Single Bridge Pier Scour in Uniform and Non-uniform Sediment Beds Under Steady and Unsteady Flow

Document Type : Research Article

Authors

1 دانش آموخته کارشناسی ارشد

2 Department of Water Engineering, College of Agriculture, Bu-Ali Sina University, 3869565178, Hamadan, Iran.

3 Department Water Science Engineering Faculty of Agriculture BU-Ali Sina University- Hamedan-Iran.

Abstract

Introduction:
Bridge scour is one of the most important challenges in river engineering. Research into local scour has primarily focused on investigating the impact of different hydrodynamic conditions on scour in a bed of uniformly graded material. However, local scour investigations in a bed of uniformly well-graded material provided knowledge of the underlying processes, field sediment beds are much more complex consisting of non-uniform sediment mixtures ("σ" _"g" ">1.4" ). In the case of complex sediment beds, selective transport of the finer particles due to unequal mobility can make the bed surface to be armored. There have been relatively few studies reported in the literature relating to scour in complex sediment beds, and most of these relate to quite specific situations. With regards to natural river materials (non-uniform sediments) and its great effects on the dimension and the time evolution of scour hole, the interaction of flow-structures with non-uniform sediments is very crucial due to armor layer development. The aim of this study is to improve understanding of scour development and armoring evolution in non-uniform sediment beds for estimating the scour depth in more realistic field conditions. Therefore, the rate of variation of erosion over time around single cylindrical pier is investigated in different bed sediment types.
Methodology:
The armored layer due to selective transport of the finer particles in non-uniform sediments causes complexity for predicting equilibrium scour depth. The present experiments on local bridge scour were conducted in hydraulic laboratory of the Bu-Ali Sina University (Hamadan, Iran). The pier model with a diameter of 4 cm was put inside a 0.5 m wide, 10.5 m long and 0.5 m depth rectangular tilting flume. In this study, the number of 15 experiments were organized at five different sediment beds, uniform and non-uniform in two steady flow condition (20 and 35 l/s with the same flow intensity of u/uc~0.9) along with an unsteady flow. The duration of tests was fixed at 8 hours in all runs based on the empirical method given by Ettema (1980).
Results and Discussion:
The experimental results revealed that with increasing flow rate from 20 to 35 l/s (increasing follow shallowness, h/b) at the same sediment bed, not only larger scour depths were recorded, but also the armor layer became coarser. The comparison between the bed configurations with uniform and non-uniform sediments represented dramatical reduction of the scour depth regards with increasing sediment non-uniformity. The effect of non-uniform sediments on scour in a current clear water conditions showed that maximum scour depth was less than scour depth in a uniform sand with the same d50 value. The comparison between these two mentioned bed configurations showed that the change in geometric standard deviation ("σ" _"g" ) from 1.4 to 2 (altering the uniform bed to non-uniform), decreased the maximum depth of scour by 70% and 60% in two corresponding experiments. As the armor layer coarser grains remains at upstream flow bed and at the vicinity of scour hole in the same flow intensity, the scour depth was decreased. Otherwise, there was not remarkable decrease on the scour depth by increasing non-uniformity index, since two sediment beds types were non-uniform. Also, a slight increase on scour depth has been observed by reduction of median grain size in the beds with non-uniform sediments at the same geometric standard deviation. By taking into account of the grain size of the armor layer and ice cover roughness, Wu et al. (2014) analyzed the dimensionless maximum scour depth and they found out that with an increase in grain size of the armor layer, the dimensionless maximum scour depth decreases. Singh et al. (2018) investigated the incipient motion for gravel particles in cohesionless sediment mixtures having silt and sand. The visual observations of the channel bed after the end of incipient motion indicated appearance of gravel particles at the top surface of the sediment bed. The critical shear stress for the gravel particles was found to be lower in the presence of silt. Presence of silt in the mixture affects the critical shear stress for gravel particles. They concluded from the present study that high silt content in the mixture leads to the higher deviation of critical shear stress from the revised Shields curve. They proposed an equation for the determination of critical shear stress of gravel particles in the non-uniform sediment mixture.
Conclusion:
The results showed that scour depths were reduced dramatically as sediment non-uniformity index ("σ" _"g" ) increase in clear water conditions. The larger particles form an armor layer protecting the bed from eroding. Also, the observation indicated that with increasing flow depth, the armor layer coarsens, and larger scour depths were recorded. However, scour depth increase rate was very different for the various bed sediment types.

Keywords


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  • Receive Date: 20 August 2019
  • Revise Date: 08 October 2019
  • Accept Date: 13 October 2019
  • First Publish Date: 20 February 2020