Evaluation of an analytical model for hydraulic resistance of submerged vegetation

The hydraulic resistance of vegetation can play a major role in the hydrodynamics of vegetated streams or rivers with extensive natural floodplains. Contrary to commonly used wall roughness methods, vegetation penetrates the flow field causing drag and, subsequently, additional energy losses. Klopstra et al. (1997) derived an analytical solution of the flow velocity profile over and through a field of homogeneously distributed rigid cylinders. This solution has been adopted as a standard method to determine vegetation roughness in the Netherlands as described in the handbook for hydraulic resistance of vegetation in floodplains (Van Velzen et al., 2003). Although the analytical solution gives a good description of the flow over submerged cylinders some assumptions oversimplify the problem. In this paper these assumptions are evaluated and suggestions for improvement are made.