Vegetation is an important part of the ecological channel, and flow structure over vegetation in ecological channel is rather complex. The vegetation resistance to flow is affected by many factors, and there is still no general calculation method to express it. Thus, it is necessary to explore the mechanism of flow resistance over vegetation in open channel. In this paper, the rigid cylindrical sticks arranged in the open channel were used to simulate the stems of non-submerged vegetation in order to investigate the effects of vegetation on the flow. Through theoretical analysis and experimental verification, it shows the form drag caused by vegetation is closely related to the vortex volume created by vegetation. In addition, the total resistance of vegetated channel can be determined by two methods based on vegetation density and flow resistance partitioning, respectively, and both of them are verified by the experimental results. What's more, for rigid vegetation, the drag coefficient is closely related to vegetation density, spacing between vegetation patches and the Reynolds number. The empirical formula among them was established through the experimental results. INDEX TERMS Velocity distribution, form drag, resistance partitioning, vortex structure, drag coefficient.

The flow structure over vegetation in ecological channel is rather complex and the resistance caused by vegetation is affected by many factors, such as discharge, flow depth, vegetation density and patch arrangement types, etc. In order to simplify vegetation characteristics, the rigid cylindrical sticks arranged in the open channel were used to simulate the stems of non-submerged vegetation. The volume of the vortex structure near the vegetation patch can be determined through velocity distribution in vegetated channel and the velocity distribution is measured by Acoustic Doppler Velocity meter. A new formula of equivalent roughness based on the vortex volume near the vegetation patch has been proposed and the form drag can be further determined by the new equivalent roughness. Based on theoretical analysis and experimental results, it shows the vortex volume plays an important role on flow resistance caused by vegetation. The theoretical and experimental results on form drag achieve a good agreement on various experimental conditions. The relation between the relative roughness and relative vortex length was also derived and verified by different experimental conductions. For the dependence of relative roughness on vegetation density, the empirical formula between relative roughness and vegetation density was further proposed and analyzed. INDEX TERMS Form drag, equivalent roughness, vortex structure, rigid vegetation, open channel.

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