Turbulence Properties of Smooth-Bed Open-Channel Flows with and without Collinear Gravity Waves

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On the Very-Large-Scale Motions in Smooth-Bed Open-Channel Flows

Bimodal distribution of the streamwise velocity pre-multiplied spectra in canonical turbulent flows (pipe, channel and boundary layer flow) is well documented in the literature. The two peaks of this distribution are associated with eddies of a defined size and they are called Large-Scale Motion (LSM) and Very-Large-Scale Motion (VLSM). These eddy structures are very important inasmuch they contain a significant fraction of the total kinetic energy of the flow. The LSMs and VLSMs size is proportional to the characteristic outer length scale of the flow (i.e. the radius, the channel half width and the boundary layer thickness); the former’s length is a few while the latter is some tens of . However, little is known about their size and scaling in open-channel flows. The knowledge of LSMs and VLSMs in open-channel flows (i.e. rivers, tides and marine currents) is important, not only from a theoretical point of view, but most of all for their impact on key transport and mixing processes occurring in many geophysical flows (e.g., sediment dynamics, transport of nutrients, microorganisms movement, etc.). The present study aims to shed light into the dynamics of LSMs and VLSMs in open-channel flow through a laboratory study. The experiments were conducted in a recirculating open-channel flume 50 m long, 0.61 m wide and 1 m deep with a smooth concrete bed. During the experiments, the instantaneous velocity in the streamwise and bed-normal directions was measured with the aid of a 2D Laser Doppler Anemometer (LDA). The conditions in every experiment were that of fully developed smooth turbulent flow. The experiments were designed in order to highlight the influence of various relevant non-dimensional groups (presumably) involved on the LSMs and VLSMs dynamics. The main results are that the evolution of LSMs and VLSMs seems not to be affected by the Von Karman and the Froude Number (in the range of conditions analysed). As suggested also in the literature, the results hint that the non-dimensional parameter that mostly influences these vortices seems to be the aspect ratio. For values of the aspect ratio below 5 (that represent a condition of 3D motion, with the instauration of secondary flows in the flume), the size of these vortices is reduced by more than half with respect to a situation of an aspect ratio greater than 5

ArduHydro: a low-cost device for water level measurement and monitoring

In this paper, we present ArduHydro, a low-cost device for water level measurement and monitoring designed for employment in controlled and outdoor environments. It measures the water level through an ultrasonic sensor and elaborates the signals through an Arduino microcontroller. The small size of this device, its robustness and accuracy make ArduHydro properly versatile for different applications in the field of water control and management. This article describes the design, the components, the costs, and the performance of ArduHydro. The performance was assessed with a laboratory test inside an open-channel flume and comparing ArduHydro measurements with those obtained with a traditional ultrasonic sensor. Furthermore, an example of ArduHydro application for detecting the wavefront evolution during surface irrigation of a maize crop is presented. The results revealed that ArduHydro measurements were, on average, very consistent with those obtained by the traditional ultrasonic sensor in all different flow conditions. The application of ArduHydro during a surface watering of an agricultural field allowed us to obtain important spatiotemporal information about the water depth along the longitudinal direction of the field, paying the way for a real comprehension of the dynamics of wavefront evolution in a real-world case study

The vertical velocity skewness in the atmospheric boundary layer without buoyancy and Coriolis effects

One of the main statistical features of near-neutral atmospheric boundary layer (ABL) turbulence is the positive vertical velocity skewness $Sk_w$ above the roughness sublayer or the buffer region in smooth-walls. The $Sk_w$ variations are receiving renewed interest in many climate-related parameterizations of the ABL given their significance to cloud formation and to testing sub-grid schemes for Large Eddy Simulations (LES). The vertical variations of $Sk_w$ are explored here using high Reynolds number wind tunnel and flume experiments collected above smooth, rough, and permeable-walls in the absence of buoyancy and Coriolis effects. These laboratory experiments form a necessary starting point to probe the canonical structure of $Sk_w$ as they deal with a key limiting case (i.e. near-neutral conditions) that has received much less attention compared to its convective counterpart in atmospheric turbulence studies. Diagnostic models based on cumulant expansions, realizability constraints, and the now-popular constant mass flux approach routinely employed in the convective boundary layer as well as prognostic models based on third-order budgets are used to explain variations in $Sk_w$ for the idealized laboratory conditions. The failure of flux-gradient relations to model $Sk_w$ from the gradients of the vertical velocity variance $σ_w^2$ are explained and corrections based on models of energy transport offered. Novel links between the diagnostic and prognostic models are also featured, especially for the inertial term in the third order budget of the vertical velocity fluctuation. The co-spectral properties of $w\u27/σ_w$ versus $w\u27^2/σ_w^2$ are also presented for the first time to assess the dominant scales governing $Sk_w$ in the inner and outer layers, where $w\u27$ is the fluctuating vertical velocity and $σ_w$ is the vertical velocity standard deviation

Criteria for identifying free-flowing river stretches for the EU Biodiversity Strategy for 2030

The EU Biodiversity Strategy includes the target that at least 25000 km of rivers should be restored into free-flowing rivers by 2030 through the removal of primarily obsolete barriers and the restoration of floodplains and wetlands. This document proposes criteria for identifying free-flowing rivers, taking into account longitudinal, lateral, and vertical connectivity at local and catchment scales. The aim is to provide a tool that can be used by authorities to determine the length of free-flowing rivers in their catchments. In addition, the tool can be used to predict the increase in free-flowing river length resulting from barrier removal and other restoration measures. This will help prioritising measures that can contribute to the 25,000 km target. Key elements of the method are (1) segmentation of the river into homogeneous reaches; (2) criteria for longitudinal, lateral, and vertical connectivity within a homogeneous reach; (3) a large-scale assessment taking into account sediment connectivity and migration barriers for target fish species; and (4) minimum length criteria to ensure hydromorphological processes and ecological functioning.JRC.D.2 - Ocean and Wate

Evaluating longitudinal dispersion of scalars in rural channels of agro-urban environments

AbstractIn agro-urban environments, the water resource conveyed by rural channels is susceptible to a gradual impoverishment due to the continuous combined sewer overflow release, constituting a pending and urgent issue for water management companies and the entire community. Reliable one-dimensional longitudinal dispersion coefficients D are required to model and study the hydrodynamics and water quality patterns at the scale of rural channel networks. Empirical formulas are usually adopted to estimate D but the accuracy in the prediction could be questionable. In order to identify which are the most suitable formulas to determine D in rural channels, field tracer measurements were carried out in three rural channels with typical geometry and configuration. The obtained D values were then compared with the most commonly used predicting formulas that the literature provides. The accuracy of the predictors was further checked by simulating different flow rates inside the tested channels by using a one-dimensional hydraulic model. Starting from the obtained results, indications and guidelines to choose the most suitable formulas to predict D in rural channels were provided. These indications should be followed when developing realistic quality models in the agro-urban environments, especially in those cases where direct measurements of the longitudinal dispersion coefficient D are not available.</jats:p

Investigating First Flush Occurrence in Agro-Urban Environments in Northern Italy

The first flush (FF) phenomenon is commonly associated with a relevant load of pollutants, raising concerns about water quality and environmental management in agro-urban areas. An FF event can potentially transport contaminated water into a receiving water body by activating combined sewer overflow (CSO) systems present in the drainage urban network. Therefore, accurately characterizing FF events is crucial for the effective management of sewer systems and for limiting environmental degradation. Given the ongoing controversy in the literature regarding the delineation of FF event occurrences, there is an unavoidable necessity for further investigations, especially experimental-based ones. This study presents the outcomes of an almost two-year field campaign focused on assessing the water quantity and quality of two combined sewer systems in Northern Italy. For this purpose, various hydro-meteorological variables, including precipitation, flow rate, temperature, and solar radiation, in addition to water quality analytics, were measured continuously to capture stormwater events. Throughout the monitoring period, sixteen stormwater events were identified and analyzed using five indices usually adopted in the literature to identify FF occurrences. The results indicate that there is a strong positive correlation between the mass first flush ratios calculated for nutrients and three factors, including maximum rainfall intensity, maximum flow rate, and antecedent dry weather period. Furthermore, rainfall duration was found to possess a strong negative correlation with the mass first flush ratios calculated for nutrients. However, for the same rainfall event, the occurrence of FF has never been unanimously confirmed by the indices examined in this study. Moreover, different macro-groups of pollutants can behave differently. Thus, it becomes apparent that relying solely on a priori analyses, without the support of data from experimental monitoring campaigns, poses a risk when designing actions for the mitigation of FF occurrences

Assessing bed shear stress effects on flow resistance of vegetated channel beds by means of Leaf Area Index (LAI)

Flow resistance in vegetated streams is generated by both the drag force exerted by plants and bed friction. Nevertheless, since surface friction is often neglected in the literature, drag is considered the preeminent resistance source. Flume tests were analysed in this study to assess the flow resistance component due to surface friction over a rough bed for different arrangements of artificial flexible plants that resemble plant species typical of vegetated floodplains and riparian areas. The plant drag was measured through drag force sensors and the bed friction factor was estimated by applying the linear superposition principle. It comes out that the contribution of the bed friction must be properly considered in the estimation of the total flow resistance, especially in low-density plant arrangements, where it reaches up to 25 %. Furthermore, the Leaf Area Index ( ) can serve as an indicator for assessing the variability of bed shear stress across different leaf mass conditions. This study also indicates that is a reliable predictor of plant density classification in vegetated beds, being a robust canopy biomass proxy, independent of the experimental hydrodynamic conditions, much more easily retrievable than the plant density parameter , especially for flexible plants and in reach-scale studies. Specifically, indicates sparse vegetation, represent dense vegetation, and values in between correspond to transitional plant density. Finally, it is demonstrated that the classical logarithmic and power flow resistance formulae are not sufficiently accurate to predict bed friction for plant species typical of vegetated floodplains and riparian zones, leading to substantial underestimations (up to 95 %)