One-Dimensional Hydraulic Theory Applied to Experimental Subaqueous Fans with Supercritical Distributaries

Abstract:

Subaqueous fans are distributive channel systems that form in a variety of settings including offshore marine, sub-lacustrine, and reservoirs. These distributive systems create complex sedimentation patterns through repeated avulsion to fill in a basin. Here we ran a series of experiments to explore the intrinsic controls on avulsion cycles on subaqueous fans. Experiments are a convenient way to study these systems since the time-scale of fan development is dramatically shortened compared to natural settings, all boundary conditions can be controlled, and the experimental domain can be instrumented to monitor the pertinent hydraulic and morphologic variables. Experiments in this study used saline underflows and crushed plastic sediment fed down an imposed slope covered in the sediment. Avulsion cycles are a central feature in these experiments which are characterized by: (1) channel extension and stagnation; (2) bar aggradation and hydraulic jump initiation; (3) upstream retreat; and (4) flow avulsion. Looking at and analyzing these cycles yield the following conclusions: (1) distributive channels cease progradation due to a drop in sediment transport capacity in an expanded region ahead of the channel; (2) mouth bar aggradation leads to a large flow obstacle to cause the hydraulic jump feedback; (3) hydraulic jump regions are a significant locus of deposition; and (4) the upstream retreat rate is a function of sediment supply and the strength of the jump. We found that simple one-dimensional hydraulic principles such as the choked flow condition and the sequent depth ratio help to explain hydraulic jump initiation and emplaced lobe thickness respectively.