Variations in roughness predictions (flume experiments)
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Data of flume experiments with bed forms are used to analyze and compare different roughness predictors. In this study, the hydraulic roughness consists of grain roughness and form roughness. We predict the grain roughness by means of the size of the sediment. The form roughness is predicted by three approaches: Van Rijn (1984), Vanoni & Hwang (1967) and Engelund (1966). The total roughness values (friction factors) are compared with the roughness values according to the DarcyWeisbach equation. Results show that the different methods predict different friction factors. In future research uncertainties in the hydraulic roughness will be taken into account to determine their influence on the computed water levels.Keywords:
Hydraulic roughness
Flume
Roughness length
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Flow field over macro‐roughness elements, as, for example, in flow conditions such as of steep mountain streams, is investigated by means of a theoretical model and a laboratory study with the main aim of understanding the influence of concentration and planimetric disposition of grain roughness on flow resistance. A theoretical model for the average velocity distribution profile over a bed with macro‐roughness elements is developed herein. The model is based on a two‐layer approach which takes into account the displacement of the main flow due to the drag around the roughness. The laboratory study is composed of an extensive set of flume experiments in the case of steep slopes and large relative roughness conditions. For a given flow discharge and bed slope, various experiments have been carried out with different pattern and spacing of the roughness elements constituted by river pebbles. Application of the theoretical model to the experimental results enables the determination of how flow resistance is affected by the macro‐roughness; in particular, it appears that when the concentration of the roughness is within an optimal range, flow resistance is maximum.
Roughness length
Hydraulic roughness
Flume
Flow resistance
Flow conditions
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Citations (71)
Determination of flow resistance is an essential parameter for studying and hydraulic analysis of the open channels. Manning roughness coefficient is often be used for description flow resistance or relative roughness of a channel or floodplain. This research was focused on the effect of the height, density and arrangement of the artificial cubic roughness elements on Manning roughness coefficient. In total 48 experiments were conducted with three values of roughness height (5,10 and 15 mm), two percentages of roughness density (25% and 50%), two types of roughness arrangement (regular and staggered patterns), four flow discharge values on rough bed and four experiments with smooth bed in a laboratory flume. A length of 3.7 meter of the flum bed was designed as the rough bed according to the experiments plan. The results showed that the influence of the roughness arrangement on changing the value of Manning roughness coefficient (n) is more than the influence of the height and density of roughness. Also the mean value of Manning coefficient increased from 0.009 in smooth bed to 0.0132 in regular pattern and 0.0175 in staggered pattern. At last in order to compare with the results of previous works, some relationships have been offered in order to estimate the Darcy-weisbach coefficient (f) on the basis of the roughness density percentage at the bottom (C), relative submergence ( s yk ), Froude number of flow ( r F ) and a factor related to roughness arrangement(€). Keyword: Darcy-weisbach coefficient, Open channel, Relative submergence
Froude number
Flume
Hydraulic roughness
Roughness length
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A Study on Roughness Coefficient Estimations in Gravel Bed Stream without Water Level-Discharge Data
본 연구는 자갈하천에서 하상에 분포하는 입자에 작용하는 전단력을 이용하여 등가조도를 산정할 수 있는 모형을 개발하였다. 산정된 등가조도는 수위-유량자료가 부재한 하천에서 유량에 대한 수위를 산정하고 조도계수를 산정하는데 이용하였다. 대상하천은 섬진강의 중 하류부인 구례수위표와 송정수위표 구간으로 선정하였다. 등가조도는 개발된 모형에 의해 구례수위표지점에서 0.194m가 산정되었다. 산정된 등가조도를 흐름모형에 적용하여 계산된 수위유량자료를 관측된 자료와 비교한 결과 6% 이내의 오차를 보였다. 조도계수는 대상구간에 대해 부정류 해석을 실시하여 유량규모별로 계산된 수위와 관측된 수위에 대해 산정하였다. 그 결과 관측된 수위와 계산된 수위에 의해 산정된 조도계수는 $0{\sim}0.002$ 의 오차를 보였고, 조도계수의 가변성도 고려할 수 있었다. This study developed a model that could calculate equivalent roughness using shear stress acting on distributed grains in gravel bed stream. The estimated equivalent roughness by the model developed was used for estimation of water level and roughness coefficient in the stream without water level-discharge data. The model was applied to the Gurey-Songjeong stage station section located in the Sumjin river mid-downstream. The equivalent roughness by the model developed in this study was estimated to be 0.194m at the Gurey stage station. Calculated water level which the estimated equivalent roughness was applied to the flow model was shown ewer of within 6% in comparison with observed water level. Also, roughness coefficient was estimated using observed and calculated water level about each discharge scale by unsteady flow analysis. As a result, error of roughness coefficient estimated by observed and calculated water level was shown error of $0{\sim}0.002$ and could consider variability of roughness coefficient.
Hydraulic roughness
Discharge coefficient
Roughness length
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Citations (5)
Estimates of bed roughness used for predictions of sediment transport are usually derived either from simple scalars of the physical roughness (i.e., ripple height or grain size) or from the hydrodynamic roughness length (Zo) based upon velocity gradient estimates in the benthic boundary layer. Neither parameter accounts for irregular bed features. This study re-evaluates the relation between hydrodynamic roughness and physical bed roughness using high-resolution seabed scanning in the inlet of a shallow lagoon. The statistically-robust relationship, based on a 1D statistical analysis of the seabed elevation at different locations of the Cabras lagoon. Sardinia, has been obtained between Zo and the topographical bed roughness Ks by defining Ks = 2*STD + skin friction, with STD the standard deviation of the seabed elevation variations. This correlation between Ks and Zo demonstrates that the roughness length is directly influenced by irregular bed features, and that the Reynolds number accounts for the total drag of the bed: the data points collapse on the Law of the Wall curves with a fitting factor x = 0.5. Further testing must be done in other locations and in the fully-rough domain in order to test how widely those new parameters can be applied.
Seabed
Roughness length
Hydraulic roughness
Berm
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Citations (4)
There is no accepted standard equation for predicting flow resistance in gravel rivers. This is mainly because of the lack of an effective approach for describing the roughness of the gravel surface. This paper aims at describing the roughness of gravel surfaces based on characteristics of the elevation field rather than conventionally used grain size distribution. A new parameter—the coarseness parameter—in addition to the steepness parameter and standard deviation of the elevation field is proposed for characterizing the roughness of gravel surfaces. Two groups of rough surfaces (A and B) were generated using unconditional Gaussian simulation to study the influence of the coarseness and steepness parameters on effective roughness. The effective roughness of groups A and B was calculated by a two-equation turbulence model, and a quantitative relationship between the effective roughness and proposed parameters was derived. The resulting relationship derived from groups A and B was validated using data from flume experiments (group C). The results confirm the dependence of the effective roughness on the proposed parameters.
Flume
Hydraulic roughness
Roughness length
Elevation (ballistics)
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Citations (17)
Large-scale geometric roughness elements is one of the solutions that is used to protect openchannels from erosion. It is use to change the hydraulic characteristics of the flow. It may be concrete blocksor large stone placed at the bed of the channel to impose more resistance in the bed. The height of theseroughness elements is an important parameter that can affect the hydraulic characteristics of the flow. Usinga series of tests of T-shape roughness elements at three different heights, 3, 4.5, and 6cm, arranged in thefully rough configuration in order to investigate the velocity distributions along the flume. ANSYSParametric Design Language, APDL, and Computational Fluid Dynamics, CFD, were used to simulate theflow in an open channel with roughness elements. This simulation helps to find the best height of roughnesselements that can be used to change the hydraulic characteristics of the flow. The results showed that thevelocity values are decreased near the bed by about 61%, 58%, and 64% in case of 3cm, 4.5cm, and 6cmroughness heights consequently compared with the velocity of the control case. The velocity values areincreased near the free surface by about 32% and 19% in case of roughness elements height 6cm comparedwith 3cm and 4.5cm roughness heights respectively. The case of 6cm roughness height is considered to bethe effective case for decreasing the velocity values near the bed of the flume.
Flume
Hydraulic roughness
Roughness length
Hydraulic diameter
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Based on the friction partition theory (Einstein 1942) and affections of breadth depth ratio on roughness(Knight 2002), the relationship between bed and wall roughness coefficient is researched though 28m flume experiment. The measured water level agree with the calculated one which supposed n = n b = n w . And it proves that bed roughness coefficient equal to wall roughness coefficient while the rectangle flume is made of same material in acceptable precision.
Flume
Rectangle
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The eguivalent roughness of a movable bed is considered. Relationships are given which can be used to determine the equivalent bed roughness from the bed material size and the bed-form dimensions. In the case of a plane bed the equivalent roughness is related to the D90 of the bed material. Based on flume and field data, the equivalent roughness may vary from 1-10 D90. Similar values were reported by other research-workers. In the case of bed forms the equivalent roughness is related to the average height and length of the bed forms. Both flume and field data were used to determine a functional relationship. The proposed relationship yields values which are considerably smaller than other existing relationships.
Flume
Hydraulic roughness
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Citations (163)
Tests have been done in a laboratory flume, to evaluate the influence vertical rods 3 mm dia on the hydraulic and sedimentological characteristics the flow. The first two tests have been done with some 10 cm waterdepth. In both tests the s ame flow rate and sediment transport was used, the only difference between the tests being the artificial roughness, which was applied during the second test. The following two tests were done in a similar way with a waterdepth about ~ and with a reduqed flow rate and sediment transport. For both 10 em and 5 om waterdepths, the bar resistance was then measured in the flume without bakelite. The influence the rods on the ripple factor, ~, thus on the bottom roughness, could be evaluated, after e1imination the bar resistance. In the case 10 em depth, the artificial roughness caused a reduction of bottom roughness, while with a depth 5 cm an increased bottom roughness with the bars was found. By calculating the bed load from the product the mean ripple celerity and half the meru1 ripple height a very good agreement was found with the actually measured quantities.
Flume
Hydraulic roughness
Rod
Bar (unit)
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Several expressions for characterizing bed roughness produced by a layer of saltating sediment have previously been proposed, and another is presented herein. These expressions relate the magnitude of the bottom roughness to the thickness of the near‐bed layer of saltating particles. In two previous studies, the empirically set constant of proportionality was determined using data for which the roughness associated with bed forms had to be calculated and subtracted from the total observed roughness, the remaining roughness being attributed to bed load. In this paper the expressions for bottom roughness are tested against values observed in upper plane bed flows where the transport is high and the bed is nearly flat. The results indicate that the two expressions in which the coefficient of proportionality was set using data requiring removal of bed form roughness overpredict the observed roughness by roughly 2 orders of magnitude, whereas the other two equations show good agreement with the measurements. The equations that overpredict the roughness may have attributed the roughness of small bed forms to bed load.
Hydraulic roughness
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Citations (141)