A REVIEW OF HYDRAULIC JUMP PROPERTIES ON BOTH SMOOTH AND ROUGH BEDS IN SLOPING AND ADVERSE CHANNELS

Hydraulic jump is a phenomenon which has received significant attention in recent years and it is still studied because of its capacity to dissipate a considerable amount of the flow energy. Nevertheless, the importance of the topic still requires significant efforts from the scientific community. Namely, the prediction of the main lengths of the hydraulic jump are still an open question, as the actual knowledge on the topic does not cover all the possible configurations and boundary conditions which can usually be found in practical applications. In particular, the effects of bed roughness, bed slope, channel geometry, and air concentration on the conjugate depths ratio are still not fully understood. The present paper aims to furnish a synthetic picture of the state of art regarding the hydraulic jump properties in a wide range of both boundary conditions and geometric configurations. In particular, the analysis will be focused on the effect of both relative roughness and bed slope on the conjugate depth ratio, including the effect of air entrainment on the estimation of the effective depth. Furthermore, some predicting relationships proposed by different authors will be compared and discussed

D-jump in rough sloping channels at low Froude numbers

Hydraulic jump is a phenomenon which usually occurs in rivers and in correspondence with hydraulic structures. It is characterized by a rapid transition from a super- to sub-critical flow conditions, involving a huge energy dissipation. For its characteristics and its relevance in hydraulics, it has been studied since the early years of the last century. Nevertheless, many features still require a further investigation, e.g., scale effects, air entrainment process, effect of bed roughness on conjugate depths, etc. In particular, in the last few decades, the analysis of the main geometric parameters has been further developed. Nevertheless, no studies dealing with D-jump occurring on a rough sloping channel are present in literature. Therefore, the present paper reports the results of experimental investigations, which allowed to develop a semi-theoretical approach in order to evaluate the D-jump conjugate depth in a wide range of channel bed geometric and boundary configurations

Structural defence for plunge pool scour

Plunge pool scour is a major topic in presence of hydraulic structures that foresees the production of jets. The scour hole is a function of several variables including the tailwater, the densimetric Froude number, the sediment granulometry, the water discharge. The maximum scour depth is of great interest in the design process. Aim of the paper is to analyze the effect of structures, inserted in the scour hole, in order to mitigate the scour geometry. About 300 tests have been carried out in the Hydraulic laboratory of the University of Pisa (Italy) in order to assess the advantages and the problems connected with the insertion of a rigid structures in the river bed. In this paper a first description of the phenomena is presented

Semi-theoretical approach for energy dissipation estimation at hydraulic jumps in rough sloped channels

Hydraulic jumps cause significant flow energy dissipation and generally occur in association with hydraulic structures. Nevertheless, the dissipative mechanisms occurring with this hydraulic phenomenon still require investigation. There are no systematic studies analysing the hydraulic jump energy dissipation for a large range of hydraulic and geometric conditions. To the authors’ knowledge, no studies provide a complete and exhaustive analysis of the dissipative mechanism for rough sloped channels. This study proposes a novel semi-theoretical and general approach to estimate energy dissipation at hydraulic jumps in rough sloped rectangular channels, involving a large range of both hydraulic conditions and bed configurations. Two relationships were derived and their predictive capability has been tested with approximately 500 experimental data points from different studies

Scour process caused by multiple subvertical non-crossing jets

The scour process induced by plunging jets is an important topic for hydraulic engineers. In recent decades, several researchers have developed new strategies and methodologies to control the scour morphology, including different jet arrangements and structures located in the stilling basin. It has been found that multiple jets can cause less scouring than single plunging jets. Based on this evidence, this study aimed to investigate the equilibrium morphology caused by multiple non-crossing jets. A dedicated laboratory model was built and experimental tests were carried out under different combinations of jet inclination angles, by varying the tailwater level and the virtual crossing point location, which was set below the original channel bed level. It was experimentally shown that the equilibrium scour morphology depends on the jet discharge, the differences in non-crossing jet inclination angles, the downstream water level, and the distance of the virtual crossing point from the original channel bed level. In particular, the last parameter was found to be one of the most influential parameters, because of the resulting flow patterns inside the water body. Furthermore, the analysis of experimental evidence allowed for a complete and detailed classification of the scour hole typologies. Three different scour typologies were distinguished and classified. Finally, based on previous studies, two novel relationships have been proposed to predict both the maximum scour depth and length within a large range of hydraulic and geometric parameters

Damping reduction factors for adjacent buildings connected by fluid-viscous dampers

This work investigates the dynamic behaviour of a multi-storey frame building, assumed as the structure to be controlled, connected with an adjacent support structure by means of horizontal fluid viscous dampers. The dampers connection system has two main effects: (i) energy dissipation provided by the viscous dissipative forces, (ii) energy transfer between modes leading to modes coupling effects between the two buildings without a significant dissipation of energy. The relative contribution of the two effects is highly dependent on the main properties of the coupled system. To investigate this complex behaviour, first the minimal coupled dynamic system composed by two Single-Degree-Of-Freedom systems connected by a viscous damper is analysed. The theory of complex damping is used to determine complex frequencies and damping ratios, while analytical expressions of steady state response under harmonic excitation are determined to investigate the influence of the system parameters on the maximum dynamic amplifications. The seismic response is also investigated through a wide parametric study with the aim of evaluating the trends of the damping reduction factors with respect to the main dynamic parameters of the coupled system. Minimum values of the damping reduction factors and corresponding optimal damping coefficients are determined. Then, the analyses are extended to uniform multistorey structures for which minimum damping reduction factors are provided for a wide range of the key system parameters. Comparisons between one-storey and uniform multi-storey systems are provided. The results can be useful for a preliminary design of the dampers connection system and for the evaluation of its effectiveness based on the fundamental dynamic properties of the connected buildings

Hydraulic jumps on rough and smooth beds: aggregate approach for horizontal and adverse-sloped beds

Hydraulic jumps, which frequently occur in hydraulic structures, have been extensively studied over the last century. However, only a few studies have evaluated hydraulic jumps in flows over rough beds and there are no studies that consider the air entrainment effect on conjugate depths. The current paper reports the results of an experimental investigation of hydraulic jump properties in flows over adverse-sloped rough beds, including the effect of air entrainment. Furthermore, a semi-theoretical predictive relationship is proposed to estimate jump characteristics for a wide range of hydraulic and geometric conditions covering both rough and smooth beds

Effect of unsteady flow conditions on scour features at low-head hydraulic structures

The study of scour mechanism downstream of low-head control structures is a fundamental topic for hydraulic engineers. Generally, the analysis of the scour process is conducted under steady flow conditions, assuming that the maximum discharge is occurring for sufficient time to reach the equilibrium scour configuration. Nevertheless, in rivers the scour process generally occurs in correspondence with a flood event, which is characterized by discharge varying with time. This last condition is still less studied and analyzed in terms of effects on bed morphology. Researchers mainly focused on the maximum scour depth assuming that it occurs in correspondence with the peak discharge, but they rarely took into account the evolution of the scour process under unsteady flow conditions. The aim of the present paper is to analyze the evolution of scour morphology under unsteady flow conditions, and compare it with that obtained under steady flow conditions. In particular, three structure typologies were tested: a stepped gabion weir with upstream impermeable filtering layer, a straight rock sill, and a curved rock sill. The results showed that the scour phenomenon deeply depended on inflow conditions. Nevertheless, it was also shown that the equilibrium morphology of the downstream stilling basin is essentially the same under both unsteady and steady flow conditions if the duration of the unsteady event is enough long

State of art in block ramp and downstream stilling basin design

Block ramps are hydraulic structures which are commonly used in river restoration projects. Especially in the last few decades, the use of this type of structures have become more and more popular. They furnish a correct balance between the hydraulic functioning and the environmental care, as they minimize the impact on the environment in which they are located. In addition, they can be considered flexible structures, i.e. they can easily adapt to the in situ conditions and they can be easily built to re-convert traditional concrete structures. They can be built either by loose or fixed blocks, arranged on a sloped bed. However, a correct design of this structural typology has to take into consideration several aspects. In particular, the hydraulic functioning of a block ramp is assured when the structure remains stable, i.e. when the blocks are not removed from their original position. Thus, the first step in designing block ramps has to be the structural stability. Furthermore, the analysis has to focus also on the dissipative process occurring on them, in particular it has to consider the different flow regimes that can take place and the effect of the bed roughness on the energy dissipation. Another important aspect is the stilling basin design. In fact, a block ramp has not to be considered as an isolated element in the contest in which it is located. It is part of that contest and it contributes to modify it. Thus, it is extremely important to consider the scour process occurring downstream of the structure. In particular, the maximum scour depth and length have to be carefully estimated in order to avoid structural collapse of the ramp. The scour process occurring downstream of the structure is also extremely important in terms of energy dissipation. In fact, the global dissipative process is the result of two distinct processes: one occurring on the ramp and the other in the downstream stilling basin. Finally, the analysis has to take into consideration which are the global sediment transport conditions of the river in which the structure is located. Thus, it has to be conducted in both clear water and live-bed conditions. It appears evident that a correct design of this type of structures is a complex operation which requires a particular attention in order to avoid functioning problems