Approximate methodology to account for effects of coherent structures on sediment entrainment in RANS simulations with a movable bed and applications to pier scour

Cheng, Zhengyang
Köken, Mete
Constantinescu, George
Movable-bed Reynolds-Averaged Navier-Stokes (RANS) simulations of local scour around bridge piers and abutments without ad hoc corrections tend to underestimate scour. This is generally attributed to the inability of RANS to accurately capture the unsteady dynamics of the large-scale coherent structures forming in the bed vicinity. Moreover, the characteristics of these coherent structures and their erosive capacity are expected to change as the bed evolves toward equilibrium. In the present study, Detached Eddy Simulations (DES) of flow past circular and rectangular piers are performed with bathymetry corresponding to different stages of the scour process between initial (flat-bed) conditions and equilibrium scour conditions to better understand how the coherent structures drive scour around the pier at different stages of the scouring process and to assess the capabilities of RANS to predict the flow and the turbulence statistics. Using these results, a new methodology is proposed to account for the effects of large-scale coherent structures on sediment entrainment that is directly applicable to RANS simulations with movable bed. The main idea is to improve the predictions of the local flux of sediment entrained from the bed by considering the effect of bed friction velocity fluctuations induced by the coherent structures situated near the bed, a critical effect that is generally neglected in RANS simulations. The new methodology is based on augmenting the bed friction velocity available from the time-accurate RANS calculation by a term that is proportional to the standard deviation of the bed friction velocity. This term is estimated from the turbulent kinetic energy field predicted by RANS. The model has one free parameter. Its value is calculated such that for fixed bed simulations the total entrainment flux predicted by the time-accurate RANS matches the value predicted by DES. Based on simulations conducted for flow past circular and rectangular piers, the value of the free parameter was estimated to be between 1.5 and 4, with lower values recommended to be used for cases when the time-accurate RANS simulation captures the large-scale vortex shedding behind the pier. Movable bed simulations performed using the augmented bed friction velocity model are shown to predict much more accurately the maximum scour depth around circular and rectangular piers compared to the corresponding simulations where the entrainment flux was calculated only based on the local value of the bed friction velocity calculated from the RANS velocity field.


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Özalp, Murat Can; Bozkuş, Zafer; Department of Civil Engineering (2013)
It is an important task that design engineers in practice predict the local scour around bridge piers as accurately as possible because excessive local scour around bridge piers unbalance and demolish the bridges. Many equations have been proposed previously by various researchers, based on their experimental findings, but no general method has been developed so far due to the complexity of the topic. In the present study two new bridge pier groups were employed to investigate the inclination effect of the ...
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Selamoğlu, Meriç; Yanmaz, Ali Melih; Köken, Mete; Department of Civil Engineering (2015)
Computation of temporal evolution of scour depth around bridge piers is essential for the efficient design of bridge pier footings. In this thesis, empirical scour-prediction equations and a semi-empirical model are developed to predict the temporal variation of maximum clear-water scour depth at dual cylindrical uniform piers with identical size in tandem arrangement. Experiments are conducted using different pier size, pier spacing, and flow intensities. The semi-empirical model is based on sediment conti...
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Excessive local scour around piers and abutments is known as a major cause of bridge failure induced by hydraulic deficiencies. Complexity of the scouring phenomenon and high degrees of uncertainties in governing parameters lead to an unavoidable risk in bridge pier design. In this paper, a generalized reliability model based on static resistance-loading interference is developed for the assessment of reliability of bridge scour for various pier shapes. In the model, the relative maximum scour depth and the...
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Yanmaz, Ali Melih (2001-03-01)
Reliability-based assessment of local scouring mechanism around bridge piers provides information for decision-making regarding the pier footing design. Parameter uncertainties that may arise from various sources due to the inability to precisely quantify a parameter need to be estimated in order to quantify the level of risk of pier failure during the physical life of a bridge. By examining extensive experimental data from the literature on local scour depth around various shapes of bridge pier the functio...
Clear-water scour evolution at dual bridge piers
Yilmaz, Meriç; Yanmaz, Ali Melih; Köken, Mete (2017-04-01)
For the efficient design of bridge pier footings, computation of time-evolution of scour depth around bridge piers is essential. In this paper, a semi-empirical model is developed to estimate the temporal variation of clear-water scour depth at a couple of identical cylindrical uniform piers in tandem arrangement. The experiments are carried out using different pier size, pier spacing, and flow intensities. The model development is based on sediment continuity approach and volumetric sediment transport rate...
Citation Formats
Z. Cheng, M. Köken, and G. Constantinescu, “Approximate methodology to account for effects of coherent structures on sediment entrainment in RANS simulations with a movable bed and applications to pier scour,” ADVANCES IN WATER RESOURCES, pp. 65–82, 2018, Accessed: 00, 2020. [Online]. Available: