From 3D grassy vegetation point cloud to hydraulic resistance: Application to close‐range estimation of Manning coefficients for intermittent open channels

The understanding of interrelations between biotic and abiotic processes in intermittent open channels is currently of primary importance to better assess the services and disservices they provide. A large body of literature attempts to characterize vegetation functional traits affecting hydraulic rugosity, through the introduction of the blockage factor of flow by vegetation. However, this factor has multiple definitions and is still difficult to assess in the fields with actual and diverse vegetation covers, especially for grassy plants of ditches. Our study aims at predicting flow resistance from 3D vegetation characteristics using a close‐range laser scanner. Flow resistance and vegetation 3D characteristics were defined using Manning coefficient and blockage factors, respectively. We tested combined effects of flow discharge against plant species and densities characterizing intermittent channels in a channel flume. Our results showed a variability of Manning coefficient describing flow rugosity against species and densities, with a highest rugosity for sclerophyllous species than herbaceous ones. Different blockage factors were calculated on the basis of scan clouds linked to Manning coefficients using nonlinear equation. The best relationship (R2 = .9) was found for nonlinear equation relating Manning coefficients to a simplified blockage factor figuring the mean vegetation height deduced from the projection of the scan point cloud to the channel frontal area. The introduction of a coefficient to correct underestimated values issued from herbaceous species considering their reconfiguration under hydrodynamic loading is thus discussed.