Effects of basin bottom slope on jet hydrodynamics and river mouth bar formation

Abstract

River mouth bars are strategic morphological units primarily responsible for the development of entire deltaic systems. This paper addresses the role of receiving basin slope in the hydrodynamics of an exiting sediment‐laden turbulent jet and in resulting mouth bar morphodynamics. We use Delft3D, a coupled hydrodynamic and morphodynamic numerical model, along with a theoretical formulation to reproduce the physics of the problem, characterized by a fluvially dominated inlet free of waves and tides. We propose an updated theoretical model with a slope‐dependent entrainment coefficient, showing that the rate at which ambient fluid is incorporated into a jet increases with higher basin slopes. Transient results reveal that the magnitude of a basin slope can alter the stability of a jet, favoring the formation of an unstable meandering jet. While a stable jet gives rise to “middle‐ground” bars accompanied by diverging channels, a “lunate” mouth bar results from unstable jets. Additional morphodynamic simulations demonstrate that the time required for mouth bar stagnation in its final position increases linearly with the basin slope. In contrast, the distance at which the mouth bar eventually forms decreases until reaching an asymptotic value for slopes higher than 2%. Moreover, the basin slope highly influences sedimentary processes responsible for bar formation: for milder slopes, progradation processes prevail, while in steeper basins aggradation is more relevant. Finally, the minimum relative water depth over a bar crest that forces the flow to bifurcate around a fully developed bar decreases with the basin slope.

Measuring Subaqueous Progradation of the Wax Lake Delta with a Model of Flow Direction Divergence

“…However, scaling of shallow water jets by Özsoy and Ünlüata (1982) showed that in the presence of significant basinward slopes, jets can converge or contract when the basinward bed slope ( −∇ ⋅ ⃗ ⃗ ≈ ∇ℎ ⋅ ⃗ ⃗ > 0) exceeds the dimensionless friction factor (Cf). Recent numerical modelling by Jiménez-Robles et al (2016) also shows that jets can exhibit 10 flow direction convergence when basinward slopes exceed ~1%. The bathymetric complexity and significant unchannelized flows on the Wax Lake Delta prevent us from rigorously applying the findings of either study to the delta front of the Wax Lake Delta, but note that there is a physical basis for flow direction convergence on delta fronts that supports the convergence we observe in streaklines.…”

Grain size fractionation by process‐driven sorting in sandy to muddy deltas

“…Hydrodynamically, a mouth bar develops when sedimentladen jets exit a channel mouth into a body of standing water. Some studies have also focused on the influence of various external parameters on these mouth bars such as discharge, waves, tides and basin slope (Leonardi et al, 2013;Jimenez-Robles et al, 2016;Gao et al, 2019). Wright (1977) was one of the first to describe mouth bar deposition in terms of inertial, frictional and buoyant forces, in addition to reworking forces (tides, waves).…”

“…More recently, the detailed morphodynamic mechanisms of the formation of individual mouth bars have been investigated using numerical simulations in Delft3D (Edmonds and Slingerland, 2007;Geleynse et al, 2010;Esposito et al, 2013;Mariotti et al, 2013;Canestrelli et al, 2014). Some studies have also focused on the influence of various external parameters on these mouth bars such as discharge, waves, tides and basin slope (Leonardi et al, 2013;Jimenez-Robles et al, 2016;Gao et al, 2019). This paper investigates how the upstream sediment supply composition is translated not only to the mouth bars, but across the whole range of architectural elements.…”

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

“…On the other hand, works focused on offshore plume processes are carried out in a framework of no connectivity with the issuing channel. Related fluid dynamics and sediment patterns on the oceanic region are based on a boundary condition at the channel mouth (Wright, 1977;Özsoy, 1986;Wang, 1984;Leonardi et al, 2013Leonardi et al, , 2014Fagherazzi et al, 2015;Jiménez-Robles et al, 2016).…”

“…These equations are dinamically coupled to the flow and sediment patterns in the offshore region by means of a sufficiently simple analytical model of plume hydrodynamics that reduces drastically the computational effort. In the near field, during the ebb-phase tidal plumes behave as a bounded, sediment-laden, turbulent jet (Rajaratnam, 1976;Joshi, 1982;Ortega-Sánchez et al, 2003;Jiménez-Robles et al, 2016). The approximate equations for the jet flow and sediment patterns were derived by Özsoy and Ünlüata (1982) and Özsoy (1986) respectively.…”