Impacts of backwater hydrodynamics on fluvial–deltaic stratigraphy

Abstract

The hydrodynamics of rivers approaching a receiving basin are influenced by the onset of backwater conditions that give rise to decelerating reach-average flow velocity and decreasing boundary shear stress. These changes occur across a spatial gradient over which decreasing sediment transport capacity triggers morphodynamic responses that include sediment deposition at the transition from uniform to nonuniform flow. As a consequence, the channel width-to-depth ratio and bed sediment grain size decrease downstream. While nonuniform flow and associated morphodynamic adjustments have been investigated in modern fluvial-deltaic systems, the impacts to fluvial-deltaic stratigraphy remain relatively unexplored. This represents an important unresolved gap: there are few contributions that link morphodynamic response to nonuniform flow, impacts on sediment deposition and influence on the rock record. This study uses a numerical model to explore how variable channel hydraulics influence long-term (1000s years) patterns of sediment deposition and development of stratigraphy. The model results indicate that: (a) nonuniform flow propagates upstream beyond the backwater transition that is traditionally estimated with a basic backwater length scale relationship. (b) Base-level fluctuations, especially rising, enhance the impact of nonuniform flow. (c) Sediment deposition shows large spatio-temporal variability, which ultimately contributes to unique stacking patterns of fluvial-deltaic stratigraphy. (d) Nonuniform flow imparts spatial variation in flow depth, channel bed slope and sediment grain size over the delta, and these signatures are potentially preserved and recognizable in the rock record.

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

“…However, since our theoretical framework is zero dimensional as quantified by the A / S ratio, it is capable of describing different kinds of flow regimes and morphological settings of the lower river reach, including the distorted physical models. The numerical results of Wu and Nittrouer (2020), though including sediment selective transport that could add to the concavity, show that greater progradation rate for a river emptying into a shallower receiving basin resulted in greater A / S ratio and more concave profile, whereas smaller A / S ratio and less concave profile would be attained for a river with smaller progradation rate when emptying into a deeper basin (the asterisks in Figure 3). Overall, the results from the literature are consistent with the generic hypothesis derived from our theoretical framework and numerical experiments.…”

“…Relationship between the A / S ratio and (a) the normalized bed difference, ε ; (b) the fill percentage of the river profile, FP ; and (c) the slope percentage of the river profile, SP , for representative numerical experiments, the lower reach of the Yellow River, the physical experiments in Bijkerk et al (2016), and the numerical results from Wu and Nittrouer (2020). The hollow circles, triangles, rectangles, and diamonds represent numerical scenarios with different normalized progradation rates β of 0.0005, 0.0025, 0.005, and 0.01 yr −1 , respectively .…”

“…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).…”

“…However, when downstream controls are considered, river mouth progradation and sea level rise will lead to the continuous aggradation of the riverbed to approach a new equilibrium profile and thus maintain the sediment transport capacity (Muto & Swenson, 2005; Schumm, 1993; Wang et al, 2019). In such cases, the assumption of attaining an equilibrium profile may not hold for the lower river section, as suggested by some previous experimental and numerical studies (Muto & Swenson, 2005; Sinha & Parker, 1996; Wu & Nittrouer, 2020) that river mouth progradation and sea level rise could prevent river profile from attaining the equilibrium profile, and instead a concave profile (Figure 1a) forms as a transient state (Chang, 1986; Muto & Swenson, 2005; Nones et al, 2019). Although the nearshore water depth and the progradation rate of the river mouth are considered the first‐order controls on the attainment of the equilibrium river profile (Bijkerk et al, 2016; Wang et al, 2019; Wu & Nittrouer, 2020), its actual development essentially depends on the infilling of the accommodation space between the initial and equilibrium profiles (i.e., the “progradation‐induced accommodation space” in this study).…”

“…In such cases, the assumption of attaining an equilibrium profile may not hold for the lower river section, as suggested by some previous experimental and numerical studies (Muto & Swenson, 2005; Sinha & Parker, 1996; Wu & Nittrouer, 2020) that river mouth progradation and sea level rise could prevent river profile from attaining the equilibrium profile, and instead a concave profile (Figure 1a) forms as a transient state (Chang, 1986; Muto & Swenson, 2005; Nones et al, 2019). Although the nearshore water depth and the progradation rate of the river mouth are considered the first‐order controls on the attainment of the equilibrium river profile (Bijkerk et al, 2016; Wang et al, 2019; Wu & Nittrouer, 2020), its actual development essentially depends on the infilling of the accommodation space between the initial and equilibrium profiles (i.e., the “progradation‐induced accommodation space” in this study). The infilling of the accommodation space is further dictated by the interplay between the sediment supply and the generation of accommodation space (Coe et al, 2003; Di Silvio & Nones, 2014; Muto & Steel, 1997), the processes of which still remains largely unclear.…”