Biological and physical modifications to the onset of sediment transport
Abstract
Virtually all mechanistic predictions of landscape evolution are underpinned by predictions of sediment transport rates. Sediment transport via bedload is the primary way by which coarse sediment is transported through landscapes, and thus, accurate prediction of bedload transport represents a fundamental topic in the study of earth surface processes. Specifically, predictions of coarse sediment transport are strongly dependent on the selection of the threshold for motion, or the forcing at which sediment transport begins. However, it is well known that the threshold for motion varies in space and time, and in response to a number of processes. This dissertation aims to identify and quantify the effects of biology and prior flow history on the onset of coarse particle motion through a combination of mathematical modeling, novel physical experiments, and continuous bedload transport measurements in the field. Chapter 2 develops and tests a mechanistic model for the entrainment of coarse sediment by attached kelp, highlighting the significance of biology in setting coastal sediment transport rates. In Chapter 3, we explore the effects of below-threshold, antecedent flow history on gravel bed structure, grain protrusion, and bedload transport through a series of flume experiments. We find that bedload flux, and by extension, the onset of motion, is extremely sensitive to the duration of prior below-threshold flow. This reduction in bedload transport rates is related to the reduction in the population of high protruding grains, related to the pivoting of precariously placed, highly mobile particles into nearby pockets. In Chapter 4, we carry out a systematic analysis of the temporal evolution of the threshold for motion in a natural channel though the analysis of a unique record of continuous bedload transport measurements. We observe that variations in the threshold for motion are significantly linked to previous flow magnitude, highlighting that the onset of motion is a history-dependent quantity. Collectively, these studies provide new observations and approaches towards predicting the temporal and spatial evolution of the threshold for motion, providing a basis for improved bedload transport models.