Shallow flow over a bed with a lateral change of roughness

Water Resources Research, 2011

1] Research on lateral exchange of streamwise momentum between parallel flows in open channels has mainly been focused on compound channels composed of a main channel and a floodplain, on mixing layer development downstream of river confluences, and on partially vegetated channels. This study aims to establish the mechanisms responsible for streamwise momentum exchange between concurrent parallel flows subject to different bed roughnesses. The contribution of momentum exchange of each mechanism is determined on the basis of flume experiments. For an initially uniform flow that experiences a bed with two parallel lanes of different roughnesses, three mechanisms for exchange of streamwise momentum can be distinguished: cross-channel secondary circulations, turbulent mixing resulting from vortices acting in the horizontal plane, and mass transfer from the decelerating flow over the rough-bottomed lane to the accelerating flow in the parallel smooth-bottomed lane. The mass transfer and associated momentum transfer across the channel cause a gain in longitudinal momentum. The secondary circulations are driven by turbulence anisotropy and feature a main cell that extends over the full water depth, which is centered at the smooth side of the smooth-to-rough transition. The gain of momentum corresponding to the mass transfer in the developing reach is on the same order of magnitude as the momentum exchange by turbulent mixing and of that by secondary circulations in the most downstream position, where the flow is nearly developed. The contribution of the secondary circulations to the exchange of streamwise momentum between the parallel flows gradually becomes dominant over the contribution of turbulent mixing when depth increases. Citation: Vermaas, D. A., W. S. J. Uijttewaal, and A. J. F. Hoitink (2011), Lateral transfer of streamwise momentum caused by a roughness transition across a shallow channel, Water Resour.

38th IAHR World Congress - "Water: Connecting the World", 2019

Knowledge of rough-bed flows is important for river engineers and environmental scientists alike as most natural surface water flows are considered within the hydraulically rough flow regime. This study helps our understanding of flow characteristics above a rough gravel riverbed, which in-turn, affect fine sediment and nutrient transport which can have a large impact on aquatic organisms and the ecosystem at large. High velocity flows over rough beds result in increased turbulence intensity and contribute to the suspension of fine sediment. Therefore, the main aim of this study is to validate first-order turbulence statistics in the form of spatiallyaveraged streamwise velocity profiles of the flow over a highly-porous rough bed at intermediate relative submergence. The experiments were conducted under uniform flow conditions in Cardiff University's Hydraulics Laboratory using a narrow flume, 10 m long, 0.3 m wide, and 0.3 m deep, equipped with a bed of natural gravel with an average diameter of 20 mm. A side looking Acoustic Doppler Velocimeter (ADV) was used to measure velocity profiles at 40 random locations within the sampling area. The results show that a logarithmic velocity profile is valid for rough-bed flows with a von Karman's constant of 0.41 and a Nikuradse equivalent sand roughness of between 1.5d and 2d.

2016

The analysis of contaminants mixing in natural channels is an important topic of Environmental Hydraulics. In the mid-field, the prediction of transverse mixing is complicated by other typical features of natural channels, such as bed forms and vegetation. The present experimental research investigated the effects of bedforms and banks vegetation on the structure of the turbulent flow field and on the transverse mixing process in a channel. A flume equipped with a set of ten 2D fixed dunes on the bed and rice stems at the banks was used. Detailed velocity measurements were performed using acoustic Doppler velocimetry. Mixing measurements were made using a NaCl solution as the tracer and measuring its concentration downstream the point of injection. The results showed that dunes and vegetation have a significant effect on the flow field and on transverse mixing coefficient Dt-y . Comparing with the flat bed conditions, the presence of dunes and vegetation at the channels banks signif...

Water Resources Research, 2002

1] A variety of surface roughness characterizations have emerged from nineteenth and twentieth century studies of channel hydraulics. When the water depth h is much larger than the characteristic roughness height k s , roughness formulations such as Manning's n and the friction factor f can be explicitly related to the momentum roughness height z o in the log-law formulation for turbulent boundary layers, thereby unifying roughness definitions for a given surface. However, when h is comparable to (or even smaller than) k s , the log-law need not be valid. Using a newly proposed mixing layer analogy for the inflectional velocity profile within and just above the roughness layer, a model for the flow resistance in shallow flows is developed. The key model parameter is the characteristic length scale describing the depth of the Kelvin-Helmholtz wave instability. It is shown that the new theory, originally developed for canopy turbulence, recovers much of the earlier roughness results for flume experiments and shallow gravel streams. This study is the first to provide such a unifying framework between canopy atmospheric turbulence and shallow gravel stream roughness characterization. The broader implication of this study is to support the merger of a wealth of surface roughness characterizations independently developed in nineteenth and twentieth century hydraulics and atmospheric sciences and to establish a connection between roughness formulations across traditionally distinct boundary layer types.

Earth Surface Processes and Landforms, 2006

This paper presents results of a field study designed to examine the structure of flow over mobile and fixed bedforms in a natural stream and to compare the results with findings of previous laboratory studies within the framework of double time-space averaging approach. Measurements of turbulence were obtained in a small river in Illinois, USA, over a fine spatial grid of sampling points above a mobile sandy bedform and its artificially moulded replica. Flow structure over the artificial bedform is similar to that observed in laboratory studies, but is markedly different from the flow structure over natural bedforms. These differences are most pronounced in the roughness sublayer, whereas flow in the logarithmic layer over natural and artificial sand waves is fairly similar and exhibits spatial uniformity. The double time-space averaged distributions of turbulence statistics conform to the multilayer model of flow structure over bedforms. Mean velocity distributions indicate neither classical flow recirculation nor substantial reduction of velocities in the lee of bedform crests. However, vertical patterns of turbulence statistics over depth suggest that stacked wakes similar to those observed in laboratory studies exist above the bedforms. Thus, despite the absence of flow separation, wake development seems to be induced by the systematic influence of upstream bedforms on the vertical structure of turbulence.

Journal of Hydraulic Engineering, 2001

In this paper it is suggested that the double-averaged (in temporal and in spatial domains) momentum equations should be used as a natural basis for the hydraulics of rough-bed open-channel flows, especially with small relative submergence. The relationships for the vertical distribution of the total stress for the simplest case of 2D, steady, uniform, spatially averaged flow over a rough bed with flat free surface are derived. These relationships explicitly include the form-induced stresses and form drag as components of the total stress. Using this approach, we define three types of rough-bed flows: (1) Flow with high relative submergence; (2) flow with small relative submergence; and (3) flow over a partially inundated rough bed. The relationships for the double-averaged velocity distribution and hydraulic resistance for all three flow types are derived and compared with measurements where possible. The double-averaged turbulent and form-induced intensities and stresses for the case of regular spherical-segment-type roughness show the dominant role of the double-averaged turbulence stresses and form drag in momentum transfer in the near-bed region.

Journal of Hydraulic Engineering-asce, 2005

The extent to which turbulent structure is affected by bed-load transport is investigated experimentally using a nonporous fixed planar bed comprising mixed-sized granular sediment with a d 50 of 1.95 mm. Three different sizes of sediment ͑d 50 = 0.77, 1.99, and 3.96 mm͒ were fed into the flow at two different rates ͑0.003 and 0.006 kg/m/s͒, and subsequently transported as bed load. Particle image velocimetry ͑PIV͒ was used to determine the turbulence characteristics over the fixed bed during clear water and sediment feed cases. Mean longitudinal flow velocities at any given depth were lower than their clear water counterparts for all but one of the mobile sediment cases. The exception was with the transport of fine grains at the higher feed rate. In this case, longitudinal mean flow velocities increased compared to the clear water condition. The coarse grains tended to augment bed roughness, but fine grains saturated the troughs and interstices in the bed topography, effectively causing the influence of bed irregularities to be smoothed. The PIV technique permitted examination of both temporal and spatial fluctuations in flow variables: therefore many results are presented in terms of double-averaged quantities ͑in temporal and spatial domains͒. In particular, the form-induced stress, which arises from spatially averaging the Reynolds averaged Navier-Stokes equations and is analogous to the Reynolds turbulent stress, contributed between 15 and 35% of the total measured shear stress in the roughness layer. Flow around protrusive roughness elements produced a significant proportion of the turbulent kinetic energy shear production, suggesting that this process is highly intermittent near rough beds.

Main channel and floodplain vegetation generally influence the flow behaviour in the flooding rivers or compound channels. The hydraulics of vegetated compound channels is complex and still not fully understood. The aim of the research was to enhance the knowledge on the effects of two-line staggered arrays with closed-spacing emergent vegetation or roughness elements along the riparian zone on the non-mobile bed compound straight and meandering channels flows. The experiments were carried out in the hydraulics laboratory of Faculty of Civil Engineering, Universiti Teknologi Malaysia. The experimental investigations were focused on an asymmetrical compound straight channel and a straight, vertical floodplain-walls meandering channel with a sinuosity of 1.54. In addition, the Shiono and Knight Method (SKM) was examined to simulate the transverse distributions of velocity in non-vegetated and vegetated compound straight channels. The physical processes experimentally investigated were stage-discharge, flow resistance, three-dimensional turbulent flow structures, boundary shear stress, momentum transfer and drag force for selected flood flow depths in both compound channels. The densest riparian vegetation increased 6.1% and 32% of flow depth in compound straight and meandering channels, respectively. It also generated the largest flow resistance in both channels. The secondary currents movements were limited due to the presence of floodplain vegetation and the current strength in meandering channel was stronger than in straight channel. The floodplain vegetation also lowered boundary shear stress and a maximum of 77.8% boundary shear stress drop was observed for the densest vegetation case in compound meandering channel. Momentum transfers between main channel and floodplain were quantified as transverse Reynolds and apparent shear stresses. Peak stresses were found in the shear layer zones and increased with density of riparian vegetation. The vegetative-induced drag force FD was calculated using force balance approach and the result showed that it increased with flood flow depth. In addition, the smallest riparian vegetation spacing produced the largest drag coefficient CD in both non-mobile bed compound channels. The hydraulic simulations showed that SKM was able to produce well prediction of the depth-averaged velocity in non-vegetated and vegetated compound straight channels. The findings also revealed that compound meandering channel flow was more complex since its structures were spatially varied along the meander due to channel geometrical properties.

Journal of JSCE, 2014

Momentum transfer of open-channel flows in the lateral direction is important in sediment transport, flood control, environmental issues, etc. in rivers. The lateral transfer of the streamwise momentum is known to be caused by three different mechanisms: a cross-sectional secondary current due to turbulence anisotropy (secondary current of the second kind), turbulence mixing with shear instability (K-H instability), and mass transfer from the rough-bottomed to the smooth-bottomed lane due to flow redistribution. Furthermore, Vermaas et al. (2011) 1) have shown experimentally that the relative contribution of each mechanism to the momentum transfer is closely affected by depth. Thus, to elucidate the fundamental characteristics of the lateral momentum transfer, we performed three-dimensional (3-D) computations of shallow open-channel flows in two parallel lanes with beds of different roughness. We simulated the momentum transfer on roughness transition using a 3-D RANS (Reynolds Averaged Navier Stokes) type k- turbulence model, which has significantly greater computational efficiency than DNS (Direct Numerical Simulation) or LES (Large Eddy Simulation). Two types of computations, focusing on either the well-developed or the developing process, were performed separately. The present numerical results show that although the linear k- model completely fails to capture the fundamental characteristics of the flows, a second-order non-linear k- model could estimate excellently each effect on the momentum transfer, including the dependence on flow depth.

2010

Airborne and satellite observations show that when large rivers join they can take hundreds of kilometers to mix completely but, on occasion, may mix very rapidly. Application of established semitheoretical analyses shows that long mixing lengths should be expected. The first measurements of mixing processes at a large river junction (Río Paraná and Río Paraguay, Argentina, combined width $2.8 km) are presented at two occasions: first when they mix in >400 km, and second when mixing is complete in only 8 km downstream of the junction. For the case of slower mixing, at-a-point surveys showed that mixing driven by turbulent shear associated with a near-vertical shear layer was restricted to close to the junction (to 0.272 multiples of the postconfluence width downstream). Transect surveys showed penetration of more turbid water from the Río Paraguay underneath the Río Paraná, but this was insufficient to promote more rapid mixing. There was no clear channel-scale circulation present and slow mixing was compounded by reverse topographic forcing on the mainstream Río Paraná side of the river. This kept more turbid water on the Río Paraguay side of the river, close to the bed. In the case of rapid mixing, we found clear channel-scale circulation. The momentum ratio between the combining flows reinforced the effects of the discordance in bed height between the tributaries at the confluence and allowed penetration of more turbid Río Paraguay water further across the channel width deeper within the flow. The importance of the interaction between momentum ratio and bed morphology at channel junctions makes mixing rates at the confluence dependent upon basin-scale hydrological response, which is more likely to differ between large confluent rivers than small rivers, as a result of the different climatic/topographic zones that they may capture.