Wave Controls on Deltaic Shoreline‐Channel Morphodynamics: Insights From a Coupled Model

Abstract

It is widely recognized that waves inhibit river mouth progradation and reduce the avulsion timescale of deltaic channels. Nevertheless, those effects may not apply to downdrift-deflected channels. In this study, we developed a coupled model to explore the effects of wave climate asymmetry and alongshore sediment bypassing on shoreline-channel morphodynamics. The shoreline position and channel trajectory are simulated using a "shoreline" module which drives the evolution of the river profile in a "channel" module by updating the position of river mouth boundary, whereas the channel module provides the sediment load to river mouth for the "shoreline" module. The numerical results show that regional alongshore sediment transport driven by an asymmetric wave climate can enhance the progradation of deltaic channels if sediment bypassing of the river mouth is limited, which is different from the common assumption that waves inhibit delta progradation. As such, waves can have a trade-off effect on river mouth progradation that can further influence riverbed aggradation and channel avulsion. This trade-off effect of waves is dictated by the net alongshore sediment transport, sediment bypassing at the river mouth, and wave diffusivity. Based on the numerical results, we further propose a dimensionless parameter that includes fluvial and alongshore sediment supply relative to wave diffusivity to predict the progradation and aggradation rates and avulsion timescale of deltaic channels. The improved understanding of progradation, aggradation, and avulsion timescale of deltaic channels has important implications for engineering and predicting deltaic wetland creation, particularly under changing water and sediment input to deltaic systems.

A Tale of Two Deltas: Dam-Induced Hydro-Morphological Evolution of the Volta River Delta (Ghana) and Yellow River Delta (China)

“…The river discharge and sediment load appeared to attain a quasi-equilibrium at the latter stage of the post-dam period after a significant reduction (Figures 3, 4 and 9) [19]. The changing sediment load further changed the balance between fluvial and wave forcings, which could cause delta erosion (Figures 7-9) [9,63]. Finally, it is foreseeable that the delta shoreline and area may potentially transition into a new quasi-equilibrium as the Volta River Delta (Figure 8) [19].…”

“…To quantify the annual mean discharge on the morphodynamics of the two waveinfluenced river deltas, we calculated the fluvial dominance ratio R, which is defined as the ratio of annual fluvial sediment supply (Q r ) and the combined annual maximum possible alongshore sediment transport away from the river mouth (Q s,max ) [9,62]. The fluvial dominance ratio R has been used to define deltaic morphological evolution in previous studies [9,63,64]. River deltas are considered wave-dominated if R < 1 and river-dominated if R > 1 [62].…”

The Longitudinal Profile of a Prograding River and Its Response to Sea Level Rise

“…Note that the river mouth progradation rate could be proportional to the sediment discharge (Aadland & Helland‐Hansen, 2019); however, the dependency can be mediated by nearshore water depth, baselevel changes, waves, etc. (Bijkerk et al, 2016; Gao et al, 2020; Swenson et al, 2005; Wang et al, 2019; Wu et al, 2020; Wu & Nittrouer, 2020). In deflected river mouths under waves, the progradation rate can be insensitive to fluvial sediment load but more dependent on alongshore sediment load (Gao et al, 2020; Nienhuis et al, 2016).…”

“…(Bijkerk et al, 2016; Gao et al, 2020; Swenson et al, 2005; Wang et al, 2019; Wu et al, 2020; Wu & Nittrouer, 2020). In deflected river mouths under waves, the progradation rate can be insensitive to fluvial sediment load but more dependent on alongshore sediment load (Gao et al, 2020; Nienhuis et al, 2016). Since we did not associate our framework with any specific receiving basins with a given nearshore water depth, baselevel changes, wave conditions, etc., we imposed a boundary condition of progradation rate at the river mouth as appropriate alongside the upstream sediment supply (Equation ) to focus on the evolution of the longitudinal profile as the main target of our study.…”

Wave‐Climate Asymmetry Influence on Delta Evolution and River Dynamics

“…Nienhuis et al (2016) examined the feedbacks between waves and river-mouth migration. Gao et al (2020) continued the work and explored the impact of wave climate asymmetry on deltaic river-mouth morphodynamics. Ratliff et al (2018) coupled wave-driven delta morphodynamics and river avulsion dynamics under a symmetric wave climate.…”