Drag-Coefficient Formulas for Linear Rigid Emergent Vegetation: An Assessment of Their Performance Versus Experimental Water Surface Profiles

From a hydraulic point of view, vegetation in watercourses increases flow resistance, raising water levels and the risk of flooding; its management requires a reliable computation of water surface profiles. Most drag-coefficient formulas for rigid emergent vegetation were derived under uniform flow and for staggered or random arrays, whereas linear arrays, typical of plantations and riparian reforestation, have received less attention. This experimental study assesses whether five drag-coefficient formulas can be used to compute accelerating subcritical (M2 and H2) gradually varied flow profiles in channels with linear rigid emergent vegetation. Thirty-six profiles measured in two laboratory datasets, with vegetation densities from 0.008 to 0.42, were simulated using the standard step method and evaluated with nine statistical indicators, profile-wise errors and Taylor diagrams; the sensitivity to the downstream depth and water temperature was also assessed. For densities up to 0.044, all formulas reproduced the profiles, with pooled RMSE% between 3.5% and 9.3%. For densities of 0.163 and above, the formulas derived from staggered and random arrays overestimated the depths (RMSE% of 21–40%), even within their calibrated range. A density-only formula, used with the effective flow width, gave the most consistent results over the whole range (RMSE% of 5.0% and 6.6% on the two datasets).