Author(s): G. Calvani; L. Solari; C. Shick; P. Perona

Linked Author(s): Luca Solari

Keywords: Biomorphology; Convective acceleration; Vegetation front; Ecomorphodynamic model

Abstract: In this work we focused on vegetation in rivers with narrowing channel showing a distinctive section beyond which growth is hampered by increased drag. Results show scaling relations between biological growth and decay rates and hydrological time scales. Channel planform with converging riverbanks due to incised floodplain is an ubiquitous configuration for natural rivers. As a matter of fact, the stream is convectively forced to accelerate because of the narrowing width, thus resulting in increasing velocity and shear stresses which essentially affect local morphodynamics and promote plant uprooting. Due to the intrinsic and dynamically active flow-biomass interaction, a distinctive sediment-vegetation pattern can be commonly found inside the main channel, particularly, a bare-bed area where pioneer species are on average precluded to colonize and establish. Here we stress the term “on average” to highlight that vegetation can colonize that region during long low-flow or drought period but it is likely to be uprooted during following high floods, whereas upstream region still remains vegetated. The existence and position of the vegetated front has been conjectured to depend on both river hydro-morphodynamic and vegetation biological characteristics. Following on the early conceptual model of Gurnell and Petts, Perona et al. derived the first analytical model that predicts the position of the vegetated front based on the steady state approximation of 1D ecomorphodynamic equations. The model has been tested against data from flume experiments under controlled conditions of flow discharge and growth rate of vegetation, but it still requires validation against field data in order to upscale it and be of use for practical and speculative purposes. In this work we focused on the interactions between in-channel biomass and hydro-morphodynamics of convectively accelerated rivers that lead to the formation of a characteristic vegetated pattern. We considered unlikely that different and, from an hydro-morphological point of view, uncorrelated rivers are in compliance with the model without the fundamental role played by the biological part. Rather, the resultant ecomorphodynamic pattern can be interpreted as the orchestrated dynamical actions among all the factors involved (i.e., flow, sediment and vegetation). We collected data from 37 worldwide river considering sections topography (river width at the vegetation front), geomorphological quantities (grain size distribution and sediment transport rate), hydrological characteristics (flow duration curve) and riparian vegetation parameters (species, cover percentage and carrying capacity, growth and decay rate). In order to validate the 1D ecomorphodynamic model proposed by Perona et al., we used the dataset to perform a yearly statistical analysis and slightly modified the equation to take into account the different duration times for which plant grow (tg) and are removed by flow uprooting (td).

DOI: https://doi.org/10.3850/978-981-11-2731-1_302-cd

Year: 2018