This study examines the problem of flow resistance due to rigid vegetation in open channel flow. The reliability of the conventional flow resistance equations (i.e. Keulegan, Manning and Chézy-Bazin) for vegetated flows at high submergence, i.e. h/k >5, (where h = flow depth and k = vegetation height) is assessed. Several modern flow resistance equations based on a two-layer approach are examined, showing that they transform into the conventional equations at high submergences. To compare the conventional flow resistance equations at high submergences, an experimental methodology is proposed and applied to the experimental data reported in the literature and collected for this study. The results demonstrate the reliability of the Keulegan equation in predicting the flow resistance. Based on the obtained results, a model to evaluate the Nikuradse equivalent sand-grain roughness, k N , starting from the vegetation height and density, is proposed and tested.

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Estimating the main hydrodynamic features of real vegetated water bodies is crucial to assure a balance between their hydraulic conveyance and environmental quality. Riparian vegetation stands have a high impact on vegetated channels. The present work has the aim to integrate riparian vegetation’s reflectance indices and hydrodynamics of real vegetated water flows to assess the impact of riparian vegetation morphometry on bulk drag coefficients distribution along an abandoned vegetated drainage channel fully covered by 9–10 m high Arundo donax (commonly known as giant reed) stands, starting from flow average velocities measurements at 30 cross-sections identified along the channel. A map of riparian vegetation cover was obtained through digital processing of Unnamed Aerial Vehicle (UAV)-acquired multispectral images, which represent a fast way to observe riparian plants’ traits in hardly accessible areas such as vegetated water bodies in natural conditions. In this study, the portion of riparian plants effectively interacting with flow was expressed in terms of ground-based Leaf Area Index measurements (LAI), which easily related to UAV-based Normalized Difference Vegetation Index (NDVI). The comparative analysis between Arundo donax stands NDVI and LAI map enabled the analysis of the impact of UAV-acquired multispectral imagery on bulk drag predictions along the vegetated drainage channel.

Bankfull discharge estimation is a crucial step in river basin management. Such evaluation can be carried out using hydrological and hydraulic modelling to estimate flow-depths, flow velocities and flood prone areas related to a specific return period. However, different methodological approaches are described in the scientific literature. Such approaches are typically based either on the assumption that the bankfull discharge corresponds to a narrow range of return periods, or on the correlation to the river geomorphological or local descriptors, such as vegetation. In this study, we used high-resolution topographic data and a combined hydrological-hydraulic modelling approach in order to estimate bankfull discharge in the ungauged basin of Rio Torbido River (Central Italy). The field survey of plant species made it possible to investigate the link between the riparian areas and the bankfull discharge. Our results were in line with previous studies and showed a promising agreement between the results of the hydraulic modelling and the plant species present in the investigated river cross sections. The plant species position could be indeed used for a preliminary delineation of the riparian areas to be verified more deeply with the hydrological-hydraulic approach.

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