbation by common marsh crabs on sediment erodibility: A laboratory flume investigation

Accelerating sea-level rise (SLR) threatens salt marshes globally, and thus understanding how these ecosystems respond to this stress is vital to increasing their resiliency. In order to maintain their surface elevation as sea level rises, salt marshes must add below ground biomass and accrete sediment. In many cases, biota can significantly affect marsh accretionary and erosional processes by stabilizing or helping to mobilize sediment, which in turn can profoundly affect the morphological evolution of the marsh. However, these effects are poorly understood. Bioturbation by dense populations of the marsh crab Sesarma reticulatum has been found to facilitate expansion of tidal creeks and creek bank erosion in multiple areas. The influence of S. reticulatum bioturbation on sediment erodibility has not yet been quantified, due to difficulties involved in measuring the processes in a field setting. In this study, we used a laboratory flume to examine the effects of burrowing by common marsh crab species S. reticulatum and Uca pugnax on sediment surface roughness and erodibility. Measurements and observations of surface elevation, flow velocities, and sediment movement indicate that burrowing and feeding by S. reticulatum, and to a lesser extent, Uca pugnax, increase surface roughness and decrease the threshold velocities and shear stresses required for sediment erosion. Erosion associated with observed burrowing is of a similar magnitude to mean annual marsh surface accumulation, and, consequently, this mechanism for sediment loss in heavily burrowed areas has the potential to produce large-scale morphological changes. TaxonomyEcological engineering, Coastal setting, Salt marsh, Geomorphological aspects, Soil erosion. Regional index termsUSA, Texas, Galveston.

Flume investigation of the influence of rigid barrier deflector angle on dry granular overflow mechanisms

Mass-wasting processes are often intercepted using rigid barriers, which are sometimes equipped with deflectors to prevent overspilling. Despite the engineering value of deflectors, they are currently only installed using empirical and prescriptive approaches because flow interaction mechanisms are not well understood. A 5 m flume was used to study dry granular flow deposition and overflow processes with and without deflectors of varying angles. The deflector angle was varied as 0°, 30°, 45°, and 60° with respect to the horizontal plane. For the geometric and material parameters adopted in this study, experimental results reveal that deflector angles greater than 45° develop steep ramp-like deadzones that result in effective energy dissipation as approaching flow impacts the deflector, whereas deflectors less than 45° rapidly develop shallow ramp-like deadzones, which promote high-energy overflow. Deflectors of at least 45° are required to develop upward overflow launch angles, whilst deflectors of 30° or less result in downward overflow launch angles. Upward launch angles indicate that a steep deadzone develops and effectively dissipates flow energy. An orthogonal deflector increases launch lengths by 40% compared to a bare rigid barrier.

Flume investigation of landslide granular debris and water runup mechanisms

The heights of rigid debris flow barriers are designed to provide adequate retention and prevent debris from over-spilling. Designers need to predict potential runup height against a vertical wall to account for potential over-spilling. Experimental investigations of debris flow runup have previously been conducted using dry sand. A 5 m long rectangular flume was used to conduct runup experiments using both dry sand and water, separately. A combination of high-speed imagery, photoconductive sensors and laser sensors was used to study runup along the vertical face of a rigid barrier. The effect of Froude number (Fr) on runup was examined by varying the channel inclination. Commonly adopted energy and momentum approaches for predicting runup were compared with experimental results. The results reveal that runup mechanisms are dependent on approach Fr conditions and whether the flow medium is frictional in nature. For water, subcritical flows did not exhibit significant runup (reflective wave mechanism), whereas supercritical flows led to a vertical jet runup mechanism. Supercritical sand flow resulted in a pile-up mechanism instead of distinct runup.

Flume investigation of landslide debris–resisting baffles

Landslide debris is a common occurrence in mountainous regions around the world that can potentially result in disastrous consequences to downstream facilities. Flow-impeding structures are often constructed along the flow path to impede this hazardous phenomenon. Baffles are a type of flow-impeding structure regularly installed using empirical and prescriptive design methods as the interaction mechanism and the influence of baffle configuration on flow impedance is not well understood. A series of flume experiments were carried out to investigate flows characterizing landslide debris impacting an array of baffles using dry uniform sand. The influence of baffle height, row number, and spacing between successive rows was examined. Photoconductive sensors were used to estimate flow velocity, laser sensors were installed to measure flow depth profiles, and high-speed cameras were used to capture flow kinematics. Experimental results reveal that baffles can be categorized relative to the approach flow depth (h) and increasing the baffle height from 0.75h to 1.5h leads to a 40% increase in upstream flow depths from backwater effects, more effective development of subcritical conditions, and additional energy losses of up to 9%. Increasing the number of rows of 1.5h baffles from a single row to a three-row staggered array results in up to 72% additional energy loss. The energy loss is attributed to the deflection of granular jets and additional backwater effects. Increasing the row spacing from 50 to 100 mm results in up to a 14% increase in energy loss.

Placed Rock as Protection against Erosion by Flow down Steep Slopes

The additional resistance to erosion due to flows down steep slopes that can be achieved by placing rock instead of dumping randomly has been quantified during a large-scale flume investigation. Testing was undertaken for slopes of 0.2, 0.3, and 0.4 with two layers of armor overlying a filter, consistent with conventional rock protection designs. Two sizes of sandstone with mean diameters of 76 and 109 mm and basalt with a mean diameter of 94 mm were tested. Placing rock to achieve maximum bulk density (mass per unit in situ volume) increased failure flow (flow at exposure of the filter) by 30% of that achieved with the same type of randomly dumped material but the total armor mass per unit surface area increased by 35%. A damage approach to steep erosion protection that is similar to that used for coastal breakwater design is presented. An improved method of partition between the aerated flow over as well as flow through both random and placed rock has also been quantified. Better characterization of aer...

A Two-Dimensional Wave Flume Investigation into the Effect of Multiple Vertical Steps on the Form of Breaking Waves

A two-dimensional wave flume investigation was conducted to examine the effect of multiple steps on the form of breaking waves. The study was initiated to gain a better appreciation of the design tolerances for artificial surfing reefs. The geotextile bags used to form an artificial surfing reef create multiple steps in the seabed; reducing the size of these steps improves the quality of waves for recreational surfing but comes at considerable economic cost. Sets of steps were placed on a 1 : 15 slope, and image analysis methods were used to quantify the effects of increasing step size on the form and intensity of the breaking waves. The vortex area, angle, and length-to-width ratio were quantified for various step sizes. It is shown that breaking intensity decreases with increasing step size. Variations in wave reflection were measured using a three-element surface-piercing wave gauge. Wave reflection coefficients were found to be unchanged for all step sizes implemented in this experiment, and reflection was not found to significantly affect the form of the breaking waves in the current tests. The quality of the surfing waves for recreational surfing was seen to degrade measurably with increasing step size, with a higher degree of variability and irregularity in the vortices of the plunging breakers. A qualitative assessment of the quality of the breaking waves suggests that step size should be limited to approximately 23% of the design wave height.

The influence of fish feed pellets on the stability of seabed sediment: A laboratory flume investigation

Superfluous fish food settling below fish farms can have a negative impact on the seabed. To aid in the assessment of this impact a series of flume experiments, designed to mimic seabed conditions below a fish farm, was conducted with the aim of examining the effects of fish pellets on the stability of fine sediments. Artificial beds, with varying quantities of fish pellets incorporated both within the sediment matrix and lying on the sediment surface, were allowed to consolidate for different periods of time ranging from 1 to 10 days, and then subjected to erosion experiments. In flume experiments containing fish pellets, a bacterial biofilm developed at the sediment-water interface after a few days. In the control experiments (no fish pellets), a diatom biofilm caused extensive stabilisation of the surface sediment. The erosion experiments showed that the addition of fish pellets reduced the surface erosion threshold by more than 50%. The stability decrease was more pronounced in the experiments with greater amounts of pellets. Evidence of drag reduction due to high suspended sediment concentration was also observed. This phenomenon is discussed and a correction formula is proposed for the effective shear stress experienced by the bed.

A laboratory flume investigation of the formation of fossil stem fills

ABSTRACTThe mechanism of formation of fossil plant ‘pith casts’ has been investigated experimentally using a small laboratory flume tank. The extent to which filling occurs is dependent on stem length and diameter, and on the current velocity. Stem fills produced by sediment carried in suspension show a distinct structure of two wedges of sediment deposited at either end of the stem cavity. These wedges are deposited from small flow vortices which form when the flow around the stem separates, as the flow through the stem becomes restricted. Stems filled by bedload currents do not show such a uniform structure. The experimentally produced stem fills are comparable with fossil pith casts, e.g. Catamites. The formation of cortical casts of Stigmaria is not simply explained in terms of the processes operating in open‐ended stem segments. This investigation of the mechanics of filling of hollow plant organs, and the structure of the fill produced, offers a basis for interpreting the sedimentary environment in which the fossil fills have formed, and leads to a fuller understanding of the mode of formation of such fossils.