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The fate and transport of soluble contaminants released in natural streams are strongly dependent on the spatial variations of the flow field and of the bed topography. These variations are essentially related to the presence of the channel banks and to the planform configuration of the channel. Large velocity gradients arise near to the channel banks, where the flow depth decreases to zero. Moreover, single thread alluvial rivers are seldom straight, and usually exhibit meandering planforms and a bed topography that deviates from the plane configuration. Channel axis curvature and movable bed deformations drive secondary helical currents which enhance both cross sectional velocity gradients and transverse mixing, thus crucially influencing longitudinal dispersion. The present contribution sets up a rational framework which, assuming mild sloping banks and taking advantage of the weakly meandering character often exhibited by natural streams, leads to an analytical estimate of the contribution to longitudinal dispersion associated with spatial non-uniformities of the flow field. The resulting relationship stems from a physics-based modeling of the flow in natural rivers, and expresses the bend averaged longitudinal dispersion coefficient as a function of the relevant hydraulic and morphologic parameters. The treatment of the problem is river specific, since it relies on an explicit spatial description, although linearized, of the flow field that establishes in the investigated river. Comparison with field data available from tracer tests supports the robustness of the proposed framework, given also the complexity of the processes that affect dispersion dynamics in real streams. [ABSTRACT FROM AUTHOR]

The Po River, in the last century, has undergone significant altimetrical and planimetrical changes, mostly induced by a progressively increasing human pressure. The extensive protection and regulations works carried out to reduce the risk of flooding, the narrowing of the river for improving the navigation, the local interruption of sediment transport caused by a large mobile barrage built for hydropower purposes and the intense sand mining caused huge alterations of the river morphology. These changes were initially very fast and determined a significant and generalized deepening of the middle water course. In the last few decades, however, the pressure induced by human activities on the river decreased significantly and, consequently, a dynamic equilibrium condition tended to be re-established along most of the reaches, as suggested by topographic surveys spanning a period of about twenty years. The present contribution investigates this equilibrium condition by means of a one-dimensional movable bed model, with reference to a 98 km long reach located between the confluence with the Oglio stream and the gauging section of Pontelagoscuro, for which an up to date stage-discharge relationship is available. Considering steady forcing conditions, we estimate the formative discharge producing the observed river topography and the corresponding sediment transport capacity. The field surveys of cross section geometry used to investigate the possible existence of an equilibrium morphology span a period (1982–2005) of about twenty years. In the presence of fixed banks, the rived bed morphology appears to be controlled by relatively moderate discharges, quite close to the mean yearly discharge and significantly smaller than both the ordinary flood discharge and the maximum annual discharge. Even though significant deviations from equilibrium are produced by the sediment waves triggered by larger floods, deposition occurring during lower stages and the continuous reworking of the bed due to less intense but more frequent discharges implies a tendency of the river to recover its equilibrium profile. [ABSTRACT FROM AUTHOR]

Abstract: We have monitored and analyzed, through remote sensing and ancillary field surveys, the rapid (O(1) year) development of a tidal network within a newly established artificial salt marsh in the Venice Lagoon. After the construction of the salt marsh, a network of volunteer creeks established themselves away from an artificially constructed main channel (with mean and maximum annual headward-growth rates of 11 m/yr and 18 m/yr, respectively). The rapid formation of this system of tidal creeks provides a unique opportunity to test the reliability of a model of tidal network initiation and development, previously proposed by the authors. The restored marsh presents the characteristics of a controlled environment analogous to a large-scale field laboratory, as it allows comparison of the morphologic features of real and simulated network structures under the reasonable assumption of neglecting accretion and deposition processes over the timescales of observation. Our results compare favorably with observational evidence, showing that the model proves reasonably capable of reproducing the main features of the actual channel-network patterns. The model reproduces statistical network characteristics of eco-morphodynamic and hydrodynamic relevance and captures the dominant modes of the network-incision process. [Copyright &y& Elsevier]

Abstract: In this paper we develop a finite volume model to solve the two-dimensional shallow water equations governing the propagation of two superimposed layers, with the upper water layer carrying a dilute sediment suspension, and the underlaying layer being a high concentration non-Newtonian fluid mud mixture. The model formulation contains non-conservative terms as well as source terms. We propose a scheme able to deal with varying topography and dry areas, providing well-balanced solutions when both water and fluid mud are quiescent. The model is tested against both exact solutions and numerical examples. The results show the ability of the model to deal with wetting and drying of both water and fluid mud layers, providing mass-conservative solutions. Moreover, the model solves discontinuities and steep fronts, computing accurate and oscillation-free solutions. [Copyright &y& Elsevier]

Highlights • A new model for Shallow Water flow in general terrains is proposed. • The model intrinsically contains the bottom geometry information. • A centered Godunov scheme based on the FORCE flux evaluation discretizes the model. • Numerical results on synthetic cases show that the effects of bottom curvatures on the simulation results are non negligible even under the SW assumptions. Abstract A formulation of the two-dimensional shallow water equations adapted to general and 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 numerically discretized with a first order FORCE-type Godunov Finite Volume scheme that allows straight forward implementation of spatially variable fluxes. We investigate the validity of our SW model and the effects of the geometrical characteristics of the bottom surface 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. By comparison with the numerical solution of vertically integrated models, we observe differences of almost 20%, in particular for the peak values and the shape of the hydrographs calculated at given sections of the fluid domain. [ABSTRACT FROM AUTHOR]