Showing papers by "Giovanni Seminara published in 2012"

A theoretical framework for the morphodynamics of bedrock channels

Abstract: [1] Bedrock rivers are characterized by conditions of insufficient sediment supply, wherein the amount of sediment supplied to the channel is less than the channel's sediment transport capacity. Consequently, bedrock channels generally exhibit discontinuous sediment cover, which current approaches to the morphodynamics of alluvial rivers are unable to handle. To fill this gap, we present a theoretical framework in which local sediment transport rates are proportional to the areal concentration of sediment available for transport on the bed and the sediment continuity equation is reformulated to account for temporal changes in areal concentration of sediment on nonalluviated surfaces. We then perform a linear stability analysis that shows that an initially uniform distribution of the areal concentration of sediment on the bed is unstable to small perturbations. This suggests that the sediment cover in bedrock channels tends to concentrate into regions which may eventually be alluviated, in general agreement with experimental observations.

A one‐dimensional eco‐geomorphic model of marsh response to sea level rise: Wind effects, dynamics of the marsh border and equilibrium

Abstract: [1] In this study we investigate the long term evolutionary trend of tidal marshes and their possible tendency to approach equilibrium. We account of the dynamic interaction between the marsh and its adjacent environment, allowing for both variations of marsh elevation and displacement of the marsh boundary. We thus consider a 1-D configuration consisting of a tidal channel merging into a 1-D marsh. Starting from some initial configuration of the channel we model how a salt marsh forms at the landward end of the channel and determine the long term evolution of the channel - marsh configuration under different scenarios of relative sea level rise. Previously established results on the morphodynamic evolution of a tidal channel are extended accounting for the effects of vegetation and wind driven sediment resuspension in regions where tidal stresses are too weak to mobilize sediments. Results suggest that, for sufficiently low rates of relative sea level rise the marsh platform may be able to reach an equilibrium elevation, provided wind resuspension is able to maintain a sufficiently large sediment concentration close to the marsh boundary. This is in general agreement with recent results based on zero dimensional modeling. However, we find that the marsh boundary is unstable, as progradation or retreat is generally experienced depending on a delicate balance between tidal transport and transport driven by wind setup. In this sense, actual morphodynamic equilibrium is a rather exceptional and unstable state.

Spatial width oscillations in meandering rivers at equilibrium

Abstract: [1] In canaliform rivers channel width at bankfull stage is fairly uniform though, at bend apexes, it is typically smaller than at crossings. Conversely, in sinuous point bar rivers bankfull width peaks at bend apexes. Why? Is there any mechanistic constraint that forces this different behavior? We provide an answer to these questions investigating how bankfull width must vary in a sequence of sine-generated meanders in order for the constraints of equilibrium (constant flow discharge and sediment flux) to be satisfied. With the help of a 3-D fully nonlinear analytical model of flow and bed topography in meandering rivers with variable width, we show that, in a meandering channel characterized by a constant longitudinal free-surface slope, the equilibrium width thus obtained oscillates with a frequency twice the frequency of channel curvature and experiences the maximum width close to inflection points. This pattern is typically observed in canaliform rivers. We then show that a similar pattern is observed in sinuous point bar rivers, provided the hydrodynamic width (width of the free surface) is replaced by the active width, namely the width of the portion of the cross section where transport occurs at formative conditions. Theoretical results are substantiated by a satisfactory comparison with field observations referring to the Mississippi River (United States) and to the Bollin River (United Kingdom).