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Influence of collars on reduction in scour depth at two piers in a tandem configuration

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Bridge failure, due to local scour at bridge pier foundations, has become a critical issue in river and bridge engineering, which might lead to transportation disruption, loss of lives and economic problems. A practical solution to prevent bridge collapses is the implementation of scour mitigation methods around bridge foundations. Based on an experimental perspective, this study is focused on the influence of the size and position of circular collars from the sediment bed on scour depth at two tandem piers. To meet this end, long-lasting experiments are performed under clear-water conditions using uniform sand for bed materials. Compared to the adjacent position of the collar on the bed, placing the collars below the bed would increase the delay time of scour at the piers up to four times. However, regardless of the delay time, the observations indicate that locating the collars on the initial bed surface results in maximum reduction in scour depths around the piers. It was found that diminishing the flow intensity has a dramatic impact on the scour reduction at the piers, so that maximum reduction in scour depths at piers increased on average from 20 to 70% with the reduction in the flow intensity from 0.95 to 0.9.

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\(B\) :

Width of the channel

\(D\) :

Pier diameter

\(D^{*}\) :

Equivalent pier width

\(D_{\text{proj}}\) :

Sum of the non-overlapping projected width of the piers onto a plane normal to the flow direction

\(d_{\text{s}}\) :

Depth of scour

\(d_{\text{se}}\) :

Equilibrium depth of scour

\(d_{{{\text{s}}({\text{ext}})}}\) :

Extrapolated depth of scour to an infinite time

\(d_{50}\) :

Median sediment grain size

\(g\) :

Acceleration of gravity

\(h\) :

Upstream flow depth

\(H_{\text{c}}\) :

Collar height from the sediment bed

\(K_{m}\) :

Number of aligned rows factor

\(K_{\text{sp}}\) :

Factor for the distance between the piers

\(R\) :

Reduction in scour depth

\(R_{\text{u}}\) :

Reduction in scour depth at the upstream pier

\(R_{\text{d}}\) :

Reduction in scour depth at the downstream pier

\(R_{\text{p}} = UD/\vartheta\) :

Pier Reynolds number

\(s\) :

Center-to-center spacing of the piers

\(U\) :

Mean upstream flow velocity

\(U_{\text{c}}\) :

Mean threshold velocity

\(u_{\text{c}}^{*}\) :

Shear critical velocity

\(T\) :

Dimensionless time of scour

\(t\) :

Time of scour

\(t_{\text{c}}\) :

Thickness of the collar

\(t_{\text{d}}\) :

Delay time of scour

\(t_{\text{e}}\) :

Equilibrium time of scour

\(w_{\text{c}}\) :

Width of the collar

\(Z\) :

Dimensionless scour depth

\(\Delta = \rho_{\text{s}}^{{\prime }} /\rho\) :

Relative submerged sediment density

\(\vartheta\) :

Fluid kinematic viscosity

\(\rho\) :

Density of fluid

\(\rho_{\text{s}}\) :

Sediment grain density

\(\rho_{\text{s}}^{{\prime }}\) :

Submerged sediment density

\(\sigma_{\text{g}}\) :

Geometric standard deviation of the sediment grain size distribution

\(\varphi\) :



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This experimental research was conducted at Laboratory of Hydraulic Constructions (LCH-EPFL), Lausanne, Switzerland, which supported the project financially as well.

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Correspondence to Mohammad Zounemat-Kermani.

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Memar, S., Zounemat-Kermani, M., Beheshti, A. et al. Influence of collars on reduction in scour depth at two piers in a tandem configuration. Acta Geophys. (2019). https://doi.org/10.1007/s11600-019-00393-0

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  • Bridge foundation
  • Collar
  • Countermeasure
  • Horseshoe vortex
  • Scour depth reduction