Investigation of flow resistance exerted by rigid emergent vegetation in open channel

The issue of the resistance to flow in open channels with vegetation has been considered by several researchers mainly experimentally, but the case of rigid emergent vegetation with linear stem arrangement is scarcely investigated. In the present work, the results are presented of an experimental investigation related to the case of rigid emergent vegetation that has been modeled by placing small rods on the bottom of a laboratory flume in aligned configuration. Tests have been executed by varying the flow rate, the bottom slope and the number and the diameter of the rods, by directly measuring the drag force exerted by the flow on a given number of rods, and the water-level profiles. A new expression has been devised for the drag coefficient as a function of the vegetation density, weakly dependent on the stem Reynolds number that allows the use of the former also in large-scale cases. The experimentally measured forces exerted by the flow on the rods have been also compared with the results obtained by applying the momentum equation in integral form to given control volumes, exhibiting a general agreement, but also showing that the use of this technique for the evaluation of the drag coefficients can give rise to not negligible errors. One of the experimental tests has been numerically simulated with the RANS technique (Reynolds-Averaged Navier-Stokes equations), and it is found that the results, mainly in terms of water-level profiles, confirm the ability of such a numerical technique in investigating this complex category of flow cases.