In the last decades, the presence of riparian vegetation on riverbanks and floodplains along rivers was acknowledged not only to improve water quality and heal biological diversity but also to contribute to river evolution processes. When water flow runs over vegetated areas, averaged velocity profile is affected by the presence of stem, branches and leaves, sediment transport changes according to modified turbulence and bed shear stresses and soil shear strength is altered by root binding. As a result, bed scour, bank erosion and accretion, bar migration and width adjustment processes lead to different river morphology evolution. Conversely, flow and sedimentary patterns influence vegetation dynamics, by shaping barebed deposits available for colonisation and by affecting mortality rate, through burying and uprooting processes. However, whereas recruitment, establishment and growth represent the transitional dynamics from barebed to vegetated conditions and are mainly related to species properties, plant removal is intrinsically related to species growth stage, flow magnitude and soil properties. Although vegetation uprooting only recently gathered attention from scientific community, there is a rising awareness that vegetation removal is crucial for species selection and location on exposed deposits and floodplains and carbon production and sequestration, at different spatial and temporal scales. This PhD work examines the uprooting process of both pioneer seedlings and established vegetation driven by flow and bed erosion, whose role is to reduce root anchorage, at various spatial scales ranging from a single plant to a river reach. The main purpose of this research is to illustrate the links between temporal scales regarding the hydro-morphological evolution of fluvial systems, such as bed scour development, flood duration and return period, and those proper of biological components with regards to both growth and decay rates of riparian vegetation. For this aim, various methodologies and approaches are followed. Firstly, an intensive analysis of the state of knowledge is presented and discussed. Secondly, the existence of links between growth rate and hydrological return period of flood events and between decay rate and flood duration are proved and investigated by means of an already available eco-morphodynamic equation, and illustrated according to different vegetation cover. Promising results are obtained when relationships are applied to the case studies of the Maggia River (CH) and the Tagliamento River (IT). Thirdly, the uprooting process of juvenile flexible riparian vegetation is investigated by means of flume experiments, field measurements of root resistance and numerical modelling. A new physical relationship able to predict critical conditions of bed shear stress and bed erosion is derived, validated and applied to the case study of the Ombrone Pistoiese River (IT) with good agreement. Lastly, the proposed relationship is combined to a very recent probabilistic model and a stochastic approach to flood events. A new relationship for uprooting process randomness is proposed and the correlation between vegetation removal and flood return period is evaluated, discussed and applied to the case study of the Santa Maria River, Arizona (USA) with very good results. The results of this PhD research show the existence of cross-related temporal scales between riparian vegetation and river morphodynamics and demonstrate their relationships with flood return period and event duration. The adapted comprehensive approach to study the uprooting process of riparian vegetation highlights that multidisciplinary methodology is essential to understand the mechanisms, correctly model the process and formulate the equations. The application to laboratory experiments and to various case studies proves the validity of the relationships as well as the applicability both to small and large spatial scales. As a final result, this research hints the capability for river to select species and cover according to hydrological regime and biological properties. This is crucial in fluvial environments altered by climate change, where alien species may replace native ones. It also underlines the importance of taking into account riparian vegetation dynamics, effects and interactions to guarantee the reliability of long-term river morphodynamics modelling and the success of river maintenance and restoration strategies.

Riparian vegetation in fluvial environments: linking timescales through flow uprooting / Giulio Calvani. - (2019).

Riparian vegetation in fluvial environments: linking timescales through flow uprooting

Giulio Calvani
2019

Abstract

In the last decades, the presence of riparian vegetation on riverbanks and floodplains along rivers was acknowledged not only to improve water quality and heal biological diversity but also to contribute to river evolution processes. When water flow runs over vegetated areas, averaged velocity profile is affected by the presence of stem, branches and leaves, sediment transport changes according to modified turbulence and bed shear stresses and soil shear strength is altered by root binding. As a result, bed scour, bank erosion and accretion, bar migration and width adjustment processes lead to different river morphology evolution. Conversely, flow and sedimentary patterns influence vegetation dynamics, by shaping barebed deposits available for colonisation and by affecting mortality rate, through burying and uprooting processes. However, whereas recruitment, establishment and growth represent the transitional dynamics from barebed to vegetated conditions and are mainly related to species properties, plant removal is intrinsically related to species growth stage, flow magnitude and soil properties. Although vegetation uprooting only recently gathered attention from scientific community, there is a rising awareness that vegetation removal is crucial for species selection and location on exposed deposits and floodplains and carbon production and sequestration, at different spatial and temporal scales. This PhD work examines the uprooting process of both pioneer seedlings and established vegetation driven by flow and bed erosion, whose role is to reduce root anchorage, at various spatial scales ranging from a single plant to a river reach. The main purpose of this research is to illustrate the links between temporal scales regarding the hydro-morphological evolution of fluvial systems, such as bed scour development, flood duration and return period, and those proper of biological components with regards to both growth and decay rates of riparian vegetation. For this aim, various methodologies and approaches are followed. Firstly, an intensive analysis of the state of knowledge is presented and discussed. Secondly, the existence of links between growth rate and hydrological return period of flood events and between decay rate and flood duration are proved and investigated by means of an already available eco-morphodynamic equation, and illustrated according to different vegetation cover. Promising results are obtained when relationships are applied to the case studies of the Maggia River (CH) and the Tagliamento River (IT). Thirdly, the uprooting process of juvenile flexible riparian vegetation is investigated by means of flume experiments, field measurements of root resistance and numerical modelling. A new physical relationship able to predict critical conditions of bed shear stress and bed erosion is derived, validated and applied to the case study of the Ombrone Pistoiese River (IT) with good agreement. Lastly, the proposed relationship is combined to a very recent probabilistic model and a stochastic approach to flood events. A new relationship for uprooting process randomness is proposed and the correlation between vegetation removal and flood return period is evaluated, discussed and applied to the case study of the Santa Maria River, Arizona (USA) with very good results. The results of this PhD research show the existence of cross-related temporal scales between riparian vegetation and river morphodynamics and demonstrate their relationships with flood return period and event duration. The adapted comprehensive approach to study the uprooting process of riparian vegetation highlights that multidisciplinary methodology is essential to understand the mechanisms, correctly model the process and formulate the equations. The application to laboratory experiments and to various case studies proves the validity of the relationships as well as the applicability both to small and large spatial scales. As a final result, this research hints the capability for river to select species and cover according to hydrological regime and biological properties. This is crucial in fluvial environments altered by climate change, where alien species may replace native ones. It also underlines the importance of taking into account riparian vegetation dynamics, effects and interactions to guarantee the reliability of long-term river morphodynamics modelling and the success of river maintenance and restoration strategies.
2019
Luca Solari, Hans Matthias Schöniger
ITALIA
Giulio Calvani
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Utilizza questo identificatore per citare o creare un link a questa risorsa: https://hdl.handle.net/2158/1174100
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