1,720,968 research outputs found
Processi erosivi indotti da restringimenti d'alveo
No full textNella presente memoria vengono descritti i risultati ottenuti da un’indagine di laboratorio volta allo studio dell’evoluzione morfologica d’alveo in corrispondenza e nell’intorno di tronchi ristretti. Le prove sono state condotte in condizioni di moto stazionario e di clear-water nonche' caratterizzate da lunga durata, dell’ordine di uno o piu' giorni, al fine del raggiungimento di condizioni di quasi-equilibrio. Si sono inoltre considerati restringimenti con diversi fattori di ingombro e lunghezza del tronco prismatico pari alla larghezza dell’alveo indisturbato. Sulla base dei dati sperimentali acquisiti, e' stato dapprima effettuato un confronto fra diverse formule di letteratura evidenziandone limiti e difficolta' applicative. Successivamente, sono state proposte delle equazioni per la stima della massima profondita' di scavo in funzione del tempo ed in condizioni di quasi-equilibrio
Local scour progress downstream of low-head stilling basins
No full textRivers are frequently controlled by weirs and low dams. The flow over these structures has a significant potential for scour even for comparatively low heads. Hence, stilling basins are commonly built to keep energy flows from scouring the streambed. However, the flow turbulence level within them is generally high and local bed scour can develop. This paper deals with temporal and spatial evolution of local scour downstream of low-head stilling basins. In this aim, experiments were carried out in a 1 m wide rectangular channel. Free overfall jets were caused by an ogee-crested spillway and plunged into a positive-step stilling basin. Additional runs were also carried out with an horizontal apron without endsill, as usually done in literature. Four different nearly-uniform sediments were used for the mobile bed among which lead spheres to deeply explore the dependence of local scour on the densimetric Froude number and gravel to better examine sediment grain size effect. Tests were of long durations mainly to get well pronounced scour pits and quasi-equilibrium conditions as well. Based on the experimental data, salient features of the bed morphology evolution are provided. Major effects of the endsill on the scour process are also highlighted
L'erosione localizzata nell'intorno di attraversamenti in alvei alluvionati ghiaiosi
No full textNonostante la molteplicita' degli studi reperibili in letteratura, l'analisi dei processi erosivi in prossimita' di manufatti in alvei fluviali e' a tutt’oggi oggetto di intense attivita' di ricerca. Infatti, la complessita' di tali processi, soprattutto allorquando si vogliano simulare piu' propriamente le situazioni reali, ne ha di fatto impedito una comprensione esaustiva. In tale contesto, il presente lavoro intende fornire un contributo esplicativo su alcuni aspetti specifici, quantunque non marginali. In particolare, vengono raccolti e descritti alcuni risultati, la maggior parte dei quali gia' oggetto di precedenti pubblicazioni, ottenuti nell’ambito di ricerche sperimentali condotte presso il DIFA ed in collaborazione con il Politecnico Federale di Zurigo. Tali risultati riguardano: (i) lo scalzamento localizzato nell’intorno di pile (cilindriche) e spalle di ponti in regime di clear-water. Cio' considerando sia condizioni di moto permanente che di moto vario; (ii) lo scalzamento localizzato nell’intorno di pile (cilindriche) in regime di live-bed; (iii) l’interazione tra correnti fluviali e pile fondate su pali; (iv) i processi di erosione localizzata indotti da significativi restringimenti d’alveo. Il tutto con particolare riferimento agli attraversamenti in alvei alluvionati ghiaiosi
Temporal evolution of live-bed scour at bridge piers
No full textThis paper aims to provide an empirical model for predicting the temporal development of local scour around cylindrical piers under live-bed conditions. In addition to the clear-water scour equation previously introduced by Oliveto and Hager (2002), the proposed model could also provide a more consistent assessment of the scour process under unsteady flow. Several experimental series by Sheppard and Miller (2006) and Chabert and Engeldinger (1956) have been analysed by using the same dimensionless groups as in Oliveto and Hager (2002). A new relationship is thus provided with a reasonably restricted data dispersion
Riverbed scour induced by a long constriction
No full textThis study is a part of a more comprehensive research and is focused on the bed scour that would occur when rivers are forced by manmade or natural contractions. To this purpose, some experiments, typically lasted for days, were carried out at University of Basilicata in a straight rectangular channel and under clear water scour regime. The bed material was uniform sand and three contraction models, with opening ratio (i.e. ratio between the widths of the contracted and uncontracted reaches) of 0.9, 0.8, and 0.7, were used. Starting from the monomial form of sediment transport formulas, a new relationship for the quasi-equilibrium scour depth is conjectured based on Froude’s similarity and by relating the scour depth to the difference between the actual and the entrainment densimetric Froude numbers in the contracted zone. The proposed approach is validated by the data of the present study, and verified by available literature data
Local scour downstream of stilling basins
No full textIn this study some results on local scour downstream of stilling basins for low-head structures are presented. A number of laboratory experiments were conducted in a 1 m wide and 20 m long rectangular straight channel in which an ogee-crest spillway followed by a stilling basin type USBR IV (without chute blocks) was positioned. Two nearly uniform bed sediments were tested. The runs were of long durations, typically 3 days, to achieve conditions of quasi-equilibrium scour hole. Based on the data collected, a new relationship was conjectured relating the quasi-equilibrium scour depth to the tail water densimetric Froude number
Effects of pile caps on local scour at bridge piers
Full text linkThe most existing pier-scour equations mainly apply to uniformly shaped piers. However, pier footings may often project above the streambed because of scour. This may be caused by either long-term bed degradation or local scour. In such cases, the computation of the local bridge scour depth becomes complicated because of the difficulty in choosing an appropriate pier dimension. Referring to piers founded on piles, in previous papers some outcomes were achieved, mainly on the quasi-equilibrium scour depth. Based on additional experiments, this paper would like to come up with a deepening of the previous findings and provide new insights on the temporal evolution of the river-bed morphology around the complex pier-plinth-piles
Local scour downstream of positive-step stilling basins
No full textThis note focuses on the temporal and spatial evolution of local scour below low-head spillways. Steady-flow experiments were carried out in a 1-m wide and 20-m long rectangular straight channel. The jet was generated by an ogee-crest spillway followed by a positive-step stilling basin. Nearly uniform sandy beds were generally tested, but additional tests were also performed with a special bed of lead spheres. To circumvent the combination of local and general scour phenomena, tailwater depths were set such that tailwater flow intensities were below the threshold of sediment motion. As a consequence, for each run a submerged hydraulic jump formed. Tests were of long durations (of order of days) mainly to achieve conditions of quasi-equilibrium. Based on the data collected, literature approaches are discussed. Then, empirical models are proposed to estimate: (1) the maximum scour depth at the quasi-equilibrium stage and its horizontal distance from edge of stilling basin; (2) the time variation of scour depth; and (3) the axial scour profiles. The proposed equations agree well with experimental data. Findings also highlight that affinity rather than similarity may be the typical property of low-angle eroding jets
Problemi connessi ai canali naturali di drenaggio in aree oggetto di pianificazione urbanistica
No full textI rischi naturali rivestono particolare importanza in ambito urbano date le implicazioni connesse alla densita' abitativa ed alla struttura territoriale. Le scelte urbanistiche e di pianificazione devono tenere in conto i rischi connessi ad eventi naturali estremi sia in termini di prevenzione che di difesa e di intervento in caso di evento calamitoso. Nell’ambito di recenti collaborazioni tra il Dipartimento di Ingegneria e Fisica dell’Ambiente dell’Universita' degli Studi di Basilicata (D.I.F.A.) ed alcune amministrazioni comunali lucane, e' stato possibile verificare la vulnerabilita' urbana in riferimento al rischio di allagamento per esondazione di corsi d’acqua e canali naturali che attraversano le aree urbane e ne drenano le acque di pioggia. Tale collaborazione ha consentito di sottolineare come le buone pratiche di governo di un territorio possano diventare, di fatto, un efficace strumento di mitigazione di tali rischi e, piu' in generale di tutti i rischi naturali. Le criticita' idrauliche riscontrate su fiumi, torrenti e fossi “urbani” sono state altresi' studiate utilizzando modelli di simulazione idrologica e idrodinamica al fine di individuare i possibili scenari di allagamento derivanti sia da eventi di pioggia ordinari che eccezionali
Time-dependent local scour downstream of positive-step stilling basins
No full textLocal scour downstream of spillways is an important problem in hydraulic engineering. The objective of this experimental study was to investigate the temporal evolution of local scour downstream of a spillway followed by a positive-step stilling basin. Experiments were conducted in a rectangular channel using four different sediments for the mobile bed. Additional tests concerned the incipient motion of sediment particles. The main parameters governing the scour process were found to be the tailwater densimetric Froude number and the relative tailwater depth. No experimental evidence was found on similar scour profiles. Scour starts at a much reduced tailwater densimetric Froude number when compared with Shields. The temporal variation of the scour hole geometry was estimated using simple power-law equations relating the scour geometry to the controlling scour parameters
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