A Regime-State Framework for Morphodynamic Modeling of Seabed Roughness

Seabed roughness in the dynamic near-shore environment is continuously modified by the interplay of physical and biological processes including bedform morphodynamics and bioturbation. We introduce a novel regime-state framework to determine the dominant modifier of seabed roughness based on observed or estimated physical forcing, which are classified by critical Shields parameter or wave mobility thresholds. The regime-state framework is applied to a set of field observations containing time-lapse sonar imagery of ripple growth, decay, and bioturbation. The rate of ripple decay varies in both physical and biological processes. The decay rate is sensitive to oscillatory forcing magnitude and direction relative to relict ripple orientation and time-evolving ripple height. We found two distinct episodes of ripples decayed by 20%-30% within 4-5 days. Ripple decay models and observations did not agree in rate of decay up to and following the same time period. By varying decay rate using a time-dependent diffusion coefficient determined by the dominant decay mechanism, the regime-state framework resulted in better agreement to field observations. Benthic megafauna are also shown to create seabed roughness, excavating dense fields (>1 pit/m(2)) of pits (<= 84 cm in diameter) that introduce acoustic roughness comparable to 8-12 cm high ripples. The spatiotemporal variability of megafaunal pit formation suggests diffusion-based bioturbation models are not adequate and megafaunal pits require additional parameterization to mechanistically model. The regime-state framework presented in this study serves as a framework for future investigations and modeling of ripple morphodynamics, bioturbation, and roughness.

Automated summary

This study introduces a novel regime-state framework to determine the dominant modifier of seabed roughness and investigates ripple decay and bioturbation. By analyzing field observations and models, the study identifies mechanisms of seabed roughness variation, providing a direction for future research.