Bedform Erosion and Roughness: Decoupling Biological and Physical Processes

The influence of bedforms on seabed roughness has driven extensive research on ripples bedforms. While many studies have focused on accurately predicting ripple formation, only recently has research identified the importance of ripple erosion when considering seabed roughness. Of the multitude of potential mechanisms driving ripple erosion, a growing number of field studies have identified two primary agents: bioturbation and weak physical forcing. However, wide variability exists within each mechanism. In bioturbation, smaller bioturbators may drive broad-scale diffusion of sediments, while larger bioturbators serve as stochastic excavators of large seabed pits. Likewise, physical processes for erosion may be driven by either weak oscillatory currents or turbulence, and modified by the degree of forcing and relative direction to antecedent ripple morphology. Moreover, previous studies have typically dealt with physical and biological processes for erosion in isolation, and a cohesive framework linking these processes is lacking. Therefore, this study introduces a simple categorical model to determine what erosion process is dominant based on observed or estimated physical forcing. This categorical process is incorporated into a simple, iterative ripple erosion model based on a Fickian diffusion equation, with the introduction of a time-dependent diffusion coefficient based on evolving near-bed forcing. This study found that, while existing models accounting for either physically- or biologically-driven ripple decay may achieve similar agreement to field observations over certain time scales, the rate of decay is variable and dependent upon local physical and biological processes. If erosion mechanism and variability is not addressed, agreement may be poor over short (e.g. inter-storm) or long (e.g. seasonal) time scales. Further, this study demonstrates that bioturbators are not only significant mechanisms for erosion of seabed morphology, but also contributors to seabed roughness on spatial and temporal scales vital to seabed modeling.