Investigating the flow field approaching the bridge deck in compound open channels with vegetation

With the occurrence of flood, the velocity and depth of the flow in the river increases and the flow enters the flood plains. The velocity difference between the deeper section and the shallow area causes the transfer of momentum between these areas and complicates the flow structure. The formation process of secondary flows and its pattern in compound channels have been investigated by researchers such as:Tominaga & Nezu, 1991. The presence of vegetation on flood plains causes complexity in the analysis of hydraulic problems of compound channels. For example, Hamidifar et al. (2012, 2014), using laboratory measurements, showed that the presence of vegetation reduces the flow through the cross section by about 30%. At the same time as the water level rises during the flood, the deck of the bridges will go under water and the current passing under it will be pressurized. In this condition, the flow field is affected by the presence of vegetation, compound channel and pressurized flow. In this research, the laboratory investigation of these complex conditions has been done.

The experiments of this research were done with 3 geometric ratios of the compound cross-section, 3 relative depths, 3 vegetation densities, and control experiments in a compound channel with a length of 10 meters and a width of 1.5 meters. The measurement of the flow velocity parameter, the scouring rate of the bridge pier in the conditions of pressurized flow has been done according to the variables mentioned above.

Comparison of depth velocity and logarithmic velocity distribution in the condition without vegetation on the flood plain, the sign shows that in all sections, the distance between the channel bed and the water surface, the difference between the measured velocity values with the logarithmic distribution of the velocity increases. This difference is due to the presence of the bridge deck and the flow retardation. Also, vegetation causes the depth distribution profile of flow velocity to deviate from the curve of logarithmic flow velocity, and the biggest difference will occur in the upstream area between the interface of main channel and flood plain. This phenomenon increases the amount of apparent shear stress between the main channel and the floodplain.With the increase in the density of vegetation, the percentage of floodplain participation in the total discharge is reduced by 20%. The highest participation percentage of floodplain is about 30% in the state without vegetation. In different densities of vegetation with an increase in relative depth from 0.3 to 0.5, the percentage of floodplain participation in the total discharge is less than 10%. With the increase in the density of vegetation, the difference between the percentage of floodplain participation in different cross section widths has decreased.

The findings of recent research to check hydraulic parameters can be summarized as follows:- Increasing the density of vegetation increases the longitudinal velocity in the main channel and decreases it in the floodplain.- Longitudinal velocity and averaged- depth velocity in the main channel in the case without vegetation is lower than the case with vegetation.- Increasing the relative depth increases the percentage of floodplain participation by an average of 5%, and the increase in vegetation density causes a decrease of 17% in the floodplain participation.- With the increase in the vegetation density of the floodplain, the velocity changes in the floodplain decrease compared to the main channel.- Examining the profiles of the depth distribution of the flow shows that due to the presence of the bridge deck and the retardation of the flow, the depth distribution differs greatly from the logarithmic distribution of the velocity . This is despite the fact that in the conditions without the presence of the bridge deck, this amount of difference reaches its minimum.- The presence of the bridge deck and the creation of backwaters reduce the difference in flow velocity in the main channel and floodplain upstream of the bridge, and this in turn reduces the strength of the secondary currents between floodplain and the main channel.- The difference between the global average velocity of the flow and the local velocities increases the slope of the (a-1) and (b-1) lines due to the flow retardation.