Modeling Deltaic Lobe‐Building Cycles and Channel Avulsions for the Yellow River Delta, China

Abstract

River deltas grow by repeating cycles of lobe development punctuated by channel avulsions, so that over time, lobes amalgamate to produce a composite landform. Existing models have shown that backwater hydrodynamics are important in avulsion dynamics, but the effect of lobe progradation on avulsion frequency and location has yet to be explored. Herein, a quasi‐2‐D numerical model incorporating channel avulsion and lobe development cycles is developed. The model is validated by the well‐constrained case of a prograding lobe on the Yellow River delta, China. It is determined that with lobe progradation, avulsion frequency decreases, and avulsion length increases, relative to conditions where a delta lobe does not prograde. Lobe progradation lowers the channel bed gradient, which results in channel aggradation over the delta topset that is focused farther upstream, shifting the avulsion location upstream. Furthermore, the frequency and location of channel avulsions are sensitive to the threshold in channel bed superelevation that triggers an avulsion. For example, avulsions occur less frequently with a larger superelevation threshold, resulting in greater lobe progradation and avulsions that occur farther upstream. When the delta lobe length prior to avulsion is a moderate fraction of the backwater length (0.3– 0.5trueL‾b), the interplay between variable water discharge and lobe progradation together set the avulsion location, and a model capturing both processes is necessary to predict avulsion timing and location. While this study is validated by data from the Yellow River delta, the numerical framework is rooted in physical relationships and can therefore be extended to other deltaic systems.

Accelerated river avulsion frequency on lowland deltas due to sea-level rise

“…2). The backwater-scaled avulsion node moves seaward or landward in tandem with shoreline progradation or retreat, consistent with field observations (31,56). Consequently, lobe length remains constant, and the competition between shoreline progradation, relative sea-level rise, and sediment supply controls avulsion frequency.…”

“…A spin-up phase was shown to eliminate bias in predicted avulsion locations associated with the assumed initial topography, by reworking the initial topography for at least one avulsion cycle per delta lobe (55). Recent models for backwater-scaled avulsions have included quasi-2D nonuniform flow, lobe aggradation and progradation, multiple cycles of lobe growth and abandonment, and a spin-up phase (55,56). These models provided new insight into the controls on avulsion location on deltas (55) and were validated against field observations from the Huanghe delta (56); however, they have yet to be used to explore how relative sea-level rise affects avulsion frequency.…”

“…Subsequently, the active channel was rerouted downstream and the lowest-elevation abandoned lobe became the new active lobe (SI Appendix, section A). Inactive lobes were unchanged, except for inundation due to relative sea-level rise, approximating a river-dominated delta with negligible reworking from waves or currents (54,56). Every model run began with a spin-up phase in which the river occupied each lobe at least once, thereby eliminating the bias of the imposed initial conditions (55) (SI Appendix, section B).…”

Effect of Small Tidal Fluctuations on the Stability and Equilibrium Configurations of Bifurcations

“…More precisely, the length of the branches modulates how the presence of a tidal fluctuation is felt by the upstream junction. Let us recall that the distance between the bifurcation and the open sea is scaled through a backwater length (i.e., the extent of channel impacted by nonuniform flow, Moodie et al, 2019), given by the ratio between the reference water depth D * ua and the channel slope S a . In order to substantiate our findings, we test the model with some literature data sets listed in Table S1 of the supporting information, comprehensive of bifurcations both found in natural deltas and modeled numerically.…”

Infilling Abandoned Deltaic Distributary Channels Through Landward Sediment Transport

“…The particular lobe and channel that is the focus of this study is the Qingshuigou lobe (Figures 1 and 2). When the Qingshuigou lobe was active, the subaerial deposit rapidly prograded into the Bohai at a rate exceeding 1 km/yr (Chu et al, 2006;Moodie et al, 2019;Wright & Nittrouer, 1995). Winter storms were highly effective at suspending sediment delivered to the delta front, and these storms redistributed sediment along the coastline via wave-and wind-driven currents and downslope by way of gravity flows and slope failures (Wright & Nittrouer, 1995, Wright et al, 1986.…”