No abstract

[1] Depositional patterns characteristic of river mouths are controlled by the sediment-laden turbulent jet exiting from rivers. Here we show that jet instability, developing at river mouths with small width-to-depth ratio, can affect these patterns. Numerical simulations indicate that sediment eddy diffusivity, which is a measure of the spreading of sediments out of the jet core, depends on the interplay between sediment settling and large coherent flow structures, which are associated with unstable jets. Our results suggest that optimal conditions for eddy diffusivity are met when the time scale of sediment settling is equal to the characteristic eddy time scale. In this condition, sediments are deposited at the farthest distance from the jet centerline, thus promoting the formation of lateral deposits. Settling time scales much greater or much smaller than the eddy time scale is associated with low eddy diffusivity, which promotes the formation of a central deposit. Such eddy-mediated sediment transport is more evident in the Zone Of Established Flow than in the Zone Of Flow Establishment, since the former is characterized by fully developed vortices. A simplified model is proposed to estimate the settling and eddy time scales and to predict the optimal condition for levee formation.

Deltas are sensitive indicators of coastal processes (e.g., waves and tides) and show dynamic changes in shoreline morphology, distributary channel network, and stratigraphic architecture in response to coastal forcing. Numerical modeling has long been used to show delta evolution associated with a single dominant coastal process, but rarely to examine the sensitivity of deltas to mixed processes. Physics-based morphodynamic simulations (Delft3D) are used to investigate the influence of tidal currents on deltas. Tidal amplitude and the sand:mud ratio of subsurface sediment have been varied in the model. The results show that increasing tidal amplitude causes deeper and more stable distributary channels and more rugose planform shoreline patterns. A new metric for channel geometry quantifies tidal influence on the distributary channel network. Stable distributary channels act as an efficient mechanism for ebb-enhanced currents to (1) bypass sediment across the delta plain, and ( 2) extend channel tips seaward through mouth bar erosion. The basinward channel extension leads to sandier deposits in the tide-influenced deltas than in their river-dominated counterparts. The deltafront bathymetry also reflects sediment redistribution, changing the delta-front profile from concave to convex with compound geometries as tidal amplitude increases. These results suggest that channel overdeepening is a possible tidal signature that should be considered when interpreting ancient systems, and that sand may be bypassed much farther basinward in tide-influenced than in purely river-dominated deltas.

Abstract. In a companion paper (Liang et al., 2015) we introduced a reduced-complexity model (RCM) for river delta formation, developed using a parcel-based "weighted random walk" method for routing water and sediment flux. This model (referred to as DeltaRCM) consists of a flow routing scheme as the hydrodynamic component (referred to as FlowRCM) and a set of sediment transport rules as the morphodynamic component. In this work, we assess the performance of FlowRCM via a series of hydrodynamic tests by comparing the model outputs to Delft3D and theoretical predictions. These tests are designed to reveal the capability of FlowRCM to resolve flow field features that are critical to delta dynamics at the level of channel processes. In particular, we focus on (1) backwater profile, (2) flow around a mouth bar, (3) flow through a single bifurcation, and (4) flow through a distributary channel network. We show that while the simple rules are not able to reproduce all fine-scale flow structures, FlowRCM captures flow field features that are essential to deltaic processes such as bifurcations and avulsions, the partitioning of flux between channels and inundated islands, and the instability of flux distribution at channel mouths which is responsible for mouth-bar growth. Finally, we discuss advantages and limitations of FlowRCM and identify environments most suitable for it.