Temporal Evolution Of Scour Research Articles

Temporal Scour Variations at Permeable and Angled Spur Dikes under Steady and Unsteady Flows

Spur dikes are river protection structures typically used for flow diversion from erodible banks. However, scouring might be a severe problem that compromises their stability and, consequently, their hydraulic function. This paper aims to study the maximum scour depth at permeable and angled spur dikes under hydrographs of different duration. Experiments were carried out in a rectangular channel 10 m long, 0.76 m wide, and 0.6 m deep. The mobile bed was made of nearly uniform sand with a median grain size of 0.8 mm. A total of 36 new experiments were performed with a detailed data collection over the time (i.e., 216 datasets), which could provide a useful contribution to the topic. The impact of the spur dike orientation angle, θ, and the degree of permeability, φ, on the temporal scour evolution were explored. Results were found physically consistent and revealed that the spur dike permeability implies a significant attenuation of the scouring processes in comparison to the impermeable spur dikes and generally its effect is more beneficial than that from a favorable orientation angle. The differences in percentage between the maximum scour depth for impermeable spur dikes and the maximum scour depths for various degrees of spur dike permeability were found ranging from 44% (at φ = 33% and θ = 60°) up to 88% (at φ = 66% and θ = 120°). Other results include the effect of the hydrograph base-time on the scour depth and the comparison between scouring processes under steady and unsteady flow conditions. By quadrupling the hydrograph base-time, keeping constant the peak and base flood discharges, the maximum scour depths increased by about 29%, 42%, and 25% in case of impermeable spur dike, spur dike with 33% degree of permeability, and spur dike with 66% degree of permeability. Furthermore, starting from dimensional analysis a new empirical model (with coefficient of determination R2 equal to 0.94) is introduced to predict the time-dependent scour depth due to the passage of a flood wave. The model suggests that the main independent dimensionless variables which control local scour processes are: the densimetric Froude number, the time t normalized to the hydrograph base-time, the degree of permeability, and the orientation angle. These dimensionless variables would generalize the laboratory results to the real-world scenarios, although caution should always be taken because of possible scale effects.

Temporal evolution of scour at bridge abutments in compound channels

Bridge abutment-induced sediment scour in compound river channels is time-sensitive during floods, and, thus, estimating the scour rate has significance in design. In the current study, large-scale laboratory experiments were done to examine the influence of multiple factors on scour evolution, including channel geometry, bed mobility (clear-water or live-bed), and lateral (embankment intrusion) and vertical (deck submergence) flow contractions. A new method is applied for scour measurement and determination of characteristic scour trends. Results show that the non-dimensional time scale is related to flow intensity and may peak before reaching the clear-water scour threshold; the corresponding equations also are derived. It is found that the use of scour countermeasures has a significant impact on the scour rate, and the existing equations for unprotected abutments have limitations. Under clear-water conditions, the scour rate is closely related to the overall flow contraction, which can be better reflected by the ratio of the approaching unit discharge to that at the bridge section. Specifically, the effect of vertical flow contraction on scour rate is more dominant than other factors. Two scour rate curves are derived to suit different structural and flow contraction types for clear-water scour. In contrast, the live-bed scour rate data are generally consistent and fall in a narrow band, showing less dependence on other factors. Finally, a new design approach is established to estimate the time scale and scour rate using the new equations proposed in this paper.

Temporal Evolution of Scour at Submerged Circular Cylinders

Temporal evolutions of scour at submerged circular cylinders were investigated. Flow visualization was carried out around the cylinders over plane, under developed and equilibrium scour holes. Video analysis technique was used to formulate the equations for determining the diameter of the horseshoe vortex around the submerged cylinders, which is also verified from the vector diagrams drawn using the velocity measurements. The scour process similar to live bed scour was noticed around the downstream cylinder. The diameter of the horseshoe vortex is found to depend on the diameter of respective cylinder, submergence ratio, spacing between the cylinders and skew angle. This formulation along with the dislodgement and transportation of a single sediment particle is further incorporated in the proposed model for determining the time variation of scour around the submerged cylinders. It is evident from the results that the upstream cylinder shelters the downstream cylinder and thereby reduces the scour at the downstream cylinder. Proposed model is further extended to incorporate the effect of non-uniformity of the sediment particles on the time variation of scour depth. The results indicate significant reduction of scour depth of around 6% and 35% for upstream and downstream cylinders respectively due to the formation of the armor layer. The model is also compared with the local scour component of field data around cylindrical bridge piers to establish the differences in the scour process around a partially submerged cylinder and fully submerged tandem and skewed cylinders.

Quantification of Modelling Uncertainties in Bridge Scour Risk Assessment under Multiple Flood Events

Local scour is a dynamic process evolving during the lifetime of bridges as a result of the changes in hydrologic and hydraulic conditions. Current approaches for scour risk assessment are generally based on the evaluation of the equilibrium scour depth for a flood event with a prefixed return period. The temporal evolution of the bridge-pier scour process is usually disregarded, by assuming that equilibrium conditions are always attained, regardless of the flood properties. However, recent studies have highlighted the importance of accounting for the contribution of multiple flood events and their exact hydrograph shape. This study aims at quantifying the epistemic uncertainty related to the modelling of the temporal evolution of scour under multiple consecutive flood events in clear-water conditions. A simple numerical case study is considered, using a Markovian framework to describe probabilistically the progression of scour. Well-known time-dependent scour models are used to estimate the temporal evolution of the scour-depth under each flood hydrograph, and the scour estimates are compared with those obtained using widely employed equilibrium scour formulas. Results show that the expected scour depth is influenced by the parameters used to describe the flood hydrograph and that the probability distribution of the scour depth is highly sensitive to the choice of the time-dependent scour model. The uncertainty in the scour estimation stemming from the formula adopted in this study for describing the temporal evolution of the scour depth can be higher than those related to the formula adopted for equilibrium scour.

Temporal Evolution of Seabed Scour Induced by Darrieus-Type Tidal Current Turbine

The temporal evolution of seabed scour was investigated to prevent damage around a monopile foundation for Darrieus-type tidal current turbine. Temporal scour depths and profiles at various turbine radius and tip clearances were studied by using the experimental measurements. Experiments were carried out in a purpose-built recirculating water flume associated with 3D printed turbines. The scour hole was developed rapidly in the initial process and grew gradually. The ultimate equilibrium of scour hole was reached after 180 min. The scour speed increased with the existence of a rotating turbine on top of the monopile. The findings suggested that monopile foundation and the rotating turbine are two significant considerations for the temporal evolution of scour. The scour depth is inversely correlated to the tip-bed clearance between the turbine and seabed. Empirical equations were proposed to predict the temporal scour depth around turbine. These equations were in good agreement with the experimental data.

Temporal scour evolution at non-uniform bridge piers

Scour at uniform cylindrical piers has received considerable attention in the past. However, bridge foundations can be subjected to the flow due to single or combined effects of long-term riverbed degradation, lateral shifting of the stream, contraction scour, local scour or other factors. This paper focuses on bed morphology changes due to local scour at piers founded on piles or caissons when the foundation is located at the initial bed level. Experiments were carried out in a 1 m wide 20 m long rectangular channel under clear-water and steady flow conditions. The majority of runs were of long duration in order to guarantee sufficient interaction between the flow and pier foundation and to approach the equilibrium phase. Two nearly uniform bed sediments were considered to assess grain size effects. The experimental data obtained provided new information on the temporal variation of scour depth, the areal extent of the scour hole and the scour volume, which are useful for validation of advanced numerical models. In this paper, scour depths are compared with the HEC-18 approach and agreements appear suitable only at the equilibrium phase. Moreover, scour progress is compared to that of uniform cylindrical piers and foundation effects are emphasised. A generalised scour equation is proposed, accounting for such effects.

Flow and scour around vertical submerged structures

The safety of the foundations of submerged hydraulic structures due to excessive local scour is threatened by the erosive action of the waves and currents passing around these structures. Fish and aquatic habitat is seriously affected due to the modification of the flow field caused by these submerged structures. Hence, the problems of flow characteristics and erosion around submerged structures were investigated by various researchers. A comprehensive discussion of the investigations on flow characteristics and local scour due to steady currents and waves around vertical submerged structures are presented, which comprises scour process, dimensional analysis, parameters influencing scour, temporal evolution of scour, flow field, flow visualization techniques, variation of bed shear stress, scour depth determination formulas and scour countermeasures. Although past investigations establish the effect of various parameters on scour around vertical submerged structures for live and clear water condition, yet further studies are required to analyze the scour around group of submerged structures for various bed sediments, understand the flow physics around the group and upscale the model results for the prototype.

Scour control at bridge piers using macro-roughness elements

A new approach involving macro-roughness elements is proposed as a scour countermeasure at circular bridge piers subject to debris accumulation. Experiments were carried out under clear water conditions with the approaching flow velocity set at the threshold of sediment motion. The efficiency of the method was investigated in terms of scour morphology reduction and temporal scour evolution. The results show the effectiveness of the method in reducing maximum scour depth. In some cases, the elements did not reduce the maximum scour depth but were effective in reducing the volume of the scour hole. Based on the experimental data, empirical equations were derived to estimate the temporal local scour depth in the presence of debris accumulation and macro-roughness elements.

Influence of large woody debris on sediment scour at bridge piers

Large woody debris (LWD) reduces the flow area, deviate the flow and increases the velocity in correspondence of the bridge pier, therefore increases the maximum scour hole depth and accelerates sediment removal. Logs and drifts accumulated on bridge piers are of different dimensions. According to logs characteristics and river morphology, drift accumulations can either extend downstream the bridge pier or they can accumulate totally upstream. This paper aims to analyze the effect of drift accumulation planimetry on bridge pier scour. The experimental investigation has been carried out at the PITLAB hydraulic centre of Civil Engineering Department, University of Pisa, Italy. Drift accumulation was characterized by different relative longitudinal lengths, flow area occlusions, length of longitudinal drift and downstream planimetrical positions relative to the pier center. The experimental investigation has been carried out in clear-water conditions. Several pier sizes, channel widths and sediment materials have been tested. Maximum scour hole in presence of drift accumulation have been compared to the maximum scour hole for an isolated pier. Finally, data were compared with previous literature findings, which highlight the effect of the downstream extension of drift accumulation on bridge pier scour. New relationships have been proposed to predict the effect of drift accumulation on bridge pier scour, both in terms of relative maximum scour and temporal scour evolution.

Temporal Development of Scour Holes around Submerged Stream Deflectors

Although the deflector structures used in many fish habitat rehabilitation schemes are frequently overtopped, few studies have examined the scour patterns created around submerged models. Furthermore, laboratory studies typically test smooth-surfaced structures, when those installed in natural rivers are generally made of logs or boulders. This study uses rough-surfaced, paired deflectors to investigate the temporal evolution of scour for three overtopping ratios in identical approach flow conditions in a flume. Results show that when maintaining identical discharge, raising deflector height and thus reducing the overtopping ratio (flow depth / structure height), an increased depth and volume of scour is generated next to the structures. The location of maximum depth and the rate of scouring with time is similar for the two highest deflectors (overtopping ratios of 1.22 and 1.83), but different for the lowest deflector model (overtopping ratio of 3.67). In order to improve the success rate of river restoration projects using in-stream structures, the overtopping ratio should be used in equations that predict scour depth evolution with time.

Temporal scour evolution at bridge piers: effect of wood debris roughness and porosity

Large wood debris transported by floods affects the scour morphology at bridge piers, thus increasing the bridge failure potential. The characteristic size and shape of the riparian vegetation includes various roughness and permeability conditions of the debris surface. The interaction between two-dimensional flow and rough debris accumulations increases the shear stress, the turbulence and consequently affects the scour evolution process at bridge piers. An experimental study on the bridge pier clear-water scour evolution in the presence of wood debris was conducted at the PITLAB research centre, University of Pisa, Italy. A debris accumulation is characterized by roughness, shape and porosity. Flow intensities range from 65 to 100% of the threshold velocity and included up to 18% of the total flow area. Flow depths were varied from 2.67 to 5.67 times the pier diameter. The effects of wood debris roughness and porosity were analysed in terms of scour temporal evolution and scour morphology.

Generalized Approach for Clear-Water Scour at Bridge Foundation Elements

Extensive studies on the temporal evolution of scour were recently conducted at Versuchsanstalt fur Wasserbau, Hydrologie und Glaziologie VAW, ETH Zurich, Switzerland, along with a computational model for the scour depth estimation. The VAW scour data are revisited in the present study to present a further refinement. Additional experiments conducted on the scour entrainment at piers, at rectangular and sloping abutments, as well as at singular and multiple spur dikes are reported. A general criterion is proposed to determine the densimetric particle Froude number for scour entrainment. A new relationship for the temporal scour evolution at bridge foundation elements is then developed based on the similitude of Froude by relating the scour depth to the difference between the actual and the entrainment densimetric particle Froude numbers. The new relationship is validated by the complete VAW scour data set, and verified by the available literature data. The combination of sediment characteristics, approach...

Temporal Evolution of Clear-Water Pier and Abutment Scour

Scour related to bridge hydraulics received much attention in the past decade, including its relation to flood hydrology and hydraulic processes in addition to steady flow. This paper presents new research on bridge pier and abutment scour based on a large data set collected at ETH Zurich, Switzerland. In total six different sediments were tested, of which three were uniform. Also a large variety of scour elements were considered, from 1 to 60% of the channel width, and flow depths ranging from 1 to about 40% of the channel width. Using similarity arguments and the analogy to flow resistance, an equation for temporal scour evolution is proposed and verified with the available literature data. The agreement of the present scour equation with both the VAW data and the literature data were considered sufficient in terms of river engineering accuracy, provided limitations relating to hydraulic, granulometric, and geometrical parameters are satisfied. These limitations are discussed and refer particularly to e...