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Biogeomorphology


 

riparian4The riparian system is a transitional zone (ecotone), complex and very interesting, between the fluvial and terrestrial environment. This dynamic system is characterized by a high biodiversity degree, and to its formation concur hydrologic, geomorphologic and ecologic processes. The riparian vegetation, in particular, is one of the main element of the riparian ecosystem, because it influences the fluvial processes and in turn is influenced by them. The vegetation, in fact, acts like a passive element, by diminishing the bank erosion and increasing the hydrodynamic resistance and the sedimentation processes. Afterwards, the vegetation plays also an active role, and its growth depends on the river for seeds, water, sediment and nutrients.


In the past years the biologic aspect has usually been studied only from a qualitative point of view, constraining the vegetation to a pure static element in the hydraulic models. But recently, researchers of many different scientific fields, from geomorphologists to hydraulic engineers, from biologists to hydrologists, have started to collaborate to propose a quantitative modeling of the mutual interplay between vegetation and river. The development of such models is still at embrional stage so the world research is currently very active on this topic.

The models developed by the hydraulic group of the Politecnico di Torino are basically of two types: stochastic minimalistic and “complete” models. In the minimalistic models, just the fundamental aspects which characterize the growth and the decay of the vegetation are accounted for, with particular emphasis to the stochasticity of the river discharges. The resulti is a simple model able to catch the main features of the vegetation-river interplay. In the complete models, instead, every single aspect of vegetation activity and their interaction with the water course is modeled. This approach needs many parameters, but it is able to provide a variety of remarkable results and to trace the validity of application of the simplified models.

The three main topics we deal with the riparian vegetation are:

Fluvial biomorphodynamicsriparian1

The spatio-temporal dynamics of the riparian vegetation is influenced by the fluvial geomorphology and, at the same time, fluvial geomorphology is influenced by the vegetation, which is able to change the hydraulic characteristics of a transect. Our aim is to develop mathematical models able to describe the interactions between water, sediment and vegetation, in order to explain the link between fluvial morphodynamics and the riparian vegetation. This would allow to establish how much the morphology of a river is influenced by the vegetation quantitatively, and to determine the possible consequences of the anthropic action on the riparian ecosystem and on fluvial processes.
Some interesting questions that can find an answer with these models are: how river meandering can influence the vegetation distribution along the banks? which is the impact of river damming on vegetation and on river morphology? which are the most important feedbacks which must be considered between vegetation and river cross section?

Transport processes

riparian2Transport phenomena play a key role in fluvial systems. In fact advection, diffusion, and dispersion are very important for both morphological and biological processes. In particular, longitudinal dispersion has an important impact on the transport of sediments, pollutants, chemicals, nutrients, wood debris, and seeds along a river. Nevertheless, riparian vegetation is able to modify the water flow field by slowing flow velocity. A part of our research is devoted to understand how the dispersion varies in presence of riparian vegetation when medium/high discharges flow in the river using stochastic modeling.

Our aim is to model how discharge stochasticity, riparian vegetation and hydraulic characteristics of the river transect interact,  and to understand how much the vegetation is able to alter the longitudinal dispersion to its advantage. The increased efficiency of the dispersion in fact leads to an increase in the longitudinal transport of nutrients, wood debris, and seeds, all of which are fundamental components for the growth, maintenance, and site colonization of the vegetation. Further, some future investigations will be lead to understand if there is synchrony (or lack of synchrony) between the discharge seasonality and vegetation bio-ecological processes, e.g. seed dispersal or species competition.


 



Riparian vegetation dynamics
 

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Riparian vegetation lives in a dynamic equilibrium determined by both allogenic and autogenic factors. In fact, riparia has to respond continuously to a complex array of hydrologic (e.g., floods, sediment transport, deposition of large woody debris, etc), non-hydrologic (e.g., fires, anthropic interferences, etc), and biotic (e.g., plant competition/cooperation, epidemics, etc) forcings. Allogenic factors play a fundamental role in determining the plant distribution along the river transects, especially the hydrologic ones. In addition to allogenic forcing, autogenic factors contribute to the condition of the quasi-equilibrium between species. Species, in fact, compete for nutrients, light and water and plants with competitive advantage in size or growth are able to eliminate other plants from the community. On the other hand, plants are able to cooperate because of their effect on the flow field.
The aim is to develop models able to consider both abiotic and biotic factors, and to understand which is the role of the first and the latter in the spatio-temporal dynamics of different vegetation species.

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List of publications on this topic