Coupled Coastal and fluvial controls on river-delta avulsion dynamics

Abstract

River deltas are among Earth's most dynamic and densely populated landforms, yet the processes that govern their long-term morphological evolution remain incompletely understood. This dissertation investigates three interconnected problems governing delta morphodynamics and river avulsion behavior: (1) how asymmetric wave climates influence delta plan-view morphology and river avulsion dynamics, (2) how geometric differences between river and floodplain longitudinal profiles localize backwater-scaled avulsions on natural deltas, and (3) how geometric and backwater hydrodynamic mechanisms jointly control avulsion localization. Together, these three studies conducted through numerical modeling and remote-sensing analysis reveal that delta evolution emerges from the interaction of fluvial sediment supply, wave-driven alongshore sediment transport, sea-level change, and episodic river avulsions.Chapter 1 examines how wave-climate asymmetry controls delta planform morphology and avulsion tendency on wave-influenced deltas. On wave-influenced river deltas, wave-driven sediment redistribution affects river progradation, and therefore avulsions, while avulsions change where sediment is delivered to the coastline, affecting coastline shape. Coastline shape, in turn, affects sediment redistribution rates and patterns. Here I use a numerical model to investigate how the asymmetry of wave climates affects delta avulsion behaviors, which are coupled with delta shape evolution. Increasing wave-climate asymmetry tends to reduce (increase) the curvature of updrift (downdrift) delta flanks, by increasing (decreasing) local shoreline diffusivity. In the model experiments, reduced shoreline curvature restricts the possible updrift post-avulsion river mouth locations, while increased curvature expands the possible downdrift locations, favoring ‘downdrift avulsions’. However, under some wave climates, local diffusivity on the downdrift flank can become negative, leading to convexity and shoreline accretion, inhibiting downdrift avulsions. Increasing wave heights and decreasing superelevation threshold for avulsions both tend to reduce delta morphologic asymmetry, and therefore avulsion tendency. Chapter 2 investigates whether geometric differences between river and floodplain longitudinal profiles can explain the backwater-scaled localization of avulsions on natural deltas, which is a mechanism proposed theoretically but not previously tested in the field. Avulsions on deltas often occur at distances from the river mouth that scale with the backwater length (channel depth divided by river slope). This backwater scaling could arise from hydrodynamic variability between flood and inter-flood periods that promotes net in-channel sediment deposition, or from geometric differences between river and floodplain profiles. In this ‘geometric mechanism’, a high-curvature zone in floodplain profiles leads to local elevation of the river above the floodplains. A new framework to analyze profile curvatures from digital elevation datasets, applied to the Mississippi Delta, reveals curvature patterns that support the geometric mechanism. An analysis of the timescales for creating and burying high-curvature zones suggests that the geometric mechanism is relevant on deltas built by large, low-slope rivers for which floodplain aggradation is weakly coupled to channel-belt aggradation, producing relatively narrow alluvial ridges. Chapter 3 builds upon the preceding chapters by constructing a unified reduced-complexity modeling framework that allows both the geometric mechanism and backwater hydrodynamic effects to operate simultaneously, enabling their interactions and relative dominance to be systematically explored. I present a numerical modeling investigation using an extended version of the River Avulsion and Floodplain Evolution Model (RAFEM) that can span the conditions expected to favor each mechanism, by incorporating variable floodplain aggradation and enhanced in-channel deposition representing backwater hydrodynamic effects. Backwater-scaled avulsions occur in all experiments, but their spatial distribution depends on the width of floodplain deposition, the magnitude of backwater-enhanced deposition, and the rate of sea-level rise. With narrow floodplain deposition, weakly coupled channel and floodplain aggradation preserves geometric differences in longitudinal profiles, promoting avulsions near a characteristic geometric-avulsion location. When enhanced backwater deposition is added, avulsions localize at the deposition maximum, often producing bimodal distributions that reflect coexistence of geometric and hydrodynamic controls, with the two mechanisms likely augmenting each other. Observations suggest that the two mechanisms jointly govern avulsion localization on the Mississippi River Delta. With wider floodplain deposition, accumulated depositional lobes can elevate floodplains over a broad area, promoting upstream “lobe-complex-switching” avulsions, in addition to backwater-scaled “lobe-switching” avulsions. Sea-level rise, if sufficiently rapid, suppresses lobe-complex-switching avulsions.