Dissertations / Theses on the topic 'LOCAL SCOURING'

Designers are often required to produce safe and economic structures in rivers with erodible beds, which may frequently induce scouring phenomena as they interfere with the natural stream. In CHAPTER 1, an extensive literature is illustrated on scouring by jets at high and low head structures, and predictive formulae are discussed that estimate the equilibrium scour depth of the scour hole, which are most obtained empirically from field and laboratory data. It is also discussed that different stages occur during local scour development. These stages basically include an initial rapid phase of severe scouring, followed by a stabilization phase approaching equilibrium conditions after a long time. CHAPTER 2 describes the theoretical basis to deal with long-term local scouring at bed sills under clear-water and steady-flow conditions. In gravel bed rivers, bed sills are used to limit bed degradation. Local scouring takes place downstream of each sill in addition to the general erosion pattern, and scour hole dimensions increase with the distance between sills. While overall aggradation can be estimated by using 1D morphological models, local scouring requires a more empirical approach. In fact most scouring phenomena are induced by turbulent jets that diffuse within the scour hole, by resulting in extremely complex flow fields. Many approaches are fully empirical, being based on curve fitting of experimental data that link scour depth to hydraulic quantities and sediment properties. In the most recent attempts, a semi-empirical approach has been followed, based on the identification of appropriate dimensionless groups using dimensional analysis before employing best-fitting procedures. The theoretical derivations proposed, which are based on the assumptions of the Buckingham's ­?-theorem, are discussed by showing some further insights on the nature and role of the dimensionless parameters that ensued form dimensional analysis. CHAPTER 3 presents the results of experimental tests carried out by the author in 2003 about the pattern of local scouring generated at the toe of bed sills. The aim of this study is to evaluate the effect of upstream sediment supply on the scour depth and shape. The experiments simulated conditions of a steady upstream sediment supply which had the same grain size composition as the sediment deposit placed on the bed of the flume. The geometric characteristics of three scour holes developed under conditions of steady-flow discharge and steady upstream sediment supply were measured during 48 different tests. It is shown that the imposed sediment transport does not require the introduction of new dimensionless parameters into existing scour depth and length prediction formulae. The effects of sediment feed are shown to be incorporated in the existing dimensionless parameters. The new data set is used to re-calibrate existing scour depth formula. The influence of jet erosion on scour geometry is also discussed. Most research efforts have focused on predicting scour depth, which may affect the structure at the free overfall, as opposed to volumetric scour dimensions and sediment yield contribution due to local scour, which may affect downstream morphology and water quality as well as the structure but are much more difficult to measure in an actively eroding plunge pool. In CHAPTER 4, shape and volume of equilibrium local scour holes at bed sills in high-gradient streams are investigated by analyzing steady-flow scour profiles at the equilibrium stage. Geometric properties of the scour profile are analytically discussed and verified on the basis of new data from experimental tests described in CHAPTER 3 and data from literature. A model is presented to organize data analysis into a conceptual frame, which is based on the formal relationships for scour depth and length ensued from dimensional analysis, and on the assumption that scours exhibit definite geometric properties. Results show evidence of a universal geometric affinity of the scour. Geometric similarity is also found to occur, but only in a limited range of physical conditions. Prediction formulae are proposed which link the eroded volume to the geometric, hydraulic and sediment properties. Evidence on the scaling nature of the scour hole shapes have important implications on the prediction of the eroded volume. In turn, a correct evaluation of the eroded volume is necessary for the optimization of the design of the inter-sill distance. The temporal development of clear-water local scour depth at bed sills in uniform gravel beds is considered in CHAPTER 5. A new experimental program has been carried out by the author, which started at the end of 2006 and was concluded in March 2007 at the Fluid Dynamics Laboratory of The University of Auckland (NZ). Experiments are presented on the development of scour holes under unsteady hydraulic conditions, with the triangular-shaped hydrographs tested being of different durations and different rates of flow variation. Preliminary observations are discussed on the behavior of scour development, which is compared with the scour evolution for the steady-state case. Based on the experimental results and a theoretical framework, a method is given for the definition and prediction of the scouring process under unsteady flows in terms of a dimensionless temporal parameter. A "flash flood" is defined as an event for which the scour doesn't attain its potential magnitude, i.e. the equilibrium value for the peak hydrograph flow rate. It is shown that this flood nature is dependent on both the characteristics of the flood event itself and the characteristics of the stream. A quantitative measure of what constitutes a flash flood is given in terms of the identified temporal parameter, which represents one of the main goal of this study.

This study examines the local scouring phenomenon associated with bridge abutments exposed to combined main-channel (MC) and floodplain (FP) flows. The principal goal of the study was to develop and validate, based on a comprehensive physical-model testing programme that considered different abutment shapes, an improved relationship for predicting maximum scour depth. A relationship was established among sediment size, flow depth and longitudinal channel slope for initiating the threshold condition of bed material movement. This relationship facilitated the selection of sediment size and channel bed slope for the various experiments, which were performed in a compound channel under interacting (combined MC and FP flows) and non-interacting (isolated FP flow) conditions. Clear-water approach-flow conditions were utilized throughout the testing programme. A flow visualization technique was employed to study the various patterns of the combining flows in the MC/FP junction regions. It determined both the flow deflection angle and the extent of the influence zone (a portion of the channel width beyond the abutment) whose discharge, $Q\sb{w}$, contributes to the scour development. Maximum scour depths produced under interacting conditions were observed to be 15% to 30% greater than those observed for non-interacting conditions. This important observation clearly demonstrated the need to properly account for compound-channel effects in the simulation exercise. The scour prediction relationship presented herein is based on a dimensional analysis of the relevant parameters and incorporates the data obtained from the laboratory studies performed in a compound channel using vertical-wall, wing-wall, semi-circular, and spill-through abutment shapes. The model indicates a very good agreement between predicted and observed maximum scour depths for bridge abutments terminating in the FP zone.

The threat of local scour around bridge piers has been in research for many years. According to the various studies, local scour around the bridge pier is the prime cause for most of the bridge failures. The main objective of the present study was to investigate the flow behavior and the scour phenomenon around the bridge piers of various shapes namely Circular, Elliptical, Square and Streamlined. Local scouring depends on various factors like depth of flow, upstream flow conditions, pier shape and dimensions. Here, we have taken only pier shape as the primary factor and kept other factors constant. The numerical simulations were even carried out using CFD- Fluent, eulerian, k–epsilon turbulence model, to elaborate the physics behind the scour formation. CFD simulation tool can be used for wide understanding of the flow behavior around the bridge piers even without physical model studies because it saves time and money as compared to experimental studies. Three dimensional simulation of flow behavior around four pier shapes indicates that the streamlined pier is the most efficient pier to use as it allows the flow to pass smoothly around it creating less obstruction to the flow and hence creating less chances of local scouring near the pier toe.

碩士
國立中央大學
土木工程研究所
97
Scour is always the primary threat to bridges in Taiwan area. Since bridges will experience floods which may cause damage from scour holes forming around bridge piers and abutments. As scour changes the geometry of the bridge pier embedded in the soil foundation, the effects due to other loads, such as live load, wind load, debris collision, etc., are amplified. In this condition, bridge may have settlement or tipping even collapse. Therefore it is demanded by the bridge authority agencies to have a monitor sensor to determine the real-time scour depth. Besides, with internet it is easily to handle the real-time information at bridge site to alarm the siren in time from control center through the predefined safety warning algorithm. A piezoelectric fluoropolymer film type real-time scouring monitoring sensor installed along the bridge pier is presented. The core idea of developing this scouring sensing system is based on the physical character that output voltage can be generated as the piezoelectric thin film is deformed by the flowing media. Therefore, a sensing device can be manufactured by mounting piezoelectric thin films along the rod with a fixed distance and insert it into a borehole beside the bridge pier. The piezoelectric film embedded in the soil of riverbed is undeformed and the output voltage is much smaller than the one disturbed by the water current. From the output signals of all the piezoelectric sensors with known locations along the depth of the pier foundation, one can trace the variation of the soil/water interface before, during and after a flood. Compared with other types of scouring sensors, this piezoelectric type scouring sensor is working without power, durable, sensitive, real-time and cost effective. In this paper, the basis theory and the performance that in laboratory of this novel piezoelectric film type scouring device are introduced. The installation of the system in field is also addressed. In this study, it also brings up three methods for readers to analyze the safety of bridge, which can be taken into consideration of blocking a bridge.

碩士
國立成功大學
水利及海洋工程學系
86
The aim of this experiment studies is undertaken to know the directions of sand motion and the positions of local scouring due to the effect of differentflow-training panels. It is known by experiments that the sand motion is caused by helix in a clockwise direction of the angle of attack is smallerthan -π/2, no matter how large the opening of the surface panel. The bottompanels have a critical height is about 0.5 times local water depth at designstage. In other words, the panel''s height is above or under the critical depthof bottom panels, while the sand-training is quite different. The middlepanels, not close to the surface and the bottom, only in the case the panel''s bottom is close to the river bed have the nearby phenomenon of depositing and scouring with the bottom panels. In the same angle of attack with flow, the bottom panels with the height greater than half local water depth at design stage have the same phenomenon of depositing and scouring with the surface panels. The open space under the surface panel allows the flow passing, then the local scour will take place at downstream, but the bottom panel has the local scour at upstream nearby.

碩士
國立中興大學
土木工程學系所
105
For river management and water acquirement, drop structures are commonly set up in the river course. Although the stream channels can be stabilized, the gravel will not move downwards and its deposition near the drop structures increases the slope of upstream channel, consequently leads to a steep slope and induces fast flow. As a result, the large impact forces destroy the downstream protection work of drop structures. This study aims at the scouring rate of local scour and relevant scouring behavior by sloping drop structure. From the analyzed results, it indicates that (1). The maximum stable scouring depth Yse is proportional to the drop number D, but it is inversely proportional to the dimensionless diameter of bed material G and the bed slope of upstream channel S. The distance from toe to the maximum stable scouring depth Lse and the maximum scour length Lei are proportional to both D and S, but they are inversely proportional to G. The maximum scouring volume per unit width of flow quantity is proportional to the dimensionless impact force F ̅d, but it is inversely proportional to the G and S; (2). The characteristic time te to reach the stability of scour increases with the increase of D and decrease of G, but it is almost indifference to S; (3). The maximum time-dependent scouring depth Yst , scouring distance Lst, scouring length Lit and scouring volume per unit width of flow quantity Ast is proportional to time in an exponential manner, but it is inversely proportional to the G and S; (4). The bed scour is caused by drop nape in a great scouring rate. The souring pit is almost completed in a quarter time of te and the scour will be developed completely when the characteristic time te is arrived.

碩士
國立中興大學
土木工程學系
89
According to the related research about the scouring effects of free over-fall will be used in this study. The situation will be discussed in steady flow by using different energy dissipators — apron, water cushion and secondary dam. Facilities, the protected effect and the comparative characteristics of scouring hole will be found (such as the max. depth, max. length, shape factor and volume) in downstream of the energy dissipators. The result would be determined by energy dissipating theory, which is the relations between the scale of dissipators and energy variation. In view of energy disspators, the water cushion had the result of protection better than apron, because of the current hits mutually at the water pond. And the height factorβ=0.13 is the best. The situation of the secondary dam is the moving bed, therefore analysis of the scouring characteristic to make use energy variety of system. In both the rigid and soft energy disspators, the energy loss of scouring increased with slop. The degree of scouring depth and volume could be lowered effectively in rigid energy disspators. In water cushion, the average decreased rate of the scouring length about is 40 percent, and the scouring volume quantity decreases rate reaches 40 percent at least. Although, scouring length can’t be decreased at apron disspators. The scour phenomenon be mollified to a downstream region for protective effects. Scouring is more serious between major and secondary dam if the distance is too short from both of them. Character factorα>1.56 is the suggested application. In this experiment condition, a simple formula is proposed for a relevant establishment. It is expected to be helpful in designs of energy disspators.

碩士
中興大學
水土保持學系所
99
To prevent the local scouring, collar is adopted to protect piers. Therefore, this study designed a mobile experiment flume with different heights of hooked-collar to understand that how the position of hooked-collar effected the local scouring around piers. The velocity fields around piers were measured with ADV (Acoustic Doppler Velocimeter). Furthermore, we also measured the topography of scour holes by laser range finder. The hooked-collar can reduce scour depth and the range of scour hole. When the hooked-collar was on the best position, it can completely reduce scour depth in front of the pier. When the hooked-collar set on 1cm from the bed, it can reduce 32% maximum scour depth. Scour hole diameter also reduced 45%. And the sediment deposition is also close to the pier. In addition, the hooked-collar reduce about 76% down flow. The horseshoe vortex is guided to the downstream successfully far away from the bridge pier. The hooked-collar can accelerate the sediment deposition behind the piers.

Dottorato di Ricerca in Ingegneria Civile e Industriale. Ciclo XXXI
The phenomenological theory of turbulence is here applied to the scouring phenomenon at bridge piers and abutments. In the last ve decades many researches have been devoted to the development of predictive formulae able to quantify the maximum scour depth for both design and risk assessment needs of hydraulic structures. Owing to the complexity of the problem, most of the proposed formulae were developed on an empirical basis, which made them susceptible to scale issues and not fully consistent with the physics underpinning the scouring phenomenon. Recently, some studies of Gioia & Bombardelli (2005), Bombardelli & Gioia (2006), Manes & Brocchini (2015) and Ali & Dey (2018) have proposed a di erent approach, which exploits a theory to derive scaling relations between the equilibrium scour depth and non-dimensional parameters. Their work presented the phenomenological theory of turbulence and the paradigms of the sediment incipient motion theory assuming rough ow conditions, meaning that the momentum transport near the sediment-water interface was dominated by eddies belonging to the turbulent energy spectrum inertial range and scaling with the sediment diameter. In order to provide more general models and on the basis of the ndings of Bonetti et al. (2017), the present work relaxes this as sumption by exploring the scaling of the equilibrium scour depth in cases where momentum transport is a ected by eddies belonging to the dissipation and production range. This improvements were applied to the scouring phenomenon at bridge piers, to derive a predictive formula for the maximum scour depth, and to the scouring phenomenon at bridge abutments, to derive a scaling law that does not allow a directly assessment of the maximum scour depth, but provides new avenues for the development of general predictive formulae that are founded more on physical than empirical bases. In both cases, the proposed theory includes the relevant non-dimensional parameters controlling the scouring process and, contrary to commonly employed empirical formulae, it is free from scale issues.
Università degli Studi della Calabria

碩士
國立臺灣大學
生物環境系統工程學研究所
95
The height from top of check dam to bed of river, makes water impacting bed of river. And further, it endangers the safety of structure possibly. There have been many theses about local scouring. This research uses model of check dam with four kinds of height, and two kinds of mix grain, and several kinds of discharge of flow to confer impact force of over-fall flow and, depth of scour hole, and the location where water drops, and the reduce of energy by scouring. And compare with the formulas of former literature.

碩士
國立中興大學
土木工程學系
87
ABSTRACT Most failures of hydraulic structures (such as Sabo dam) in Taiwan were due to the scouring of downstream protection works. According to the past research of other scholars and the experiments conducted here at the laboratory of the NCHU (National Chung-Hsing University), this study is to investigate the effect of local scouring by free over-fall flow on the fixed apron, which is constructed on downstream. Starting with the experiments without the fixed apron to see the behavior of free over-fall flow and the characteristics of scouring hole on downstream bed, then with the fixed apron to investigate how them protect the downstream bed. At last, change the flume-slope to see how it affects the scouring. According to the experimental results, the behavior of free over-fall flow is close to the situation of fixed bed, the maximum scouring depth and the scouring volume of the scouring hole decrease; the scouring also farther from the Sabo dam. Those show that the fixed apron could decreases the scouring energy and the force of free over-flow effectively to protect the element of Sabo dam. When the flume-slope is steeper, the scouring energy and the force of free over-flow are larger, and it easier to transmit the sand downstream, and the scouring become even worse.