HYDRAULIC RESISTANCE OF VEGETATION IN RIVER FLOW APPLICATIONS

When vegetated regions become part of a river’s flow field, the hydraulic resistance of vegetation affects the overall conveyance. Several descriptions exist to describe this type of flow, among which are empirical relations and relations that are physics-based. Empirical relations are often simple in form but have the drawback that their use is limited to the situation where they were determined. On the other hand, theoretical descriptions become increasingly complex (large computational efforts) and pose practical difficulties when gathering necessary input data. In the current work a new scaling expression of the average flow field is put forward that combines advantages of existing flow equations: (i) it is physics-based, (ii) it is dependent on readily measurable quantities and (iii) it requires little computational effort. The performance of the new expression is compared to existing expressions, and limits are given for its practical use. Vegetation that penetrates a flow field causes drag in the layer occupied by the vegetation. Based on scaling considerations of the forces involved, depth-averaged flow velocities within the vegetation layer and in the free flowing layer above the roughness elements are estimated. The conditions in the two separate flow layers yield a new description of the overall depth-averaged flow velocity (U), which is entirely determined by measurable geometrical boundaries