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.

Wildfires are an increasingly alarming phenomenon that affects forests and agro-ecosystems, generating several cascade effects among which soil erosion is one of the most deleterious. A robust body of data-based evidence on post-fire soil erosion and sediment yield at watershed scale is thus required, especially dealing with areas where wildfires are particularly frequent, such as the Mediterranean Basin. This study analyses the impact of the first rains after a large wildfire in terms of soil erosion and sediment yield at watershed scale in a Mediterranean area, the Pisan Mountains, Central Italy. Here about 1,000 ha of olive groves, maquis, maritime pine and chestnut forests burned. Fire severity was mapped by remote sensing and checked by a field survey. Sediment yield was assessed by sampling the earthy material deposited upstream a check dam at the outlet of the watershed. Finally, a hydrological model was developed in HEC-HMS environment for exploring the relationship between the erosion-deposition events observed in the watershed and the rainfall-induced hydrological processes. The first two post-fire rainy events relocated a high amount of sediments, mostly non-organic, perhaps already in the stream before the fire, while the subsequent four rains deposited materials rich in pyrogenic organic matter. Overall, the soil erosion caused by such six main post-fire rains – the larger of which had a return time of one year – was estimated to amount to 7.85 t ha -1, corresponding to 42% of the watershed average annual potential erosion rate in normal conditions. This value is lower than expected and, overall, moderate if compared to other Mediterranean case studies, possibly because of the nature of soils in the studied watershed, i.e. shallow and quite stony, thus poor in fines prone to erosion.