Flume Test Section Length and Sediment Erodibility

This paper analyzes the effect of flume test section length on sediment erodibility measurements. A modular flume was constructed and experiments were conducted with two test section lengths: 0.15 and 1.10?m. The internal height and width of the flume were 0.11 and 0.13?m, respectively. A fine (7?μm) commercially available quartz sediment was used for the tests. The expectation was that the shorter flume test section would experience a significantly higher erosion rate (per unit surface area) due to its greater sensitivity to edge effects (i.e., scour) at the entrance and exit of the flume test section. However, the measured erosion rates at comparable bottom stresses were only 35% greater in the short test-section tests. These results were consistent with the lack of significant scour development at the entrance or exits of the test sections. Hence, flume test section length alone does not appear to significantly affect erodibility measurements provided edge effects (i.e., scour) are minor.     

Measurements of Sediment Erosion and Transport with the Adjustable Shear Stress Erosion and Transport Flume

Soil and sediments play an important role in water management and water quality. Issues such as water turbidity, associated contaminants, reservoir sedimentation, undesirable erosion and scour, and aquatic habitat are all linked to sediment properties and behaviors. In situ analysis is necessary to develop an understanding of the erosion and transport of sediments. Sandia National Laboratories has recently patented the Adjustable Shear Stress Erosion and Transport (ASSET) Flume that quantifies in situ erosion of a sediment core with depth while affording simultaneous examination of transport modes (bedload versus suspended load) of the eroded material. Core erosion rates and ratios of bedload to suspended load transport of quartz sediments were studied with the ASSET Flume. The erosion and transport of a fine-grained natural cohesive sediment were also observed. Experiments using quartz sands revealed that the ratio of suspended load to bedload sediment transport is a function of grain diameter and shear stress at the sediment surface. Data collected from the ASSET Flume were used to formulate a novel empirical relation for predicting the ratio of bedload to suspended load as a function of shear stress and grain diameter for noncohesive sediments.     

Comparison of Two Techniques to Measure Sediment Erodibility in the Fox River, Wisconsin

This paper compares the results of two sediment erodibility test methods that have been applied on surficial sediments at a number of locations on the Fox River in Wisconsin. The methods include a straight flume that is deployed in situ (the FLUME) and a straight laboratory flume (the SEDFLUME). The sediment erosion rates measured near the surface (in the top four centimeters) as a function of bottom stress were compared. On average, the erodibility measured by the SEDFLUME was about 5.5 times greater than that measured by the FLUME. A possible reason for the difference is the relatively short test section of the SEDFLUME.     

Straight Benthic Flow-Through Flume for In Situ Measurement of Cohesive Sediment Dynamics

The design of a straight benthic flow-through flume for in situ studies of cohesive sediment dynamics is described including the flume structure and probes installed for routine measurements of suspended sediments and flow velocity. The flume was calibrated for two roughness types covering the range of possible cohesive bed roughnesses. The calibration included a set of three-dimensional velocity measurements using acoustic Doppler velocimeter. These measurements were used to develop calibration relationships between the bed shear stress (which is difficult to measure directly in routine deployments) and the flume centerline flow velocity, which is routinely measured. An example of a successful deployment of the flume is presented. The limitations and potential for further improvements are also briefly discussed.     

Simulating Sediment Transport in a Flume with Forced Pool-Riffle Morphology: Examinations of Two One-Dimensional Numerical Models

One-dimensional numerical sediment transport models (DREAM-1 and DREAM-2) are used to simulate seven experimental runs designed to examine sediment pulse dynamics in a physical model of forced pool-riffle morphology. Comparisons with measured data indicate that DREAM-1 and -2 closely reproduce the sediment transport flux and channel bed adjustments following the introduction of fine and coarse sediment pulses, respectively. The cumulative sediment transport at the flume exit in a DREAM-1 simulation is within 10% of the measured values, and cumulative sediment transport at flume exit in a DREAM-2 simulation is within a factor of 2 of the measured values. Comparison of simulated and measured reach-averaged aggradation and degradation indicates that 84% of DREAM-1 simulation results have errors less than 3.3?mm, which is approximately 77% of the bed material geometric mean grain size or 3.7% of the average water depth. A similar reach-averaged comparison indicates that 84% of DREAM-2 simulation results have errors less than 7.0?mm, which is approximately 1.7 times the bed material geometric mean grain size or 11% of the average water depth. Simulations using measured thalweg profiles as the input for the initial model profile produced results with larger errors and unrealistic aggradation and degradation patterns, demonstrating that one-dimensional numerical sediment transport models need to be applied on a reach-averaged basis.     

Data Interpretation for In Situ Measurements of Cohesive Sediment Erosion

Methods for interpreting data from in situ flume measurements of cohesive sediment dynamics are evaluated and a technique for estimating various erosion parameters using in situ measurements is proposed. There is currently a lack of uniformity in analysis techniques for cohesive erosion data collected in flumes and with in situ instruments and the proposed technique resolves some of these inconsistencies. The data set used in this study was derived from field experiments conducted with a straight benthic in situ flume in different aquatic environments in New Zealand. The experiments with stepwise increases in flow velocity revealed that peaks in the erosion rate at the beginning of each velocity step are most likely associated with heterogeneous bed structure, as transient hydrodynamic effects due to the experimental procedure were found to be insignificant. The field data showed an exponential decay of the erosion rate with time that is indicative of depth-limited erosion. These data are used to illustrate methods for the parameterization of the proposed semiempirical erosion equation, taking into account the time dependency of the erosion process.     

Cohesive Sediment Characterization by Combined Sedimentation and Rheological Measurements

A laboratory characterization of cohesive sediment has been carried out in which data obtained from standard sedimentation and rheological measurements were combined in a determination of the critical solid concentration for the detection of elasticity in a weakly cohesive suspension. The corresponding storage modulus and shear stress are very critical in any in situ rheometry of sediments, especially in the study of mud-water surface erosion in a flume. Sedimentation results showed that particle size distribution rather than surface treatment controlled the rheological behavior of the suspension while the critical solid concentration for the appearance of three-dimensional space-filling network, showing some measurable elasticity in the suspension, occurred in the region of 0.015. This parallel between the consolidation behavior and shear rheology development for the flocculating system has been established. This technique could be an adjunct to the laboratory characterization of cohesive sediments for the estimation of critical shear stress for surface erosion, especially in a typical flume experiment under water wave pressure.     

Correcting Acoustic Doppler Current Profiler Discharge Measurements Biased by Sediment Transport

A negative bias in discharge measurements made with an acoustic Doppler current profiler (ADCP) is attributed to the movement of sediment on or near the streambed, and is an issue widely acknowledged by the scientific community. The integration of a differentially corrected global positioning system (DGPS) to track the movement of the ADCP can be used to avoid the systematic bias associated with a moving bed. DGPS, however, cannot provide consistently accurate positions because of multipath errors and satellite signal reception problems on waterways with dense tree canopy along the banks, in deep valleys or canyons, and near bridges. An alternative method of correcting for the moving-bed bias, based on the closure error resulting from a two-way crossing of the river, is presented. The uncertainty in the mean moving-bed velocity measured by the loop method is shown to be approximately 0.6?cm/s. For the 13 field measurements presented, the loop method resulted in corrected discharges that were within 5% of discharges measured utilizing DGPS to compensate for moving-bed conditions.     

Simple Flume for Flow Measurement in Open Channel

There are various types of water measurement devices available. The type of flume used in a particular application depends on site-specific factors or limitations and the need for precision and cost. In measuring water in open drains, it is often desirable to prefabricate the flume to avoid the need for dewatering prior to installation. Traditionally, cutthroat flumes have been used in open drain systems or ditches due to ease of fabrication and installation. However, due to the extended transitional length and width associated with the cutthroat flume, transportation of the flume requires special facilitation or permits. A simple Venturi flume was proposed and tested that has several advantages over the traditional cutthroat flume. These advantages include lower construction cost, simplicity in fabrication, ease of transportation, and less width requirements where large flows are to be measured.     

Distribution of Bed Shear Stress in Rotating Circular Flume

Distributions of bed shear stress across the width of a rotating circular flume with smooth and rough bed surfaces were obtained by measurement and model prediction. Results with flows over smooth beds showed that the flow in the central part may be considered to be two-dimensional and that effects of flow depth over the operating range of the flume are minor for flow depths not exceeding 0.14 m. For rough beds, the bed shear stress distributions were found to be skewed toward the inner wall. This can be corrected if a compensating roughness is added to the bottom of the ring. Such measures are also effective for flumes with smooth beds. Measured bed shear stress distributions agreed well with the predicted distributions for smooth beds and reasonably well for rough beds. The modified Preston tube, for measurement of bed shear stress in flows over rough beds, was found to give promising results. Further tests are required to completely define the uncertainty in bed shear stress measurements made with this instrument.     

水系沉积物地球化学测量在青海哈图地区的找矿应用及成矿远景预测

水系沉积物地球化学找矿具有见效快,成本低的特点,与其相关的找矿预测是当前人们普遍感兴趣的课题,也将是未来找矿发展方向。本文通过综合研究青海省哈图地区的地球化学背景,结合水系沉积物地球化学异常,筛选出有利成矿地段,开展野外矿点检查和异常查证,对该地区水系沉积物测量成果进行综合研究及评价,划分找矿远景区及找矿靶区,为多种有色金属矿产靶区优选、资源潜力评价和下一步找矿地质工作提供了依据。     

Laboratory Experiments on Frazil-Size Characteristics in a Counterrotating Flume

In an attempt to increase the fundamental understanding of the formation and evolution of frazil ice, laboratory experiments have been conducted in a counterrotating flume at the Hydraulics Research and Testing Facility, University of Manitoba. A high-precision thermometer and recently improved digital image processing system were used to acquire data from these experiments, which will hopefully aid in the development of numerical models. The image processing system is described, and results from a series of experiments are presented. The effects of air temperature, water velocity, and bottom roughness on the formation of frazil ice are discussed. A lognormal distribution appears to provide a reasonable fit to the observed frazil particle size distributions.     

Flume Tests for Scour in Clay at Circular Piers

Local scour at circular piers founded on clay was studied experimentally in the laboratory to compare the depth of scour in sand and in clay and to investigate the effects of the Reynolds number, Froude number, and approach flow depth on scour depth. The depths of scour in front, at the side, and in the back of the piers were measured as a function of time under steady, gradually varied flow conditions. The measured scour-depth-versus-time curves were fitted with a hyperbola to estimate the equilibrium scour depths. The extrapolated equilibrium scour depths were compared with values predicted by the Federal Highway Administration equation. The results showed that although the rates of scour were much slower in clay than in sand, equilibrium scour in clay was about the same as in sand. It was found that the shape of the scour hole correlates with the pier Reynolds number. At low Reynolds numbers, the depth of scour was about the same all around the piers. At higher Reynolds numbers, the scour holes developed mainly behind the piers with much less scour in front of the piers. It was also found that the extrapolated equilibrium scour depth correlates well with the pier Reynolds number and that the Froude number and relative water depth did not have a significant effect on the scour depth for these experimental conditions.     

Simple Flume for Flow Measurement in Sloping Open Channel

First, this paper presents a new flume for measuring flow discharge in sloping channels, originally proposed by Samani and Magallanez for use in a horizontal channel. The flume is obtained by inserting two semicylinders in a rectangular cross section. Then, using dimensional analysis and the self-similarity theory, the stage-discharge relationship of the flume is theoretically deduced. For determining the two coefficients of the power stage-discharge equation, some experimental runs are carried out using flumes characterized by different values of the contraction ratio (ranging from 0.17 to 0.81) and of the flume slope (ranging from 0.5 to 3.5%). Finally, for a given range of the contraction ratio, the relationships for estimating the two coefficients of the stage-discharge equation are obtained.     

Experimental Observations of Flow and Bed Processes in Large-Amplitude Meandering Flume

Meanders of large amplitude often exhibit asymmetric planform shape or subsidiary bends. The present work is aimed at improving on understanding of the morphodynamic phenomena affecting the bed evolution of large amplitude meandering channels. Attention is focused on the development of the steady point bar-pool configuration and of the superimposed large-scale migrating bed forms; of particular interest is the role of the changing channel curvature and bed topography variation on flow pattern. A series of experiments was carried out in a sine-generated large-amplitude meandering flume, for two values of width-to-depth ratio. Maps documenting the bed topography and the flow pattern along the meandering bends are reported. Two point bars per bend were observed and seem to be part of a series of damped oscillations developing in response to the changing channel curvature. In response to the bed deformation, the maximum flow velocity moves at the outer bank at the entrance of the bend.     

Field Measurement of Ship Waves in Victoria Harbor

Field measurement of ship waves has been carried out in Victoria Harbor of Hong Kong. The wave data were collected at 18 monitoring stations by the underwater pressure/current instrument S4ADW and a floating wave rider TYPE 1156. It is found that the wave conditions in the study area are predominantly affected by the ship-generated waves under normal weather conditions. By comparing with the wind-generated waves in the open region which are irregular but with dominant directional characteristics, the existing harbor–wave field is much more complex. The irregular ship–wave field is not statistically stationary. It is also found that the ship–wave spectra are either of multiple peaks or have wide crests without distinct peaks.     

Adsorption of Polycyclic Aromatic Hydrocarbons in Aged Harbor Sediments

Polycyclic aromatic hydrocarbons (PAHs) are a group of hydrophobic organic contaminants which have low aqueous solubilities and are common pollutants in harbor sediments. Adsorption and desorption isotherms for PAHs are conducted to study the abiotic sorption of PAHs in uncontaminated harbor sediments in contact with the natural overlaying water. Representative 2-, 3-, and 4-ring PAHs are used to obtain PAH adsorption/desorption data. Linear adsorption onto sediment is obtained for the following PAHs: Naphthalene and 2-methyl naphthalene (2 ring), acenaphthene, anthracene, and phenanthrene (3 ring), and fluoranthene and pyrene (4 ring). Linear adsorption is followed by a significant hysteresis in desorption from sediment, due to strong retention by the aged sediment organic carbon. Sediment organic carbon–water partition coefficients (log?Koc) for the seven PAHs range from 2.49 to 4.63. Based on the sorption data for these representative PAHs, sediment organic carbon–water partition coefficients may be predicted for other PAH compounds, particularly the less soluble and the more hydrophobic PAHs (5 or more rings).     

Effect of Covers for Soil and Water Conservation Using Tilting Flume

The effect of stone and vegetative covers was evaluated for soil and water conservation in a waterway on alfisols. Experiments were conducted on a hydraulic tilting flume under simulated flow (93 and 40 cm2?s?1) and slope (0.1, 1.0, 3.0, and 5.0%) conditions. The depth of soil was maintained at 0.35 m over a perforated bed to facilitate deep drainage. A comparative study of bare soil, stone cover (50%), and vegetative cover (50%) is made to evaluate soil loss, deep drainage, Manning’s roughness coefficient, and the Froude number. The study has revealed that stone cover is more effective than vegetative cover at lower discharge in reducing the flow velocity, and thereby soil erosion. Deep drainage has been reduced from lower to higher discharge for all the slopes with cover measures, including bare soil. It is found that cover measures are necessary beyond 3% slope in order to prevent rill erosion in alfisols.