Journal of Hydraulic Engineering

Despite substantial research on various aspects of bed deformation in meandering streams, no systematic effort has yet been made to establish the timescales of the process. Additionally, in past experiments, as a rule, the bed was not monitored throughout the development, which has left undocumented essential features of the growth of meandering pool-bar complexes. In this paper, an effort is made to penetrate some of these aspects. More specifically, the five laboratory runs presented by the writers in their 2009 paper in this journal are revisited with the following two objectives: (1) to reveal the rate of growth of the resulting meandering pool-bar complexes; and (2) to document the extent, if any, of upstream or downstream migration of developing pool-bar complexes in examples of sufficiently wide streams in which the bed deformation is primarily because of the convective behavior of flow. The runs were conducted in a 70° sine-generated channel having rigid banks. In each run, the bed was allowed to deform from an initially flat bed to the equilibrium state, the deformation monitored throughout the runs. It was found that the bed development was very rapid at the early stages of the runs, with approximately 70% and 90% of the development accomplished when only one-quarter and one-half, respectively, of the total bed development time had been reached. An equation describing the growth rate of the pool–bar complexes is proposed. The plan location of these complexes remained invariant throughout each run. However, the location of maximum erosion within the pools noticeably shifted upstream during the development.

An 8-km-long and 250-m-wide reach of the Po River (Italy) was surveyed with two vessels, aiming to investigate the channel morphology and flow field characterizing this major Italian river and to provide data for the calibration of a numerical model. The first vessel was equipped with a multibeam ecosounder (MBES), whereas an acoustic doppler current profiler (ADCP) was located on the second one. Fixed-vessel ADCP measurements were used to preliminarily investigate the turbulence variability affecting the reliability of moving-vessel measurements. To evaluate the friction velocity by using moving-vessel measurements, the vertical variance of velocity profiles as an alternative to logarithmic fitting of ADCP profiles was estimated. An average of 30-m cells of depth-averaged velocity, friction velocity, and surface-bottom velocity angle are presented in maps as the most relevant flow field features. The flow field and channel morphology at bed-form or channel scales showed noticeable correlations (0.9). In particular, a correlation between the friction velocity and small-scale bed forms (4–5 m mean wavelength) was observed.

Systematic experimental investigations have been performed under steady flow conditions in a channel whose banks are equipped with large-scale rectangular roughness elements. The purpose was to determine the flow resistance owing to such macrorough banks. The practical motivation of the study is to see how morphological restoration of banks in channelized rivers, such as lateral cavities, influences the steady flow characteristics. The experiments performed in 40 different geometrical configurations revealed various two-dimensional flow characteristics in the bank cavities created by the roughness elements compared to the prismatic reference channel. The overall head loss of the flow is governed by the existence of different phenomena, such as vertical mixing layers, wake zones, recirculation gyres, coherent structures, and skin friction. The analysis of the experiments for steady flow conditions showed that the flow resistance is significantly increased in the macrorough configurations because of the disturbance of the bank geometry inducing large-scale depressions. The additional flow resistance attributable to macroroughness has been related to the forms of the banks. By separating the observed flow behavior into a normal recirculating, a reattachment and a square-grooved flow type, macrorough flow-resistance formulas according two different approaches could be developed that are in good agreement with the laboratory experiments.

This paper presents the results of a series of flume experiments to evaluate the interactions between pier-scour and abutment-scour in a noncohesive boundary. To facilitate optimal structural design and minimize pier presence in a channel, many bridges have a pier located close to an abutment. Scant information exists as to how bridge pier proximity to abutment affects scour depth at an abutment and at a pier. The existing relationships for predicting scour at a pier or at an abutment in isolation do not estimate scour depth at a pier or an abutment when both are in close proximity to one another. During this study, scour depth, scour location, and bathymetry data were recorded for two common abutment forms: spill-through and wing-wall. For the abutment and pier configurations investigated, the results show that pier presence does not lead to substantial increases in abutment scour depth, although it may reduce abutment-scour depth when the pier is close to the spill-slope toe of a spill-through abutment; the pier acts to retain riprap stone and spill-slope soil in the scour region. However, scour depth at a pier located close to an abutment is determined predominantly by scour at the abutment and, therefore, may substantially exceed the depth estimated for an isolated pier.

This study improves total sediment load calculations on the basis of depth-integrated sediment concentration measurements for channels with significant sediment transport in suspension. The series expansion of the modified Einstein procedure (SEMEP) removes most of the empiricism found in the existing modified Einstein procedures (MEP). SEMEP calculations require field measurements of flow discharge, depth-integrated suspended sediment (SS) concentration, and suspended particle sizes. SEMEP calculates the Rouse number, Ro, from the median particle size measured in suspension $d_{50ss}$. On the basis of the sediment discharge measurements collected from 14 rivers, the accuracy of sediment discharge calculations depend on the ratio of the shear velocity $u_{*}$ to the settling velocity $\omega$. SEMEP performs accurately (error less than 25%) and without bias when $u_{*}/\omega >5$. Calculations are also acceptable, but less accurate when $u_{*}/\omega$ is between two and five. Both SEMEP and MEP should not be used when the value of $u_{*}/\omega <2$, and a simplified formulation on the basis of bed sediment discharge is recommended when $u_{*}/\omega <2$.

Models for the deposition of cohesive sediment from turbulent plumes (or “buoyant jets”), gravity currents, and turbidity currents are provided in this paper. The cohesive sediment is made up of small particles that aggregate together to form larger flocs, which are in turn broken up by turbulent shear. The equilibrium mean floc size (and thus the equilibrium mean fall speed) is a function of the turbulent dissipation rate and the sediment concentration. The flows are modeled by using integral and box models, with dissipation related to bulk flow properties. For plumes it is shown that there is a well-defined equilibrium fall speed at the virtual origin and that the fall speed changes relatively slowly in the momentum-dominated part of the flow (within one jet length or so of the source). If the flocs are assumed to adjust instantaneously to their equilibrium size, an integral model for a turbulent plume carrying cohesive sediment can be described in terms of two parameters: the angle between the plume and the horizontal at the virtual origin and the (nondimensional) fall speed there. Next, a typical time scale for flocs to adjust to their equilibrium size is identified, and the model is extended to include an equation for the rate of change of the mean floc size along the plume. The time scale over which the mean floc size changes can be compared with a natural time scale for the plume (the time taken for a particle traveling at the mean plume speed to travel a jet length). Thus, in this nonequilibrium model, a further nondimensional parameter is identified, $B$, which is proportional to the ratio of a typical plume time scale to the typical floc size adjustment time scale. When $B$ is large, the flocs adjust almost instantaneously to the equilibrium size, whereas when $B$ is very small, the flocs remain close to their size at the source. However, whatever the value of $B$ (which is in terms of typical time scales), the local adjustment time scale always tends to zero approaching an idealized source (virtual origin) so that the equilibrium model is always valid there. For plumes injected horizontally, the equilibrium floc size tends to reduce with distance from the source, with any reduction in turbulent shear more than compensated for by the reduction in sediment concentration. The equilibrium model is then applied to two-dimensional and axisymmetric gravity currents and turbidity currents. The gravity currents are assumed to be steady flows driven by a constant source of dense fluid with the sediment having a negligible effect on the fluid density. In contrast, the turbidity currents modeled are initiated by the release of a finite volume of fluid containing the sediment, with the sediment concentration providing the density difference from the ambient fluid. For these flows, the basic scales are identified, and the concentration and deposition distributions given.

Previous work has sought to investigate flow resistance caused by vegetation in open channels. However, many existing flow resistance models are based on data from artificial plant mimics in laboratory channels and are untested with live, aging vegetation and may neglect some of the key effects. This paper presents results from a study using two types of live vegetation grown within a laboratory channel. A series of flow resistance tests were conducted over a time period sufficient to observe changes caused by growth. The effects of the vegetation on bulk flow resistance are presented and discussed. The results are compared with predictions made by existing practical momentum balance based models for flow through emergent vegetation, and new empirical relationships are presented. The work shows that momentum balance methods can provide an accurate prediction of flow depth in live vegetation provided that the relationship between bulk drag coefficient and flow is known. This relationship has been shown to vary with changes in plant properties during growth.

This study investigates the characteristics of reattached plane turbulent offset jets in channels with rough beds and shallow tailwater depths. The flow consists of a deflecting free jet caused by the Coanda effect and the evolving wall jet past reattachment. In limited tailwater conditions, the jet flow is affected by the relative tailwater depth or the submergence parameter, which is defined with respect to the maximum B-jump at a negative step. The results show that for an offset height larger than the jet thickness, the forward-flow momentum, local maximum velocity, and wall shear stress decrease faster in the longitudinal direction in the offset jet than in plane turbulent wall jets. The influence of roughness on reattached offset jets appears to be less than that in submerged wall jets on a similar rough bed. The presented results and the comparative analysis with respect to wall jets in hydraulic jumps are significant for controlling and implementing similar flows on rough beds with variable downstream water levels.

This paper proposes a holistic unsteady-friction model for the transient simulation of the condition of laminar initial flow. The effect of wall shear stress is considered through the introduction of radial-velocity distribution with kinematic viscosity from a simplified two-dimensional Navier-Stokes equation. The model simultaneously addressed the frequency-dependent friction and the unsteady friction associated with local and convective acceleration on the platform of the impulse-response method. A widely used hypothetical example demonstrated that the proposed model appropriately and simultaneously expressed the effect of pressure damping from both the radial-velocity variation with viscosity and acceleration terms. The nonlinear mutual effects were examined between two distinct energy-dissipation mechanisms by comparing the behavior of the pressure head between the proposed and existing models. Comparisons of the experimental results illustrated that the combined model exhibited predictive and fitting capabilities for the pressure-head oscillation and shape for both the end and middle positions of a reservoir-pipeline-valve system. The proposed model showed significantly improved predictability over the frequency-dependent friction model for the early pressure associated with a water hammer. The comparison of the performance between the developed model and the traditional method of characteristics approach demonstrated the robustness of the proposed method in both its computational efficiency and its representation of a real-life system.

Closed-form theoretical solutions for steady seepage below a horizontal impervious apron with equal end cutoffs are obtained using Schwarz-Christoffel transformation. The resulting implicit equations involving elliptic integrals are used to obtain various seepage characteristics in nondimensional floor-profile ratios. These ratios are further expressed in the form of design charts and as algebraic equations. The results have been validated by comparing the solutions with the available exact solutions for infinite-depth cases.

In flood protection engineering, side weirs or overflow dams are used to divert water in a controlled way into flood plains as soon as the discharge capacity of the main-channel is reached. Because of the lateral loss of water, the sediment transport capacity is reduced, resulting in local sediment deposition near the side overflow. Moreover, bed form characteristics such as length, height, steepness, and stoss and lee slope angle are affected by the lateral water withdrawal. Both phenomena are responsible for an increased side overflow intensity compared with plane bed conditions. The results from a systematic flume study show that the shape of observed bed forms is highly three-dimensional and that three distinct regions along the channel axis can be identified. The first one extends from the channel entrance to the upstream weir corner, the second one comprises the reach of the weir, and the third one represents the reach downstream of the weir. The description of bed form shape by approaches from literature shows reasonable agreement with measured bed form geometry.

This work presents results on riprap sizing for pile group protection against local scour. Shear failure mechanism is addressed, avoiding edge, bed-form undermining, and winnowing modes. The influences of the flow conditions and the distance between the piles are analyzed. As a result of a laboratory experimental campaign, and within a detailed dimensional analysis framework, the writers propose expressions for riprap block size to resist shear failure. The influence of the distance between the piles on the block size is shown to be small, and the proposed expressions compare well to some of the literature expressions for single piers.

The free-surface flows in rectangular basins are studied by the lattice Boltzmann model for shallow-water equations including flow turbulence (LABSWE). Asymmetrical flows occurring in the basins with different ratios of length to width are simulated. The predicted results are compared with corresponding experiments and show reasonable agreement. This indicates that the present model can produce solutions with accuracy similar to the conventional method with an algebraic model for flow turbulence. Furthermore, the effects of the Froude number and bed friction on flow asymmetry and reattachment length are investigated. The results show that the flow becomes less asymmetrical and the reattachment length grows with the increase of either the Froude number or bed friction.

The pressure variations associated with a filling undulating pipeline containing an entrapped air pocket are investigated both experimentally and numerically. The influence of entrapped air on abnormal transient pressures is often ambiguous because the compressibility of the air pocket permits the liquid flow to accelerate but also partly cushions the system, with the balance of these tendencies being associated with the initial void fraction of the air pocket. Earlier experimental research involved systems with an initial void fraction greater than 5.8%; this paper focuses on initial void fractions ranging from 0 to 10% to more completely characterize the transient response. Experimental results show that the maximum pressure increases and then decreases as the initial void fraction decreases. A simplified model is developed by neglecting the liquid inertia and energy loss of a short water column near the air-water interface. Comparisons of the calculated and observed results show that the model is able to accurately predict peak pressures as a function of void fraction and filling conditions. Rigid water column models, however, perform poorly with small void fractions.