Abstract
Over the last centuries streams and rivers were managed with the main
purpose of achieving drainage and flood control, with the reduction of
hydraulic roughness and minimally obstruction of the flow, removing and
impoverishing native riparian vegetation, causing flood peaks increase
and flood wave travel time decrease. This study focuses on the
evaluation of the hydraulic effects caused by the application of
different management techniques carried out in a vegetated river; the
considered scenarios regard radical or severe cut and an alternative or
selective cut, with the aim of simulating the interaction between flow
hydrodynamics and riparian vegetation, in particular in terms of peak
discharge and flood conveyance. The proposed methodology is applied and
tested along the Ombrone Pistoiese River in Tuscany. Water flow
resistance caused by rigid riparian vegetation along the reach were
calculated on the base of measurements collected during two field
campaigns, conducted in 2018 and 2022, before and after the severe
management cut. The main morphometrical vegetation features (i.e., stem
diameters and overstorey density) have been measured at different cross
sections of the vegetated reach. Hydraulic simulations in the 4.4 km
modeled reach showed a wave celerity decrease up to 15%, due to the
presence of surveyed riparian vegetation, potentially causing a delay of
the flood up to 20 minutes. Hence results suggest that there is
considerable advantage in managing riparian vegetation with a less
impacting technique, both to alleviate downstream flooding and also
preserve riparian ecosystem. We adopted three different formulations for
each type of vegetation. For shrubby vegetation we used Jarvela’s (2004)
equation, that requires the Leaf Area Index (LAI) as input parameter.
For herbaceous vegetation we adopted the formulation in the case of
fully submerged herbaceous vegetation developed by Nepf (2012). Finally,
for woody, rigid vegetation we considered the formulation developed by
Baptist et al. (2007), which considers the rigid stems contribution
calculating the roughness coefficient as a function of stem diameters,
spatial density and water depth. Roughness coefficients obtained for
cross-subsections were subsequently used to calculate the equivalent
Manning’s value for each cross-section. Results showed that the higher
intensity management techniques worsened the flood risk after few years
after the cut, where low intensity or selective thinning would have had
a lower impact.