[1] This paper tackles the problem of morphodynamic equilibrium of tidal channels and tidal inlets. We report a laboratory investigation of the process whereby an equilibrium morphology is established in a tidal system consisting of an erodible channel connected through an inlet to a tidal sea. Observations suggest that a morphodynamic equilibrium is eventually established both in the inlet region and in the channel. The latter exhibits a weakly concave bed profile seaward, a weakly convex profile landward, and the formation of a ''beach'' close to the landward end of the channel. A second set of observations concerns the formation and development of both small-and large-scale bed forms. In particular, small-scale forms are found to develop in the channel and in the basin, while larger-scale forms, i.e., tidal bars, develop in the channel. A last observation concerns the formation of an outer delta in the ''sea'' basin. Results concerning the long-term equilibrium of the bed profile in the channel compare fairly satisfactorily with recent theoretical results. The nature and characteristics of the observed small-scale forms appear to be consistent with theoretical predictions and field observations concerning ''fluvial'' ripples and tidal dunes; bars show features in general accordance with recent results of a stability theory developed for tidal bars. The hydrodynamics of the inlet region exhibits a strongly asymmetric character, as observed in the field and predicted in early theoretical works, while the overall characteristics of the outer delta conform to available empirical relationships.Citation: Tambroni, N., M. Bolla Pittaluga, and G. Seminara (2005), Laboratory observations of the morphodynamic evolution of tidal channels and tidal inlets,

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.

Do tidal channels have a characteristic length? Given the sediment characteristics, the inlet conditions and the degree of channel convergence, can we predict it? And how is this length affected by the presence of tidal flats adjacent to the channel? We answer the above questions on the basis of a fully analytical treatment, appropriate for the short channels typically observed in coastal wetlands. The equilibrium length of non-convergent tidal channels is found to be proportional to the critical flow speed for channel erosion. Channel convergence causes concavity of the bed profile. Tidal flats shorten equilibrium channels significantly. Laboratory and field observations substantiate our findings

[1] Through the centuries, Venice Lagoon has undergone morphological changes that can be attributed to both natural events and human actions. The lagoon has progressively deepened, and it is claimed to lose roughly one million cubic meters of sediments each year. In the ongoing debate concerning the possible means to counteract this morphodynamic degradation, inlet geometry is considered a major factor controlling the exchange of sediments. Our aim is to explore the causes of this loss. We focus first on sand, as this is the type of sediment present on the bottom of the near-inlet regions.We employ a simple model of the inlet hydrodynamics to estimate the net exchange of sand associated with the sequence of tidal events recorded for several years. Results suggest that in the absence of an excess supply from the sea, the yearly loss of sand through Venice inlets is an order of magnitude smaller than the total sediment loss usually claimed. We then show that this estimate is only slightly affected by the sand supply from wave resuspension in the far field whose effect is simply to store sediments in the near-inlet region. We finally argue that most of the sediment loss is wash load carried by the ebb currents overloaded by very fine sediments resuspended by wind in the inner lagoon and unable to settle within the channel network.

Deltas are fascinating landforms subject to fluvial and marine forcing. Bifurcations are common features in deltas, governing the distribution of water and sediment fluxes among the distributary channel network. Recently, it has been observed that tide‐influenced deltas tend to display less numerous but more stable branches in comparison to their riverine counterparts. River bifurcations subject to unidirectional flow have been widely studied in the last decades. In contrast, the acting physical mechanisms and factors controlling the stability of bifurcations in tide‐influenced deltas are still not well understood, and a theoretical framework is still lacking. In order to fill this gap and understand how the stability and evolution of bifurcations in distributary deltaic systems could be affected by the tides, we investigate, through an analytical model, the equilibrium configurations and stability of tidal bifurcations under the hypothesis of small monochromatic tidal oscillations. In particular, we build on previous works on river bifurcations, incorporating the solution for the equilibrium of a single‐river‐dominated estuary. We find that higher tidal amplitudes and a closer proximity of the junction node to the sea tend to hamper the development of unbalanced solutions, reducing the asymmetries in water and sediment fluxes between branches. This stabilizing effect exerted by the tidal action is associated with the erosive character of the tidal currents that promotes channel deepening and increases the capacity of the system to keep morphodynamically active both bifurcates in comparison with the purely fluvial case. Preliminary field observations of natural deltas corroborate our findings.