Granular bed consolidation, creep and armoring under subcritical fluid flow

TL;DR

This study investigates how granular beds under subcritical fluid flow gradually settle, creep, and become armored over time, revealing long-term relaxation processes and the development of resistance to erosion.

Contribution

It demonstrates that granular beds undergo slow, long-term rearrangements and armoring under subcritical shear stress, linking these processes to deep shear-induced relaxation and volume fraction changes.

Findings

Granular beds creep and settle over millions of times longer than individual grain sedimentation.

Surface roughness remains unchanged despite compaction and creep.

Higher shear stress is needed to erode a bed that has been subjected to subcritical shear, indicating armoring.

Abstract

We show that a freshly sedimented granular bed settles and creeps forward over extended periods of time under an applied hydrodynamic shear stress, which is below the critical value for bedload transport. The rearrangements are found to last over a time scale which is millions of times the sedimentation time scale of a grain in the fluid. Compaction occurs uniformly throughout the bed, but creep is observed to decay exponentially with depth, and decreases over time. The granular volume fraction in the bed is found to increase logarithmically, saturating at the random close packing value $\phi_{rcp} \approx 0.64$, while the surface roughness is observed to remain essentially unchanged. We demonstrate that an increasingly higher shear stress is required to erode the bed after a sub-critical shear is applied which results in an increase in its volume fraction. Thus, we find that bed armoring occurs due to a deep shear-induced relaxation of the bed towards the volume fraction associated with the glass transition.