Modeling memory in gravel bed rivers: consideration of a discharge-dependent threshold for motion

Abstract:

Predictions of bedload transport are underpinned by the specification of a threshold for particle motion. While typically assumed to be constant, numerous studies have documented temporal variations in the threshold for motion, of up to an order of magnitude. In particular, the evolution of the threshold for particle motion has been shown to integrate some memory of past conditions, such that accounting for flow history and an evolving threshold may improve predictions of bedload transport. Previously, Johnson (2016) presented a model to describe the evolution of critical Shields stress as a function of net erosion/deposition, which, with some calibration, was able to produced trends with good agreement to experimental datasets.

The evolution of the threshold for motion has been shown by numerous studies to be closely linked to previous flow magnitude. In particular, Masteller et al. (2019) used a high-resolution, multi-year bedload transport time series to highlight this dependence, such that flows that transport little to no bedload result in increases in the threshold for motion, whereas higher magnitude flows that reliably transport sediment ultimately reduce the threshold for motion. Similar dependencies of critical Shields stress on past flow magnitude have been observed experimentally.

In this contribution, we present a discharge-dependent state function to capture the evolution of the threshold for motion under unsteady flows. We discuss the formulation of the model, and in particular, a feedback, or memory, parameter, which modifies the effects of flow magnitude based on the current state of the threshold. We find that a calibrated version of the model captures the general behavior of the threshold for motion when compared to the bedload data presented in Masteller et al. (2019). Using the calibrated model, we compare the evolution of the threshold for motion in response to various, synthetic discharge histories.

We will discuss the implications of a history-dependent threshold for motion with regard to prediction of bedload transport rates, as well as connections to alluvial channel morphology.

In relation to JEDI topics, Masteller will share experiences related to a current project focused on floodplain channels, flooding, and environmental justice, if time permits.