Response of coastal plain rivers to falling relative sea-level: Allogenic controls on the aggradational phase

This study combines mathematical modelling and supporting flume experiments to address the problem of how coastal plain rivers respond to a steady fall in relative sea-level. The theoretical component of the study focuses on the development of a moving boundary model of fluviodeltaic progradation that treats rigorously the dynamics of the shoreline and alluvial - basement transition (the upstream limit of the alluvial river system). Dimensional analysis and numerical solutions to the model governing equations together suggest that, at first order, coastal plain rivers will remain aggradational on a timescale that varies with allogenic sediment and water supply and the fall rate of relative sea-level. In natural fluviodeltaic systems, this intrinsic timescale is likely to vary by several orders of magnitude, suggesting that the aggradational phase of river response can be geologically long-lived. At second order, the duration of alluvial aggradation is controlled by two dimensionless numbers that embody system geometry and the kinematics of alluvial sediment transport. Model predictions were tested in a series of carefully scaled flume experiments. The level of agreement between predicted and measured trajectories for the shoreline and alluvial - basement transition strongly suggests that the moving boundary theory developed here successfully captures the response of small-scale fluviodeltaic systems to falling sea-level. The results of this study have several sequence-stratigraphic implications: a fall in relative sea-level at the shoreline is not a sufficient condition for river incision; the onset of alluvial degradation and sequence-boundary formation need not coincide with a maximum in the rate of sea-level fall; and the onset of sequence-boundary formation is sensitive to allogenic sediment supply.