Flow resistance law in channels with fully submerged and rigid vegetation

The estimate of flow resistance in vegetated channels is a challenging topic for programming riparian vegetation management, controlling channel conveyance and flooding propensity, for designing soil bioengineering practices. In this paper, measurements collected by Gualtieri et al. (2018), in a flume where rigid cylinders were set in two arrangements (staggered, aligned) at high submergence ratios (ratio between the water depth and the vegetation height greater than 5), were used to study the effect of rigid submerged vegetation on estimating flow resistance. The theoretical flow resistance equation, obtained by integrating the power flow velocity distribution, was first summarized. Then, this flow resistance equation was calibrated and tested by measurements of Gualtieri et al. (2018). In particular, a relationship between the Γ function of the power velocity distribution, the channel slope, the flow Froude number, and the submergence ratio was established by using the available measurements carried out for the two arrangements with different stem concentrations. The calibration of this relationship was carried out by (i) distinguishing measurements corresponding to different vegetation arrangements (staggered, aligned), (ii) joining all available data, and (iii) using only a scale factor representing the effect of vegetation arrangements. For the cases (ii) and (iii), the analysis demonstrated that the theoretical flow resistance equation allows an accurate estimate of the Darcy–Weisbach friction factor, which is characterized by errors that are always less than 5% and less than or equal to 2.5% for 88% of the investigated cases

Resistenze al moto e caratteri della turbolenza in correnti defluenti in alvei vegetati

E' oramai diffusa nella comunità scientifica idraulica come tematica di ricerca quella inerente la creazione di un adeguato approccio circa lo studio dell’interazione acqua-vegetazione allo scopo di definire una modellistica tesa alla trasformazione e al miglioramento delle formulazioni attualmente disponibili per le correnti fluviali. Nell'ambito della suddetta tematica i nostri studi teorici, sperimentali e infine appunto modellistici hanno seguito due segmenti principali di ricerca, atti a definire e perseguire meglio due scopi finali: come primo obiettivo ci siamo posti quello di riuscire a determinare la resistenza al moto indotta dalla vegetazione rispetto alle correnti idrauliche, mentre come secondo obiettivo abbiamo analizzato le caratteristiche statistiche della turbolenza sempre per correnti defluenti su alveo vegetato. In particolare il primo obiettivo della nostra ricerca è consistito nell'individuazione della grandezza che potesse al meglio caratterizzare la scabrezza del fondo; ci siamo focalizzati in particolare sulla possibilità di applicare rispetto al nostro case-study le ordinarie scabrezze idrauliche (Nikuradse, Strickler, Bazin) analizzando i numerosi lavori presenti in letteratura e cercando di determinare per quali condizioni al contorno fosse valida la nostra ipotesi; siamo giunti alla conclusione che tutti i modelli presenti in letteratura potessero essere approssimati alle ordinarie formule di resistenza in condizioni di alta sommergenza (rapporto tra il tirante idrico h e altezza di vegetazione k maggiore di 5, h/k>5). Successivamente è stato possibile determinare, tramite indagini di laboratorio, i valori di tali scabrezze per diverse tipologie di vegetazione (a diverse densità) e infine, definire un modello che consentisse il calcolo teorico di quella che viene ritenuta quella scabrezza che tra le tre scabrezze classiche fosse più adatta in condizioni di h/k>5. Si dimostra che la scabrezza ritenuta più affidabile in queste particolari condizioni idrodinamiche sia quella di Nikuradse. Il secondo scopo è consistito invece nello studio delle grandezze statistiche della turbolenza della corrente e come esse variano a causa della presenza della vegetazione. Sono stati analizzati i numerosi modelli presenti in letteratura, soprattutto quelli in cui fosse esplicata la condizione di “mixing layer analogy” ovverosia l’analogia tra le caratteristiche della turbolenza di correnti defluenti su alveo vegetato e quelle generate dall'incontro di due correnti orizzontali parallele aventi diverse velocità

Flow resistance of submerged vegetation: focus and validation on two layer approach

Vegetation behavior in riparian environment represent an important research topic in hydraulics and ecohydraulics. Therefore, one of the most important research challenge is to deepen interactions between flow and vegetation, in order to better understand its effects on flow resistance and turbulent characteristics of the flow. In vegetated flow models, the flow regime is often separated in two layers: vegetated layer and surface layer. Flow resistance models, based on two-layer approach, are often tested with experimental data relative to vegetation density and submergence ratio ((i.e., the ratio between h=flow depth and hv=vegetation height, h/hv) varying in wide ranges. The aim of the paper is to deepen how variability of submergence ratio and the non-dimensional vegetation density can affect the reliability of the flow resistance models. In particular, in this paper, the results of the statistical test applied to the most cited and used literature models for evaluating the flow resistance of rigid vegetation, are shown. The test have been carried out with experimental data from literature, by the mean of different statistical parameters, considering the different ranges of vegetation density and submergence ratio. The results shows that vegetation density and submergence ratio can affect the reliability of the model

On Evaluating Flow Resistance of Rigid Vegetation Using Classic Hydraulic Roughness at High Submergence Levels: An Experimental Work

Vegetation resistance is generally evaluated using drag coefficient CD related to friction factor f; however, some authors examined a possibility of employing classic hydraulic roughness coefficients (i.e., Nikuradse’s or Strickler’s) to calculate vegetation resistance in case of high submergence (h/k > 5, in which h is flow height and k represents vegetation height). In order to compare conventional roughness at high submergence levels, an experimental methodology was developed, focused, in particular, on fully submerged and rigid vegetation, for different hydraulic conditions and varying non-dimensional vegetation density

Preliminary experimental results of a masonry building prototype subject to harmonic forces of different intensity

In the present paper an experimental study has been performed on a masonry building prototype and some preliminary experimental results are illustrated for analyzing the structural behavior of a masonry building prototype subject to harmonic forces of different intensity. The physical model used for the laboratory test is a two-story masonry building characterized by a regular floor plan. The structure test is subjected to a harmonic force input supplied by a vibrodyne. The experimental results are aimed at characterizing the dynamic response of the masonry building subject to harmonic forces in order to describe the bahaviour of the masonry building under the predominant actions of a seismic inpu

Displacement based approach for the seismic retrofitting of a RC existing building designed for only gravitational loads

The present paper deals with the evaluation of the level of seismic vulnerability of a Reinforced Concrete (RC) building by using a Displacement Based Approach (DBA), in the context of a performance methodology, and by adopting the N-2 method, according to Italian seismic code NTC 2008. The RC building is located in Southern Italy in an area classified as high seismicity zone and designed, in the past, only for gravitational loads. During the evaluation phase the effect of masonry infills is considered by modeling with a non linear analysis all the panels considered in effective interaction with the structural frame. It is highlighted that in the examined case the masonry infills determine a worsening of the seismic behavior of the existing structure. So that it is illusory that by neglecting these panels a general beneficial effect would be obtained. Moreover in such case the neglect of these panels would have guided to a retrofitting strategy which is completely different and would have not answered to the real needs of the structure. The correct evaluation of the seismic response is finalized to the seismic retrofitting by using a traditional strategy, designed through a displacement based approach, for reducing the displacement demand on the existing structure. The adopted retrofitting strategy is represented by steel braces with steel reinforcement elements for the columns. In the non linear model for these elements it has been considered their confinement action for the reinforced concrete columns. The vulnerability level of the structure and the Safety Factor (SF), defined as the ratio between the displacement capacity and the displacement demand, is calculated for the building before and after the seismic retrofitting

Finite element modelling for characterizing some dynamical properties of a masonry building prototype

A finite element modelling of a masonry building prototype is illustrated in this paper. A computational analysis describing the dynamic behavior of the structure is presented in order to characterize some dynamical features of the system under the effect of harmonic forces of different intensity. The structure is represented by a two-story masonry building characterized by a regular floor plan. The finite element modelling of the structure intends to reproduce an experimental test of the masonry building subject to a harmonic force input supplied by a vibrodyne. Frequency response functions corresponding to the frequency load inputs are reported for different monitored nodal points of the finite element mesh. In this way by comparison with the experimental results obtained in a parallel study it is possible to suitably assess the calibration of the parameters for characterizing the dynamical behaviour of the masonry structure

Characterization of an autoclaved aerated concrete building with respect to a similar unreinforced masonry structure

In the present work some characteristical features of autoclaved aerated concrete structures are illustrated in the seismic and nonseismic design for residential and industrial buildings. Besides the properties of this material with regards to fire resistance and thermal and acoustical insulation, the use of autoclaved aerated concrete for engineering structures may have the advantage of a confined structure with reinforced concrete bond elements that are disposed horizontally and vertically. In the present work the dynamical behavior of a building prototype realized with autoclaved aerated concrete is analyzed, a finite element modelling of the structure has been calibrated according to an experimental modal analysis carried by loading the structure with a vibrodyne located on top of the building and by monitoring the building outputs due to horizontal harmonic forces. The finite element modelling of the dynamical behavior of the autoclaved aerated concrete structure has been compared with a similar tuff masonry building whose characteristical behavior has already been the object of experimental and numerical analysis in a previous work

Condizioni Preliminari Necessarie per Effettuare un Confronto tra Scabrezza di Strickler e Scabrezza di Nikuradse

Nel seguente lavoro gli Autori si sono soffermati sulle scabrezze presenti nelle formule di resistenza al moto idraulico di una corrente frequentemente usate dalla comunità tecnica e scientifica. La nascita di un filone di pensiero che vede una riaccesa attenzione alle espressioni delle resistenze al moto in alvei fluviali, ha fatto sì che gli i Autori abbiano cercato di inserirsi nel “solco” che divide due scuole di pensiero diverse. Prima di tutto verranno presentati i sostrati teorici delle formule di Colebrook e White nonché di Manning; successivamente si mostra come entrambi i modelli di resistenze al moto facciano emergere l’esistenza del concetto di scabrezza di parete (Scabrezza equivalente di Nikuradse e Scabrezza di Strickler). Nei due modelli peraltro la definizione di scabrezza è differente: basata sulla equivalenza di effetti idrodinamici la prima, semplicemente basata sulla geometria dei grani la seconda. Nel lavoro viene mostrata una modalità per ridefinire anche la Scabrezza di Strickler attraverso un concetto di equivalenza idrodinamica. Tale modalità potrà consentire confronti tra le due scabrezze meglio basati fisicamente

Use of conventional flow resistance equations and a model for the Nikuradse roughness in vegetated flows at high submergence

This study examines the problem of flow resistance due to rigid vegetation in open channel flow. The reliability of the conventional flow resistance equations (i.e. Keulegan, Manning and Chézy-Bazin) for vegetated flows at high submergence, i.e. h/k >5, (where h = flow depth and k = vegetation height) is assessed. Several modern flow resistance equations based on a two-layer approach are examined, showing that they transform into the conventional equations at high submergences. To compare the conventional flow resistance equations at high submergences, an experimental methodology is proposed and applied to the experimental data reported in the literature and collected for this study. The results demonstrate the reliability of the Keulegan equation in predicting the flow resistance. Based on the obtained results, a model to evaluate the Nikuradse equivalent sand-grain roughness, kN, starting from the vegetation height and density, is proposed and tested