The effect of flexible vegetation on flow in drainage channels: Estimation of roughness coefficients at the real scale

An experimental campaign was implemented along a 300 m stretch of a drainage channel, with the purpose of estimating the roughness coefficients of vegetated channels at the real scale for different management scenarios. The study was carried out in an agricultural area where the ordinary vegetation management consists in the total removal of natural vegetation along the whole wetted perimeter; thus, significantly impacting the aquatic ecosystem. To determine the sustainability of a less-impactful management, three scenarios were tested: a full-vegetated scenario, a partial removal, and the traditional total removal. The vegetation distribution and the channel morphology were determined for modelling purposes. Four increasing discharges were controlled by a pumping system up to the bankfull conditions, while hydraulic parameters were being monitored. Measuring the flow characteristics on three cross-sections, roughness coefficients were obtained by inverting the 1D equation of gradually varied steady flow. Results show how the presence of an undisturbed vegetation community significantly rises the water levels, thus possibly increasing the flood risk. The flow resistance resulted decreasing for increasing velocities, suggesting a considerable effect of plants reconfiguration due to drag. The intermediate scenario, represented by the preservation of an untouched vegetated buffer on one bank, resulted to be less impactful on flow and not far from the total removal scenario. The experimental results represent a reference for flow resistance models development and validation. Moreover, the findings of the present work provide useful information for a more focused drainage channel management, supporting the reduction of the ecological impact on aquatic communities.