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

Abstract

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.

Tidal controls on river delta morphology

“…A formal equilibrium for tidal channel morphology has only been found adopting rigorous simplifications, from one-dimensional analytical or numerical models 59,60 . These idealized models are of conceptual relevance and provide a basic understanding about how tides increase the tide-average sediment transport capacity in case of uniform sediment.…”

Exploring the impacts of multiple tidal constituents and varying river flow on long‐term, large‐scale estuarine morphodynamics by means of a 1‐D model

“…[], and Bolla Pittaluga et al . [] had modeled long‐term bed profile (reflecting along‐channel variations of cross‐sectionally averaged depth) development in 1‐D estuaries forced by river and tides. River‐tide current interaction is found to be an important mechanism driving TRST and flushing sediment seaward [ Guo et al ., ].…”

Implications of Plume Discharge for Tidal Channels Morphodynamics: A Coupled Onshore and Offshore System

“…Previous works focused on the long-term evolution of tidal channels imposed a fixed boundary condition at the tidal mouth (i.e. (Lanzoni and Seminara, 2002;Todeschini et al, 2008;Yu et al, 2012;Bolla Pittaluga et al, 2015). During the flood phase they prescribed a sediment load in equilibrium with the local hydrodynamics at this boundary, whereas during the ebb phase the sediment load computed at the inlet is simply assumed to leave the estuary.…”

“…Tidal channel morphodynamics is dictated by asymmetries in surface elevations and tidal currents that lead to a net sediment flux directed landward within a tidal cycle (De Swart and Zimmerman, 2009). Laboratory experiments (Tambroni et al, 2005), analytical methods Swart, 1996, 2000;Prandle, 2003;Seminara et al, 2010) and numerical techniques (Lanzoni and Seminara, 2002;Hibma et al, 2003;Yu et al, 2012;Bolla Pittaluga et al, 2015) found that, starting from an arbitrary initial bottom profile, the system asymptotically tends to a long-term morphodynamics equilibrium state with a nearly constant value of the maximum flood/ebb speed. This dynamic equilibrium state is characterized by a vanishing net (tidally-averaged) transport in the entire embayment that results in a shoaling bed with water depths decreasing from the inlet to an emerging shore landward.…”