INFLUENCE OF POSIDONIA OCEANICA MEADOW DENSITY ON FLOW RESISTANCE IN SHALLOW WATERS

Aquatic vegetation considerably affects circulation in water bodies with influence increasing as the depth decreases. On the other hand, increasing use of mathematical circulation models for management of coastal sea waters, lagoons and marshes requires determination of such effects and in particular of flow resistance. A typical plant of inshore sandy beds, less than 40 m deep, in the Mediterranean Sea is Posidonia oceanica, which is constituted by a tuft of very thin and flexible ribbon-like leaves about 1 cm wide and up to 1.5 m long. This plant forms meadows with areal density usually ranging between 500 and 1000 plants/m2. Because of its characteristics, Posidonia oceanica constitutes a particular roughness, since, as the flow velocity increases, the leaves bend more and more until they lie down on the bottom, already for velocities of some decimetre per second. Previous writers’ experimental investigations concerned two plant areal densities (682 and 1037 plants/m2) and a specific resistance law was determined for each one. In the present paper, the two experimental data sets are re-processed altogether. By introduction of new parameters, a sole resistance law is obtained, which expresses the friction factor of Darcy-Weisbach’s formula as a function of the vegetation Reynolds number, the plant density and the leaf width. This result, of course, is consistent with those of the previous works related to single plant densities as well as it confirms the plant hydraulic behaviour as the Reynolds number and the leaf length to flow depth ratio vary; moreover, it specifies how the plant density and the leaf width affect flow resistance.