Investigating the flow field approaching the bridge deck in compound open channels with vegetation | ||
| تحقیقات مهندسی سازه های آبیاری و زهکشی | ||
| Volume 24, Issue 92, March 2024, Pages 87-69 PDF (1.83 M) | ||
| Document Type: Original Article | ||
| DOI: 10.22092/idser.2024.364964.1573 | ||
| Authors | ||
| ali dankoo1; Hojjat Allah Yonesi* 2; Mojtaba Saneie3 | ||
| 1Phd student, Faculty of Agriculture, Department of water Engineering, Lorestan University, Lorestan , Iran | ||
| 2Associate Professor,, Faculty of Agriculture, Department of water Engineering, Lorestan University, Lorestan , Iran | ||
| 3Associate Professor, Department of River and Coastal Engineering, Soil Conservation and Watershed Management Research Institute, Tehran, Iran. | ||
| Abstract | ||
| Extended Abstract Introduction 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. Methodology 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. Results and Discussion 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. Conclusions 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 average depth of flow 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 decreases the floodplain participation by 17% on average. - With the increase in the vegetation density of the floodplain, the speed changes in the floodplain decrease compared to the main channel. -The increase in relative depth and density of vegetation increases the non-uniformity of flow lines and finally increases the correction coefficients of kinetic energy and movement size. -In high-density vegetation, the interference effect of eddies prevents the formation of large secondary currents on the floodplain. | ||
| Keywords | ||
| Depth average velocity; compound channel; bridge pier; Energy correction coefficient; momentum correction coefficient | ||
| References | ||
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Ackers, P. (1993). Flow Formulae for straight two-stage channels. Journal of Hydraulic Research, 31(4), 509- Chow, V. T. (1959). Open channel hydraulics, McGraw-Hill, London, International Student Edition. Pasche, E. & Rouve, G. (1985). Overbank flow with vegetatively roughened flood plains. Journal of Hydraulic | ||
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