Laboratory experiments for analysing the impact of herbaceous vegetation on riverbank erosion

Planting of herbaceous vegetation on riverbanks is a measure for reducing river flooding occurrence, through the protection of the bank face from fluvial erosion. In fact, mitigating the erosive action of the water flow and improving soil resistance by increasing the strength of the bank material with their roots, such nature-based solution reduces the risk of local and shallow instability mechanisms that may lead to the collapse of levees and riverbanks during flood events.

While there is nowadays a wide experience on the use of vegetation over hill slopes and other ‘mainly dry’ soil conditions, a carefully calibrated design approach to understand the vegetation impact inside the river bed and banks, under flood flow forcing, represents a much less explored research field, which is investigated in the Open Air Lab-Italy in the EU H2020 project OPERANDUM. To address this important and complex problem, a combined use of laboratory experiments, site monitoring and numerical analysis is required to improve actual procedures and standards.

In the present work, the principal focus is on the design, preparation and deployment of the laboratory activities, extremely rare in the literature, with a discussion on the first experimental findings and observations. A set of experiments in a recirculating, tilting hydraulic flume are designed and implemented, in order to gain, in a controlled environment, information on the influence of the vegetation on both hydraulic and erosive processes.

During the experimental tests, water flow depth and velocity are monitored through UPV Ultrasound Velocity Profilers and Particle Tracking Velocimetry, in order to estimate the tangential stress at the soil-water interface. The main challenge resulted to be the estimation of the volumes of soil eroded during the experiments, due to the very limited quantities that are eroded and to the particularly fine-grained texture of the soil (that was collected from a real embankment of the river Panaro, reference case for the Open Air Lab).

The laboratory experiments allowed to compare the impact of different flow regimes (varying the channel slope, different flow velocity fields were tested) over soils vegetated with both shallow-rooted and deep-rooted perennial herbaceous species, and the results will successively help to analyse the hydraulic and erosive processes on the riverbanks, where such vegetation cover will be installed.