Bulk drag of a regular array of emergent blade-type vegetation stems under gradually varied flow

• Effect of distribution pattern of stems on mean bulk drag coefficient Cd is investigated.
• Cd decreases with increasing solid volume fraction at fixed lateral stem spacing.
• Cd increases with increasing solid volume fraction at fixed longitudinal stem spacing.
• An empirical equation is obtained relating Cd to stem spacing.

The drag induced by flow through vegetation is affected by the velocity, shape of vegetation stems and wake interference among stems. To account for the interference effects, previous works generally related the bulk drag coefficient of vegetation to the solid volume fraction ϕ of the vegetated zone, and the trends of the results were found to be inconsistent. In this work, a systematic laboratory study has been carried out to investigate the effect of the distribution pattern of vegetation stems on the hydrodynamics of gradually varied flow through emergent blade-type vegetation. The finite artificial vegetation patches of solid volume fractions ranging from 0.005 to 0.121 have been used and the stem Reynolds number ranges from 500–2600. The longitudinal water surface profiles have been measured, and the effect of increasing roughness density with respect to varying longitudinal and lateral spacing under the flow conditions is examined. The momentum equation that relates the vegetation resistant force and water surface profile has been used to obtain the mean bulk drag coefficient Cd of the canopy. The results show that Cd decreases with increasing stem Reynolds number, decreases with increasing ϕ at fixed lateral spacing due to sheltering effect, and increases with ϕ at fixed longitudinal spacing due to channeling effect. An empirical equation has been obtained relating Cd to the lateral and longitudinal spacing instead of ϕ.