Self-Similar Morphodynamics of Gilbert and Hyperpycnal Deltas Over Segmented Two-Slope Bedrock Channels

Delta progradation over segmented two-slope bedrocks is prevalent in nature, where the bedrock slope upstream or downstream of the slope-break knickpoint is steepened by continued tectonic uplift or subsidence. Understanding the morphodynamics of the most common types of delta in different slope settings is important for interpreting the observed delta progradation into reservoir and predicting delta evolutions under future tectonic/climate scenarios or anthropogenic interventions. Here, we present an experimental and analytical study demonstrating the morphological responses of Gilbert-type and hyperpycnal deltas to variations of bedrock slopes. Steepening the upstream slope accelerates shoreline migration; steepening the downstream slope decelerates shoreline migration. In either case, the subaqueous volume is enhanced yet the subaerial volume is reduced. Both types of delta exhibit self-similar morphologies when evolving over segmented bedrocks. Hyperpycnal deltas, through enhanced sediment fluxes driven by dense underflows, develop larger subaqueous volumes. We further demonstrate the underlying self-similarities in sediment flux and bed growth rate that come into play for attaining the self-similar morphologies. The combined effect of flowrate (Q) and sediment supply rate (I) may be characterized by a dimensionless Q/I ratio. Increasing Q/I advances the entire delta. For Gilbert deltas, shoreline migration accelerates with Q/I. For hyperpycnal deltas, shoreline migration exhibits a "first-accelerate-then-decelerate" trend with Q/I in a limited range of slope combinations close to the single-slope setting, indicating that the effect of Q/I emerges only when the two-slope effect is weak or absent. Away from this near-single-slope range, the two-slope effect becomes dominant, thus suppressing the effect of Q/I.