Bed Roughness Height Research Articles

Sediment deposition characteristics in shallow saturation excess overland flow for urban Stormwater applications

AbstractOver the last several decades, urban stormwater management has grown to include green infrastructure, such as bioswales. However, little is known about the fate of particulate contaminants after depositing in bioswales and similar systems, in part due to limited understanding of sediment transport dynamics in the shallow (water depths comparable to bed roughness height), near‐saturated flow conditions typical of such systems. To better understand the sediment deposition characteristics, a flume study was performed with natural sediment and flow characteristics similar to those encountered in bioswales. A fluorescent, paramagnetic sediment tracer was used to mimic silt sized particles carried by stormwater flows, and a photographic hood was used to image the tracers deposited on the flume surface after subsequent pulses of tracer slurry, where the image intensity was established to be a reliable proxy for surface tracer concentration. These data were analysed to calculate the sediment trapping efficiency, that is, the rate of sediment deposition onto the soil surface per unit time, and samples of the flume effluent were collected to provide an independent mass balance of the deposited tracer. Experimentally determined sediment trapping efficiencies were found to decrease with increasing bed slope angle, which indicates soils at greater bed slopes will be less capable of retaining particulate‐associated contaminants, and were found to decrease over time as the soil surface became saturated with fine particles. These trapping efficiencies were also compared with mathematical models from the literature, augmented with a formula representing the observed surface‐saturation effect. The results of this study can lead to the development of more realistic models for predicting the removal performance of current stormwater remediation designs in regard to fine sediment associated contaminants.

Response of Flow and Saltating Particle Characteristics to Bed Roughness and Particle Spatial Density

AbstractWith the goal to explore the effects of natural bed roughness on bedload transport, numerical simulations of flow and particle saltation are carried out with varying bed roughness and particle spatial density. A combination of Eulerian and Lagrangian point‐particle methods is applied to solve the equations of motion of the fluid and the particles within the large‐eddy simulation framework. Flows over smooth and rough beds with four particle densities are considered. As the bed roughness increases, there is a leftward shift of the double‐averaged streamwise velocity profiles against dimensionless vertical distance, which is scaled with the bed roughness height, an upward shift of the peak values of double‐averaged Reynolds stresses, and a fragmentation and disappearance of coherent structures in the form of high‐speed and low‐speed near‐bed streaks. These observations are consistent with those of previous studies. As the bed roughness increases, the mean resting time of saltating particles increases, however the particles' saltation length, velocity, and angular velocity decrease, while their saltation height remains almost unchanged. Saltation height, saltation length, particle angular velocity, and resting time exhibit linear, gamma, normal, and exponential distributions, respectively. Further, as the bed roughness increases, the kurtosis and skewness of some particle parameters vary, and the particle velocity shifts from a symmetrical normal to an asymmetrical gamma distribution.

Hydrodynamics of flow over a gradually varied bed roughness

Turbulence characteristics in a fully developed flow over a gradually varied bed roughness are investigated. The results of the Reynolds stress profiles indicate that they increase with an increase in bed roughness height. Their peaks occur within the wall-shear layer close to the bed. Besides, the bed shear stress rises in accordance with the roughness height. The roughness-induced layer grows as the roughness height increases with the streamwise distance. The velocity profiles fitted with the logarithmic law reveal that the zero-velocity level is elevated as the roughness height increases, but the zero-plane displacement is not influenced by the roughness. The turbulent kinetic energy (TKE) flux results indicate that an inrush of faster moving fluid parcels composing the sweep event is the dominant mechanism in the near-bed flow zone. The magnitude of the sweep event escalates, as the roughness height increases. On the other hand, a process of slowly moving fluid parcels forming the ejection event prevails in the outer flow layer. The TKE flux results agree with those obtained from the bursting analysis. Concerning the TKE budget, the peaks of the TKE production, dissipation, and pressure energy diffusion rates being positive appear near the bed and grow as the roughness height increases, whereas the peak of the TKE diffusion rate being negative behaves in the similar way as the other terms of the TKE budget behave.

Experimental and numerical study of bed roughness effect on longitudinal dispersion

Abstract The effects of bed roughness on the longitudinal dispersion coefficient (DL) were experimentally and numerically investigated in the present study. The tracer experiments were first carried out in a circular flume with a diameter of 1.6 m over both smooth and rough beds (coarse sand) with four sizes (ks = d65) of 1.04, 2.09, 3.01, and 4.24 mm. In addition, the one-dimensional advection–dispersion equation was numerically solved. The longitudinal dispersion coefficient was calculated by comparing the numerical and experimental breakthrough curves. The results showed that by increasing the bed roughness height (from zero to 4.24 mm), the longitudinal dispersion coefficient increased by 34%. In addition, the longitudinal dispersivity (λ = DL/V) increased with increasing relative roughness (ks/h), so that the range of longitudinal dispersivities in smooth bed experiments were 0.037–0.049 m and for rough bed (ks = 4.24 mm) were 0.07–0.084 m. In other words, with increasing the bed roughness height from zero (smooth bed) to 4.24 mm, the longitudinal dispersivities increased from 0.037 to 0.077 m, indicating an increase of about 108%. Furthermore, a relationship was developed using non-dimensional longitudinal dispersion (DL/(Vh)) as a function of relative roughness (ks/h). It can be concluded that taking into consideration bed roughness as the driving force of shear dispersion would improve predictive equations of the longitudinal dispersion in the rivers. As the bottom of all natural rivers has roughness elements with different sizes, the results of this study will definitely be useful in estimating the longitudinal dispersion coefficient in natural rivers and quantifying the effect of roughness in the longitudinal dispersion coefficient equations.

Numerical and Physical Modeling of the Effect of Roughness Height on Cavitation Index in Chute Spillways

This study presents the results of physical and numerical modeling of the effect of bed roughness height of chute spillways on the cavitation index. A 1:50-scale physical hydraulic model of the chute spillway of Surk Dam was constructed at the hydraulic laboratory of Shahrekord University, Iran. The experiments were conducted for different flow rates and the parameters of pressure, velocity, and flow depth in 26 positions along the chute. Finally, the ANSYS-FLUENT model was calibrated in the chute spillway using the experimental data by assumptions of two-phase volume of fluid and k–e (RNG) turbulence models. The cavitation index in different sections of the chute spillway was calculated for different values of bed roughness including the roughness heights of 1, 2, and 2.5 mm. Results showed that the minimum values of the cavitation index were 0.2906, 0.2733, and 0.2471 for the roughness heights of 1, 2, and 2.5 mm, respectively. The statistical significance analysis showed that reducing the roughness height from 2.5 to 1 mm would not change significantly the value of the cavitation index at 95% confidence interval.

Predicting Hydraulic Jump Length on Rough Beds Using Data-Driven Models

The hydraulic jump can be used for some purpose such as dissipating the flow energy in order to prevent bed erosion; aerating water and facilitating the mixing procedure of chemical that used for water purification. In this paper, various artificial intelligence (AI) models including gene expression programming (GEP), adaptive-neuro-fuzzy inference system with grid partition (ANFIS-GP), and neural networks (ANNs) were used to estimate developed and non-developed hydraulic jump length. Four various GEP, ANFIS-GP and ANN models including different combinations of Froude number, bed roughness height, upstream and downstream flow depth based on measured experimental data-set were developed to estimate hydraulic jump length variations. The root mean squared error (RMSE) and determination coefficient (R2) indices were applied for testing models’ accuracy. Regarding the comparison results, it was seen that the ANFIS-GP, ANN, and GEP models could be employed successfully in estimating hydraulic jump length. The comparison between three AI approaches emphasized the superiority of ANNs and ANFIS-GP over the other intelligent models for modeling developed and non-developed hydraulic jump length, respectively. For non-developed hydraulic jump, the R2 and RMSE values obtained as 0.87 and 2.84 for ANFIS-GP model.

Prediction of Glossosoma biomass spatial distribution in Valley Creek by field measurements and a three‐dimensional turbulent open‐channel flow model

AbstractThe fluid flow environment associated with high Glossosoma abundance is predicted by large‐eddy simulation of a natural turbulent open‐channel flow. The spatial distribution of Glossosoma was depicted by high resolution physical variables described by fluid flow and streambed topography. Variogram analysis of the streambed topography revealed a characteristic length scale of the streambed of the order 0.2 m over which bed roughness height was correlated. Flow simulation output was spatially and temporally averaged over the streambed characteristic length scale and linked to Glossosoma spatial density. A dimensionless scaling relationship between Glossosoma spatial distribution and streamwise velocity averaged in the longitudinal and transverse direction, spatial velocity fluctuation, and spanwise vorticity from the computational fluid dynamics simulation output explained 79% of the variation in observed dimensionless Glossosoma spatial density. The analysis demonstrated that computational fluid mechanics and high resolution bed topography could be instrumental in predicting benthic macroinvertebrate spatial distribution in streams and rivers.

Average Velocity of Solitary Coarse Grain in Flows over Smooth and Rough Beds

AbstractA formula is derived for the prediction of the average velocity of a solitary coarse grain moving over a rigid flat channel bed. To verify the theoretical consideration, data collected in the present study and reported in the literature (811 sets in total) are analyzed for typical bed roughness configurations. The results show significant improvements in the prediction of solitary grain velocity for both smooth- and rough-bed conditions in comparison with other formulas available in the literature. From the proposed formula, it follows that for rough-bed conditions, the dimensionless grain velocity depends on the Shields number and also the ratio of bed-roughness height to the grain diameter. The present study is limited to motion of solitary grains, but the result obtained implies that bed-load transport rate could be also affected by the equivalent bed-roughness height even under mobile bed conditions and such effects may become pronounced for nonuniform sediments.

Experimental study on coarse grain saltation dynamics in bedrock channels

AbstractSaltation of bed load particles on bedrock surfaces is important for landscape evolution and bedrock incision in steep landscapes. However, few studies have investigated saltation in bedrock channels where, unlike alluvial channels, the bed roughness height and the sediment size may be independent. To address this data gap, we measured the saltation hop height, hop length, and velocity of gravel saltating over a planar bed using 80–160 readings from high‐speed photography and direct measurements. Two separate dimensional analyses are used: one leading to a bed shear stress scaling and another leading to a Froude number (Fr) scaling. Our new saltation data coupled with numerous data from previous studies suggest that both shear stress and Fr‐scaling analyses are valid in characterizing bed load saltation dynamics with bed roughness ranging from smooth to alluvial beds. However, the Fr approach has the advantages that (1) there is no need to estimate a critical Shields stress , which alone can vary up to 2 orders of magnitude (e.g., 0.001–0.1) due to changes in relative bed roughness and slope and (2) the Fr‐based scaling fits the saltation data set better in a least squares sense. Results show that the saltation velocity of bed load is independent of grain density and grain size and is linearly proportional to flow velocity. Saltation height has a nonlinear dependence on grain size. Saltation length increases primarily with flow velocity, and it is inversely proportional to submerged specific density. Our results suggest that either or bed roughness coefficient must be properly estimated to yield accurate results in saltation‐abrasion models.

Predicting ripple geometry and bed roughness under combined waves and currents in a continental shelf environment

Waves, current and seabed response data collected by an instrumented tripod deployed on the Scotian Shelf during the winter of 1993/94 are analyzed to derive a ripple predictor for combined flows and to evaluate the applicability of existing ripple- and bedload-roughness algorithms under combined waves and current. Wave-dominant ripples developed during storms were generally higher and steeper than current-dominant ones. The ratio of the skin-friction wave shear velocity to that of the steady current, u ∗ ws /u ∗ cs , can be used to define the various types of ripples under combined flows. By comparing the measured ripple geometry and the predictions by existing ripple predictors, the wave-ripple predictors of Nielsen (1981), and Grant and Madsen (1982) are found to over-predict ripple height and ripple roughness for combined flows under the conditions of the present study. These methods also neglect the enhancement of shear stress at the ripple crest. A new empirical ripple predictor is proposed and it uses the combined shear velocity and the ratio u ∗ ws /u ∗ cs to predict the heights and wavelengths of ripples and their dynamic transition under combined flows. The effect of enhanced shear velocity at the ripple crest is also incorporated for the prediction of ripples in the weaktransport range. A simplified logarithmic profile method and the values of the bedload shear velocity due to the combined grain size and bedload roughnesses are used to evaluate the applicability of various ripple- and bedload-roughness height algorithms under combined flows. While the ripple roughness height algorithm of Grant and Madsen (1982) is found to give good predictions of the total current shear velocity u ∗ c and apparent bottom roughness z 0c, the algorithm of Nielsen (1992), tends to underpredict both parameters. The bedload roughness algorithms of Nielsen (1992) and Li et al. (1997) are both found to give reasonable predictions under combined flows. The total bed roughness height under combined flows can be expressed as k b=2.5 D+27.7 η 2/ λ+170 D( θ cws− θ cr) 0.5.

Turbulence Modulation and Particle Velocities over Flat Sand Beds at Low Transport Rates

The presence of sand moving at low transport rates over a flat bed modulates the production of turbulence when compared to clearwater flow at similar mean flow conditions. Phase Doppler anemometry is used to discriminate the turbulence characteristics of the carrier fluid from the sediment grains (0.22 mm diameter) and shows that the presence of mobile sediment increases the near-wall velocity gradient and shear velocity when compared with the clearwater values. This increased shear velocity is associated with a greater bed roughness height and near-bed turbulence intensities and smaller mixing lengths. Quantification of slip velocities between the fluid and sediment phases reveals particle Reynolds numbers that range from 1 to 30. Turbulence enhancement is shown to occur at lower values of both the Stokes number and ratio of the particle size-to-turbulent length scale than in past work. Several mechanisms of turbulence modulation may be invoked to explain these changes, including increased bed roughness, eddy shedding from large grains, grain inertial effects, and particle-coherent structure interactions. These mechanisms may be significantly influenced by both particle-particle and particle-wall interactions. Since mobile sediment modulates the carrier fluid turbulence, there is a need for modification of existing theories of sediment suspension and for caution when interpreting velocity profiles that are obtained without discriminating the fluid and sediment phases.

Initiation of Bed Forms on a Flat Sand Bed

The results of a series of 47 bed development experiments are combined with appropriate experimental results of other authors. The generation of initial bed waves, termed sand-wavelets, on a plane sand bed is found to be instigated by the occurrence of a random pileup of sediment, this pileup subsequently growing as further sediment is trapped by the pileup. When the height of the pileup approaches the bed roughness height, further pileups are generated at preferred spacings downstream. The wavelength l of the sand-wavelets so generated is relatively insensitive to the applied bed shear stress and primarily a function of the size d g of the sediment. This wavelength can be described by the empirical formula (l/dg)R*c0.2= 10 2.5. From a theoretical viewpoint, this wavelength is approximately described by a reassessment of the flow system instability analyses of Richards. Both ripples and dunes are found to develop from the sand-wavelets.

Field measurements of the hydrodynamic roughness of the deep-sea boundary

Measurements were made of the velocity profile within a meter of the deep-sea floor at four locations off the California coast. Photographs of the boundary were taken to provide a means of evaluating the bed configuration. From these data it was possible to estimate the boundary shear stress, the drag coefficient, and the approximate height of the bed deformities. It is concluded that the deep-sea boundary-layer flow is hydrodynamically smooth only during periods of relatively low velocity and that it is generally transitional in nature. The boundary-layer flow is characterized by a drag coefficient that is highly variable and is related to the flow conditions as well as the bed configuration. The commonly used roughness Reynolds number criterion appears adequate to predict the transition from hydrodynamically smooth boundary conditions as long as the bed-roughness height, rather than the sediments diameter, is used as the characteristic length.