2019 CSCE Annual Conference - Laval (Greater Montreal)

2019 CSCE Annual Conference - Laval (Greater Montreal) Conference

Evaluation of channel blockage as a governing influence in local scour experiments

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Ms. Priscilla Williams, University of Windsor (Presenter)
Dr. Ram Balachandar, University of Windsor
Dr. Tirupati Bolisetti, University of Windsor
Dr. Vesselina Roussinova, University of Windsor
Ms. Mia Marrocco, University of Windsor

Previous evaluation of dimensionless parameters used in governing pier scour depth prediction methods has indicated that channel blockage influences local scour geometry in laboratory experiments. The majority of prediction methods are functions of previously-defined scour-governing parameters (Williams et al. 2017), and do not include parameters related to so-called blockage effects. The effect of blockage ratio, defined as the ratio between pier diameter (D) and channel width (b), has been erroneously defined in literature as negligible when blockage ratio is less than 10 percent. Recent investigations have determined that changes in values of blockage ratio as minimal as 0.5 percent influence scour geometry (Hodi 2009, D’Alessandro 2013), even when blockage ratio is less than the stated threshold of 10 percent.

Sidewall influence on the flow characteristics around a bluff body can be reduced using Us, the magnitude of streamwise velocity along the separating streamline (Ramamurthy and Lee 1973). Using a similar procedure, the present study investigates the influence of blockage ratio on bridge pier scour. Scour experiments for cylindrical piers were carried out at low values of blockage ratio with all other scour-governing parameters held constant in order to isolate the effect of sidewall influence. Experimental work was conducted in a recirculating horizontal flume at the University of Windsor under model conditions. Results indicated that scour depth increases with increasing blockage ratio; furthermore, as scour progresses downstream of the scour hole, dune formations reach the sidewalls at a shorter distance downstream for a higher blockage ratio. Experimental results from the current investigation were analysed together with benchmark results from literature in order to develop a new scour depth prediction method based on Us.  The present study also includes a discussion on influence of blockage ratio on scour geometry around triangular piers, through analysis of literature results.

References: 1. D’Alessandro, C. 2013. Effect of blockage on circular bridge pier local scour. M.A.Sc. thesis, Department of Civil and Environmental Engineering, University of Windsor, Windsor, Ont. 2. Hodi, B. 2009. Effect of blockage and densimetric Froude number on circular bridge pier scour. M.A.Sc. Thesis, Department of Civil and Environmental Engineering, Faculty of Engineering, University of Windsor, Windsor, Ont. 3. Ramamurthy, A.S., and Lee, P.M. 1973. Wall effects on flow past bluff bodies. Journal of Sound and Vibration, 31(4): 443–451. doi:10.1016/S0022-460X(73) 80259-7. 4. Williams, P., Bolisetti, T., and Balachandar, R. 2017. Evaluation of governing parameters on pier scour geometry. Canadian Journal of Civil Engineering, 44(1): 48–58. doi:10.1139/cjce-2016-0133.