Effect of air injection pipe position on local scour reduction around the cylindrical bridge pier with experimental investigation

Document Type : Research Article


1 Hydraulic Structure Department. Faculty of Water Science Engineering. Shahid Chamran University of Ahvaz,

2 Shahid Chamran University


Extended Abstract
Local scour is one of the main factors of the destruction of bridge piers in rivers before the life expectancy of these structures. One of the newest indirect methods for reducing the erosion and scour around the bridge piers is the air injection method which is based on counteracting with downward flow to create such conditions to prevent or reduce the flow impact on the bed and the destruction of bridge piers.
Tipireddy and Barkdoll (2019) studied the effect of air injection in controlling bridge pier scour. The results of this study showed that the maximum scour depth decreased with increasing air injection rate until the ratio of air outflow velocity from the pores to flow velocity was 57.1 and then increased.
The performance of air injection to reduce bridge pier scour has been investigated in only one study, considering all parameters except constant air outflow rate. The main objective of this study was to investigate the effect of air injection on reducing bridge pier scour considering the number and the location of injection pipes.

Research Methodology
Experiments were carried out at the Hydraulic Laboratory of the Faculty of Water Engineering, Shahid Chamran University of Ahvaz in a flume with 10 m long and 0.74 m wide and 0.6 m deep. The bottom of the flume was covered by sediments with a mean diameter of 0.7 mm with a standard deviation of 1.21 indicating uniform sand. All experiments were performed in four Froude numbers of 0.18, 0.2, 0.22, and 0.24 at a constant depth of 15 cm under clear-water conditions. To reach these conditions, the flow depth was adjusted so that the ratio of flow velocity to the critical velocity is less than 95%. In order to determine the appropriate time for the experiments, a 12-hour test at a Froude number of 0.2 was performed and it was observed that about 90% of the scour occurred in the first 3 hours. In order to create an air injection system, porous pipes were selected with an internal diameter of 4 mm. Each pipe had 24 pores with a diameter of 2.2 mm at fixed intervals mounted on the upstream half of the pier and two ends were connected to the air compressor for injection. Experiments were carried out at air injection rate of 100 lit/min and in 7 different number and position of aeration pipes on the pier. The pipes were mounted in three general modes: one pipe, two pipes and three pipes. The installation level of the pipe on the pier was considered as three heights of 1/3h, 1/2h, and 2/3h from the bed.

Results and Discussion
A number of four control experiments (without the air injection system) was first performed at four specified Froude numbers. A total of 28 experiments were performed in the presence of the air injection system at the air injection rate of 100 lit/min. The aeration pipe at on the upstream half of the pier provides some protection for the sediments against the horseshoe vortices.

Effect of aeration pipe position on scour depth and scour development around bridge pier
To investigate the effect of the position of the aeration pipe on the pier, the pipes were installed in single-pipe mode at three different heights on the pier and in the simultaneous usage of two pipes, the pipes were installed in three modes at the same three height. The transverse expansion of the scour hole for the bridge pier were plotted in two general modes using a single aeration pipe and two aeration pipes in four Froude numbers. In single-pipe mode, the maximum decrease in the scour hole depth in front of the bridge pier (25.5%) was observed at the pipe installation depth of 5 cm beneath the bed and for Froude number of 0.18. For the use of two pipes with the same injection rate and similar hydraulic conditions, the maximum decrease in scour depth of 11.8% occurred when the pipes were installed at depths of 7.5 and 10 cm beneath the bed.

Effect of aeration pipe on scour depth in scour development around bridge pier
In order to investigate the effect of the number of pipes on the scour depth, the transverse expansion of the scour hole around the bridge pier was plotted in three general modes: single pipe (three mode), two pipes (three modes) and three pipes (one mode). The results showed that the lowest efficiency was observed to the state of the three pipes and the efficiency is to the one state..

Effect of Froude Number on Scour Depth and Scour Expansion around Bridge Pier
The shape of the relative scour depth was plotted against the Froude number for the use of an aeration pipe. The results showed that there is a direct relationship between the scour hole depth and the Froude number and the scour hole depth increases with increasing Froude number.

In this study, the effect of air injection system on the scour pattern and sedimentation around the bridge pier under clear-water conditions in a straight flume was experimentally investigated. All experiments were performed with four Froude numbers of 0.18, 0.2, 0.22, and 0.24 and 7 different numbers and installation levels of the aeration pipe at air injection rate of 100 lit/min. The results showed that aeration structure in the flume reduced the scour depth around the pier. The reduction of the scour depth at the front of the pier in the single-pipe test was 25.5% with an installation level of 5 cm, Froude number of 0.18, and air injection rate of 100 lit/min. The results of comparisons made to investigate the effect of the number of aeration pipes on the pier showed that the three-pipe mode had the lowest efficiency. This system reduced the volume of scour hole around the pier. Therefore, it can be concluded that the application of this structure can be an appropriate alternative in comparison with the commonly used protective structures, therefore, further studies are suggested.


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  • Receive Date: 19 March 2020
  • Revise Date: 23 May 2020
  • Accept Date: 24 May 2020
  • First Publish Date: 22 September 2020