چكيده به لاتين
Large proportion of flow resistance is due to vegetation in open flow channel. In this research, the Drag coefficient is selected as the parameter for measuring the flow resistance. The field study was performed on the Deryouk River in Mazandaran Province. The chosen site was 28 meter. After dimensional analysis, all dimensionless parameters affecting the drag coefficient were obtained. laboratory experiments were performed in the channel with 13-m- long by 0.45-m- wide and flow depth 0.2-m-. Emergent vegetation -was simulated by wooden cylinders with 8-mm- diameter and a height of 30-cm.
The results shown that in the same conditions, the drag coefficient of irregular vegetated patch is larger than the coefficient of regular patch. The wake structure of each wooden cylinder affects the drag coefficient of the patch. Therefore, the stem Reynolds number is one of the most important parameters affecting drag coefficient of patch. Also, the drag coefficient has direct relation with density. However, the slope of this relation varies for the dense and sparse patches. so that, the drag coefficient increases strongly after the boundary of the conversion of the sparse vegetation to the dense vegetated. Adverse pressure gradient dominated in the wake zone of dense vegetation that is suitable for sediment accumulation. The length of this area depends on the density and drag coefficient of the patch. The amount of penetration of stream into the circular patch depends on its density. circulation flow is formed at upstream of the circular patch. In all patches, the initial rows have the highest contribution of flow resistance, so the drag coefficient decreases throughout the patch. In laboratory study, the Saint-venant equation method and in field study, the boundary layer theory was used to calculate shear velocity. The use of double-averaged velocity increased the accuracy of calculating the shear velocity of the patch. The phenomenon of dip occurs because of the secondary flow. Also, due to the drag force generated by the bedforms and vegetation, all of the velocity profiles of Deryouk cross sections were S-shaped. The presence of dense vegetation on the bank increased river bank stability. So that the width of the river increased at some cross sections during the field experiments due to flooding. While This was not observed in cross sections with dense vegetation.
Keywords:
Drag coefficient, Vegetated patch, Shear velocity, Double averaging method, Stem Reynolds number
