Department of Civil & Infrastructure Engineering

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An experiment was conducted in a laboratory flume to measure the three-dimensional turbulent flow field, using an acoustic Doppler velocimeter, in a scour hole at a vertical-wall abutment under a clear water scour condition. The distributions of time-averaged velocity components, turbulence intensity components and Reynolds stresses at different vertical sections are presented. In the upstream, a large primary vortex associated with the downflow exists inside the scour hole. On the other hand, in the downstream, the flow field is chaotic due to the vortex shedding. The data presented in this paper would be useful for the development and validation of a flow field model, which can be used for the estimation of scour depth at bridge abutments. © 2005 Elsevier Ltd. All rights reserved.

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Flow Measurement and Instrumentation

Velocity and turbulence in a scour hole at a vertical-wall abutment

Journal of Hydraulic Engineering

Closure to "time variation of scour at abutments" by Subhasish Dey and Abdul Karim Barbhuiya

Expressions for the Reynolds stress and bed shear stress are developed for nonuniform unsteady flow in open channels with streamwise sloping beds, assuming universal (logarithmic) velocity distribution law and using the Reynolds and continuity equations of two-dimensional open-channel flow. The computed Reynolds stress distributions are in agreement with experimental data. © ASCE.

Reynolds stress and bed shear in nonuniform unsteady open-channel flow

Results of an experimental investigation into local scour at piers and abutments in uniform gravels under a clear water scour condition are presented. Two types of pier, namely circular and square, and two types of short abutment (abutment length/upstream flow depth < I, for short abutments), namely wing-wall and vertical-wall, were tested. Although scour at piers and abutments has been studied widely in sand beds, it remains almost unexplored in gravel beds. Therefore, different gravel sizes, ranging from 4-1 mm to 14-25 mm, were used in this experimental study. The influence of gravel size on equilibrium scour depth at piers and abutments is prominent. From the experimental results, it is observed that the equilibrium scour depth increases with decrease in gravel size and with increase in pier width and abutment length. The variations of equilibrium scour depth with gravel size for both pier and abutment scour depart considerably from the variations of equilibrium scour depth with sand size. Consequently, for scour at piers and abutments, the resulting sediment size factors for gravels, evaluated by fitting envelope curves, are significantly different from the existing sediment size factor for sands. Combining pier and abutment scour data, an integrated approach to estimating the equilibrium scour depth at a bridge foundation is proposed. Scaling of distorted experimental model results to prototype is also discussed.

Proceedings of the Institution of Civil Engineers: Water Management

Scour of gravel beds at bridge piers and abutments

A semiempirical model is presented to compute the time variation of scour depth in an evolving scour hole at short abutments (abutment length/flow depth ≤ 1), namely the vertical wall, 45° wing wall, and semicircular, in uniform and nonuniform sediments under a clear water scour condition. The methodology developed for computing the time variation of scour depth is based on the concept of the conservation of the mass of sediment, considering the primary vortex system as the main agent of scouring, and assuming a layer-by-layer scouring process. For an equilibrium scour hole, the characteristic parameters affecting the nondimensional equilibrium scour depth (scour depth/abutment length), identified based on the physical reasoning and dimensional analysis, are excess abutment Froude number, flow depth-abutment length ratio, and abutment length-sediment diameter ratio. Experiments were conducted for time variation and equilibrium scour depths at different sizes of vertical walls, 45° wing walls and semicircular abutments in uniform and nonuniform sediments under limiting clear water scour conditions (approaching flow velocity nearly equal to the critical velocity for bed sediments). The present model corresponds closely with the data of time variation of scour depth in uniform and nonuniform sediments obtained from the present experiments and reported by different investigators. © ASCE.

Time variation of scour at abutments

Nonuniform-unsteady flow in open channels with streamwise sloping beds having uniform upward seepage is theoretically analyzed. Expressions for the Reynolds stress and bed shear stress are developed, assuming a modified logarithmic law of velocity profile due to upward seepage, and using the Reynolds and continuity equations of two-dimensional open channel flow. The computed Reynolds stress profiles are in agreement with the experimental data. © ASCE.

Journal of Engineering Mechanics

Reynolds stress in open channel flow with upward seepage

The characteristics of scour holes in uniform and nonuniform sediments downstream of an apron due to a submerged horizontal jet issuing from a sluice opening are studied experimentally. Attempts are made to explain the similarity existing in the scour profiles (including the dune downstream of the scour hole). From the scour profiles, the characteristic lengths of the scour hole which are determined are: horizontal distance of dune crest from the edge of the apron; dune height; horizontal distance of maximum scour depth from the edge of the apron; horizontal extension of scour hole from the edge of the apron and maximum equilibrium scour depth. For uniform sediments, the horizontal distance of the dune crest from the edge of the apron, the horizontal distance of maximum scour depth from the edge of the apron, the horizontal extension of scour hole from the edge of the apron and the maximum equilibrium scour depth were all found to increase with increase in densimetric Froude number; whereas the dune height decreases with increase in densimetric Froude number. In addition, for nonuniform sediments, the aforementioned characteristic lengths of the scour hole are related to the geometric standard deviation of sediments. The characteristic lengths of the scour hole decrease with an increase in geometric standard deviation of sediments. The application of the findings of the paper to the prototype is discussed, which would assist the practising civil engineer to ascertain the dimensions of a scour hole (scour depth, position of maximum scour depth, extension of scour hole etc.) in uniform and nonuniform sediments downstream of an apron. Importantly, these are the key parameters for designing apron foundations and protective measures for scour downstream of an apron. Future work will focus on field work to prove the scalability of the derived relationship.

Scour downstream of aprons caused by sluices

Experimental results on local scour in long contractions for uniform and nonuniform sediments (gravels and sands) under clear-water scour are presented. An emphasis was given to conduct the experiments on scour in long contractions for gravels. The findings of the experiments are used to describe the effects of various parameters (obtained from dimensional analysis) on equilibrium scour depth under clear-water scour. The equilibrium scour depth increases with decrease in opening ratio and with increase in sediment size for gravels. But the curves of scour depth versus sediment size have considerable sag at the transition of sand and gravel. The scour depth decreases with increase in densimetric Froude number, for larger opening ratios, and increases with increase in approaching flow depth at lower depths. However, it becomes independent of approaching flow depth at higher flow depths. The effect of sediment gradation on scour depth is pronounced for nonuniform sediments, which reduce scour depth significantly due to the formation of armor layer in the scour hole. Using the continuity and energy equations, a simple analytical model for the computation of clear-water scour depth in long contractions is developed with and without sidewall correction for contracted zone. The models agree satisfactorily with the present and other experimental data. Also, a new empirical equation of maximum equilibrium scour depth, which is based on the experimental data at the limiting stability of sediments in approaching channel under clear-water scour, is proposed. The potential predictors of the maximum equilibrium scour depth in long contractions are compared with the experimental data. The comparisons indicate that the equations given by Komura and Lim are the best predictors among those examined. © ASCE.

Scour in long contractions

An experimental investigation on scour at circular and square piers in uniform and non-uniform gravels (fine and medium sizes) under clear-water scour at limiting stability of gravels is presented. From the experimental results, it is observed that the equilibrium scour depth increases with decrease in gravel size. The variation of equilibrium scour depth with gravel sizes departures considerably from that with sand sizes. Consequently, the resulting sediment size factors for gravels, obtained from envelope curve fitting, are significantly different from the existing sediment size factor for sands. The influence of gravel gradation on scour depth is also prominent in non-uniform gravels. The time scales to represent the time variation of scour depth in uniform and non-uniform gravels are determined. For uniform gravels, the non-dimensional time scale increases with increase in pier Froude number and gravel size, whereas for non-uniform gravels, it decreases with increase in geometric standard deviation of particle size distribution of gravels. © 2005 NRC Canada.

Canadian Journal of Civil Engineering

Clear-water scour at bridge piers in fine and medium gravel beds

Experiments were conducted to measure the three-dimensional turbulent flow field, using the acoustic Doppler velocimeter, at a short vertical-wall abutment (ratio of abutment length to approach flow depth less than unity) before and after the development of a scour hole under a clear water scour condition. In the upstream, the presentation of flow field through vectors at vertical sections shows a primary vortex associated with the downflow. In the downstream, the upward flow is comprised of with irregularities owing to the vortex shedding. The flow separation near the bed and within the scour hole is evident from the turbulent kinetic energy distribution. Using Reynolds stresses, the bed shear stresses are calculated. © ASCE.