Median Sediment Diameter Research Articles

Predicting the sediment transport capacity from flow condition and particle size in the presence of vegetation cover

AbstractThe sediment transport capacity plays a pivotal role in erosion research, and is usually predicted using hydraulic variables. The transport capacity and hydraulic variables are affected by vegetation cover. Our understanding of the effect of vegetation cover, including the size, density, and arrangement of vegetation stems, on the relationship between the sediment transport capacity and hydraulic variables were rather limited. The objectives of this study were to investigate the effect vegetation stem cover on the relationship between hydraulic variables and the sediment transport capacity and to derive an equation for predicting the sediment transport capacity in the presence of vegetation cover. Five data sets from 288 flume experiments with a wide range of discharge (0.25–2 × 10−3 m3 s−1), slope (8.8–42.3%), median sediment diameter (0.11–1.16 × 10−3 m), stem cover (0–30%), stem diameter (2–36 mm), and stem arrangement (bead, tessellation, zigzag, random, and banding) were compiled for this study. Extensive regression analysis has shown that the sediment transport capacity could be expressed as a power function of flow velocity, shear stress, stream power, or unit stream power. Predictors of the sediment transport capacity were ranked from the unit stream power as the strongest, followed by the stream power, flow velocity, and the shear stress. Vegetation stem cover had no apparent and direct effect on the relationship between hydraulic variables and the sediment transport capacity so long as the unit stream power or stream power was used as its predictor. Vegetation cover became a significant factor only when the shear stress was used to predict the sediment transport capacity. Finally, a new equation involving the slope gradient, flow velocity, and median sediment diameter in a nondimensional form was shown to be a superior predictor of the sediment transport capacity with the Nash–Sutcliffe coefficient of efficiency of 0.92. The product of slope and flow velocity, that is, the unit stream power, captures the effect of vegetation stem cover and surface roughness and was shown to be an effective predictor of the transport capacity in the presence of vegetation cover.

Bedload transport rate prediction: Application of novel hybrid data mining techniques

The accurate prediction of bedload transport in gravel-bed rivers remains a significant challenge in river science. However the potential for data mining algorithms to provide models of bedload transport have yet to be explored. This study provides the first quantification of the predictive power of a range of standalone and hybrid data mining models. Using bedload transport data collected in laboratory flume experiments, the performance of four types of recently developed standalone data mining techniques - the M5P, random tree (RT), random forest (RF) and the reduced error pruning tree (REPT) - are assessed, along with four types of hybrid algorithms trained with a Bagging (BA) data mining algorithm (BA-M5P, BA-RF, BA-RT and BA-REPT). The main findings are four-fold. First, the BA-M5P model had the highest prediction power (R2 = 0.943; RMSE = 0.061 kg m−1 s−1; MAE = 0.040 kg m−1 s−1; NSE = 0.945; PBIAS = −1.60) followed by M5P, BA-RT, RT, BA-RF, RF, BA-REPT, and REPT. All models displayed ‘very good’ performance except the BA-REPT and REPT model, which were ‘satisfactory’. Second, the M5P, BA-RT, and RT models underestimated, and the BA-M5P, BA-RF, RF, BA-REPT and REPT models overestimated, bedload transport rates. Third, flow velocity had the most significant impact on bedload transport rate (PCC = 0.760) followed by shear stress (PCC = 0.709), discharge (PCC = 0.668), bed shear velocity (PCC = 0.663), bed slope (PCC = 0.490), flow depth (PCC = 0.303), median sediment diameter (PCC = 0.247), and relative roughness (PCC = 0.003). Fourth, the maximum depth of tree was the most sensitive operator in decision tree-based algorithms, and batch size, number of execution slots and number of decimal places did not have any impact on model’ prediction power. Overall the results revealed that hybrid data mining techniques provide more accurate predictions of bedload transport rate than standalone data mining models. In particular, M5P models, trained with a Bagging data mining algorithm, have great potential to produce robust predictions of bedload transport in gravel-bed rivers.

Examination of Blockage Effects on the Progression of Local Scour around a Circular Cylinder

An evaluation of scour estimation methods has indicated that the effects of blockage ratio are neglected in both scour modelling and development of new predictive methods. The role of channel blockage on the mechanism and progression of local scour is not well understood, and further analysis is required in order to incorporate this effect into scour estimation. In the present investigation, local scour experiments were carried out under varying blockage ratio. The results were compared with data from literature in order to explore the effects of blockage ratio (D/b, where D is the pier diameter, and b is the channel width) on equilibrium scour depth (dse/D, where dse is the depth of scour at equilibrium). It was determined that D/b had a small influence on both dse/D and the progression of scour depth (ds/D) when relative coarseness D/d50 < 100 (where d50 is the median diameter of sediment), and that the influence appeared to be amplified when D/d50 > 100. The efficacy of scour estimation methods used to predict the progression of local scour was also dependent on D/d50. A method of scour estimation used to predict dse/D was evaluated, and it was similarly found to be particularly effective when D/d50 < 100. In future work, further experiments and analysis in the range of D/d50 > 100 are required in order to establish the role of D/b under prototype conditions and to refine existing scour estimation methods.

Empirical Sediment Transport Models Based on Indoor Rainfall Simulator and Erosion Flume Experimental Data

AbstractLand degradation processes start with accelerated runoff and sediment delivery. In this study, rainfall‐runoff induced sediment transport is investigated using data from an indoor laboratory experimental setup consisting of a rainfall simulator and an erosion flume. The data are analysed to develop empirical models using sediment discharge, slope, flow discharge, rainfall intensity and sediment size. Fine and medium sands are considered as bare soil in experiments. Four rainfall intensities (45, 65, 85 and 105 mm h−1) are applied with combinations of lateral and longitudinal slopes of 5%, 10%, 15% and 20%. Eighty experiments are conducted. Flow is measured, and sediment within flow is separated and weighted. Experimental data are used for developing empirical models through multiple regression with parameters optimized by genetic algorithm. Results show that slope is the main contributing variable to the sediment transport over hillslopes. Accommodating variables among slope, rainfall intensity, flow discharge and median diameter of sediment as independent variables, one‐variable, two‐variable and four‐variable models are developed considering also that higher number of parameters increases the performance of the model with higher cost of parameterization. Copyright © 2016 John Wiley & Sons, Ltd.

Prediction of cross-shore sandbar volumes using neural network approach

Correct estimation of bar volumes, wave height, wave period and median sediment diameter is crucial for the designing of coastal structures and water quality problem. In this study, bar volumes caused by cross-shore sediment transport were investigated using a physical model and obtained 64 experimental data considering the wave steepness (H0/L0) and period (T), the bed slope (m) and the sediment diameter (d50). Artificial neural network (ANN) and multi-linear regression (MLR) are used for predicting the bar volumes. A multi layer perceptron is used as the ANN structure. The results show that the ANN model estimates are much closer to the experimental data than the MLR model estimates.

FIELD INVESTIGATION OF SIZE DISTRIBUTION OF SUSPENDED SEDIMENT USING LISST-100

To investigate size distributions of suspended sediments in the surf zone, a field measurement using LISST (Laser In-Situ Scatter and Transmissometer) was conducted at Hasaki, Japan. The time series of suspended sediment concentration (SSC) for a grain size in the sand range (63 to 500 ï­m) had strong correlations with those for other sand grain sizes, and the strong correlations were also observed in the silt range (2.5 to 28 ï­m). However, at zero time lag, the time series of SSC for sand grain sizes had little correlations with those for silt sizes. With considering time lag, the time series of SSC for sand sizes had weak correlations with those for silt sizes, but which time series lagged behind the others, sand particles or silt particles, was not clear. When the total SSC CLISST was larger than 0.2 g/l, the median sediment diameter d50 was scattered around 185 ï­m. However, at CLISST < 0.2 g/l, d50 increased as CLISST increased. With the increase in d50 from 150 to 200 ï­m, the sorting coefficient decreased from 2.4 to 1.2 and the skewness increased from 0.7 to 1.0.

Comparison of two stream gauging systems for measuring runoff and sediment yield for a semi‐arid watershed

ABSTRACTOur ability to understand erosion processes in semi‐arid ecosystems depends on establishing relationships between rainfall and runoff. This requires collection of extensive and accurate hydrologic and sediment data sets. A supercritical flume with a total load traversing slot sediment sampler used on several sites at the Walnut Gulch Experimental Watershed (WGEW) near Tombstone, AZ has proven to be a reliable way to measure flow and sediment discharge from small watersheds. However, it requires installation of a costly structure that is only suitable for relatively small flows. A more commonly used method based on ease of installation and expense is the pump sampler. One example of this is a set of instrumentation developed by the Australian Commonwealth Scientific and Industrial Research Organization (CSIRO), in which the pump sediment sampler is part of an in‐channel, fully automated system for measuring water velocity, depth, turbidity and collecting runoff samples. A 3.7 ha arid watershed at WGEW was instrumented with both systems and hydrologic and sediment data were collected and compared during a 2 year period. Total sediment yield for the entire period measured by the CSIRO pump sampler (11.6 t ha‐1) was similar to that by traversing slot sampler (11.5 t ha‐1). The pump sampler accurately estimated the amount of fine (< 0.5 mm) sediment fractions exported, but consistently underestimated the coarse (>0.5 mm) sediment fractions. Median sediment diameter of samples collected by traversing slot and pump sampler were 0.32 and 0.22 mm, respectively. This study outlines the benefits and limitations of the pump sampler based system for monitoring sediment concentration and yield in high‐energy headwater catchments, and makes recommendations for improvement of its performance. Copyright © 2012 John Wiley & Sons, Ltd.

Dynamic Routing of Flow Resistance and Alluvial Bed-Form Changes from the Lower to the Upper Regime

On the basis of a Strouhal number and the definition of the control factor, m, a new routing to calculate the energy slope in the lower and upper alluvial regimes is proposed. The control factor, m, representing the interactions in alluvial rivers, is reckoned as a bed-form index: while the flow evolves through transition, the control factor, m, decreases from m=2, associated with two-dimensional fully developed dunes, to m=1, associated primarily with in-phase waves. The way to predict the value of the control factor, m, is drawn from a previously published criterion for delineating the upper regime and is calibrated with experimental data. On several data from flumes and rivers, the routing is tested and compared with other methods from the literature. It appears that the new routing is the most robust because it allows researchers to obtain low averages of the discrepancy ratio for a wide range of ratios between the water depth and the median sediment diameter. On a selection of contrasted freshet events, the new routing allows for the capture of the primary dynamic of the flow resistance decrease.

Influence of Meandering on the Estimation of Velocity and Shear Velocity in Cobble-Bed Channels

Field experiments carried out on five reaches of the meandering cobble-bed Beheshtabad River in central Iran showed that the position of the maximum velocity was independent of the relative submergence ( h/ d50 less or more than 4) and relative curvature ( R/W ) (where h is the flow depth, d50 is the median diameter of sediment, R is the radius of curvature, and W is the river width). A new method, called the boundary-layer characteristic method, was employed for the determination of shear velocity. The shear velocity values estimated with this method were in agreement with those obtained using the parabolic law. The law of the wall was valid for low and high relative curvatures in the inner, outer, and central zones of the meandering reaches. However, this law was not suitable for determining the shear velocity for the relative submergence range of 2.7<h/ d50 <9.6 due to its high sensitivity to near-bed velocities. Interestingly, it was possible to predict the flow velocity in meandering reaches using th...

Studies on the community structures of meiofauna and marine nematode at six stations in the Southern Yellow Sea, China

Meiofauna (0.031–0.5 mm) were sampled and studied at a grid of six stations during the cruise of investigation for over winter ground of anchovy (Engraulis japonicus) in the Southern Yellow Sea, China in January 2003. Total meiofauna individuals averaged (1.162±0.347) × 106 ind m−2 and the corresponding biomass 1.748 ± 0.72 g dwt m−2 with the maximum 2.35g dwt m−2 at Station 9594 and the minimum 0.537g dwt m−2 at Station 11794. A total of fourteen taxa were identified. Free living marine nematodes were the most dominant group with an average abundance of (1.026 ± 0.206) × 106 ind m−2, accounting for 88.5% of total numbers, and an average of 85.4% lived in the surface layer (0–2 cm) of sediments. The individual dry weight was calculated, which ranges from O.Ol µg to 31.32f*g with an average of 0.261 µg. One hundred and forty two species or taxa of free-living marine nematodes belonging to 89 genera, 24 families and 4 orders were identified. Six of them were new species. The common dominant species were Dorylaimopsis rabalaisi, Sphaerolaimus balticus, Sphaerolaimus minitus, Metalinhomoeus longicauda, Prochromadorella attenuata., Campylaimus sp. 1, Vasostoma sp. 1, Daptonema normandicum, Paramonohystera riemanni, and Promonhystera sp.1. According to cluster analysis of Primer 5.0, the six stations were clustered into 2 groups with similar species composition at 40% similarity. The species diversity index (J), evenness index (d) and species richness index (d) of nematode were calculated and analyzed. Correlation analysis with respect to environmental factors showed that the abundance of meiofauna had a significant correlation with the median diameter of sediment (Mdo). The species richness of nematodes had a highly significant correlation with Chl-a and significant correlations with Pha-a and Chl-a + Pha-a.

Resistance to overland flow due to bed‐load transport on plane mobile beds

AbstractDuring bed‐load transport by overland flow, momentum is transferred from the flow to the bed via grain collisions, resulting in a decrease in flow velocity and an increase in flow resistance, herein termed bed‐load transport resistance. In overland flow on mobile plane beds, total flow resistance f consists of grain resistance fg and bed‐load transport resistance fbt. In order to identify and evaluate the relative importance of the factors controlling fbt, 38 flume experiments were performed on slopes of 2·7 and 5·5° using sediment with median diameters of 0·74 and 1·16 mm. All flows were supercritical and turbulent.This study is an extension of a recent study by Gao and Abrahams (Earth Surface Processes and Landforms 2004, vol. 29, pp. 423–435). These authors found that fbt is controlled by three factors: sediment concentration C, dimensionless sediment diameter D*, and relative submergence h/D, where h is flow depth, D is median sediment diameter. However, a new dimensional analysis identifies two additional factors: Froude number F and slope S. Multiple regression analyses reveal (1) that these five factors together explain 97 per cent of the variance of fbt, and (2) that S controls fbt entirely through C. The variable C is therefore redundant, and a new functional equation relating fbt to D*, h/D, S and F is developed. This equation may be used to predict fbt. An advantage of this equation is that it may be used to predict fbt without measuring bed‐load transport rate. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Simple Model for Downstream Variation of Median Sediment Size in Chilean Rivers

A simple theory is developed to account for the observed downstream variation of the median sediment size in Chilean rivers based on a reach-wise equilibrium sediment transport concept. The theory makes use of a bedload transport equation linked with a resistance equation to estimate the median sediment diameter as a function of channel slope, with the flow Reynolds number and bedload concentration as parameters. Both, Meyer-Peter and Müller’s and Ackers and White’s formulas are used alternatively as bedload equations. A Manning–Strickler type of formulation is used as a resistance relationship. The resulting model is validated against field data corresponding to 150 rivers in Central Chile, covering slopes in the range of 0.04–8.61%, with median sediment size in the range of 0.3–250 mm. Despite the simplicity of the present theory and the somewhat bold assumptions made in its derivation, the estimated variation of the median sediment size with channel slope follows the same trend as the field data. Most of the scatter of these data falls within the theoretical limits given by the estimated range of values of the parameters of the model.

Wind stress, bed roughness and sediment suspension on the inner shelf during an extreme storm event

Instrumented bottom boundary layer tripods were deployed on the inner shelf at depths of 13 and 8 m off the U.S. Army Corps of Engineers Field Research Facility at Duck, NC, U.S.A., over a 2-week period that included the severe and prolonged “Halloween Storm” of late October 1991. The storm persisted for 5 days and generated waves with heights and periods of up to 6 m and 22 s. Although the instrumentation was destroyed, current profile and suspended sediment concentration profile data were recovered from the 13 m site. Mean currents attained speeds of nearly 0.5 m s −1 at 0.29 m above the bed and were directed about 10° offshore from shore-parallel. These strong currents are shown to be wind driven and result in predictions of a wind-drag coefficient, C a = 4.7 × 10 −3. The currents were recorded simultaneously with root-mean-square (rms) wave orbital velocity amplitudes in the 0.6–1.0 m s −1 range. During the peak of the storm suspended sediment concentrations exceeded 1 kg m −3 throughout the lower 1 m of the water column. Analysis of current profiles, accounting for the presence of waves, is performed to obtain an equivalent bottom roughness, k n, of approximately 15 times the median sediment diameter, i.e. k n ⋍ 15 d 50. Analysis of the suspended sediment concentration profiles, using the experimentally obtained hydrodynamic characteristics, results in a value of 4 × 10 −4 for the resuspension parameter, γ 0, with the reference concentration taken 7 d 50 above the bed. From the severity of the storm condition it is inferred that our estimates of k n and γ 0 correspond to sheet flow conditions.

Transport of gravel by flows up to 500 m3/s, Ohau River, Otago, New Zealand

Detailed measurements of velocity, and of sediment transport inferred from delta build up, in a river scoured by flows up to 500 m3/s to form a 1.4 hm3 delta, are used to calibrate a bed-load formula. According to this calibration, “jet zones” which occupy less than 6% of the channel area convey approximately half the load. The resulting ratings for bedload were: a. locally q s = 11 × 10-6 v 4.5 for v > 1.35, otherwise q s = 0; b. river Q s = 6.6 × lO-6 Q 1.5 where qs (m2/s) and Q s (m3/s) are sediment loads, v (m/s) is the velocity and Q (m3/s) is the flow. The mean discharges of gravel and water over a period of approximately two years were 0.0165 and 218 nr/s respectively; a ”concentration“ of 204 g/m3. The median sediment diameter was 0.02 m and the channel slope 0.006. An attempt to reconcile field and laboratory data using the conventional scaling of transport with shear stress failed, but scaling with velocity was successful.

Oscillation Ripplemarks and Orbital Diameter

Ripplemark wavelength, L, is related to median sediment diameter, M, and orbital diameter, D, of waves at the sea floor by the empirical equation L = .00136 M 1.6575 D −.3837 . This indicates an inverse relationship between ripplemark wavelength and orbital diameter, a result contrary to expectation.