Propagation and deposition of stony debris flows at channel confluences

The fluid dynamics of stony debris flows generated in two small tributaries adjacent to each other and flowing into a main receiving channel was analyzed experimentally at a laboratory scale. The analysis on the propagation along the tributaries and deposition in the main channel provide information about sediment-water mobility, dangerous damming, and potential hazard. Debris flows were generated by releasing a preset water discharge over an erodible layer of saturated gravels material. As a consequence, the debris flow sediment concentration varied accordingly to the entrainment rate which, in turn, was strongly controlled by the tributary slope. The data collected by acoustic level sensors, pore fluid pressure transducers, and a load cell were used to characterize the evolution of bulk density and solid concentration of the sediment-water mixture. These two parameters were relevant to assess the stony debris flow mobility which contributes to determine the shape of sediment deposits in the main channel. The detailed bed topography surveys carried out in the main channel at the end of each experiment provided information on the morphology of these deposits and on the interplay of adjacent confluences. The influences of conflu- ence angle, tributary slopes, and triggering conditions have been investigated, for a total of 18 different configurations. Within the investigated range of parameters, the slope angle was the parameter that mainly influences the stony debris flow mobility while, for adjacent confluences, the degree of obstruction within the receiving channel was strongly influenced by the triggering scenario

Modeling Shallow Water Flows on General Terrains

A formulation of the shallow water equations adapted to general complex terrains is proposed. Its derivation starts from the observation that the typical approach of depth integrating the Navier-Stokes equations along the direction of gravity forces is not exact in the general case of a tilted curved bottom. We claim that an integration path that better adapts to the shallow water hypotheses follows the "cross-flow" surface, i.e., a surface that is normal to the velocity field at any point of the domain. Because of the implicitness of this definition, we approximate this "cross-flow" path by performing depth integration along a local direction normal to the bottom surface, and propose a rigorous derivation of this approximation and its numerical solution as an essential step for the future development of the full "cross-flow" integration procedure. We start by defining a local coordinate system, anchored on the bottom surface to derive a covariant form of the Navier-Stokes equations. Depth integration along the local normals yields a covariant version of the shallow water equations, which is characterized by flux functions and source terms that vary in space because of the surface metric coefficients and related derivatives. The proposed model is discretized with a first order FORCE-type Godunov Finite Volume scheme that allows implementation of spatially variable fluxes. We investigate the validity of our SW model and the effects of the bottom geometry by means of three synthetic test cases that exhibit non negligible slopes and surface curvatures. The results show the importance of taking into consideration bottom geometry even for relatively mild and slowly varying curvatures

Meandering evolution and width variation, a physics-statistical based modeling approach

Many models have been proposed to simulate and understand the long-term evolution of meandering rivers. These models analyze the hydraulics of the in-channel flow and the river bank movement (erosion \u2013 accretion) process in different ways, but some gap still remain, e.g. the stability of long-term simulations when width variations are accounted for. Here we proposed a physics-statistical based approach to simulate the river bank evolution, that erosion and deposition processes act independently, with a specific shear stress threshold for each of them. In addition, we link the width evolution with a parametric probability distribution (PPD) based on a mean characteristic channel width. We are thus able to obtaining stable long-term simulations with realistic and reasonable spatio-temporal distribution of the along channel width

Effetti dell'eterogeneit\ue0 della piana alluvionale sulla migrazione di meandri fluviali

Le piane alluvionali e i relativi fiumi costituiscono complessi sistemi dinamici che coinvolgono processi idrodinamici, morfodinamici, geologici, geomorfologici, chimici e biologici agenti su un vasto intervallo di scale temporali (dai giorni ai secoli), con reciproche interazioni. La morfologia superficiale e la stratigrafia di una piana alluvionale sono modellate dalla mutua interazione di portata liquida, portata solida, processi di erosione e di deposito (Howard, 1996). L\u2019effetto a lungo termine \ue8 controllato dalla configurazione morfologica del fiume (meandriforme, intrecciato, anastomizzato), dal regime fluviale e dalle peculiarit\ue0 del trasporto solido. Confinamenti della piana alluvionale, la meccanica delle faglie e la distribuzione di biomassa possono inoltre assumere un ruolo apprezzabile nel determinare l\u2019evoluzione della piana alluvionale. L\u2019obiettivo del presente lavoro \ue8 analizzare un particolare aspetto della dinamica fiume-piana alluvionale: il ruolo dell\u2019eterogeneit\ue0 della superficie alluvionale nel determinare la configurazione spaziale di un fiume meandriforme la cui dinamica, a sua volta, contribuisce a formare la struttura sedimentologica della piana alluvionale stessa. Questa interazione mutua \ue8 strettamente correlata alla formazione di oxbow lakes e scroll bars (Figura 1), i quali introducono modificazioni nella superficie della piana alluvionale e, pertanto, influiscono sull\u2019evoluzione morfologica dei meandri (Lazarus & Constantine, 2013). Gli oxbow lakes rappresentano recipienti naturali per l\u2019immagazzinamento di sedimenti e producono singolarit\ue0 morfologiche, ecosistemi locali, serbatoi di idrocarburi ed inquinanti (Lewin and Ashworth, 2014). D\u2019altra parte, la continua formazione di cutoffs, con la conseguente rimozione dei lobi di meandri pienamente sviluppati e la creazione di nuovi oxbow lakes, introduce un limite nello sviluppo della complessit\ue0 geometrica del fiume e, di conseguenza, assicura una condizione di evoluzione stabile dal punto di vista statistico (Camporeale et al, 2005; Frascati & Lanzoni, 2010). Le unit\ue0 geomorfologiche formate da scroll bars e oxbow lakes assumono quindi un ruolo di primaria importanza sia nel passato del fiume, rappresentando un archivio della sua storia morfodinamica (Schwenk et al., 2015), sia per prevenire la sua evoluzione futura, operando come filtro sullo sviluppo planimetrico a breve e lungo termine

Field migration rates of tidal meanders recapitulate fluvial morphodynamics

The majority of tidal channels display marked meandering features. Despite their importance in oil-reservoir formation and tidal landscape morphology, questions remain on whether tidalmeander dynamics could be understood in terms of fluvial processes and theory. Key differences suggest otherwise, like the periodic reversal of landscape-forming tidal flows and the widely accepted empirical notion that tidal meanders are stable landscape features, in stark contrast with their migrating fluvial counterparts. On the contrary, here we show that, once properly normalized, observed migration rates of tidal and fluvial meanders are remarkably similar. Key to normalization is the role of tidal channel width that responds to the strong spatial gradients of landscape-forming flow rates and tidal prisms. We find that migration dynamics of tidal meanders agree with nonlinear theories for river meander evolution. Our results challenge the conventional view of tidal channels as stable landscape features and suggest that meandering tidal channels recapitulate many fluvial counterparts owing to large gradients of tidal prisms across meander wavelengths

Coarse-grained debris flow dynamics on erodible beds.

A systematic set of flume experiments is used to investigate the features of velocity profiles within the body of coarse-grained debris flows and the dependence of the transport sediment concentration on the relevant parameters (runoff discharge, bed slope, grain size and form). The flows are generated in a 10 m long laboratory flume, initially filled with a layer consisting of loose debris. After saturation, a prescribed water discharge is suddenly supplied over the granular the runoff triggers a debris flow wave that reaches nearly steady conditions. Three types of material have been used in the tests: gravel with mean grain size of 3 and 5 mm, and 3 mm glass spheres. Measured parameters included: triggering water discharge, volumetric sediment discharges, sediment concentration, flow depth and velocity profiles. The dynamic similarity with full-sized debris flows is discussed on the basis of the relevant dimensionless parameters. Concentration data highlight the dependence on the slope angle and the importance of the quasi-static friction angle. The effects of flow rheology on the shape of velocity profiles are analyzed with attention to the role of different stress generating mechanisms. A remarkable collapse of the dimensionless profiles is obtained by scaling the debris flow velocity with the runoff velocity, and a power law characterization is proposed following a heuristic approach. The shape of the profiles suggests a smooth transition between the different rheological regimes (collisional and frictional) that establish in the upper and lower regions of the flow, and is compatible with the presence of multiple length scales

On the O'Brien-Jarrett-Marchi law

The relationship between the total water volume entering a lagoon during a characteristic tidal cycle (i.e., the prism) and the size of its inlet is well established empirically since the classic work of O'Brien and Jarrett widely cited in the geomorphic and hydrodynamic literature. Less known is a rather deep theoretical explanation proposed by Marchi. This paper reviews the empirical and theoretical evidence on which the relation is based, setting the various theoretical approaches so far pursued within the general framework ensured by Marchi's theoretical treatment of the problem. We conclude that the depth of the empirical and theoretical validations and the breadth and the importance of its implications suggest that the O'Brien-Jarrett-Marchi law relating the minimum inlet cross-sectional area and the tidal prism flowing through it may be referred to thereinafte

Intertwined Eco‐Morphodynamic Evolution of Salt Marshes and Emerging Tidal Channel Networks

The formation and development of tidal channels and salt marshes are controlled by complex interactions between hydrodynamics, sediment transport, and vegetation dynamics. Tidal channels affect and, at the same time, are affected by the growth of salt marshes fringing them. The coupled evolution of these morphological units, mediated by vegetation growth, is thus a key ingredient for simulating the behavior of tidal environments. Considering these two factors, we developed a mathematical model to investigate the eco-morphodynamic evolution of intertidal areas fringing a main channel and of the tidal creeks cutting through them. Model results indicate that vegetation promotes the development of channel networks, leading to more complex channel structures and higher drainage efficiency. Vegetation encroachment influences sediment deposition patterns by trapping sediment in the seaward and middle intertidal areas, while reducing the amount of sediment delivered to landward areas. In the presence of sea level rise, this deficit of sediment enhances the landward-decreasing trend of the intertidal platform and leads to more isolated vegetation patches. Overall, sea level rise restricts the extension of salt marshes and consequently reduces the effect of vegetation on channel network form and function