Michele Bolla Pittaluga

Where river and tide meet: The morphodynamic equilibrium of alluvial estuaries

We investigate the morphodynamic equilibrium of tidally dominated alluvial estuaries, extending previous works concerning the purely tidal case and the combined tidal-fluvial case with a small tidal forcing. We relax the latter assumption and seek the equilibrium bed profile of the estuary, for a given planform configuration with various degrees of funneling, solving numerically the 1-D governing equation. The results show that with steady fluvial and tidal forcings, an equilibrium bed profile of estuaries exists. In the case of constant width estuaries, a concave down equilibrium profile develops through most of the estuary. Increasing the amplitude of the tidal oscillation, progressively higher bed slopes are experienced at the mouth while the river-dominated portion of the estuary experiences an increasing bed degradation. The fluvial-marine transition is identified by a “tidal length” that increases monotonically as the river discharge and the corresponding sediment supply are increased while the river attains a new morphological equilibrium configuration. Tidal length also increases if, for a fixed river discharge and tidal amplitude, the sediment flux is progressively reduced with respect to the transport capacity. In the case of funnel-shaped estuaries the tidal length strongly decreases, aggradation is triggered by channel widening, and tidal effects are such to enhance the slope at the inlet and the net degradation of the river bed. Finally, results suggest that alluvial estuaries in morphological equilibrium cannot experience any amplification of the tidal wave propagating landward. Hence, hypersynchronous alluvial estuaries cannot be in equilibrium.

A unified framework for stability of channel bifurcations in gravel and sand fluvial systems

Bifurcating rivers shape natural landscapes by distributing water and sediments on fluvial plains and in deltas. Symmetrical bifurcations were often found to be unstable so that one branch downstream of the bifurcation enlarged while the other dwindled. A unified theory able to predict bifurcation stability in both gravel bed and sand bed rivers is still lacking. Here we develop a new theory for the stability of bifurcations for the entire range of gravel bed to sand bed rivers. The theory indicates opposite behavior of gravel bed and sand bed rivers: we predict that symmetrical bifurcations are inherently stable for intermediate Shields stresses but are inherently unstable for the low and high Shields stresses found in the majority of rivers on Earth. In the latter conditions asymmetrical bifurcations are stable. These predictions are corroborated by observations and have ramifications for many environmental problems in fluviodeltaic settings.

On the migration of tidal free bars

We derive and employ a depth-averaged model for the formation of free bars in infinitely long tidal channels in order to investigate the mechanism whereby tidal bars may experience a net migration over a tidal cycle. The flux of the suspended sediment is modeled by means of an analytical relationship derived by Bolla Pittaluga and Seminara [M. Bolla Pittaluga and G. Seminara, Water Resour. Res. 39, 5 (2003)] for slowly varying flows. The model is validated by performing a linear stability analysis of flow and bed topography in a rectangular channel with an erodible bed, subject to the propagation of a symmetric tidal wave of small amplitude. The results of the present depth averaged model show a fairly satisfactory agreement with previous results based on a three-dimensional model [G. Seminara and M. Tubino, J. Fluid Mech. 440, 49 (2001)]. We then investigate the role of overtides, showing that a flood or ebb asymmetry of the basic flow gives rise to a net migration of bars. The mechanism is due to the no...

Finite amplitude bars in mixed bedrock‐alluvial channels

We present a nonlinear asymptotic theory of fully developed flow and bed topography in a wide channel of constant curvature to describe finite amplitude perturbations of bottom topography, subject to an inerodible bedrock layer. The flow field is evaluated at the leading order of approximation as a slowly varying sequence of locally uniform flows, slightly perturbed by a weak curvature-induced secondary flow. Using the constraint of constant fluid discharge and sediment flux, we calculate an analytical solution for the cross-sectional profile of flow depth and bed topography, and we determine the average slope in the bend necessary to transport the sediment supplied from a straight, alluvial, upstream reach. Both fully alluvial bends and bends with partial bedrock exposure are shown to require a larger average slope than a straight upstream reach; the relative slope increase is much larger for mixed bedrock-alluvial bends. Curvature and sediment supply are shown to have a strong effect on the characteristics of the point bars in mixed bedrock-alluvial channels. Higher curvature bends produce bars of larger amplitude and more bedrock exposure through the cross section, and increasing the sediment supply leads to taller and wider point bars. Differences in the relative roughness of sediment and bedrock have a smaller, secondary effect on point bar characteristics. Our analytical approach can potentially be extended to the case of arbitrary, yet slowly varying, curvature, and should ultimately lead to an improved understanding of the formation of meanders in bedrock channels.