Bed Roughness Research Articles

Characteristic Analysis of Vertical Tidal Profile Parameters at Tidal Current Energy Site

Many mathematical models have been proposed to estimate vertical tidal current profiles. However, as previous studies have shown that tidal current energy sites have different characteristics in their vertical tidal current profiles, it is necessary to estimate the profiles from field-measured data for practical purposes. In this study, we measured layered tidal currents over two months using an acoustic Doppler current profiler (ADCP) to analyze the characteristics of vertical tidal current profiles at the Jangjuk Strait, a candidate site for tidal current energy. As a result, the power law parameter α and bed roughness β were estimated as 4.51–12.41 and 0.38–0.42, respectively. Additionally, the maximum roughness length representing seabed roughness in the logarithmic profile was estimated as 0.221 m, and the estimated friction velocity was 0.038–0.194 m/s. Furthermore, a high correlation was observed between the depth-averaged tidal current velocity and friction velocities at all sites during flood and ebb tide conditions. A high correlation was also found between the bed roughness, roughness length, and power law exponent at relatively deeper sites. Tidal current energy sites display distinct characteristics compared to other sea areas. Therefore, it is essential to account for field conditions when conducting numerical modeling and design.

Distribution of turbulent eddies under wave-current coexisting flow over hemispherical rough bed

Distribution of turbulent eddies under wave-current coexisting flow over hemispherical rough bed

Characteristics of dynamic thickness change across diverse outlet glacier geometries and basal conditions

Abstract Outlet glaciers in Greenland are undergoing retreat and diffusive thinning in response to external forcings, but the rates and magnitudes of these responses differ from glacier to glacier for unclear reasons. We test how changes in ice overburden pressure and basal lubrication affect diffusive thinning rates and their spatial patterns by conducting numerical experiments over various idealized Greenland-like glacier domains. We find that ~10 km frontal retreat over a decade can produce sustained thinning rates as large as 16 m a−1 due to ice overburden pressure changes, at outlet glaciers with high basal drag (>60 kPa) and lateral resistive stress (>70 kPa). Localized basal lubrication perturbations induce upstream thinning and downstream thickening up to 12 m a−1; the duration of the lubrication forcing generally has a greater effect than its intensity on induced thickness changes. Lastly, episodic grounding line retreats over a rough bed produce a stepped time series of thinning broadly consistent with observations of dynamic elevation change on multiple Greenland glaciers. Our findings highlight the critical role of the total grounding zone – not ice front position – through the resistive stress change in relation to total glacier thinning.

Turbulence characteristics of non-spherical isolated particles with different flatness on a rough bed surface

The transport of non-spherical particles on bed is significantly influenced by flow dynamics and particle shape. To study the impact of changes in the flatness of isolated bed particles on the surrounding flow characteristics, we prefabricated three isovolumetric particles with varying levels of flatness and analyzed the flow field around them using two-dimensional particle image velocimetry measurement methods. The results show that the presence of the particles significantly alters the turbulence parameters of the surrounding flow. Specifically, the influence extends up to 1D upstream of the particles, 0.5D above the particle tops, and throughout most of the downstream region, where D represents the equivalent particle diameter. The particles cause a reduction in Reynolds stress and turbulent kinetic energy in the near-bed upstream region, with a more pronounced effect observed for particles with lower flatness. Increased particle flatness is associated with a smaller difference in the longitudinal time-averaged velocity between upstream and downstream regions. In the wake region, particle flatness shows a negative correlation with Reynolds stress, longitudinal turbulence intensity, and turbulent kinetic energy, with this effect being more pronounced closer to the particles. Quadrant analysis indicates that ejection (Q2) and sweep (Q4) quadrant events remain dominant in the presence of particles, while particle flatness positively correlates with the frequency of outward (Q1) and inward (Q3) events in the wake region and negatively correlates with the frequency of Q2 and Q4 events. This study enhances our understanding of particle-fluid interactions, particularly the response relationship between non-spherical particles and turbulent mechanisms.

Large and very large scale motions in open-channel flows over rough bed in the presence and absence of a mixing layer

Large and very large scale motions in open-channel flows over rough bed in the presence and absence of a mixing layer

Impact of Fenton aging on the incipient motion of microplastic particles in open-channel flow

Upon entering the environment, Microplastics (MPs) experience aging processes that modify their properties and integrity. Previous methods for predicting the incipient motion of MPs have been validated using pristine plastics, which do not account for the effects of aging. This can lead to uncertainties in both quantification and characterization. This study investigates the effect of aging on the incipient motion of MPs with different bed roughness (smooth and rough beds) and MP properties (e.g., grain sizes and densities) in an open-channel flow. Five types of MPs were subjected to four different degrees of aging using the Fenton reagent, and their incipient velocities were tested on beds with two distinct roughness. The results suggest that the incipient velocity of MPs increases linearly with aging. However, this increase is not uniform across different particles and bed roughness. Upon comparing various commonly employed sediment incipient velocity equations, experimental results are in agreement with Ruijin Zhang's equation as the most precise. The parameters in Ruijin Zhang's equation are modified to enhance its applicability for predicting the incipient velocity of aged MPs. This study provides novel insights into the incipient motion of aged MPs in an open-channel flow, highlighting the intricate interaction between aged MP characteristics and bed roughness.

Revisiting wave friction factors for rough turbulent flow

The wave friction factor 𝑓𝑤 and the phase lead of the seabed shear stress over the free stream velocity 𝜑 for rough turbulent flow are revisited by utilizing the similarity theory used by Myrhaug (1989). Results are obtained for 𝑓𝑤 and 𝜑 by determining the similarity coefficients using the Dixen et al. (2008) data for large bed roughness. Comparisons are made with other experimental data and wave friction factor formulae. As a result, an approximation for 𝑓𝑤 by disregarding the phase 𝜑 is recommended covering a wide range of amplitude-to-roughness ratios.

Exploring coupling effects of rainfall and surface roughness on the sheet flow velocity

Accurately describing the path of sheet flow (SF) is crucial in soil erosion. Raindrop impact and underlying surface conditions can affect the SF velocity by changing the velocity profile. However, since this information is rarely known, the estimation of SF velocity is inaccurate. A series of upstream inflow and rainfall experiments were carried out on an impermeable flume to determine the coupling effects of rainfall and rough bed surfaces on the SF velocity and correction factor (α). The results showed that the roughness of the bed surface had a more pronounced effect on reducing the mean velocity compared to the surface velocity in both cases with and without raindrop impact. The raindrop impact notably reduced the flow velocity near the water surface, while the mean velocity slightly decreased or remained almost constant with increasing rainfall intensity. The reduction in SF velocity can be explained by the combined effects of the roughness reducing the mean velocity (up to 33.52%) and the raindrop impact reducing the surface velocity (up to 25.43%). In addition, α was not a constant when the SF was subjected to raindrop impact. The rainfall was found to increase α, while the roughness of the bed surface reduced α for all cases. Finally, a model was created to forecast α based on the ratio of water depth to roughness height, hydraulic slope, and rain Reynolds number. The results are valuable in soil erosion by providing accurate α for estimating the surface and mean velocities of SF.

Bed shear stress distribution across a meander path

ABSTRACT Laboratory experimentation for bed shear stress distribution has been carried out in two sets of meandering channels. The channels have crossover angles of 110° and 60° constructed by ‘sine-generated’ curves over a flume of 4 m width. Variations in bed roughness were studied for the meandering main channel. Bed shear stress distribution across a meandering length for the 110° and 60° channels was examined for different sinuosities and roughnesses. The boundary shear stress study illustrated the position of maximum shear along the apex section and across the meandering path. These variations were observed for different flow depths. A comparison of the bed shear among the three experimental channels was conducted, and the results were analyzed.

Bed Roughness Incorporated Lift Force Formulation for Sediment Concentration Solutions

Within the field of sediment transport dynamics, there is an emphasis on the need for providing and improving simplified analytical solutions from which the findings can be extended upon. Typically, solutions have been developed from the understanding and incorporation of vertical velocity components/distribution; however, there remains opportunity for analytical enhancement. Under uniform open channel dilute sediment laden flow conditions, the type II concentration profile reflects the maximum concentration at a distance from the bed. To analytically depict the type II profile, it is essential to describe the vertical velocity components acting within the flow hydrodynamics. Yet, the impact of bed roughness on the vertical velocity distribution influencing a sediment particle has not been articulated. Hence, in this research, the impact of bed roughness is incorporated into the vertical velocity distribution to formulate the lift force acting upon a particle, to provide a sediment concentration solution. The proposed analytical vertical velocity distribution solution is validated against existing formulations, followed by the proposed sediment concentration being validated against existing solutions. Here, the results indicate that analytically incorporating the bed roughness alongside the secondary current induced vortices provides a lift force solution that can depict the type II concentration profile. Under relatively low shear velocity, increasing the sediment diameter allows for a clearer depiction of the near surface region and profile curvature. Furthermore, the proposed sediment concentration solution can accurately depict the near surface region, curvature of concentration, and maximum concentration under relatively large shear velocity data. Importantly, under conditions where both the shear velocity and sediment diameter are relatively large, the maximum concentration is captured with greater accuracy; however, the curvature of the profile and the near surface region accuracy are hindered. It is to be noted, that due to the formulation of the proposed velocity distribution the accuracy of the solution towards the bed decreases due to instability caused as the characteristic depth decreases to zero.

Evaluating the Hydraulic Performance of USBR II Stilling Basin with Rough Bed

Evaluating the Hydraulic Performance of USBR II Stilling Basin with Rough Bed

Observing and Modeling Short‐Term Changes in Basal Friction During Rain‐Induced Speed‐Ups on an Alpine Glacier

AbstractBasal shear stress on hard‐bedded glaciers results from normal stress against bed roughness, which depends on basal water pressure and cavity size. These quantities are related in a steady state but are expected to behave differently under rapid changes in water input, which may lead to a transient frictional response not captured by existing friction laws. Here, we investigate transient friction using Global Positioning System vertical displacement and horizontal velocity observations, basal water pressure measurements, and cavitation model predictions during rain‐induced speed‐up events at Glacier d'Argentière, French Alps. We observe up to a threefold increase in horizontal surface velocity, spatially migrating at rates consistent with subglacial flow drainage, and associated with surface uplift and increased water pressure. We show that frictional changes are mainly driven by changes in water pressure at nearly constant cavity size. We propose a generalized friction law capable of capturing observations in both the transient and steady‐state regimes.

Rough bed hydrodynamics under wave-current interactional flow field: double averaging approach

ABSTRACT The effect of surface wave on the double-averaged (DA) current-only flow properties is examined over two different types of rough beds comprising hemispherical elements with regular and staggered arrangements. For each case, the relative roughness spacing of p/r = 4 is maintained, where p = pitch distance and r = hemisphere height. The changes in turbulence characteristics against the water depth are analysed for superimposed surface waves propagating along the current with 1 and 2 Hz frequencies. The effect of regular and staggered roughness arrangements on different turbulence characteristics is explored. Results demonstrate that the DA stream-wise velocity decreases and increases at the outer region and within the roughness region, respectively, due to wave propagation. Owing to the surface wave, the DA intensity and form-induced intensity increase above the roughness layer and decrease within the bottom boundary layer. The values of form-induced intensities are smaller (greater) than the DA turbulence intensities at each considered flow depth within the roughness (outer) layer. For the entire flow depth, the values of DA Reynolds stress decrease due to wave interactions along a steady current. Again, the turbulence is observed to be close to isotropic for the current-only flow at the outer zone, while the flow transits to an anisotropic state with wave imposition on current. Further, the computed values of the turbulence indicator and dispersion functions are used for the identification of the level of turbulence and dispersion at each flow depth for the locations upstream and downstream of a hemisphere in the hemispherical bed.

Design of Hydraulic Jump Stilling Basin over Rough Beds Using CFD

Design of Hydraulic Jump Stilling Basin over Rough Beds Using CFD

Experiments on flow velocity profiles in mountain river channels

ABSTRACT Research on the vertical profiles of flow velocity in mountainous river channels is limited, particularly in scenarios where complex bed geometries are absent. Due to the coarse roughness and seepage flow on streambeds composed of gravel, the conventional formulae for flow velocity profiles derived from fluvial river channels do not apply to mountainous river channels. Based on flume experiments with a bed packed with natural gravel and a slope ranging from 0.006 to 0.16, we derived a theoretical formula for flow velocity profiles. This new formula integrates the influence of the subsurface flow and velocity reduction near the water surface, demonstrating a strong alignment with measurements. Our findings indicate that for shallow water flow over rough bed surfaces, the turbulence intensity diminishes along the vertical direction in the near-bed region while remaining relatively constant in the upper water body. Contrary to conventional theories which attribute the increase in flow resistance and the decrease in sediment transport rates in mountainous river channels to form drag, our study emphasizes that the subsurface flow plays a significant role in the overall flow resistance of mountainous river channels and should not be overlooked.

Conservation equations for open-channel flow: effects of bed roughness and secondary currents

AbstractTime-averaged velocity fields in uniform open-channel flows over rough beds may exhibit spatial heterogeneities due to the effects of bed roughness and secondary currents (SCs). The latter typically originate from the turbulence anisotropy and spatial heterogeneity introduced by the solid and mixed corners (i.e., between sidewalls and water surface), but may also appear due to roughness spanwise heterogeneities, e.g., associated with patchy vegetation distributions or streamwise sediment ridges on the channel bed. In this paper, we propose rigorous conservation equations for momentum, kinetic energy and fluid stresses accounting for the contributions of bed roughness and SCs, separately. Particular attention is given to the terms regulating the energy exchanges between roughness-induced and SC-related motions, which are expected to provide information on the physical mechanisms leading to the generation of roughness-induced SCs. The proposed approach is illustrated using a large-eddy simulation of a rough-bed open-channel flow.

Sediment transport modelling in densly populated urban areas due to earthfill dam break

This study simulates two dimensional sediment transport, as a result of overtopping earthfill dam break, in urban areas. The model can consider breaching, removal of sediment from dam body, and transport of sediment. The model is first validated by simulating laboratory experimental data that involved measurements of levels, and longitudinal sediment profiles, and sediment distribution. Downstream side of the experimental canal is designed as; (1) smooth bed, and (2) rough bed with concrete blocks. For both cases, the model simulations are found to be satisfactory. The model is then applied to simulate artificial overtopping break scenarios of two real earthfill dams. The AW3D30 is used as the source data for representing the topographic surfaces and the LULC dataset is generated from the ESA’s Sentinel-2 imagery. The results reveal that downstream of the dams can be subject to both scour, at onset of the dam break, and substantial deposition after having reservoir completely emptied. The cut can go all the way to dam bed. The scoured areas can be refilled after peak discharge recedes. Sediment depths can reach up to 1.5 m in the case of Urkmez Dam break in some areas in the vicinity of Urkmez Town and up to 3 m at the downstream area of Alibey Dam in Istanbul, implying disastrous consequences for the settlement areas.

Acclimatize experimental approach to adjudicate hydraulic coefficients under different bed material configurations and slopes with and without weir

The primary purpose of this study was to evaluate the hydraulic coefficient of coarse aggregate grain size beds and hydraulic parameters under random and perpendicular bed configurations, as well as to explore the discharge coefficient for rectangular weirs. The research objectives were to compare flow resistance coefficients, evaluate the discharge coefficient for rectangular weirs, investigate the relationship between roughness coefficient and hydraulic parameters, and validate the theoretical hydraulic equation for the rectangular weir. This was achieved by analysing different bed configurations, bed slopes, and 20 and 30-mm bed materials. Sieve analysis was conducted on bed materials using American-standard sieves to determine their particle size distribution. The experiment was performed in a rectangular flume measuring 12 m in length, 0.31 m in width, and 0.45 m in depth. In a laboratory experiment, water was pumped into a flume using centrifugal pumps, and a rectangular weir was attached downstream for discharge measurement. The experiment investigated factors such as Manning roughness coefficient, bed material geometry, bed slope, and weir shapes. Approximately 1680 tests were conducted to analysed the impact of these factors on discharge and the coefficient of discharge. The average Manning's roughness coefficients for a grain size of 20 mm were 0.019 (with weir) and 0.019 (without weir) in a random bed configuration, and 0.028 (with weir) and 0.027 (without weir) in a perpendicular flow bed configuration. For a grain size of 30 mm, the coefficients were 0.023 (with weir) and 0.022 (without weir) in a random bed configuration, and 0.033 (with weir) and 0.026 (without weir) in a perpendicular flow bed configuration. The presence of a weir has affected Manning's roughness coefficients and discharge coefficients. With a weir, the roughness coefficients have generally been higher compared to without a weir, indicating increased roughness in the channel. The discharge coefficient for a rectangular weir with a grain size of 20 mm ranged from 0.39 to 0.84 (random bed) and 0.27 to 0.68 (perpendicular flow bed), while for a grain size of 30 mm it ranged from 0.31 to 0.81 (random bed) and 0.23 to 0.48 (perpendicular flow bed). The discharge coefficients have varied depending on the grain size and bed configuration, reflecting different flow efficiencies over the weir. Rough particles influenced flow and Manning's roughness coefficient value, then reduced discharge and velocity values. Under two bed configurations and slopes, beds with a grain size of 30 mm have higher roughness coefficients compared to those with a grain size of 20 mm. The models have shown that the roughness coefficient is inversely proportional to the discharge and directly proportional to the tailgate water levels. The coefficient of roughness and discharge coefficient are mainly influenced by the channel slopes, bed roughness, bed grain size, and bed configuration. A randomly configured bed with a 20 mm grain size gravel bed is preferred over a perpendicular bed configuration to handle high discharges. Using a 20 mm grain-size gravel bed in open-channel flow is more suitable than a 30 mm grain-size bed. Relying on the constant friction factor, Manning's n, is not recommended as it may result in design errors. These findings have the potential to improve hydraulic engineering design practices, enhancing the accuracy and efficiency of open-channel flow systems.

Efficient prediction of tidal turbine fatigue loading using turbulent onset flow from Large Eddy Simulations

To maximise the availability of power extraction from a tidal stream site, tidal turbines need to be able to operate reliably when located within arrays. This requires a thorough understanding of the operating conditions, which include turbulence, velocity shear due to bed proximity and roughness, ocean waves and due to upstream turbine wakes, over the range of flow speeds that contribute to the loading experienced by the devices. High-fidelity models such as Large Eddy Simulation (LES) can be used to represent these complex flow conditions and turbine device models can be embedded to predict loading. However, to inform micro-siting of multiple turbines with an array, the computational cost of performing multiple simulations of this type is impractical. Unsteady onset conditions can be generated from the LES to be used in an offline coupling fashion as input to lower-fidelity load prediction models to enable computationally efficient array design. In this study, an in-house Blade Element Momentum (BEM) method is assessed for prediction of the unsteady loads on the turbines of a floating tidal device with unsteady inflow developed with the in-house LES solver DOFAS. Load predictions are compared to those obtained using the same unsteady inflow to the commercial tool Tidal Bladed and from an Actuator Line Model (ALM) embedded in the LES solver. Estimates of fatigue loads differ by up to 3% for mean thrust and 11% for blade root bending moment for a turbine subject to a turbulent channel flow. When subjected to more complex flows typical of a turbine wake, the predictions of rotor thrust fatigue differ by up to 10%, with loads reduced by the inclusion of a pitch controller.

Advp Measurements Of Flow Over Low-Angle Bedforms In A Laboratory Flume Setup

In many coastal and estuarine environments, bedforms develop due to the complex interactions between hydrodynamics, sediment transport and morphology which affect hydro-morphodynamic processes at various spatio-temporal scales. Furthermore, bedforms are considered main drivers of flow resistance via turbulence and affect near bottom processes such as bed roughness, bottom shear stress and turbulent structures. A detailed study and characterisation of flow over bedforms is therefore significant to many fundamental and engineering applications such as sediment transport calculation and prediction, channel managements, or burial of submarine cables. Asymmetric dunes with an angle-of-repose (30°) lee side have been well studied (Best, 2005; Venditti, 2013). They are representative of dunes commonly found in laboratory flumes and small rivers. Over such bedforms, flow is characterised by a permanent flow separation zone and intense turbulence over their lee side. Recently, it was shown that bedforms in large rivers are mainly low- to intermediate-angle dunes (mean lee side ca. 5-20°) having their steepest slope located close to the trough (Cisneros et al., 2020). Over such low- and intermediate-angle river dunes, flow separation is inexistent or intermittent and only little turbulence is generated (e.g. Kwoll et al., 2016; Lefebvre and Cisneros, 2023). Estuarine bedforms also possess mostly low- to intermediate-angle mean lee slopes between 5° and 20° (Dalrymple and Rhodes, 1995; Lefebvre et al., 2021). However, contrary to river bedforms, estuarine bedforms usually have a sharp pointed crest with the steepest slope situated near the crest and a relatively flat trough (Aliotta and Perillo, 1987; Lefebvre et al., 2021). It is not clear yet how much flow properties vary between high-angle flat-crested dunes and low-angle sharp-crested dunes. In particular, it is unknown whether a permanent or an intermittent flow separation can be observed over some segments of the lee side. Furthermore, the relation between the reversing tidal flow and natural estuarine morphology is not well understood yet. In this study, laboratory flume experiments will be conducted to measure the mean flow and turbulence over estuarine bedforms. Previous experiments performed by Carstensen and Holzwarth (2023) demonstrated the potential of high-resolution measurements over a large-scale estuarine dune in a laboratory flume. Building on this, experiments will be carried out over a fixed concrete low- and intermediate- angle estuarine bedform field. The results from this study can be used to characterise in detail the flow dynamics over estuarine bedforms.