This review describes mean and turbulent flow and mass transport in the presence of aquatic vegetation. Within emergent canopies, the turbulent length scales are set by the stem diameter and spacing, and the mean flow is determined by the distribution of the canopy frontal area. Near sparse submerged canopies, the bed roughness and near-bed turbulence are enhanced, but the velocity profile remains logarithmic. For dense submerged canopies, the drag discontinuity at the top of the canopy generates a shear layer, which contains canopy-scale vortices that control the exchange of mass and momentum between the canopy and the overflow. The canopy-scale vortices penetrate a finite distance into the canopy, δe, set by the canopy drag. This length scale segregates the canopy into two regions: The upper canopy experiences energetic turbulent transport, controlled by canopy-scale vortices, whereas the lower canopy experiences diminished transport, associated with the smaller stem-scale turbulence. The canopy-scale vortices induce a waving motion in flexible blades, called a monami.

Flow and Transport in Regions with Aquatic Vegetation

Leakage in water distribution systems is an important issue which is affecting water companies and their customers worldwide. It is therefore no surprise that it has attracted a lot of attention by both practitioners and researchers over the past years. Most of the leakage management related methods developed so far can be broadly classified as follows: (1) leakage assessment methods which are focusing on quantifying the amount of water lost; (2) leakage detection methods which are primarily concerned with the detection of leakage hotspots and (3) leakage control models which are focused on the effective control of current and future leakage levels. This paper provides a comprehensive review of the above methods with the objective to identify the current state-of-the-art in the field and to then make recommendations for future work. The review ends with the main conclusion that despite all the advancements made in the past, there is still a lot of scope and need for further work, especially in area of rea...

A review of methods for leakage management in pipe networks

An empirical model to estimate the propagation of random breaking and nonbreaking waves over vegetation fields

Data from laboratory flumes and natural streams show that the critical Shields stress for initial sediment motion increases with channel slope, which indicates that particles of the same size are more stable on steeper slopes. This observation is contrary to standard models that predict reduced stability with increasing slope due to the added downstream gravitational force. Processes that might explain this discrepancy are explored using a simple force-balance model, including increased drag from channel walls and bed morphology, variable friction angles, grain emergence, flow aeration, and changes to the local flow velocity and turbulent fluctuations. Surprisingly, increased drag due to changes in bed morphology does not appear to be the cause of the slope dependency because both the magnitude and trend of the critical Shields stress are similar for flume experiments and natural streams, and significant variations in bed morphology in flumes is unlikely. Instead, grain emergence and changes in local flow velocity and turbulent fluctuations seem to be responsible for the slope dependency due to the coincident increase in the ratio of bed-roughness scale to flow depth (i.e., relative roughness). A model for the local velocity within the grain-roughness layer is proposed based on a 1-D eddy viscosity with wake mixing. In addition, the magnitude of near-bed turbulent fluctuations is shown to depend on the depth-averaged flow velocity and the relative roughness. Extension of the model to mixed grain sizes indicates that the coarser fraction becomes increasingly difficult to transport on steeper slopes.

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Is the critical Shields stress for incipient sediment motion dependent on channel‐bed slope?

Understanding of the hydraulics of flow over vegetation is very important to support the management of fluvial processes. In this paper the flow over flexible bottom vegetation is experimentally studied. A two-dimensional-acoustic doppler velocimeter is used to measure the local flow velocities for different vegetation concentrations, discharges, and flume slopes. The influence of vegetation concentration and the depth/vegetation height ratio on the measured velocity profiles is analyzed. All measured velocity distributions are S-shaped and exhibit a three-zone profile. The relationship between the velocity distribution and the turbulence intensity distribution is also analyzed. The characteristics ~inflection point, maximum value, asymptotes ! of the measured velocity distributions and the results of the previous investigators are used to select an analytical expression for the shape of the velocity profile. A theoretical velocity profile is deduced using the classical Prandtl's mixing length approach with a new expression for the mixing length. The deduced four-coefficient profile allows description of the flow both inside and above the vegetation. The physical and geometrical meaning of the four coefficients are also shown. DOI: 10.1061/~ASCE!0733-9429~2002!128:7~664! CE Database keywords: Velocity profile; Vegetation; Channel flow.

Flow Velocity Measurements in Vegetated Channels

Estimation of water loss at small leaks and openings in different areas of a pipe network is handled by means of an iterative two-step procedure. The first step is estimation of the parameters in the network simulation model. It is achieved with the maximization of a likelihood function, equal to the sum of the squares of the difference between the observed water heads and flow rates and their computed values in a given number of nodes and pipes. The estimated parameters include the loss factors and the loss exponent. They provide the water loss per unit time in each area of the network, called zone. The second step is optimization of the openings of the valves included in the network. It is constrained by the minimum pressure head allowed in the nodes. Closure of the valves enhances the different response of the network with respect to the water loss and the load in each node. The robustness of the procedure is tested with numerical experiments by relaxing most of the assumptions made in the model. The water loss estimation in a real network with fictitious measures also shows the robustness of the procedure.

Leak Analysis in Pipeline Systems by Means of Optimal Valve Regulation

In this paper, experimental data collected in a straight flume having a bed covered by grasslike vegetation have been used to analyze flow resistance for flexible submerged elements At first, the measurements are used to test the applicability of Kouwen's method Then, a calibration of two coefficients appearing in the semilogarithmic flow resistance equation is carried out Finally, applying the P-theorem and the incomplete self-similarity condition, a flow resistance equation linking the friction factor with the shear Reynolds number, the depth-vegetation height ratio and the inflection degree is deduced

Flow Resistance Law in Channels with Flexible Submerged Vegetation

Flow in meandering bends is characterized by the formation of a large cross-sectional central-region circulation cell. The width-to-depth ratio is one of the most important parameters affecting the entity of the cross-circulation motion. In steep outside bends, beside the central-region cell, a counter-rotating circulation cell often forms in the upper part of the outer-bank. In spite of its practical importance, the evolving mechanisms of both the circulation cells and their role on boundary shear stress distribution in bends are not yet fully understood. The aim of the present paper is to gain some insight into how cross-sectional flow motion evolves along meandering bends. Experiments have been carried out in a laboratory meandering channel of large amplitude, over a deformed-rigid bed, for two values of the width-to-depth ratio. The three-dimensional flow velocity field has been measured in detail at five cross-sections, almost equally spaced along the channel reach between two consecutive apex sections. The measurements have been carried out on a fine grid by an acoustic Doppler velocity profiler. The distributions of the cross-sectional flow (e.g. cross-sectional flow velocity, net transversal flux) and turbulent kinetic energy are analyzed in each investigated section. Measurements show that the counter-rotating circulation cell is evident only in the case of ‘small’ width-to-depth ratio. Such circulation cell begins at the bend entrance and it is fully developed at the bend apex; then it decays. At the bend apex, the core of maximum velocity is found near the bed at about the separation between the central and the outer-bank circulation cells. Moreover, the presence of the counter-rotating circulation cell allows the bank shear stress to maintain low values in the outer-side of the bend. Copyright © 2010 John Wiley & Sons, Ltd.

Experimental analysis of cross-sectional flow motion in a large amplitude meandering bend

Meanders of large amplitude often exhibit asymmetric planform shape or subsidiary bends. The present work is aimed at improving on understanding of the morphodynamic phenomena affecting the bed evolution of large amplitude meandering channels. Attention is focused on the development of the steady point bar-pool configuration and of the superimposed large-scale migrating bed forms; of particular interest is the role of the changing channel curvature and bed topography variation on flow pattern. A series of experiments was carried out in a sine-generated large-amplitude meandering flume, for two values of width-to-depth ratio. Maps documenting the bed topography and the flow pattern along the meandering bends are reported. Two point bars per bend were observed and seem to be part of a series of damped oscillations developing in response to the changing channel curvature. In response to the bed deformation, the maximum flow velocity moves at the outer bank at the entrance of the bend.

Donatella Termini

Papers

Flow Velocity Measurements in Vegetated Channels

Leak Analysis in Pipeline Systems by Means of Optimal Valve Regulation

Flow Resistance Law in Channels with Flexible Submerged Vegetation

Experimental analysis of cross-sectional flow motion in a large amplitude meandering bend

Experimental Observations of Flow and Bed Processes in Large-Amplitude Meandering Flume