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Influence of foliage on flow resistance of emergent vegetation

Resistance equations for flow through emergent vegetation require estimation of the stem drag coefficient, which depends on the vegetation characteristics and the flow conditions. Values of the drag coefficient inferred from flow measurements are significantly larger than accepted values for long cylinders, and show greater variation with the stem Reynolds number. Experiments with real reeds and bulrushes show that representative values of the drag coefficient for single stem bulk vegetation can be determined from laboratory measurements. The variation of the drag coefficient with Reynolds number depends on the stem shape and the degree of foliation. Experiments using artificial stems with leaves set in fixed, deformed shapes show that this variation is also due to the change in projected area of the foliage as the leaves are deflected by the flow. Defining the drag coefficient in terms of the true projected area results in more constant values, but the true area cannot be estimated in practice. Equations, derived in terms of the rigid stem area, are therefore recommended for use, together with drag coefficients that account for the influence of the flexible foliage.