River - Riparian Vegetation Interactions

The river and the riparian ecosystem are two dynamical systems which exhibit complex spatio-temporal interactions. Because of the possible implications for engineering works and environmental management, in recent times a number of studies has focused on this topic, considering both large- and small-scale problems. Riparius is a latin word which derives from ripa (riverbank) and means 'that frequents riverbanks'. The riparian ecosystem is thus a community of individuals which lives close to the river and is regularly influenced by fresh water. Both the flora and the fauna are part of this highly dynamic community, and the research in this years has produced results for both the groups. In this thesis the attention will be focused on plants. Riparian vegetation is of great interest because of its double role, passive and active. In the first role (i.e. the passive one) the biological activity is not considered and only the effect of vegetation morphological and mechanical characteristics on river dynamics is discussed. From this point of view vegetation merely affects the roughness, hydraulic resistance, and bank erodibility in the same manner as any a-biotic element with the same mechanical and morphological characteristics. The active role is due to the biotic changes intrinsic to living vegetation. From this point of view, vegetation is no longer seen as a static element, but as an element with temporal and spatial dynamics, which can influence the geomorphology of the river. In fact, if the vegetation distribution along the banks changes, or the native species are substituted by exotic ones, the mechanical effects of vegetation change, leading then to possible changes of river geometry. Riparian vegetation is closely linked to the hydrology of the river, in particular to the stochastic water level fluctuations and to the erosion/sedimentation processes. Floods and erosion/ sedimentation processes, in fact, influence the growth of plants to a great extent. Flood events are able to destroy the vegetation because they can cause physical damage, uprooting and sediment removal, anoxia and burial. The stochastic nature of floods is therefore a key point for the vegetation dynamics. In fact, the random water stage fluctuations drive the alternation of periods of exposure and submersion, which cause the alternation of growth and decay of vegetation. Erosion/sedimentation processes are also fundamental for the vegetation dynamics, because they can cause extirpation (erosion) or provide protection and nutrients during the growth phase (sedimentation). In turn, erosion and sedimentation rates are modified by the vegetation, because the vegetation protects the site from erosion and increases hydraulic roughness, enhancing fine sediment deposition and contributing to the aggradation of the site. In this complex scenario, in which vegetation influences and is influenced by hydrological forcings, it is important to develop analytical models able to consider all these interactions. These models can be used as tools for river management and restoration projects, in order to carefully plan the anthropic disturbances on the river and, thus, reduce the impacts on the riparian ecosystem. In fact, several works in literature report drastic changes of vegetation (shifts, invasion of exotic species in spice of native ones, reduction of biomasses etc.) and of river geometry (narrowing/widening processes, degradation/aggradation of the bed quote etc.), which perhaps could have been reduced with the use of a proper model. The first part of this thesis is devoted to the study of the interplay between river meandering, discharge stochasticity and riparian vegetation. In particular it will be studied the impact of river sinuosity on the total vegetation biomass. In the second part of this thesis it will be studied how the coefficient of dispersion varies in presence of riparian vegetation when medium/high discharges flow in the river. In the third part of this thesis it will be analyzed how the effect of cooperation/competions impacts the distribution of the vegetation along the transect. In the fourth part of this thesis a model will be developed in order to quantitatively analyze how the construction of a dam along a river is able to influence the vegetation. In the fifth part of this thesis a model will be developed in order to quantitatively evaluate the amount of the changes in cross-section after hydrological changes.