Hydraulic resistance of submerged cylindrical elements: a two-layer scaling approach

The hydraulic resistance of vegetation can play a major role in the hydrodynamics of rivers with extensive natural floodplains. Contrary to commonly used wall roughness methods, vegetation penetrates the flow field and thereby causes drag and, subsequently, additional energy losses. In this study, these processes are treated in an idealized form by replacing vegetation with cylindrical elements with homogeneous geometrical dimensions. Based on scaling considerations of the forces involved, depth-averaged flow velocities within the roughness layer and in the free flowing layer above the roughness elements are estimated. Consequently, conditions in the two separate flow layers yield a new description of the overall average flow field, which is entirely determined by measurable geometrical boundaries. The new description shows to give good agreement with laboratory flume experiments